COMPARISON OF CLOUD FREQUENCY DATA FROM THE HIGH-RESOLUTION INFRARED RADIOMETER SOUNDER AND THE STRATOSPHERIC AEROSOL AND GAS EXPERIMENT-II

Climatologies of cloud frequencies were compared from two satellite se nsors, the high-resolution infrared radiometer sounder (HIRS) and the Stratospheric Aerosol and Gas Experiment II (SAGE II), during two summ ers (June, July, and August 1989 and 1990) and two winters (December, January, and February 1989-1990 and 1990-1991). Both sensors were oper ating simultaneously during these periods, which spanned 12 months. Th e two data sets generally agreed on the locations of clouds; however, SAGE II reported higher frequencies of clouds by 12-22 percentage poin ts. The differences in the frequencies of clouds appear to be caused b y differences in sensor sensitivity and scanning field of view sizes, both of which would have the same effect. SAGE II has a higher sensiti vity to thinner clouds and a larger field of view than the HIRS. The v ertical distributions of clouds also show close agreement except for t he low levels (below 3 km) and the upper troposphere where SAGE II rep orted more clouds. The very thin subvisual cirrus detected only by SAG E II also were generally found in the same locations where HIRS report ed denser clouds, indicating a relationship between subvisual cirrus a nd other cloud forms.