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Roach et al. These waves are particularly energetic at intraseasonal periods [ Spillane et al. Recently, Jacobs et al. Jacobs et al. The wave is characterized by coherent oscillations in SST, sea level pressure, meridional winds, and sea ice extent. The tropical Atlantic is characterized by a prominent mean seasonal cycle in surface winds, sea level upper ocean currents, and temperatures [e. In addition, two important modes of interannual-to-decadal variability are evident around this seasonal cycle, one of which consists of warm events with variability concentrated near the equator [ Philander , ; Houghton , ; Zebiak , ; Carton and Huang , ] and another of which consists of interhemispheric variations in tropical SST [ Moura and Shukla , ; Servain , ; Houghton , ; Houghton and Tourre , ].

Dynamics intrinsic to the ocean-atmosphere-land system in the Atlantic basin are important in determining the variability associated with these low-frequency climate signals. However, ENSO teleconnections through the atmosphere influence their evolution as well, as discussed by Servain [] , Delecluse et al. Variability in the Indian Ocean is dominated by a pronounced seasonal cycle related to monsoon wind forcing [ Rao et al. However, interannual anomalies on ENSO timescales are detectable as well [e. Tourre and White's [] simultaneous analysis of upper ocean thermal data in all three tropical ocean basins indicated what appeared to be a coherent eastward propagating interannual wave in upper ocean heat content near the equator.

On the strength of this result they suggested the possibility of oceanic precursors to ENSO in the Indian Ocean thermal field, in addition to atmospheric precursors believed to be important in association with the monsoons [ Webster and Yang , ]. Latif and Barnett [] , on the other hand, argued that the Pacific forces the tropical Indian and Atlantic Oceans remotely through atmospheric teleconnections on ENSO timescales and that this forcing accounts for a significant percentage of the observed thermal variability described by Tourre and White [].

Lukas and Lindstrom [] proposed that salinity variability of the upper ocean may be an important determinant in the evolution of ENSO. They hypothesized that in regions of heavy rainfall, thin surface mixed layers form which are isolated from the upper thermocline by salt stratified "barrier layers. Thus, barrier layer formation would favor warm SSTs in regions of heavy rainfall, thereby coupling the hydrologic cycle to the upper ocean heat balance.

Barrier layers have been detected in all three tropical oceans. Reynolds, and D. In other words, the atmospheric response to changing patterns of sea surface temperature extends to high altitudes owing to the influence of tropical convection. Diabatic heating associated with latent heat release and radiative effects of clouds have a profound influence on even the largest-scale circulation systems in the atmosphere [ Hartmann et al. While the mean zonal winds in the tropics are usually easterly, we observe substantial westerlies recurring periodically in the upper troposphere. These westerlies develop on an annual basis during the northern hemisphere winter months.

These observations are consistent with a strengthening of the Walker circulation during cold events and a weakening of the Walker circulation during warm events. After Gage et al. Zonal winds over Christmas Island are typically easterly at all heights during the northern summer. The annual variation of the zonal winds observed at Christmas Island is in phase with the annual cycle of tropical convection over the western Pacific and is consistent with a strengthening and weakening of the Walker circulation driven by convective heating over the western Pacific warm pool region [ Gage et al.

The depth of the upper tropospheric westerlies is likely due to the deep tropical heating associated with mesoscale convective systems [ Hartmann et al. Figure Low-pass-filtered composite annual cycle of zonal winds observed at Christmas Island. Vertical motions are rarely observed directly in the atmosphere [ Balsley et al. This is partly due to the difficulty in measuring very small motions, but the measurement problem is complicated by the presence of internal gravity waves that can mask the small long-term mean vertical motions or otherwise bias observations [ Nastrom and VanZandt , ].

Wind profiler direct measurements of vertical velocities in the tropics have confirmed some expectations at the same time they have raised new questions. The adiabatic warming consistent with the observed magnitude of subsidence is what is required to balance radiative cooling to space [ Gage et al. While they have not been in use as long as the VHF profilers, the UHF profilers have already proven to be valuable tools for atmospheric research [ Angevine et al.

High-resolution time and height observations by UHF profilers have improved our knowledge of vertical structure and temporal variability of lower tropospheric winds in the tropics [ Gutzler et al. For example, Deser [] and Gutzler and Hartten [] have used the profiler observations to obtain a more complete picture of the daily variability of the lower tropospheric winds at a number of locations in the Pacific. Recently, it has become evident that UHF profilers can provide valuable information about precipitating cloud systems [ Gossard , ; Rogers et al.

In the presence of precipitating loud systems the height coverage of the profilers is greatly increased. With the large amounts of data obtained from the tropics using UHF profilers at a number of locations, it is now possible to begin to construct the climatology of precipitating cloud systems in the western Pacific.

Used in conjunction with a VHF profiler, the UHF profiler can provide precipitation fall speeds relative to background vertical air motions [ Currier et al. TAO data have also been of value in studies of atmospheric dynamics. For example, Hayes et al. Accounting for these variations in vertical stability in diagnostic studies allows for a more dynamically consistent interpretation of oceanic effects on boundary layer winds in the equatorial Pacific [ Nigam and Chao , ].

Zhang [] used TAO data to document surface manifestations of the Madden and Julian Oscillation in the atmospheric boundary layer of the western Pacific. He found inconsistencies, as did Jones and Gautier [] and Flatau et al. As a result, Flatau et al. Their modified theory allowed for time varying SST feedbacks to the atmosphere in response to intraseasonal heat flux forcing of the ocean, which led to a better simulation of the Madden and Julian Oscillation in a simple coupled ocean-atmosphere model.

TAO data have been used to examine the role of mesoscale enhancement of surface turbulent fluxes [ Zhang , ; Esbensen and McPhaden , ] and the related issue of convection-evaporation feedbacks [ Zhang et al. The TOGA observing system provided a broad geographical perspective and long time history to aid in the interpretation of the measurements from these shorter-duration, regional-scale field programs.

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In many cases the TOGA observing system was enhanced to facilitate these process studies. Several TAO moorings in the western Pacific were also equipped with special sensors to measure salinity, rainfall, and incoming shortwave radiation in an effort to better understand surface fluxes in relation to upper ocean variability [ Cronin and McPhaden , ]. During TIWE, moored data were used to estimate diurnally varying vertical heat fluxes associated with that mixing [ Bond and McPhaden , ], and a large number of drifters were deployed to provide additional information on the structure of instability waves [ Flament et al.

Moored and drifting buoys were deployed with bio-optical sensors during JGOFS to document physical controls on primary productivity in the equatorial Pacific [ Foley et al. However, as the program matured, a number of factors contributed to the development of a mutual dependency between TOGA models and observations. At a very basic level, data were needed for the development and validation of oceanic, atmospheric, and coupled models.

Moreover, as experimental forecasts of ENSO became more routine and as initialization and assimilation techniques for coupled models took on greater importance, the modeling and observational components of TOGA developed a more intricate relationship. Overviews of the interaction between tropical models and data can be found in work by Latif et al. We concentrate here specifically on the evolution of this partnership toward improved model-based analyses and better coupled model initial conditions for predictions.

When considering the initialization of tropical ocean models and coupled prediction models, there are several factors that are critical. First, the tropical oceans are to a certain extent deterministic, by which we mean that adequate knowledge of past forcing in principle allows us to largely determine the state of the ocean. Knowledge of the surface wind stress is paramount in this determination. For example, Busalacchi and O'Brien [, ] demonstrated that, with a reduced gravity model and surface stress, one could capture key aspects of sea level variability associated with ENSO.

Studies with ocean general circulation models OGCMs [e. This fact makes the analysis and initialization problem quite different from that of numerical weather prediction, where there is no counterpart to the external forcing and its associated errors that are imposed through surface wind stress. While in theory it is feasible that coupled tropical forecast models could be initialized with wind stress alone, practical considerations suggest that ocean thermal data will also be important.

This is because wind stress and upper ocean thermal structure are partially redundant, so that observing and initializing baroclinic equatorial wave modes with subsurface temperature data could help correct some of the deficiencies in the imposed wind forcing. SST observations, either through assimilation or via surface boundary constraints, have also been important for the development of both the atmospheric and oceanic components of coupled prediction models.

The ready availability, spatial coverage, and accuracy of SST analyses makes this variable particularly valuable for model validation and development [e. From a historical perspective, sea level data has made one of the more significant contributions to ocean model development, particularly as equatorial theory was developing prior to TOGA.

In situ sea level data continue to provide important model validation, particularly as sea level variations represent an integral, low baroclinic mode response to wind forcing and thermodynamic adjustments. With the advent of satellite altimetry, giving the spatial coverage not possible with in situ instrumentation, sea level may well assume far greater importance for model initialization. Early studies in TOGA pointed to the advantages of thermal mass information vis-a-vis velocity information for ocean model initialization [ Moore et al.

Hence, in a modeling context, velocity data have been used mostly for validation purposes [e. Various other data sets, such as those for salinity and surface heat fluxes, have also played important though somewhat less critical roles in model development. Consistent with these considerations and with the discussion in section 2. It was known that VOS winds would be useful but likely inadequate, but it was not immediately clear whether improved analysis techniques and improved numerical weather prediction schemes would make up for some of these inadequacies [e. Harrison et al.

Overall, the research analyses [ Goldenberg and O'Brien , ; Sadler and Kilonsky , ] produced more realistic dynamic responses but less convincing SST results for the equatorial waveguide. Details aside, one of the most important conclusions of these studies as far as TOGA was concerned was that improved knowledge of the surface wind stress was essential. Operational atmospheric weather analysis and forecast models routinely merge observations of different parameters e. These analysis and forecast systems produce a dynamically consistent model atmosphere with high temporal and spatial resolution.

