Clouds as seen by satellite sounders (3I) and imagers (ISCCP). Part I: Evaluation of cloud parameters

The improved initialization inversion (3I) algorithms convert TIROS-N Opera tional Vertical Sounder observations from the National Oceanic and Atmosphe ric Administration (NOAA) polar-orbiting environmental satellites into atmo spheric temperature and water vapor profiles, together with cloud and surfa ce properties. Their relatively good spectral resolution and coverage make IR sounders a very useful tool for the determination of cloud properties bo th day and night. The iterative process of detailed comparisons between clo ud parameters obtained from this global dataset, which is available in the framework of the NOAA-National Aeronautics and Space Administration Pathfin der Program, with time-space-collocated observations of clouds from the rec ently reprocessed international Satellite Cloud Climatology Project (ISCCP) dataset has led to an improved 3I cloud analysis scheme based on a weighte d-chi(2) method described in the second article of this series. This proces s also provides a first evaluation of the ISCCP reanalysis. The new 3I clou d scheme obtains cloud properties very similar to those from ISCCP for homo geneous cloud scenes. Improvement is especially notable in the stratocumulu s regimes where the new 3I scheme detects much more of the low-level cloudi ness. Remaining discrepancies in cloud classification can now be explained by differences in cloud detection sensitivity, differences in temperature p rofiles used, and inhomogeneous or partly cloudy fields. Cirrus cloud ident ification during the daytime in the recent ISCCP dataset is improved relati ve to the first version of ISCCP, but is still an underestimate. At night o nly multispectral IR analyses like 3I can provide cirrus information. The r eprocessed ISCCP dataset also shows considerable improvement in cloud cover at higher latitudes. Differences in 3I and ISCCP summertime cloud cover ov er deserts may be caused by different sensitivities to dust storms.