INFLUENCE OF TROPICAL CLOUD SYSTEMS ON THE RELATIVE-HUMIDITY IN THE UPPER TROPOSPHERE

The influence of tropical clouds on the humidity in the upper troposphere is investigated. Upper tropospheric humidity (UTH) in the 550-200 mbar layer is retrieved from GOES 7 6.7-mu m brightness temperatures and validated against humidity profiles from radio soundings and European Centre for Medium-Range Weather Forecasts analyses. UTH decreases from values of 44-74% near the edges of upper tropospheric clouds to 11-15% at a distance of about 500 km from the cloud edge and remains relatively constant for greater distances. The average UTH at the cloud edge is lower in land regions and in regions of suppressed convection than in the Intertropical Convergence Zone (ITCZ). The spread of UTH from cloudy to clear regions is least in the regions of suppressed convection and greatest in the ITCZ. It is shown that the UTH at the cloud edge and its spread into cloud-free regions both increase with the average cloud top altitude. Using particular geographical regions as examples, it is demonstrated that UTH seems to increase linearly with the amount of upper level cloud. A diurnal cycle of UTH in cloud-free areas is observed and related to diurnal variations in cloud amount. The diurnal variation in UTH lags behind the diurnal cycle in upper tropospheric clouds by about 8 hours.