CLOUD FIELD IDENTIFICATION FOR EARTH RADIATION BUDGET STUDIES .1. CLOUD FIELD CLASSIFICATION USING HIRS-MSU SOUNDER MEASUREMENTS

Onboard the NOAA satellites, the High-Resolution Infrared Sounder (HIR S) with its 20 channels, combined with the Microwave Sounding Unit (MS U), provides a powerful tool for cloud field classification at a spati al resolution of about 100 km. The 3I (improved initialization inversi on) algorithm-developed to obtain atmospheric temperature and water va por profiles as well as cloud and surface properties-has been modified in order to extract more reliable information on cloud-top pressure a nd effective cloud amount. These cloud parameters have been compared t o cloud types identified by an operationally working threshold algorit hm based on Advanced Very High Resolution Radiometer measurements over the North Atlantic. The improved 3I cloud algorithm provides cloud pa rameters not only for high clouds but also greatly improves the determ ination of low clouds. The algorithm has also been extended to give cl oud information over partly cloudy situations. The 3I cloud field clas sification yields 11 different cloud field types for spatial elements of 100 km according to cloud height, cloud thickness, and cloud cover. The radiative effects of these different cloud field types are studie d by combining the 3I results with Earth Radiation Budget Experiment ( ERBE) fluxes. A simple radiative transfer theory can relate the ERBE o utgoing longwave flux to all 3I cloud field types to within 5 W m(-2). This encourages a detailed analysis of cloud radiative effects on a g lobal scale. Especially during night, as shown in this study, Internat ional Satellite Cloud Climatology Project (ISCCP) cloud information ca n be extended by the HIRS-MSU analysis, because the ISCCP provides inf ormation on cloud thickness only during day.