For this reason the surface wind fields from such systems are often used to force ocean models like that run at NCEP for near-real-time tropical ocean analyses. Improving the quality of operational atmospheric model-based wind analyses is therefore an issue of some importance to climate modelers. For example, the U. Phoebus et al. Gaffard and Roquet [] found that the ERS-1 and ERS-2 vector winds improved the analyses in the southern hemisphere and had some positive impact in the short-range forecast.

TAO data are also used in operational weather forecast systems. In addition, the impact of TAO observations tended to weaken significantly if the model was not reinforced with new TAO observations every 6 hours. Anderson [] pointed out that, in general, single level surface data like those from TAO buoys can be expected to have a relatively low impact on the atmospheric weather analyses.

Reynolds et al. They found that the analyses looked more like each other than like the data. However, the models themselves have problems, as pointed out in a study by Williams et al. They compared wind profile data at Christmas Island with the ECMWF forecast model and found that the model and the data were consistent above 1.

The model winds at these lower elevations were too weak and did not properly turn with height. This result suggests that there are problems in the model tropical boundary layer and that model and analysis systems need to be improved to optimize assimilation of tropical surface winds. These decade-long, internally consistent model analyses are produced using state-of-the-art numerical models, assimilation systems, and the most complete data sets available from historical archives [e.

These analyses are valuable for providing initialization and validation fields for coupled model predictability studies, for determining the sensitivity of atmospheric models to slow variations in the surface boundary conditions, and for diagnostic studies of atmospheric variability. Evaluations of these reanalyses products are currently underway [e. Two studies illustrate this approach and the impact that TAO data make on such analyses. The other baseline wind product was the FSU winds.

Wind observations at each TAO location were converted to wind stress using the stability dependent parameterization of Liu et al. For example, the point labeled represents the cross correlation from through A similar study was done by Reynolds et al. An objective analysis procedure [see Lorenc , ] was used to correct each of the wind fields with TAO data. NCEP stresses were too weak i. After Ji and Leetmaa []. However, in contrast to Menkes and Busalacchi [] , they used the general circulation model reported by Ji et al.

Results showed that assimilation was able to compensate for wind stress differences. Without assimilation, though, the ocean model was more affected by the different wind stress forcing. In particular, it was possible to clearly determine that ERS-1 zonal wind stresses were too weak in the eastern equatorial Pacific. The Menkes and Busalacchi [] and Reynolds et al. However, in combination these studies indicate that TAO data have the strongest positive impact on the wind stress fields that are most independent of the mooring data.

Keys to achieving this were the vastly improved data coverage from the TOGA observing system, more effective data management strategies allowing rapid access to observations, order-of-magnitude improvements in computing capacity and resources, and improvements in ocean models. Prior to TOGA, most oceanic observations were obtained from VOS lines, a handful of moorings and circulation drifters, and occasional research cruises. With the exception of SST, which could also be retrieved from satellite, it was essentially impossible to produce basin-scale ocean analyses from observations alone.

With increased data coverage during TOGA, including a greatly enhanced volunteer observing ship network and the TAO array in the equatorial Pacific, regular and routine subsurface ocean analyses became possible. All the data analysis and assimilation systems depend on knowledge of the amplitude and spatial and temporal scales of variability.

Scale analyses, such as those of Meyers et al. While the practical application of such information is not always straightforward, particularly when the first guess is provided by a dynamical model, it does nevertheless represent the fundamental basis for most of the applications described below. This system uses optimal interpolation and a simple statistical forecast model to produce global upper ocean temperature analyses at periods from 10 days to 2 months, utilizing data from XBTs and TOGA-TAO.

All quality control is objective [ Smith, ] on the basis of information derived from the statistical interpolation. The shorter-period analyses were shown to retain all the important low-frequency, large-scale information of the bimonthly analyses the analysis period upon which much of the TOGA observations were planned as well as much of the interesting high-frequency fluctuations [ Smith , b ].

Dynamic ocean models have been used to simulate basin-scale ocean circulations long before TOGA. While such simulations did not usually ingest ocean information, they did represent an alternative route to ocean analyses, whereby information in the applied surface boundary forcing principally the wind stress was used to indirectly infer the state of the ocean.

The main problems with model simulations were the poor quality of surface forcing, because of a lack of wind observations over the open ocean, and errors in the ocean model physical parameterizations. Limited in situ observations were primarily used for validation of model results. Although the quality of surface winds has improved steadily, especially since the TOGA observing system increased surface wind observations in the tropical Pacific, errors in the winds and in ocean models still significantly limit the accuracy of the simulations [ Ji and Smith , ].

One way to compensate for errors in wind stress forcing and ocean model physics is to use data assimilation techniques to combine observations and model fields to yield the best possible estimate of the ocean state. Data assimilation has been an active area of research from well before TOGA, although most practical applications were in the field of meteorology.

Advances in ocean data collection, communication, and modeling in the late s and early s made ocean data assimilation a feasible option. Several studies have examined the problem of ingesting ocean subsurface data into simpler, linear, shallow water models of the tropical ocean [e. All these studies showed that subsurface sampling as practiced during TOGA could be used to correct model and wind-forcing errors and that the time taken for correction was only a month or so, owing to the rapid communication of information by equatorial waves. An early attempt to produce routine ocean analyses utilizing an ocean data assimilation technique was a system developed by Leetmaa and Ji [] for the tropical Pacific.

This system used wind-forced ocean model simulation as a first guess and combined the observations collected during a period of 1 month with the model field using the optimal interpolation. The data assimilation procedure was done monthly. The main advantage to the model-based analyses is that large areas of data void are filled in by model dynamics. The main drawback to the sequential initialization method is that the data assimilation can introduce a strong shock when corrections are applied to the model fields, as discussed by Moore []. Also, for models integrated forward in time until the next data assimilation cycle without continuous constraint by observations, model fields will drift toward the model's own equilibrium state.

Hence a "sawtooth" pattern in the time history of the analyses is sometimes obvious [e. A data assimilation system developed by Derber and Rosati [] was a significant improvement over earlier ocean analyses. This system is based on a variational method in which assimilation is done continuously during the model integration. Corrections to the model are spread over a long period of time; thus change to the model temperature field during each model time step is incremental. This significantly reduces the impact to the dynamical balances of the model fields and also keeps model fields from drifting toward their own climate.

This procedure significantly increases the influence of observations to compensate for the lack of spatial and temporal data coverage in many areas. The drawback in doing this is that it tends to limit the analyses to resolving only large spatial scales and low-frequency phenomena [ Halpern and Ji , ]. An operational model-based ocean analysis system based on the data assimilation system of Derber and Rosati [] has been implemented at the NCEP [ Ji et al. Real-time observations from satellite, VOS ships, and drifting and moored buoys are assimilated into an ocean general circulation model to produce near-real-time weekly mean Pacific and Atlantic analyses.

Retrospective monthly Pacific Ocean reanalyses have also been generated at NCEP by forcing the ocean model with historical monthly wind-stress analyses produced at Florida State University [ Stricherz et al. Comparisons with in situ observations of moorings and tide gauges suggest that the model-based analyses are of higher accuracy than the wind-forced simulation [ Ji and Smith , ]. These studies show that even when using a high-quality wind stress forcing and a state-of-the-art ocean general circulation model, ocean data assimilation can still further improve the quality of analyses by compensating for errors in the forcing and model.

From Ji and Leetmaa []. There are, however, some significant differences; the NCEP analysis of the cooling is characterized by a coherent west-to-east evolution, whereas the BMRC analysis shows essentially in-place cooling. A promising way of improving tropical ocean model-based analyses is through the assimilation of altimetry data [see, e. This requires projection of sea level variability onto baroclinic ocean thermal structure, which can be readily done by developing empirical relationships between the two variables [e.

Impact studies of altimetry assimilation on ocean general circulation model-based analyses have also been performed Carton et al. Assimilation of observations obtained from the TOGA observing system not only provides means to produce much improved ocean analyses, it also provides a great opportunity for improving the definition of the initial ocean fields for prediction of ENSO using coupled models. This is discussed in section 4.

Analyses such as those described above have also found a wide range of other applications. For example, Lukas et al. The use of model-based analyses for process studies is now quite common in meteorology, and the advances in ocean analysis and assimilation during TOGA will assist in making such applications more common in climate studies. Finally, analysis systems have been used to examine the design of the TOGA subsurface observing system. Miller [] investigated the impact that ocean thermal data processed to estimates of dynamic height might have in hindcasts of sea level in the equatorial Pacific.

His results suggested that the TAO array would positively impact on hindcasts of monthly mean sea level. Three different methods are presently used for initialization of the ocean for ENSO predictions using coupled ocean-atmosphere models. The first method, used by Cane et al.

A second method uses assimilation of subsurface temperature data together with surface wind forcing to achieve better defined subsurface ocean states. A third method developed at BMRC utilizes both wind and subsurface data jointly to initialize a coupled model through an adjoint data assimilation method [ Kleeman et al. Assimilation experiments described in the previous section illustrated the need to assimilate data in such a way that initialization "shock" is minimized.

On the other hand, these studies demonstrated the potential impact of data assimilation on the forecast of eastern equatorial Pacific SSTs several seasons into the future. Ji and Leetmaa [] , for example, compared results from forecast experiments initiated from ocean initial conditions produced with data assimilation and produced with wind forcing alone, using the NCEP coupled ocean-atmosphere forecast model [ Ji et al.

