Statistical Relationship between ISCCP Cloud Type and Vertical Relative Humidity Profile

Abstract

The statistical relationship between ISCCP (International Satellite Cloud Climatology Project) cloud type, and the vertical relative humidity profile observed by radiosonde, was studied during the period from August, 1992 to July, 1994. The ISCCP-DX data of GMS (Geostationary Meteorological Satellite) were used to obtain cloud type information over the 1.5 degree grid area centered at the radiosonde station. To obtain the reliable cloud type classification, the data at 00 UTC-when the visible data were available-were used. The radiosonde observations when the 1.5 degree grid area is covered by single cloud type were used to compare the vertical relative humidity profile. Four cloud types of low-level, middle-level, cirrus-type and cumulonimbus-type cloud, and cloud-free case were selected for the comparison. As was expected, the mean relative humidity was largest at all levels when the cumulonimbus-type cloud existed, and was smallest at all levels when no cloud existed. The relative humidity was larger than that of a cloud-free case at middle- and low-level (low-level) when middle-cloud (low-cloud) existed. When cirrus cloud existed, the high level relative humidity was larger than that when low- and middle-cloud existed, and showed local maximum at 400-500 hPa. The feasibility for the use of the statistical relationship in this study in estimating relative humidity profile was shown as a case study by comparing with the current JMA statistical relationship derived from the GMS single infrared data. Cirrus clouds are difficult to be identified objectively from single infrared measurement and are often misclassified as middle- or low-level cloud. The misclassification between cirrus-type cloud and middle-/low- level cloud leads to a different vertical profile of water vapor. However, cirrus-type cloud is reasonably identified in the ISCCP with the use of visible and infrared measurements.