Cloudy and clear-sky relative humidity in the upper troposphere observed by the A-train

Cloudy and clear-sky distributions of relative humidity with respect to ice (RHI) are derived from the Atmospheric Infrared Sounder and cloud profiles from CloudSat and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellites. A peak frequency in RHI of 10–20% exists for clear sky, increasing to 75–95% within clouds detected by CloudSat. The range of values depends on the season, altitude, latitude, cloud type, and ice water content. Global distributions of RHI reveal persistent supersaturation in the clear and cloudy tropical upper troposphere and lower tropospheric polar regions, large but variable RHI in the cloudy midlatitude storm tracks, and small yet variable RHI in the clear-sky subtropics. Previous studies using satellite and in situ observations have shown a greater frequency of midlatitude supersaturation in the Southern Hemisphere (SH). Seasonal and interannual variations in RHI are also demonstrated with larger frequencies of supersaturation generally present in the winter hemisphere. An analytical method quantifies the impacts of mean temperature (T ) and temperature variance (sT) on the distribution characteristics of RHI. The seasonal variation of RHI in the Northern Hemisphere is primarily modulated by seasonal changes in sT, whereas in the SH, both T and sT modulate variations in RHI. However, variations in T and sT do not explain the presence of slightly greater supersaturation frequency in the SH, suggesting a link between anthropogenic aerosol and ice cloud processes.