The atmospheric processes condition the weather evolution, the climatic evolution and the chemical composition of the atmosphere. Insofar as they bring into play a whole range of space-time scales, in the long run dense measurements in time and space are necessary to understand and supervise the atmospheric component of the climatic system, strongly disturbed today by the human activity.

Among the topics and environmental questions, important stakes were identified by the atmospheric community for a scientific prospective of the future space observation. They are presented here by indicating for each observable necessary to their study, the potential contribution of space taking into account the European and international context, the existing missions and measurements or to come.

CLIMATE AND RADIATIVE FORCING

The first stake of search and space is currently the forecast of the evolution of the climate and the planetary heating expected following the increase in greenhouse gases. To determine the amplitude and the speed of this change is a real challenge, complicated by forcings which are added to greenhouse gases, in particular those caused by the particles of aerosols and the clouds. If one considers the radiative forcing estimated in W/m² by the models of the Intergovernmental Panel on the Climatic Change (IPCC) and according to various scenarios of future projection of emissions and industrial development (Figure 1), one observes a dependant important heating, on the one hand, due to the greenhouse gases, on the other hand, due to the effect of the variation of the solar constant and a cooling related to the aerosols and the clouds.

Figure 1 : Radiative forcing estimated at the top of the atmosphere in W/m² by the models (noted A, B,…) Intergovernmental Panel on Climate Changes (IPCC) and according to various scenarios of future projection of emissions and industrial development.
Even if the contribution of greenhouse gases that is known since the beginning of the industrial era appears the least critical, the bar of uncertainty is still of more than 0,5W/m². Among greenhouse gases, one can quote the more studied, CO₂, which one well knows the global distribution. On the other hand the estimate of net fluxes of CO₂ between the atmosphere and the biosphere from surface measurements still has many divergences. The future American mission Oco answers in the medium term this essential question of estimate on a regional scale of the CO₂ assessments to evaluate the models of evolution of carbon.

The heating related to greenhouse gases is modulated by the effect of the aerosols. This effect is either direct by reflection towards the space of part of the incidental solar radiation, or indirect resulting from their influence on the optical properties of the clouds through their role as a core of condensation or Cloud Condensation Nuclei (CCN). The particles of aerosols, which have short life times and varied chemical compositions and sizes, are emitted by various anthropic or natural sources and can also be formed in the atmosphere from precursors gases. The direct effect of these aerosols is their capacity to diffuse and absorb the solar and infra-red radiation, which depends on their abundance and distribution in the atmosphere, of their physical properties (granulometry and index of refraction), their optical properties (function of phase, clean absorption). This type of measurements already exists (Erbe, ScaRaB, Ceres, etc.) but there is a problem of continuity after 2008.

The formed clouds have, as for them, a double role on the radiative budget. Either they reflect the solar radiation and, consequently, tend to decrease absorbed energy, or, being generally strongly absorbents in the infra-red, they contribute to the greenhouse effect. How the indirect effect acts on the clouds, on their albedo, their life time and their precipitating capacity especially in the case of frozen clouds, remains an essential uncertainty in the models. According to their chemical composition and conditions of supersaturation met in the atmosphere, some aerosols, CCNs, can be activated forming droplets in cloud. The increase in the number of CCN in the atmosphere, due to increasingly important rejections of aerosols by the anthropic activities, implies an increase in the number of drops in clouds which are then smaller. This modification of the microphysical properties of the clouds will increase the albedo of the clouds modifying the radiative budget. It is the first indirect effect of the aerosols.

The second indirect effect comes from the first and represents the reduction in the precipitating capacity of the cloud whose life time will then be increased. The major parameters to measure in this case are :

altitude/pressure at the top of the cloud to consider infra-red flow,
the cloud cover, whose measurement was improved with Modis thanks to its space resolution,
the phase and granulometric distribution by polarization,
the visible optical thickness and infra-red emissivity (conventional, continuity after 2005),
contents in water and/or ice (ultra high frequency).

Calipso mission, bringing into play a backscattering lidar and an infra-red imager, the Cloudsat mission, using a radar cloud and the Parasol mission, allowing the measurement of directional reflectances of the clouds, which are added to Aqua, tackle this problem (Figure 2). But the question of the continuity of these observations arises and could be ensured by Earthcare.

Figure 2 : Artist view of the A-train.
From left to right : Aura, Parasol, Calipso, Cloudsat, Aqua and Oco.

Within this framework, it is recommended, in a more general way, to exploit experimental synergies several instruments and to include the data in the Icare pole of competence.

CLOUD SYSTEMS AND PRECIPITATIONS, METEOROLOGY

To understand and forecast the evolution of the climate, another important stake is related to the dynamic processes of transport, of the formation, of the hold or not of the cloud systems and precipitations which are associated to them.

The dynamics of average scale conditions the evolution of the intense weather phenomena. Because of the changes of the intensity, direction or horizontal and vertical variation of the wind, evolution of static stability, the fast and precise follow-up of the development of the clouds and precipitations is important to apprehend the systems convectifs average or tropical latitudes, potentially generator of extreme phenomena (Figure 3).

Figure 3 : Distribution of the monthly averages of precipitations and the temperatures of surface of the sea for two months (June and August 1998-2003) resulting from the data of TRMM mission on the African western sector. The monsoon of June seems very influenced by the temperatures of surface of the sea (ST) along the African West coast.

It is of more essential to ensure a reliable follow-up and of long duration (> 10 years) with qualified observations to apprehend intraseasonal, interannual and decennial variability, and to compare them with the forecasts of coupled climatic models. It is the only way of identifying the natural, anthropic and coupled components climatic variability. The important atmospheric parameters are the pressure, the temperature, the moisture, the clouds properties, the precipitations, the radiation, in connection with the characteristics and fluxes associated with continental and oceanic surfaces.

