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. 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.

This statistical relationship between cloud type and vertical relative humidity profile is applied to the VIRS and TMI data on TRMM. From VIRS data, we can claasify cloud types. From the coincident TMI data, we can estimate total precipitable water. With the constrain of the total precipitable water by TMI, the statistical relationship between cloud type and vertical relative humidity profile is used to estimate the vertical profile of relative humidity.