Cloud optical depth from UW-NMS and GEOS-DAS and comparisons with MODIS and ISCCP satellite observations

Cloud optical depth is a critical parameter needed to describe the radiative effects of clouds on climate and tropospheric photochemistry. Current estimates of its magnitude and distribution, however, have large uncertainties. We use the University of Wisconsin Non-hydrostatic Modeling System (UW-NMS), a regional-scale meteorological prediction model where distributions of cloud water are explicitly predicted, to simulate cloud optical depth in the Asian Pacific region in spring 2001. On the other hand, empirically diagnosed global cloud optical depth for all seasons of the same year is available from the GEOS-3 archive of the Goddard Earth Observing data assimilation system (GEOS-DAS) at the NASA Global Modeling and Assimilation Office (GMAO). The calculated cloud optical depths from these two modeling systems are compared with the satellite retrieval products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the International Satellite Cloud Climatology Project (ISCCP). Both UW-NMS and GEOS-3 reproduce the prominent features in the satellite data. In particular, both models capture the variabilities in cloud optical depth associated with synoptic-scale frontally induced cloudiness in the Asian Pacific region during spring. A preliminary comparison of model monthly zonal mean cloud optical depth with satellite observations shows that model cloud optical depths are closer to the MODIS than ISCCP retrievals in terms of both magnitude and latitudinal distribution. The discrepancies between the model and the satellite retrievals and the differences among the satellite products will also be discussed.