This comparison demonstrates convincingly that data assimilation has a significant positive impact on improving ENSO forecast skill. The results indicate significant positive impact of the TAO buoy data, largely due to the vastly improved spatial and temporal data coverage by the TAO array in the tropical Pacific. Solid dash-dot lines are for forecasts initiated from ocean initial conditions produced with without subsurface data assimilation.

Kleeman et al. In this study the adjoint for the ocean component of the coupled model was used to improve the ocean initial conditions by finding a condition that was consistent with both the wind forcing and the subsurface ocean thermal data. Two sets of experiments were performed for the period January through October In the first experiment the ocean initial conditions were obtained by forcing the ocean model with the FSU winds. This initialization procedure was consistent with that of Cane et al.

The use of the ocean data assimilation in this case led to notable increases in forecast skill. Altimetry data, in addition to upper ocean thermal data, likewise have the potential for improving the skill of short-term climate predictions. Ji, R. The sea level data improved the agreement of the model sea level with independent tide gauge data and led to a more realistic forecast of tropical Pacific SSTs.

On the other hand, predictability experiments using the Zebiak and Cane [] coupled model indicated that forecast errors were not reduced by using altimetry data for ocean model initialization [ Cassou et al. Thus, the utility of altimetry for initialization is model dependent, so that more research will be required to fully exploit altimetry for ENSO prediction. In the previous initialization studies the oceanic component was first forced by observed wind stress and adjusted by assimilating subsurface thermal observations.

Subsequently, the model-simulated SST was used to force the atmospheric component. However, a potential problem with this common approach is that since there are no interactions allowed between the oceanic and atmospheric components during initialization, the coupled system is not well balanced initially and may experience a shock when the forecast starts. Further, the imbalances between the mean states of the oceanic initial conditions and the coupled model contribute to systematic error of the forecast fields [ Leetmaa and Ji , ].

In the study by Chen et al. Initial conditions for each forecast are obtained by running the coupled model for the period from January up to the forecast starting time. At each time step prior to the forecast a simple data assimilation procedure is used whereby the coupled model wind stress anomalies are nudged toward the FSU wind stress observations. In this manner the coupled model itself is used to dynamically filter the initial conditions. Initialization shocks are reduced by providing a better balanced set of ocean-atmosphere initial conditions for the coupled forecast.

Previously, the ocean initial conditions contained considerable high-frequency energy when forced by the FSU wind stress anomalies. The influence of the coupled model in the new initialization preferentially selects the low-frequency, interannual variability. This approach also results in a shallower thermocline in the western equatorial Pacific during most ENSO warm events with important implications for improved forecasts of warm event termination.

Moreover, the coupled approach to initialization eliminates the springtime barrier to prediction that characterizes most coupled forecast schemes. Recently, decadal-scale variability in the forecast skill has been noted in coupled models. Chen et al. Balmaseda et al. It is also likely that the present generation of prediction models does not adequately represent the full range of physical processes responsible for the ENSO cycle or the interaction of ENSO with decadal time scale variations. These limitations could contribute to decadal fluctuations in predictability as well. Although most data assimilation efforts in support of coupled models have focused on improving initial conditions, data assimilation techniques such as the Kalman filter have also been used as a means of parameter estimation in simple coupled models.

In idealized versions of intermediate coupled models, there exist key parameters that govern the coupling strength between SST and the surface winds and the relation between the depth of the thermocline and the temperature of the water entrained into the ocean mixed layer.

The particular values of these coefficients tend to determine the behavior of the coupled mode characteristic of the system. Similar to the way in which assimilation techniques have been used to estimate parameters such as the phase speed in shallow water models, the work of Hao and Ghil [] demonstrated how subsurface thermal data from the TAO array could be assimilated into coupled models to guide the proper estimation of key model parameters.

Discussion and Conclusion The preceding sections have described the evolution of the TOGA observing system and how it has contributed to scientific progress in studies of short-term climate variability during the TOGA decade. Development of this observing system was a major technological achievement, which revolutionized climate monitoring programs by stimulating increased demand for real-time ocean data delivery. The data from this observing system were essential to fostering advances in many aspects of TOGA research, including the following: 1 documentation of the ENSO cycle and related phenomena, such as the mean seasonal cycle and intraseasonal variability, with unparalleled resolution and accuracy; 2 testing of ENSO theories, such as the delayed oscillator; 3 development of new theoretical concepts relating to ocean-atmosphere interactions on seasonal-to-interannual timescales; 4 development of oceanic, atmospheric, and coupled ocean-atmosphere models; and 5 development of ocean data assimilation systems for improved climate analyses and for initializing climate prediction models.

Subsequently, warm SST anomalies and associated westerly wind anomalies weaken and eventually disappear by the following May. Significant differences in duration, phasing, and spatial warming patterns observed during events of the s and early s defy easy categorization. Moreover, South American coastal warming did not generally precede maximum SST anomalies in the equatorial cold tongue, as in the Rasmusson and Carpenter [] composite. Consistent with the complexity of the observed interannual variability, tests of ENSO theories using data prior to and during the TOGA decade suggest that more than one set of mechanisms can give rise to ENSO timescale warm and cold events in the tropical Pacific.

The delayed oscillator theory, for example, can often, but not always, be invoked to explain the termination of ENSO warm events. On the other hand, delayed oscillator physics cannot generally account for the onset of warm ENSO events. New physical hypotheses are being formulated regarding the ENSO cycle, based on the failure of existing theories to explain the full range of observed variability. The persistent warm anomalies are the reflection of a decadal timescale variation that has higher latitude manifestations in North and South Pacific SSTs [e.

In either case the decadal timescale of this variation and its manifestations at higher latitudes suggest a link to decadal timescale processes that maintain the equatorial thermocline [ Fine et al. These processes involve the ocean thermohaline circulation which couples the tropical ocean to the subtropical and higher-latitude North and South Pacific Ocean [e. Decadal timescale variations in the overlying atmospheric circulation at midlatitudes [ Trenberth and Hurrell , ; Latif and Barnett , ; Zhang et al. A theory for self-sustaining decadal time scale oscillations involving ocean-atmosphere interactions and heat transports between the tropical and extra-tropical oceans has been proposed recently by Gu and Philander [].

Average SSTs in the tropical Pacific were unusually high during the s and s, at the same time that there was a trend for warmer global surface air temperature. Two recent studies [ Kumar et al. Tropical Pacific SSTs in these simulations were prescribed from observations, however. There is no consensus on this issue, and recently, Cane et al. Clearly, resolution of the questions concerning ENSO, decadal variability, and anthropogenic greenhouse gas warming will require considerably more research. TOGA demonstrated the synergy that can emerge from the combined use of data and dynamical models.

As a measure of progress, prior to TOGA, there was no system of routine data assimilation for tropical ocean climate analyses and no routine short-term climate prediction efforts. However, during TOGA, models were used to help design the observing system, and data from the observing system were then used to foster model development and to initialize models for short-term climate prediction.

Now many ENSO prediction modeling groups have been established [ National Weather Service , ], and prediction models, initialized with TOGA data sets, show significant skill for lead times of up to 1 year. The skill of these predictions is likely to improve as we learn more about the underlying dynamical processes involved in ENSO and as models and assimilation systems improve.

TOGA also demonstrated the synergy that can emerge from the combined analysis of satellite and in situ measurements. In situ measurement systems provide high-accuracy information on both surface and subsurface ocean variability, the latter of which is not directly accessible to satellites. In situ measurement systems also provide necessary data for ongoing calibration and validation of satellite retrievals.

The strength of the satellite data, on the other hand, is their near-global coverage and uniform time-space sampling characteristics. Coordination between agencies and countries sponsoring satellite missions did not always succeed because of uncertainties in funding, payload development, and launch dates. This lack of coordination led to a 2-year gap in altimeter measurements between the U. Navy Geosat mission and the ERS-1 mission.

Nonetheless, the tremendous value of those satellite data that were acquired during TOGA bodes well for the future application of satellite measurements to ocean climate studies. As a result of TOGA, we are now entering a new era of climate research and forecasting. Also, a newly instituted International Research Institute for Climate Prediction IRICP will begin to issue routine short-term ENSO forecasts, conduct research on ways to improve those forecasts, and help to coordinate the use of the forecast products for various socioeconomic applications [ International Research Institute for Climate Prediction Task Group , ].

Likewise, some national meteorological centers are already routinely issuing climate forecasts [e. The success of these research and forecasting activities requires that essential elements of the TOGA observing system be continued for the foreseeable future. Explicit guidance on the development of post-TOGA climate observing systems is contained in the reports of various planning committees that have considered the observational needs of future climate programs [e.

These reports are unanimous in their recommendations to continue the observing system developed under TOGA in support of short-term climate prediction. Effecting this transition will be challenging because there is no precedent for institutionalizing an observing system built entirely within the framework of a climate research program. These emerging international programs, modeled loosely on the World Weather Watch for weather forecasting, are intended to foster and coordinate measurements for a wide range of climate applications.

As national commitments were essential in developing the TOGA observing system, so will they be essential in maintaining the observing system after TOGA. GOOS and GCOS are at different stages of evolution in different countries involved in supporting climate observations, complicating coordination at the international level. Therefore, in the near term, it is almost inevitable that the post-TOGA observing system will be maintained under a mix of research and operational support.

