Geographical patterns of the cloud amount derived from the ISCCP and their correlation with the NCEP reanalysis datasets

The International Satellite Cloud Climatology Project (ISCCP) has so far accumulated cloud observations for over two decades, presenting the advantage to ascertain climatological characteristics of global clouds. This study focuses on the geographical patterns of high, middle, low and total cloud amount (CA), along with their seasonal variations on global scale. The total CA is found to be quite uneven, with local maximum higher than 90 and evident gradient near coastlines. The mean total CA (unit:%) is 71.6 and 55.9, over ocean and land, respectively, with a global mean of about 66.5. The zonal mean shows that the Southern midlatitudes have the maximum and Northern subtropics are covered by the minimum CA. The averaged CA over the poles is near 69, but with larger uncertainty. The oceanic CA is slightly higher during spring and summer, while it is almost changeless over land. Low-clouds mostly occur over the southeastern Pacific and northeastern Pacific near the western coast of America. The Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ) are covered by quite more high-clouds, due largely to strong active convection in these areas. The areas predominated by lowclouds or high-clouds are separated for further analysis of their respective circulation features with the result that these two category clouds are corresponding to the distinctly different vertical and horizontal wind field from each other. High-clouds are associated with strong updrafts from surface to the tropopause, while low-clouds are accompanied by combination of middle-upper downdraughts with a weak boundary-layer ascending motion. The vertical variation of divergence in the lowcloud areas is just converse to that in the high-cloud areas. Similar result is found for the vertical profiles of vapor divergence, from which the difference of vapor divergence between the two levels in the lower troposhere is shown to be instructive for the spatial pattern of low-clouds as well as high-clouds.