Onboard the NOAA satellites, the High-Resolution Infrared Sounder (HIRS) with its 20 channels, combined with the Microwave Sounding Unit (MSU), provides a powerful tool for cloud field classification at a spatial resolution of about 100 km. The 3I (improved initialization inversion) algorithm-developed to obtain atmospheric temperature and water vapor profiles as well as cloud and surface properties-has been modified in order to extract more reliable information on cloud-top pressure and effective cloud amount. These cloud parameters have been compared to cloud types identified by an operationally working threshold algorithm based on Advanced Very High Resolution Radiometer measurements over the North Atlantic. The improved 3I cloud algorithm provides cloud parameters not only for high clouds but also greatly improves the determination of low clouds. The algorithm has also been extended to give cloud information over partly cloudy situations. The 3I cloud field classification 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 studied by combining the 3I results with Earth Radiation Budget Experiment (ERBE) fluxes. A simple radiative transfer theory can relate the ERBE outgoing 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 global scale. Especially during night, as shown in this study, International Satellite Cloud Climatology Project (ISCCP) cloud information can be extended by the HIRS-MSU analysis, because the ISCCP provides information on cloud thickness only during day.

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