In the meantime it is of paramount importance that the existing data stream not be interrupted. Tremendous effort was expended in developing an adequate infrastructure to support the collection of critical data sets during TOGA. This infrastructure, involving cooperative relationships between research institutions and government agencies in several countries, was established through painstaking evaluation and oversight by the international scientific community over the course of 10 years. This infrastructure is fragile; premature curtailment or disruption of observational efforts could have disastrous and long-lived effects on the development of future climate observing systems.

Thus a conservative approach must be adopted in recommending changes to either observational strategies or to the organizational framework in which the observations are supported. Conservatism does not imply that the observing systems for post-TOGA climate studies should be static in their design, though. On the contrary, the observing system should be flexible enough to take advantage of new advances in technology. Likewise, it is essential that there be ongoing assessments of the observing system design and that these assessments be guided by scientific priorities.

Clearly, adequately observing the tropical Pacific was a sine qua non for making progress on understanding and predicting ENSO.

U.S.-Japan Technology Linkages in Transport Aircraft

In contrast, scientific questions relating to the climatic impacts of ocean-atmosphere interactions were not as thoroughly explored in the other two ocean basins, and resources were too limited to allow for uniform development of observing system components throughout the global tropics during the TOGA decade. Nonetheless, as a consequence of TOGA, our understanding of ocean-atmosphere interactions in the Indian and Atlantic Oceans has significantly improved.

The relationship of these decadal variations to ENSO and to global climate variability, in general, needs to be better understood. Thus geographic expansion of in situ observational efforts should be carefully considered as part of the post-TOGA climate research agenda.

At the time, most in situ oceanographic data were available for analysis only months, or in some cases years, after they had been collected. So only a handful of scattered reports from islands and volunteer observing ships were available to track conditions in the equatorial Pacific in real time delay of less than a day or near-real time delay of less than a month. However, they were discounted as erroneous for several reasons. These analyses indicated that the equatorial Pacific SSTs were near normal, or even slightly colder than normal, during much of These eruptions injected a cloud of aerosols into the lower stratosphere, where prevailing winds spread it around the globe at low latitudes within 3 weeks.

The aerosols, whose effects were not included in algorithms to convert observed satellite radiances to SSTs, led to cold biases of several degrees centigrade in the satellite SST retrievals. Cloud detection algorithms interpreted these retrievals as cloud contaminated and replaced them with climatological mean SSTs. In situ data were then rejected because they differed so greatly from the satellite analyses, which were strongly biased toward climatology.

Based in part on these early successes, original plans in TOGA called for a small number of moorings to be deployed near the equator and in gaps between widely spaced XBT lines [ U. TOGA Office , ]. However, significant cost savings were achieved by eliminating current meters from the suite of instrumentation and by targeting temperature rather than velocity as the primary oceanographic variable.

Elimination of current meters, whose moving parts rotors, vanes, or propellers were sensitive to mechanical wear and biofouling in the energetic and biologically productive upper layers of the equatorial Pacific, also extended the expected lifetime of the mooring from 6 months to 12 months. In , real-time winds were added to the ATLAS system, adapting earlier design concepts developed for real-time wind measurements from current meter moorings [ Halpern et al.

ATLAS sampling and data transmission schemes have evolved with time. The current generation ATLAS telemeters all data as daily averages and, in addition, as hourly values for SST and surface meteorology coincident with three to four satellite overpasses per day. Data are also internally recorded and available upon recovery of the mooring system. A recent assessment of instrumental accuracies indicates errors of about 0. The estimate of wind speed error unlike the other estimates does not take into account possible calibration drift for instruments deployed at sea for up to one year.

An assessment of this drift is presently underway, and preliminary results suggest that including it may lead to an overall accuracy of about 0. The early technical successes of the ATLAS mooring program and the recognized value of the data for short-term climate studies led to multinational plans for a basin-scale expansion of the array during the second half of TOGA [ National Research Council , ].

This expansion was feasible because the relatively low cost of the ATLAS mooring allowed for its deployment in large numbers and because the 1-year ATLAS design lifetime made for manageable long-term maintenance costs and ship time requirements. These moorings were concentrated on the equator where direct measurements would be most valuable in view of the limited applicability of the geostrophic approximation.

Siting was based in part on historical precedent i. It became apparent, however, as the ATLAS program expanded, that the current meter mooring and ATLAS mooring programs should be integrated more fully for a variety of technical, logistic, and scientific reasons. Details of current meter mooring design, sampling characteristics, and instrumental accuracies can be found in work by Halpern [a, c] , McPhaden et al. Design criteria for the TAO array were based on general circulation model simulations of wind-forced oceanic variability and on empirical studies of space-time correlation scales as described in section 2.

The array was built up over time and maintained through a series of research cruises at roughly 6-month intervals. These cruises were necessary to deploy new mooring systems and recover old mooring systems that were close to or past their design lifetimes. The data are then available to operational meteorological and oceanographic centers around the world.

The rapid growth of the TAO array during the second half of TOGA has led to improvements in the quality of several important operational climate analysis and prediction products. Worley, personal communication, Figure B1: GTS ship small solid dot and buoy open circles wind reports used in the Florida State University Pacific surface wind analysis for the month of December Legler, Development of the TAO array required an extraordinary effort from individuals and institutions in several countries, at the core of which was sustained support provided by the United States, Japan, France, Taiwan, and Korea.

As one measure of effort, accumulated over the 10 years between and , more than TAO moorings were deployed on 83 research cruises involving 17 ships from six different countries, requiring a total of 5. At present, nearly 1 year of dedicated shiptime per calendar year is required to maintain the fully implemented array of nearly 70 moorings. Scientific use of TAO data has been encouraged by the development of sophisticated data management, display, and dissemination capabilities.

Drifters In the early s the Argos Doppler ranging system became operational on National Oceanic and Atmospheric Administration NOAA polar orbiting weather satellites, and a cost-effective technique of listening to and locating radio transmitters on the global ocean surface was made available to oceanographers. This spawned the design and construction of a large number of ocean surface drifters, both for measuring ocean circulation as well as for use as platforms for deploying a variety of meteorological sensors. Small arrays of FGGE drifters with drogues were also deployed in the tropical Pacific as part of the EPOCS program between and , with the main purpose of understanding eastern tropical circulation.

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A potentially valuable tool was the Argos-tracked drifter, but several serious questions arose regarding the feasibility of designing an affordable instrument that could be deployed in global arrays. The drifters used during FGGE were too heavy to be routinely deployed from merchant ships or by air; they were very costly to build and did not retain their drogues longer than several months.

No mechanical design improvements had been made to them since There were no engineering standards or field-verified hydrodynamic models by which to design a Lagrangian drifter in order that its water-following capability could be determined to the accuracy required by TOGA. Finally, the tariffs charged by Service Argos, the firm which had the exclusive right to decode Argos location data, would severely limit the extent of a global, long-term deployment.

To meet TOGA objectives, a two-pronged program of drifter deployments was developed, as described below. Surface Velocity Program SVP In a group of oceanographers and engineers met at the National Center for Atmospheric Research to consider the challenges presented by the WCRP requirements for global ocean and atmosphere monitoring and to determine how a variety of newly designed ocean Lagrangian tools could be used to meet these needs.

It was decided that a low-cost, lightweight surface drifter should be developed. Several modeling studies of drifter behavior in steady upper layer shear and linear gravity wave fields were also done [ Chabbra et al. These studies provided a rational basis for the interpretation of the drogue slip measurements in the field. By several SVP drifter designs had emerged and were being used in research programs in the Atlantic and Pacific. Its technical objectives were to use VOS ships to maintain an array of drifters for a 3-year period and to select from the competing SVP drifter designs the most robust elements.

Its scientific objectives were to obtain a tropical Pacific basin-wide field of surface currents and SST for the purpose of studying a variety of processes that determine SST evolution. It now consists of a spherical surface float that carries the electronics, SST, barometer, and drogue-on sensors.

This float is tethered with plastic-coated wire to a holey sock drogue centered at m depth [ Sybrandy and Niiler , ; Sybrandy et al. In the subtropical oceans the mean lifetime of a buoy defined in terms of drogue retention is days; in the Southern Ocean the mean lifetime is days.

The accuracy of the water-following capability is dependent upon the winds and the "drag area ratio," the ratio of the frontal drag areas of the drogue relative to the surface float and tether [ Niiler et al. At the time of deployment the calibrated accuracy of the SST sensor is 0. To reduce Service Argos fees, these drifters transmit one third of the time in a or hour period. The GTS data are quality controlled and used on an operational basis by the meteorological agencies for weather and climate prediction and in a variety of data products that assess the nature of the variability of the oceans and lower atmosphere.

For example, NCEP uses the raw drifter barometer data in real time off the west coast of the United States to aid in marine weather broadcasts and forecasts. In the tropical Pacific most of the drifters were released near the equator. For example, there were many more deployments in the eastern Pacific equatorial waveguide than in the North Equatorial Countercurrent, although the data density was much larger in the latter because of the nature of the surface flow and its variability. The total number of 5-day observations is 81, The maximum number of 5-day observations possible in any given box is Southern Ocean drifters While the SVP drifter was being developed, the second part of the two-pronged program for drifter deployment was getting underway.

This heterogeneous reaction affects not only the nitrogen family but also those of chlorine and hydrogen. This competition should be highly dependent on latitude and season. The hydrolysis of N2O5 is one of the most important processes dictating the effect that stratospheric aircraft will have on ozone. The balloon measurements contained a fairly complete set of measurements in the reactive nitrogen family above 30 km, and observations and model calculations using gas-phase chemistry are in excellent agreement. The aircraft measurements, on the other hand, did not have enough simultaneous measurements to demonstrate with small uncertainty that the partitioning among CIO, NO, and CIONO 2 is completely understood.