They are partly provided by the meteorological observation, by the satellites envisioned up to 2020 either in geostationary orbit (MSG), or in polar orbit (Metop, Npoess). Moreover, the current or envisioned missions in the short run with instruments such as Iasi/Airs, Amsu should enable to obtain temperature and moisture profiles whose precision would approach that of in situ measurements. Indeed, the synoptic networks of observation (conventional stations and weather radars) prove insufficiently dense to represent the very strong space variability of pluviometry. Mégha-Tropiques and GPM projects, succeeding TRMM, will constitute a constellation of satellites equipped with passive ultra high frequencies sounders associated to a radar, enabling to obtain correct estimates of precipitations. These missions are essential and prepare a future operational follow-up of precipitations from space.
The interdisciplinary program Amma will mobilize most of the scientific community on the study of African monsoon. It is a major opportunity to exploit the space data and to develop synergies between the specialists in the processing of space data and the ones creating models. The Ammasat database was created in order to compare the different outputs, to cross the data obtained with fine resolution with those of the sensors with lower resolution, and especially to exploit jointly satellite products, digital simulations and ground measurements.

OXIDIZING CAPABILITIES, CHEMISTRY AND CLIMATE

The third stake relates to the atmospheric chemical composition role on the evolution of the climate and the evolution of the oxidizing capacity. It intervenes in two areas more particularly :

in stratosphere and the high troposphere where the ozone and the water vapor which are greenhouse gases control the climate,
in low troposphere, where the atmospheric composition changes because of an increase in the anthropic emissions.

In high troposphere/low stratosphere (UTLS), one finds ozone, water vapor, cirrus and aerosols which play a part in the radiative budget of the Earth and thus on the climate of our planet. This area is complexed by phenomena of transport, by multiphasic and microphysics processes influencing the concentrations in ozone and water vapor. The phenomena of dynamics taking place in this area are illustrated on Figure 4. They are transport from the boundary layer towards stratosphere by deep convection in the equatorial area or foliations of tropopause in the average latitudes, giving place to exchanges of mass of air between troposphere and stratosphere.

Figure 4 : Simplified illustration of the dynamic processes of the high troposphere/low stratosphere.

In addition, this area knows disturbances of anthropic origin related to vertical transport by major convection or due to the air traffic, which modifies the contents of the intermediate chemical species of the UTLS controlling the ozone concentrations.

For this reason, the scientific community endeavors to realize the assessment of the traces species in this area: by in situ measurements of traces gases using sounding-balloons, by airborne measurements or by satellite by using infra-red and microwaves spectrometers in order to monitor the ozone and the water vapor.

On a global scale, in low troposphere, various emissions related to the human activity, the biosphere and the bush fires generate an increase in components such as CO, the nitrogen oxides, COVs and, of course, ozone, which will intervene in the oxidizing capability of the atmosphere. Some of these components are currently measured from space by instruments such as Iasi and Gome, but often in the form of integrated column and for not very reactive species (like CO or CH₄).

The current data have a good space cover, but there is a need to define a new instrumentation to reach vertical profiles, with a better temporal and spectral resolution.

The exploitation of existing measurements must continue in parallel, in particular within the framework of the data base Ether, which contains some of the ODIN data, and associate space measurements to ground measurements in network and in situ (balloons, Mozaic).

POLLUTION

The current characteristics of space measurements in physicochemistry are used for the study of the interactions chemiistry/climate, but are insufficient to answer a new stake: the forecast of pollution. One of the major questions for the study of the air pollution is to be able to distinguish, during episodes of pollution, the share of photochemical pollutants, such as ozone, which are produced locally, on behalf of pollutants which were brought by long-distance transport. To answer this question, it is essential to understand the processes in the beginning of the peaks of pollution in the great urban centers, but also to optimize, area by area, the strategy of reduction of the emissions. Until now, several large series of measurements took place around large capitals such as Paris, Athens, Mexico City, etc But they remain specific and do not allow to extrapolate the results over greater periods of time. The recourse to modeling is the alternative which provides interesting information, but strongly dependant on the quality of the inventories of emission. To envision pollution, the atmospheric models also require a temporal resolution lower than the hour, by day or by night, a good vertical resolution with several layers in troposphere (in particular between 2 and 3 km), a good space resolution adequate and representative of the episode of pollution, peak of ozone for example (Figure 5). The infrared sounder SIFTI is under study in the framework of a scientific mission of pollution measurement, as well as a demonstrator for a future operational service for the air quality. The detection and the correction of the fogs and the clouds are necessary.

Figure 5 : Diurnal variation of several tropospheric pollutants in London measured in July 1998, with photochemical production of ozone by day by CO oxidation in the presence of NO_x.

To represent transborder transport of pollution, the space cover of measurements must go from the local scale on a regional scale, even on a continental scale.

The satellites have an important place for the determination of tendencies for a long-term follow-up of the quality of the air. The satellite data constitute essential bases for the test and the validation of the current models of transport and chemistry, the statistical studies, the constitution of emissions maps.
Their exploitation rests on the constitution of databases, on the interpretation of the results of the models, which will be able to confirm the validity of measurements by comparison with those of the networks or those resulting from intensive campaigns, and on the development of suitable techniques of assimilation of the optimal interpolation type, variational methods (4D-Var), Kalman filter.
New technologies are being studied (small sensors, lighter and consuming less energy sensors) which will simplify the calibration, integration and the operations. It is necessary to encourage the use of active sensors of lidars kind, radars with a more reliable, more effective life time and with a reduction of the weight, volume and cost and an increase in the autonomy.