Comparison of passive microwave sounding data with numerical weather prediction profiles

The Advanced Microwave Sounding Unit (AMSU), launched on May 13 1998, is a twenty-channel passive microwave radiometer designed to provide information on atmospheric temperature and humidity structure in clear and cloudy conditions, complementing existing infra-red radiometers which provide information only in clear areas. AMSU has some channels similar to those flown on existing missions and others which are new. Observed radiances from the Special Sensor Microwave Imager (SSMII) and the Microwave Sounding Unit (MSU) are compared with radiances calculated from numerical weather prediction (NWP) model profiles, and the differences discussed. A processing method for ATOVS radiances is described, and differences from previous techniques are highlighted. An initial evaluation of AMSU data is given. Finally the impact of passive microwave observations on the skill of numerical weather forecasts is discussed. Key-words: Numerical weather prediction, ATOVS, AMSU, MSU, SSM/I, preprocessing, idvar, radiance, assimilation