REQUIREMENTS AND PERSPECTIVES FOR MW/SUB-MM SOUNDING FROM GEOSTATIONARY SATELLITE

Planning for an MSG-follow-on programme, has already initiated. After a one-year preparatory work, the EUMETSAT 1 st Post-MSG User Consultation Workshop took place on 13-15 November 2001. A major requirement emerged for frequent observation of precipitation. This requirement can partially be addressed by improving the VIR/IR imagery and a possible IR sounding missions, but clearly calls for MW radiometry. In addition, there is a strong requirement to have frequent temperature/humidity sounding in overcast areas, which also calls for MW radiometry. However, it is currently felt that affordability and programmatic constraints are too tight to enable a MW radiometer being baselined for an operational post-MSG satellite. The paper shows that affordability and feasibility become closer if frequencies higher than usual are exploited (54, 118, 183, 380 and 425 GHz) to reduce antenna size; and a different principle is adopted to observe precipitation (in absorption channels for temperature and humidity profiling rather than in atmospheric windows). Temperature and humidity profiles retrieved at different frequencies are differently affected by liquid and ice water, drop size and shape, and precipitation; therefore, in principle, simultaneous retrieval of temperature/humidity profiles, cloud ice/liquid water columnar content or gross profile and precipitation is possible. Being in absorption bands, the observation is equally available over sea and land. Profiles will have coarser vertical resolution than what is achievable in IR, but will be all-weather and as frequent as dictated by the driving precipitation mission (e.g., at 15-min intervals). These ideas are supported by several airborne experiments, to continue in the near future. The paper will dwell on the physical principle highlighting potentials and limitations, and introduce the concept of GOMAS (Geostationary Observatory for Microwave Atmospheric Sounding) originated from an American concept (GEM, Geostationary Microwave Observatory). It will be shown that, if implemented as a dedicated mission, GOMAS could be a relatively small satellite and that current state-of-the-art technology is sufficient to make possible a demonstration mission before 2010, the only limitation being that the 15-min rate is possible over a regional scale as wide as 1/12 of the Earth visible disk, abundantly covering Europe.