Abstract
The propagation characteristics and spatial distributions of cloud cover associated with synoptic-scale circulation systems are studied using both satellite data processed by the International Satellite Cloud Climatology Project (ISCCP) and surface observations taken at weather stations or ships of opportunity. These two independent datasets produce comparable climatological patterns of the preferred direction and speed of the cloud movements. Such “propagation vectors” based on the ISCCP products depict cloud motions at a higher altitude than that of the corresponding movements deduced from surface synoptic reports.
By application of a similar composite procedure to the ISCCP and surface datasets, a detailed comparison is made between the satellite and surface observations of the organization of various cloud types in midlatitude cyclones. The relationships of these cloud patterns with the concurrent atmospheric circulation are illustrated by superposing the composites of wind and geopotential height on the cloud data. Both the satellite and surface observations yield well-defined cloud patterns organized about frontal zones. These cloud patterns are consistent with the composite distributions of weather types as coded in the surface reports. Combination of findings from the surface reports (which provide information on the altitude of the cloud base) and those from the satellite observations (which offer estimates of cloud-top heights) is found to be helpful for portraying the vertical distribution of cloud cover in various sectors of the extratropical cyclones. The ISCCP dataset underestimates the amount of low stratiform clouds lying under the high, optically thick cloud shield in the warm sector. Comparison between the composite results for cyclones occurring over the North Atlantic and those over the North American continent indicates the occurrence of larger anomalous cloud amounts in the continental systems. The more widespread cover by nimbostratus cloud in the northwestern sector of the surface low pressure center over land is particularly noteworthy. The distinction between the cloud organization in maritime and continental cyclones is seen to be related to the stronger, frictionally induced cross-isobaric flow over land surfaces.
Analogous composite charts are constructed for the convective disturbances occurring over the tropical western Pacific by using both satellite and surface cloud observations. Some degree of consistency is again discernible among the cloud patterns based on the two datasets. The ISCCP data tend to underestimate the amount of cumulus clouds situated below the anvils at the trailing end of these convective systems.
Corresponding author address: Dr. Ngar-Cheung Lau, GFDL/NOAA, Princeton University, P.O. Box 308, Princeton, NJ 08542.
Email: gl@gfdl.gov
