Scientific background for CLOUDS - a Cloud and Radiation monitoring satellite

CLOUDS is a project co-funded by the EC under FP-4 (Environment and Climate), conducted by 12 European partners (7 scientific institutes and 5 industrial companies), also cooperating with NOAA/ETL. It is the mission study of a monitoring satellite to perform measurements necessary to describe cloud-radiation interaction in operational models for climate and long-term weather prediction. Complementary to missions for process study (Picasso-Cena, CloudSat,...), CLOUDS addresses the monitoring aspect. As such, it has to comply with requirements of sufficiently frequent observing cycle, and operational sustainability. This prevents using active systems (radar and lidar) and leads to consider passive radiometry only, however exploiting as much as possible of the e.m, spectrum, with more polarisations and more viewing geometries. The paper reports on the effort to demonstrate that, by these means, the main ingredients of the cloud-radiation interaction mechanism may be observed with sufficient accuracy. The optimal channels (central wavelength or frequency, bandwidth, radiometric accuracy, absolute calibration, IFOV, polarisations, viewing geometry) are determined. Clouds, aerosol radiation and precipitation are observed under identical geometry (conical scanning with fore- and aft- viewing) within a range of the e.m. spectrum spanning from 340 nm to 4.3 cm, i.e. over five orders of magnitudes, for a true multi-spectral approach.