Diurnal variation of upper tropospheric humidity over the tropics and its relations to convective activities

Diurnal variation of upper tropospheric humidity (UTH) and its relations to deep convection and high clouds were studied over the tropical Indian Ocean and tropical Africa using European Geostationary Meteorological Satellites (i.e. Meteosat 5 and 8) measurements. Distinct diurnal variation of UTH is noted over regions where tropical deep convective cloud systems are commonly observed. The amplitude appeared larger over land. By contrast convectively inactive subtropical regions do not show meaningful diurnal variations of UTH. The land-sea contrast is also found in diurnal phases. UTH tends to peak around 0300 LT over land, lagging deep convection (~1800 LT) and high cloudiness (1800-2400 LT). In contrast, considering that deep convection and high cloud maximums over the ocean exist around 0600-0900 LT and 1500-1800 LT, respectively, the time lag of maximum UTH (around midnight) appears larger in comparison to those over the land. Phase differences between deep convection, high clouds and UTH suggest that the tropical upper troposphere is moistened by detrainment of moist air from deep convection core or evaporation of high clouds. This is somewhat contradictory to the recent finding that the contribution of direct evaporation of cloud condensates to the UTH is minor (Soden, 2004).