Comparison of cloud frequency data from the high-resolution infrared radiometer sounder and the Stratospheric Aerosol and Gas Experiment II

Abstract

Climatologies of cloud frequencies were compared from two satellite sensors, the high-resolution infrared radiometer sounder (HIRS) and the Stratospheric Aerosol and Gas Experiment II (SAGE II), during two summers (June, July, and August 1989 and 1990) and two winters (December, January, and February 1989-1990 and 1990-1991). Both sensors were operating simultaneously during these periods, which spanned 12 months. The two data sets generally agreed on the locations of clouds; however, SAGE II reported higher frequencies of clouds by 12-22 percentage points. The differences in the frequencies of clouds appear to be caused by differences in sensor sensitivity and scanning field of view sizes, both of which would have the same effect. SAGE II has a higher sensitivity to thinner clouds and a larger field of view than the HIRS. The vertical distributions of clouds also show close agreement except for the low levels (below 3 km) and the upper troposphere where SAGE II reported more clouds. The very thin subvisual cirrus detected only by SAGE II also were generally found in the same locations where HIRS reported denser clouds, indicating a relationship between subvisual cirrus and other cloud forms.