However, the results from the sunrise flight have not yet been fully analyzed. However, the instrument complements for these two experiments were insufficient to answer the questions that we now pose for MAESA. As a result, we have an extremely limited data set of reactive trace gas abundances for the tropical region, and even less seasonal information. These tropical measurements are important for establishing and testing the current ideas about gas-phase chemistry.

The situation for the northern middle latitudes and the high latitudes in winter and spring is much better than that for the tropics. We have many measurements that span the middle latitudes for many of the chemical species, and observations from instruments placed on balloons, the space shuttle, and satellites contribute to our understanding of the abundances at these latitudes.

The number of aircraft observations in the summer is small however. Observations of reactive nitrogen and chlorine are inconsistent with models that contain only gas-phase chemistry. They support an important role for the hydrolysis of N 2 O 5 in middle latitudes. No measurements of species in the nitrogen family are in conflict with the concept of hydrolysis of N 2 O 5. However, some observations, particularly in the background levels of sulfate aerosol that existed prior to the Mt. Pinatubo eruption, indicate that the reaction efficiencies measured in the laboratory may not be the same for the stratospheric sulfate aerosols under all circumstances Considine et al.

Webster, private communication, The HC1 abundances observed for ER-2 altitudes at middle to high latitudes in winter were substantially 33 lower than predicted by photochemical models, whether they included heterogeneous chemistry or not. This observation implies that HC1 was not the major inorganic chlorine reservoir in the lower stratosphere. Three possible causes, if no problem with the ER-2 instrument is uncovered during ongoing tests, have been suggested for the discrepancies in HC1 abundances.

First, the representation of the heterogeneous chemistry in the models is either incomplete or incorrect. Effects due to temperature, aerosol impurities, aerosol phase, or photochemistry on the reaction rates may not be properly characterized. A second possibility is that the abundances of OH are substantially different from model predictions.

A third possibility is that key photolysis rates are in error. If we cannot simulate the observations of reactive and reservoir species in the chlorine and nitrogen families, then we can have no confidence that the photochemistry included in the HSRP assessment models is complete and accurate. Only more data with tighter constraints on the possible mechanisms with more and better simultaneous measurements over a wider range of conditions , combined with additional laboratory studies of the heterogeneous chemistry and all of its nuances, can resolve these issues. Measurement Collection Strategy The goal is to gather as much data as possible on the abundances of reservoir and reactive species in the nitrogen, chlorine, and hydrogen chemical families.

These measurements must be made along with tracer, aerosol, and meteorological measurements to put them in an understandable reference frame. To collect these data, we will need both ER-2 and Perseus flights. The ER-2 can give us good spatial and seasonal coverage, but over a limited altitude range. The Perseus can give us fewer simultaneous measurements, but over a greater altitude range at a few latitudes and seasons. These flights will also provide the opportunity to examine the photochemical state of the middle latitudes in through the measurement of trace gases and aerosol abundances.

The transit flights will provide data about the seasonal and latitudinal variations of these trace gases, and hence the photochemistry, because the four transit flights occur in March, June, July, and October. During these transit flights, the aircraft will be fully loaded with instruments and fuel, and probably will not be able to exceed a pressure altitude of 64, ft J.

Barrilleaux, private communication, , which is roughly equivalent to a potential temperature of K. At these altitudes in the tropics the abundances of reactive trace gases will be quite low and the range of useful measurements will be highly restricted. A greater altitude range, up to roughly 68, ft, can be obtained if the aircraft takes off and lands at Hawaii because it can be loaded with less fuel and still reach alternate landing sites.

Thus, to extend the altitude range of the measurements in the tropics, flights could be made from Hawaii. The tropical region should be accessible from Hawaii, and if not, the possibility of stop-over flights from Fiji will need to be examined. The Perseus A aircraft will also be used to address our concerns about the photochemistry.

Because it can attain an altitude of 25 km, but has limited horizontal range, we will stage it for 34 flights from only a few locations. We have chosen Darwin, Australia as the tropical deployment site for Perseus and the ground-based lidar. The observations generated from the limited number of instruments on the Perseus does not supplant the need to cover a greater altitude range with the ER-2 and its more complete instrument package.

It does extend the altitude range considerably, however, giving us a view of CIO and NO and a tracer to higher altitudes, and allowing us a better view of the photochemistry in a different environment. We need the greater altitude capability of Perseus, even with the limited payload, for several reasons. Perseus, on the other hand, will be able to sample N 2 O to about ppbv and NOy to about 6 ppbv.

It measures exactly the quantity that HSCTs will perturb the most. It has been used to indicate denitrification in the polar vortices, and displays some interesting differences between hemispheres. This observable difference may indicate the photodestruction of NO and thus NO y above about 30 km.

However, we need a measure of this relationship over a sufficient range in N 2 O in order to ensure the use of this diagnostic. Perseus provides us with this range. Third, the amounts of trace gases change rapidly in the lower stratosphere. These steep gradients result from changes in both trace gases and photochemical environment. Specific Measurements The enhanced instrument capabilities give us the tools to make many measurements that have never been made before Figure 3.

However, each measurement has an instrumental absolute uncertainty and a limit to the precision. The precision of these measurements is generally a few percent for a few minutes or less of integration time. With this uncertainty in mind, we can consider combining measurements from instruments in a way that tests photochemical and heterogeneous mechanisms that involve the nitrogen, chlorine, and hydrogen chemical families and the interactions among them. Many tests are possible with the ER-2 payload. The uncertainties in these tests are estimates.

Tests over a range of altitudes and latitudes in differing photochemical environments will expose inconsistencies in measurements and will reduce the uncertainties. Other specific studies are listed under other questions. How do the abundances of these same chemical species vary as a function of aerosol loading of the lower stratosphere? How will these amounts change as the aerosol loading slowly decreases over the next few years? Observations of the aerosols injected by volcanoes into the stratosphere suggest that aerosols have a stratospheric lifetime of 1 to 2 years WMO, When these measurements are combined with those taken prior to the Mt.

Pinatubo eruption, we will have observed the effects that different aerosol surface areas have on the trace gas distributions. Importance to A ESA The revelation that heterogeneous chemistry on sulfate aerosols was changing the NO x abundances in the stratosphere has significantly altered the assessment of the impact of high-speed aircraft on the stratosphere. From the comparison between observations and model results, we know that heterogeneous chemistry on sulfate aerosols needs to be included in our assessment models.

But questions remain. First, have we accounted correctly for the heterogeneous processes and do we understand their effects on trace gas abundances? Second, how much of the detailed heterogeneous mechanism do we need to consider for making the assessment? Measurements of trace gas abundances that are affected by heterogeneous chemistry and the variations of those abundances as a result of different aerosol loadings should give us some indication of the complexity of the problem.

Current Observations and Calculations As discussed for the first question, we have large uncertainties in the detailed processes that are occurring on sulfate aerosols in the lower stratosphere. Pinatubo resulted in a less-than-proportional reduction in NO x due to the hydrolysis of N 2 O 5 Fahey et al. This saturation effect, which occurs because the formation and gas-phase destruction of N 2 O 5 is slower than the hydrolysis of N 2 O 5 even with background aerosols, apparently reduces the sensitivity of stratospheric chemistry to the observed variability of stratospheric aerosol loading.

The model fits the observations better if the heterogeneous reaction efficiency is reduced a factor of two. As significant, no measurements have been made of NO 2 , OH, and HO 2 in the lower stratosphere where the effects are the greatest. Because the hydrolysis of N 2 O 5 has such a powerful effect on reactive nitrogen photochemistry and the injection of additional NO x by HSCTs, we need to have many more measurements over a wider range on conditions than are currently available.

The measurement strategy is to collect observations of those species most affected by this reaction over a range of seasons, latitudes, and altitudes. Measurements toward the Arctic during the test flights in February and from New Zealand during ASHOE will augment the existing measurement set for middle to high latitudes, but at a smaller sulfate aerosol surface area. Measurements in the tropics will test the assumption that gas-phase chemistry dominates in the tropics. Measurements to higher altitudes with Perseus will do the same. Specific Measurements A test with high priority is the measurement of ratios of species that are most susceptable to heterogeneous chemistry.

The decrease in the sulfate aerosol surface area with time permits a good test of the saturation effect. Pinatubo eruption, and is now about 2 to 3 times smaller. In , the surface area will have decreased even more. The combination of these measurements in middle latitudes for a number of reactive species over these 3 years will permit us study the trace gas ratios given above as a function of sulfate aerosol surface area and HNO 3 photolysis rate. We can establish the relative importance of these cycles and compare relative destruction rates with expectations for computer models. The inclusion of the hydrolysis of N 2 O 5 into photochemical models results in a larger role for the hydrogen-catalysis in the destruction of ozone McElroy et al.

In fact, these model calculations suggest that HO x catalysis dominates over NO x catalysis of ozone up to as high as 23 km altitude. Thus, observations of OH and HO 2 in the lower stratosphere appear to be essential for determining the rates of ozone catalysis in the lower stratosphere. Previously, attempts have been made to infer abundances of reactive species that were not directly measured. These exercises have been valuable, but do not provide low enough uncertainty to test our understanding of the ozone destruction rates in middle or low latitudes.

