a. Development of appropriate data assimilation techniques
Satellites can observe the Earth with nearly complete global coverage every day, and also provide continuous observations of particular regions. They also offer the only practical method of obtaining valuable data over much of the world, especially the oceans and remote land areas. It is therefore prudent to make sure that we can use these data as effectively as possible.
The aim of data assimilation is the combination of our theoretical understanding encapsulated in a model, with measurements (which may be sparse), and any a priori information that we might already have (e.g. climatologies). In this way we can analyse and/or forecast the state of the atmosphere at any given time or place. Assessment of model and observation errors will guide model and instrumental algorithm improvement [e.g. Lary 1999]. Data assimilation techniques enable us to exploit information elegantly that is simply not available by using models or observations independently. Due to the complexity of the system, and the spatial inhomogeneity of the observations, the analysis of chemical trace species has received little attention in comparison with the analysis of meteorological variables. Satellite observations of multiple species have only become available relatively recently, while meteorological satellite observations were among the first to be made.
Data assimilation analysis can provide guidance on how, when, and where additional measurements are necessary (see also task 3) and point to possible redundancy. It can answer the question which, how many, and where instruments should be employed. Software already available includes Optimum interpolation, 4D-Var, and the Kalman filter data assimilation tools.
b. Specification of requirements for new satellite instruments
Measuring chemical species in the troposphere from space is at the forefront of current science and technology and a clear need is for the development of new observing techniques, concepts, and strategies for space borne instruments. In order to define future requirements for satellite measurements of the troposphere, there is a need to understand the fundamental science, policy and also the technological limitations. As questions arise about the changing nature of the troposphere, it is essential to develop and to take advantage of these new methods and techniques and to integrate them with both scientific and policy requirements. Within the next few years there will be opportunities to propose new satellite missions, some of which are likely to address the requirements of measuring the troposphere from space.
TROPOSAT will provide a European forum for development of concepts for instruments and platforms aimed at observing tropospheric parameters from space. Specifically, new concepts such as GEO-SCIA, observing the troposphere from a geostationary orbit, require an understanding of the scientific and technological advantages and limitations of making diurnal rather than sun-synchronous measurements from space. New options are likely to be available for space borne instrumentation with the growing commercial use of space and the use of small satellites as in the IRIDIUM and GLOBALSTAR networks, which will require smaller and lighter instrumentation developed over shorter time scales. Industry and the scientific community should be encouraged to participate at an early stage in the definition of and priority for new satellite instrumentation for measuring the troposphere and so be allowed to influence the future prospects for future tropospheric measurements and science. Within the bounds of EUROTRAC-2, TROPOSAT will develop a European wide approach for specifying the requirements for new instrumentation and techniques.