Measurement Collection Strategies Studies of the catalytic cycles is a high priority for AESA because it is through these cycles that the HSCT effluents and their photochemical by-products will interact with ozone. The flights proposed for the other photochemical studies above and for the dynamical studies below will give us a good look at this issue because we will be measuring these species all the time. The overall catalytic destruction rate should be only slightly more uncertain. This level of uncertainty, while not sufficient to eliminate the possiblity of any other unknown catalytic cycles, is nonetheless a significant advance over present conditions, where lack of measurements of one or more of the key radicals has prevented any reasonable attempts at determining the ozone destruction rate.

Do the characteristics of sulfate aerosols vary with temperature in a way that is consistent with ideas of liquid aerosol growth? The phase of the stratospheric sulfate aerosols— be it liquid or solid — is important to both the global heterogeneous chemistry and the nucleation of PSCs in the cold polar regions. If the sulfate aerosol is liquid, then hydrolysis of N 2 O 5 is efficient but hydrolysis of CIONO 2 is relatively inefficient, except at temperatures below about K.

The nucleation of PSCs is thought to occur on frozen sulfate aerosols, so that in the wintertime polar regions, the phase of the sulfate aerosol may be important to PSC formation. These species are all condensables that can affect the partitioning of the nitrogen, hydrogen, and chlorine chemical species that control ozone. The increase in water vapor in particular could increase the occurrence and duration of PSCs. The presence of accumulations of aircraft-generated sulfate aerosols in corridors that stretch through cold regions could increase the possibility of heterogeneous processing in the nitrogen and chlorine chemical families.

These issues must be resolved before we can have any assurance that HSCTs will not cause substantial depletion of stratospheric ozone. If they are liquid, then the hydrolysis of N 2 O 5 is relatively fast and the conversion of reservoir chlorine to reactive chlorine is relatively slow. In addition, the sulfate aerosols must be frozen to act as good condensation nuclei for the formation of PSCs.

Current Observations and Calculations No direct stratospheric measurements of the phase of the sulfate aerosol exist. Some measurements of the change in sulfate aerosol size as a function of temperature suggest that sulfate aerosols can stay liquid to temperatures as low as K Dye et al. The lack of depolarization in the lidar backscattered light indicates that the most sulfate aerosol particles are spherical, but they do not necessarily have to be liquid.

This issue of aerosol phase remains to be resolved. Measurement Collection Strategies The observations of the change in aerosol characteristics with changes in temperature is an indirect method to distinguish liquid from solid sulfate aerosol particles. This observation also provides a test of the laboratory measurements and theoretical calculations for how the liquid aerosols should change with temperature.

Despite the low amounts of water vapor available in the tropics, the variation in temperature just above the tropical tropopause see Figure 4 should provide a test of this relationship between aerosol size and temperature. Observations during SPADE in May should also provide an opportunity to examine this issue at middle latitudes after the break-up of the Arctic polar vortex, which in has experienced substantial low temperatures. Transport of Trace Species The ER-2 and Perseus are better suited for studies of photochemistry than they are for studies of dynamics.

Nonetheless, very limited aircraft observations have had a role in shaping the discussions about transport within the stratosphere and between the stratosphere and troposphere. Can relatively undiluted tropospheric air be found within a few kilometers above the tropopause in the tropics? Do these measurements indicate restricted exchange between the tropics and the middle latitudes? What is the character of the exchange of trace gases between the stratosphere and troposphere?

For dynamical studies, however, we are most interested in those trace gases that have lifetimes that are longer than or comparable to the dynamical time constants N 2 O, CH 4 , CFCs, H 2 O, O 3 which is roughly the same as O x in the lower stratosphere , CO 2 , and NO y. Analyses of the distributions of trace gas abundances measured during MAESA and other aircraft programs, along with the satellite observations of trace gases and volcanic debris, will help develop our understanding of how stratospheric transport occurs in the tropical regions.

Because all the issue about transport of trace species have the same importance to AESA and require the same measurement strategy, we consider them all at once. We must know what the distribution of aircraft exhaust is likely to be. If any exhaust emissions that are released in middle latitudes are mixed into the tropics, they might be lofted to higher altitudes before they descend and enter the troposphere at middle latitudes. Once NO x emissions rise to higher altitudes, they have a longer stratospheric residence time and they pass through a region of the stratosphere where increased NO x catalytically destroys ozone.

In addition, we need to know when and where transport and exchange might occur and whether it is fairly constant or sporadic. The character of the transport will affect the distribution of the aircraft exhaust. Thus, the seasonal and quasi- biennial dynamical effects must be understood.

Current Observations and Calculations Few measurements of stratospheric tracers exist for the tropics, and simultaneous measure- ments of tracers are even rarer. Some satellite and shuttle measurements exist, with some simultaneous measurements of tracers, but the increasing uncertainty and poorer horizontal resolution of these measurements degrades the value of the correlations of the observations.

The concept of correlations among tracer abundances has been used as a test of the occurrence of PSCs in the polar stratosphere Fahey et al. An explanation for these observed compact correlation curves is that rapid mixing occurs on surfaces that slant poleward with respect to isentropic surfaces Plumb and Ko, The question is, then, what do these correlations look like in the tropics, and can the assessment models, or the 3-D models for that matter, reproduce them?

If we examine the modeled correlations of long-lived species with respect to N 2 O Prather and Remsberg, , we see that the correlation curves for CH 4 , the CFCs, and NO y are calculated to be relatively compact. However, they are not perfectly compact for most models. For species such as CFC-1 1 and CCI 4 , the abundances are predicted to be more than five times lower in the tropics than in middle latitudes for N 2 O abundances of ppbv Figure 6.

At present, we have no measurements to test these predictions. Two features of correlations of NO y remain to be fully explained. First is the observation that the NO y -N 2 0 correlation is measured to be slightly different in the two hemisphere at middle to high latitudes Fahey et al. This first observation may give us a hint about either the different sources and sinks of NOy in the two hemispheres and help provide another limit to the transport time between the two hemispheres across the tropics. These abrupt shifts cannot be simulated by the 2-D models.

The observations that show evidence of restricted exchange between the tropics and the middle latitudes are the satellite maps of the extinction by the aerosols released by Mt. Pinatubo in June, McCormick and Veiga, More observations are required from aircraft, perhaps balloons, and UARS for us to better understand this issue. The transport of trace gases between the tropics and middle latitudes appears to vary depending on the time of the year and the phase of the quasi-biennial oscillation QBO.

Eruptions from two different tropical volcanoes in and occurred during a different phase of the QBO, and the resultant spread of the volcanic aerosol was dramatically different during the easterly and westerly phases, as in Figure 7 Trepte and Hitchman, When the QBO was easterly below 23 km and westerly above, the aerosol distribution suggested that the air was descending at 25 km, causing a lateral spread in the air near 23 km.

When the QBO was easterly above 23 km and westerly below, less lateral spreading and more vertical lofting in the tropics occurred. These satellite observations also suggest that transport between the tropics and middle latitudes is rapid within a few kilometers of the tropopause. These observations may have implications for the spread of aircraft exhaust to higher altitudes. Recent work Rood et al. Even though these calculations indicate that the amount of exhaust being lofted to 30 km altitude is only a few hundredths of a ppbv, this question is so critically important to the assessment of aircraft effects that additional observations and calculations are required.

No large gradients in trace species are evident in the tropics in the 2-D assessment models. Thus, these models do not represent correctly some of the important dynamical features of the lower stratosphere. We must understand the underlying physical processes that create these gradients in order to evaluate the importance of these failings in the models to the assessment of stratospheric aircraft effects. This result indicates that tropospheric air can enter the stratosphere from regions other than the energetic convection regions over Micronesia.

Either tropospheric air can enter the stratosphere more globally, or the lower stratosphere above the tropopause is all connected by rapid quasi-horizontal transport. These two models predict significantly different behavior for the transport of HSCT effluents. We will be able to observe differences in the abundances of trace gases and their correlations over a range of seasons. However, in the tropics, the ER-2 can fly less than 5 km above the tropopause on the transit flights.

Any greater altitude that can be gained by stop-over flights from Hawaii into the tropics, projected to be 0. They will also allow us to examine tropical correlations over more substantial ranges in N 2 O. Perseus, or balloons, have an important role in the study of tropical dynamics. Measurements from these platforms permit us to tie together measurements from the ER-2 aircraft and the UARS satellite because the altitude range of this aircraft bridges those of the other two platforms. An entirely separate mission must be planned if such a study is thought to be important.

However, it is doubtful that heterogeneous chemistry on these ice clouds can be very effective at repartitioning the trace gas constituents of the tropical stratosphere. It is also doubtful that chasing such clouds would yield much, or any, unambiguous information about the processes Murphy et al. However, Perseus will be deployed in Darwin in October, and should encounter some very low temperatures near the tropopause. Any opportunity to sample this cold air will be taken. Does air from middle latitudes, rich in NO y and H2O, pass through the cold regions on the margins of the tropics? We now understand that if clouds form in the stratosphere, they exert a large nonlinear perturbation on the photochemistry of a region.

Thus, the possibility of tropical stratospheric clouds, and if they are occurring, the possibility that they will be more frequent and intense with the addition of aircraft emissions, are good reasons to search for their existence under the aegis of AESA. The large nonlinear perturbations of increased occurrences of stratospheric clouds could easily offset any amelioration that heterogeneous chemistry might have on the NO x catalytic cycles in the lower stratosphere. In the tropics, will the tracer abundances and correlations differ during the easterly and westerly phases of the Quasi-Biennial Oscillation QBO?

No provisions are being made to look at this interesting phenomenon. The ER-2 can fly only up to 20 km, which is at least 3 km below the region where restricted exchange between the tropics and middle latitudes occurs. Because of the uncertainty of the changes in the phase of the QBO, no plans can be made at this time to even use Perseus, which can sample into the restricted region of the tropics.

The chosen deployment site for Perseus — Darwin, Australia — is at a latitude that does not experience oscillations from the QBO. This issue will have to wait for future studies. The combin- ation of all the measurements from all the platforms is far more powerful than the individual measurements for testing the assessment models and providing an improved understanding of stratospheric processes. Thus, we are actively seeking out possible links with the other sources of observations. Intercomparisons of Instruments on Different Platforms During MAESA The ability to combine observations by different instruments on different platforms and perhaps at different times relies on the intercomparison of those different instruments.

For tropospheric measurements, a good method for intercomparing instruments has been to make them sample the same standard gases on the ground, before they are flown. However, for the stratospheric measurements, the platforms are as important as the instruments in determining the measured values, and the platforms and measurement techniques are both remote and in situ. The best inter- comparison for these is to measure the same trace gases in roughly the same volume of air at roughly the same time.

We will endeavor to perform such intercomparisons whenever and where ever possible. Some examples of intercomparisons are: 1. Such intercomparisons can only reduce the uncertainty in our total observational data set.

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  • We will need to relate the observations of these species during to those made by instruments on the ER-2 and Perseus in This observational data set includes radicals, reservoir species, and tracers. The global observations over several seasons are more easily compared to assessment model results than in situ observations are. For this reason, such observations are preferred by assessment modelers, even though satellite observations tend to have lower accuracy and altitude 43 resolution in the lower stratosphere than in situ observations do.

    One of our goals is to learn to best use these two types of observations. ER-2 Flights Flights will be out of four locations: 1. Flights will consist of: 1. Four transits 3 legs between Moffett Field and Christchurch; The estimated flight path is shown in Figure 8. A stop-over flight from either Hawaii or Fiji during each transit: The purpose of these flights, which may be only a few hours long, is to attain measurements at higher altitudes in the tropics than are possible on the transit legs. These flights will define the trace gas distributions in the southern middle latitudes — just as the flights south from Bangor did during AASE II.

    The estimated flight times for the transit flight legs are J. Barrilleaux, private communication, : 1. Moffett Field to Barber's Point: 6 hrs 2. Barber's Point to Nadi: 7. Nadi to Christchurch: 4. Facilities Hangar facilities are available for all the sites. Hangar space only will be provided for the Hawaii and Fiji transit stops, even though a stop-over flight will occur on each of the four transits between Moffett Field and Christchurch.

    The scientists' activities will thus be limited to maintenance and preparation of the instruments for either the stop-over or the transit flight. Go-no- go criteria for the transit flights will be developed with the Pis prior to The change in the temperatures at the tropical tropopause and the southward movement of the ITCZ toward Darwin are the greatest during this period.

    Thus, observations over this 5- week period permits sampling of a wide cross section of the tropical lower stratosphere from one site. Perseus will be deployed in middle latitudes at Dryden to coincide with flights of the ER-2 out of Moffett Field. Four flights two of each payload from Dryden in mid-July; 3.

    Ten to fifteen flights from Darwin, Australia. Because Perseus A must remain in radio contact with the control station, it can cover only about 2 degrees of latitude from the launch site. Perseus operations will thus be conducted as if it were a balloon platform, and flights will consist of a scan up to 25 km and back. This simplified flight planning in the tropics results from the expected homogeneity of the trace gas distributions over the 2 degrees of latitude that can be covered by Perseus; by the lack of a good observational meteorological network to guide the meteorological forecasts accurately; and by the need to keep operations simple for the new Perseus platform and new instruments on a foreign deployment.

    Some flight planning considerations for Perseus flights in the tropics are: 1. The flights at middle latitudes will also consist only of vertical scans to 25 km altitude. However, the flight planning considerations will include: 1. Sites and Facilities These criteria for the tropical deployment of Perseus are met by the site at Darwin, Australia. It was the location of the STEP mission in , and is known to have good facilities. Furthermore, convective storms that could endanger Perseus can be tracked by the excellent radar facility there.

    The facilities at Dryden should meet the needs of the aircraft and the investigators for the two deployments in middle latitudes. The Helium-Filled Balloon Option If for some reason the Perseus aircraft or its key instruments are not ready for deployment in , then helium-filled balloons will have to be used for the platforms for the higher altitude measurements.

    We will not know if this option is necessary until after the Perseus test flights in 45 mid-to-late Thus, we will pursue any details of the planning until after these test flights. However, we have some preliminary ideas about deployment. The flights will consist of: 1.

    Sumner, New Mexico in July; 2. Sites and Facilities Complete balloon and laboratory facilities already exist in Ft. Sumner, New Mexico. The site in Brazil was to be used for UARS correlative measurements balloon flights in March , but this activity was canceled. The personnel from the National Scientific Balloon Facility have examined this site, however, and could develop it if necessary.

    The timing of the Perseus component is dependent on the coincidences with the ER-2 flights and the stratospheric conditions in October in Darwin. Meteorological Support ER-2 The timing of the transit flights is dictated by the requirements of ASHOE, and the flight plans, maximum altitude cruise climbs, are dictated by the requirements of flight operations. Meteorological support for these flights is in the form of satellite imagery, trajectories, and other dynamical and meteorological analyses. Perseus The required meteorological support for Perseus is similar to that for the ER-2, only perhaps not quite as extensive.

    Additional Meteorological Support The need for additional radiosonde launches must be assessed, particularly for the tropical flights of all the platforms where observational meteorological data will be otherwise scarce. Additional sondes may be necessary for flight planning for the ER-2 flying north out of Christchurch and out of Hawaii or Fiji.

    Otherwise, they may be necessary for the analyses of the meteorological conditions that will be required for all the flights. They may also be necessary for the flight planning and meteorological analyses for either Perseus or balloons in the tropics. Data Reduction and Submission to the Archives Traditionally, the data submission protocol has been relaxed for transit flights to the polar missions. On occasion, data from these flights have not been submitted to the archives until well after the mission was completed. Thus, a plan for submission of data to the archives needs to be developed by the principal investigators.

    The most reasonable possibility is the submission of transit flight data to the archives within a week of arrival at either Christchurch or Moffett Field. Although no immediate action is required on this subject, it needs some careful thought. We see no need to have real-time data transmission capabilities from the Perseus or balloon sites of this program to either Moffett Field or Christchurch. Data from Perseus or from balloon flights would be submitted to the archives on the same schedule as is developed for the ER Thus, Perseus Pis should be involved in the discussions on data submission protocol.

    However, the HSRP program has some special, pressing requirements that will require additional analytical capabilities on site. First, the AES A assessment is mandated for early Data from MAESA must be analyzed, understood, and incorporated into the assessment models, which then must be run in a very short time. At the appropriate time after the flights as determined by the science team , this person can then help speed the assimilation of these data sets by the other groups involved in the assessment process.

    Very likely, more process-oriented models will be required for analysis of the data and the improvement of the assessment models. These scientists who use such models may also be required to be present in the field. Second, the MAESA mission may also need modeling capability associated with the Perseus or balloon part of the mission.

    However, it is not clear that such capability must be present with Perseus or the balloons. The wedge-shaped box is covered by the ER-2; the two vertical boxes by Perseus or balloons. The shaded area indicates measurements from JARS - the bottom is jagged to indicate the uncertainty in the lower altitude limits for the UARS instruments. A schematic of stratospheric photochemistry, showing the links among the oxygen, nitrogen, halogen, and hydrogen chemical families.

    ER-2 instruments can measure the chemical species shown in bold print. These measurements enable the study of the photochemical processes that are shown with bold arrows. The central, dotted area indicates heterogeneous chemistry. The highlighted ellipse indicates the importance of the nitrogen family in assessing the effects of HSCTs. March , b June , and c September, Vertical scales are pressure altitude km and pressure mb. Transit flights between Moffett Field and Christchurch will occur in these months at longitudes near 'E. Note that the ER-2 transit flight ceiling of 64, ft Newman, private communication , The ER-2 observations come from all seasons and potential temperatures greater than K.

    ATMOS observations diamonds are added for comparison. Model results come from Solomon and Garcia and Ko et al.

    The Tropical Ocean-Global Atmosphere observing system: A decade of progress

    Murphy et al. Note the large modeled differences in the correlation at low latitudes and at the middle to high latitudes. Remsberg and Prather, Hathaway, private communication, Toohey, W. Brune, R. Salawitch, In situ measurements of CIO and ozone: implications for heterogeneous chemistry and midlatitude ozone loss, submitted to Geophys. Brune, W. Toohey, S. Lloyd, and J. Considine, D. Douglass, and R. Douglass, A. Rood, C. Weaver, M. Cernglia, and K. Brueske, Implications of 3-D tracer studies for 2-D assessments of the impact of supersonic aircraft on stratospheric ozone, to appear in J.

    Dye, J. Baumgardner, B. Gandrud, S. Kawa, K. Kelly, M. Loewenstein, G. Ferry, K. Chan, and B. Gary, Particle size distributions in Arctic polar stratospheric clouds, growth and freezing of sulfuric acid droplets, and implications for cloud formation, J. Solomon, S. Kawa, M. Loewenstein, J. Podolske, S. Strahan, and K.

    Chan, A diagnostic for denitrification in the winter polar stratosphere, Nature, , , Kawa, and K. Chan, Nitric oxide measurements in the Arctic winter stratosphere, Geophys. Kawa, E. Woodbridge, P. Tin, J. Wilson, H. Jonsson, J. Dye, D. Baumgardner, S. Bormann, D. Toohey, L. Avallone, M. Proffitt, J. Margitan, R. Salawitch, S. Wofsy, M. Ko, D. Anderson, M. Schoeberl, and K. Chan, In situ measurements constraining the role of reactive nitrogen and sulphate aerosols in mid-latitude ozone depletion, to appear in Nature, Kawa, S. Fahey, S. Solomon, W.

    Brune, M. Proffitt, D. Toohey, D. Anderson, L. Anderson, and K. Fahey, L. Heidt, W. Pollock, S. Solomon D. E Anderson, M. Loewenstein, M. Margitan, K. Chan, Photochemical partitioning of the reactive nitrogen and chlorine reservoirs in the high-latitude stratosphere, J. Plumb, and U. Prather and E. Kelly, K. Tuck, D. Murphy, M. Jones, D. McKenna, M. Strahan, G. Chan, J. Vedder, G. Gregory, W.

    Soaring into Atmospheric Science | Science | AAAS

    Hypes, M. McCormick, E. Browell, and L. Heidt, Dehydration in the lower Antarctic stratosphere during late winter and early spring , J. Brune, D. Toohey, J. Rodriguez, W. Starr, and J. McCormick, M. McElroy, M. Salawitch, and K. Minschwaner, The changing stratosphere, Planet. Space Sci. Murphy, D. Proffitt, S. Liu, K. Chan, C. Eubank, S. K Kelly, Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere, J.

    Plumb, R. Ko, Interrelationships between mixing ratios of long-lived stratospheric constituents, J. Remsberg, E. Prather, Eds. Rood, R. Douglass, and C. Weaver, Tracer exchange between tropics and middle latitudes, Geophys. Schoeberl, P. Newman, L. Lait, D. Fahey, E.

    Woodbridge, C. Webster, R. May, and J. Anderson, Observations of the temporal evolution of reactive chlorine in the northern hemisphere stratosphere, submitted to Science, Trepte, C. Hitchman, Tropical stratospheric circulation deduced from satellite aerosol data. Nature, , , Tuck, A. Davies, S. Hovde, M. Noguer-Alba, D. Kelly, D. Margitan, M. Chan, Polar stratospheric cloud processed air and potential vorticity in the northern hemisphere lower stratosphere at mid-latitudes during winter, J.

    Webster, C. May, R. Toumi, and J. Jonsson, C. Brock, D. Baumgardner, J. Dye, S. Borrmann, L. Poole, D. Woods, R. DeCoursey, M. Osborn, M. Pitts, K. Chan, G. Ferry, M. Podolske, A. Weaver, D. Toohey, and L. Avallone, In situ observations of the stratospheric aerosol following the eruption of Mt. Pinatubo, submitted to Science, Seals, Jr. Subchapter provides an overview of the scenarios developed and reviews the basic methodology and requirements for preparation of the database.

    Subchapter describes the model and scenarios database development at Boeing Commercial Airplane Group. Subchapter similarly describes the model and scenarios database development at McDonnell Douglas. Subchapter 3- 4 describes the combined scenarios database, describes validation studies and consistency checks on the database, evaluates the strengths and weaknesses in the database developed, and describes future needs. The development of a detailed three-dimensional database that accurately represents the integration of all aircraft emissions along realistic flight paths for such scenarios requires complex computational modeling capabilities.

    However, the goal in the scenario development was to represent flight patterns and aircraft specifications as realistically as possible. Prior scenario databases for aircraft emissions have greatly simplified the emissions from aircraft. For example, the earlier scenarios evaluated by Boeing determined emissions at cruise altitudes only. The scenarios developed for the interim assessment are described in Table Fuel burned and emissions of nitrogen oxides NO x , hydrocarbons HC , and carbon monoxide CO were to be developed for fleets of subsonic aircraft operating in and , and for assumed fleets of HSCTs in flying at either Mach 1.

    From the fuel burned, emissions of water vapor and carbon dioxide can also be determined. The subsonic scenarios evaluate the fuel burned and emissions for scheduled airliner jet aircraft , scheduled cargo, scheduled turboprop, military, charter, and nonscheduled air traffic. It was necessary to limit the scenarios evaluated to those shown in Table All but the Mach 2. Although not directly relevant to evaluating HSCTs, the emissions database does provide a relative basis for evaluating the future environmental effects from fleets of these aircraft.

    Boeing was responsible for evaluating the scheduled Official Airline Guide airline, cargo, and turboprop emissions for the and subsonic fleets and for the Mach 2. McDonnell Douglas was responsible for the military, charter, and nonscheduled non-OAG flights within Russia and China emissions for the and subsonic fleets and for the Mach 1. Two versions of the subsonic emissions were developed. A modified subsonic case was developed to reflect the reduced number of subsonic flights when HSCTs are included in the scenarios.

    The modified subsonic emissions were used in all of the scenarios that include HSCTs. At the request of the assessment modeling group, a special Mach 2. This Scenario G was developed to examine the outer envelope of aircraft emissions effects on ozone. The guidelines developed for the emissions database recommend that it be properly documented, publicly available, continuous in space and time, open ended, flexible, and well scrutinized.

    In order to generate the emissions for each scenario, it is necessary to account for the aircraft performance, engine characteristics, and marketing forecasts traffic projections, flight frequencies, city-pairs, routing. For example, the flight altitude of an HSCT will vary with its cruise Mach number, increasing with higher speeds. The cruise altitude will also increase during the flight as fuel is burned and the aircraft becomes lighter.

    Total Passenger Demand Passenger demand, which forms the basis of the year route system emissions analysis, was projected by Boeing and McDonnell Douglas. Data regarding growth rate forecasts were exchanged, and a single growth scenario was devised which resulted in a common forecast for passenger demand.

    Both companies produce passenger demand projections as part of normal business activity. After exchanging forecast growth rate data, Boeing and McDonnell Douglas agreed that a simple averaging of growth rates by regional market would suffice to create a common forecast. Table Projected passenger demand, which for is shown in Figure Based on these criteria, McDonnell Douglas and Boeing both produced a set of candidate city-pairs and route paths.

    After much negotiation and several iterations, a single set of city-pairs and flight frequencies was agreed upon which met the criteria described above and met the further requirement that the HSCT route system and market penetration as devised, would need about Mach 2.

    The passenger demand estimate for the year was partitioned between the different city- pairs to create a single universal airline network. Flights were scheduled to satisfy local airport curfews. The results are summarized for different HSCTs and for the subsonic aircraft they replace in Table The higher speed aircraft would be able to fly more trips and thereby carry more people per day. The average stage length was nautical miles with an average diversion from great circle routing of 4. These high utilization rates are consistent with the scheduling guidelines; they probably represent an upper limit utilization for Mach 2.

    These calculations result in a Mach 2. While Mach 2. The average fleet utilization would likely be lower than this as additional aircraft would be needed for spares, replacement aircraft during periodic maintenance, etc. The table lists origin, destination, and "via" cities refueling stops required when the origin-destination distance is greater than the nautical mile nominal range for the HSCT designs now contemplated. Also listed are flights per day and great circle paths and flight-path distances between cities.

    Since it was assumed for this study that supersonic flight over land will be prohibited, the flight path distances are greater than the great circle paths due to the routings that have been defined to minimize subsonic overland flight. This resulted in HSCT service between city-pairs.

    Because some HSCT flights are routed through the same cities, mission profiles were calculated to fly this network. Flight Profiles Actual flight profiles between city-pairs were used to distribute emissions during takeoff, subsonic and supersonic climb and cruise, and descent. Based on these mission profiles, the fuel burned and emissions were then calculated onto the database grid. Two missions which are representative of the way in which an actual HSCT would be flown are shown in Figures The simplest mission Figure The HSCT would take off and climb subsonically and then supersonically to a supersonic cruise altitude.

    It would then fly at supersonic cruise at the optimum altitude determined by its gross weight. As it approached Tokyo, it would descend and land. The cumulative fraction of the total NO x emissions is plotted on the right axis. A more complicated but still common mission is a flight in which one leg would be flown subsonically over land. This is illustrated in Figure The HSCT would take off and climb to subsonic cruise altitudes.

    It would then cruise at subsonic speeds until reaching Hudson Bay where it would begin to climb supersonically. It would then cruise at supersonic speeds altitude determined by the optimum performance until descending near London. A substantial amount of the NO x emissions would occur during the subsonic climb, subsonic cruise, and supersonic climb. An example would be Madrid, Spain, to Mexico City. In this case, the HSCT would fly subsonically over Spain, supersonically over the Atlantic, subsonically over Florida, supersonically over the Gulf of Mexico, and then subsonically inland over Mexico.

    Because of the extensive fuel required for supersonic climb, such flight profiles were kept to a minimum in the scenario development. Other Considerations An analysis of the potential importance of considering seasonal variations in emissions by MDC indicated that emissions from commercial jet flights from show very strong variations in the subsonic traffic with season see Wuebbles et al. However, the HSCT fleets may be more dependent on business traffic and therefore less seasonal. Although the effect of seasonality still needs to be evaluated, there was insufficient time to do further analysis for the Interim Assessment and the effects of seasonality are not included in the database developed for any of the scenarios.

    For each mission, fuel consumption and emissions are calculated including all the flight segments taxi out, takeoff, climb, cruise, descent, landing, taxi in , distributing the emissions as a function of space along the route between city-pairs. The emissions are then combined for all flights into the resulting three-dimensional database. The details of the calculations are described in Subchapters and which follow this section. Summary results are presented in Subchapter