Relationship between upper tropospheric humidity and deep convection

Abstract

Satellite measurements of upper tropospheric humidity (UTH) are used together with global cloud imagery to study the relationship between deep convection and moisture in the upper troposphere. Throughout the tropics, regions of cold cloud coincide with regions of enhanced UTH,in time-mean distributions, in daily synoptic maps, and even on the short scales that characterize individual convective systems. Cold-cloud fraction and UTH have very similar instantaneous structure. The horizontal correlation scales are narrow for both cloud fraction and UTH at 315 hPa and 215 hPa. The correlation scale of UTH expands at 146 hPa, signaling an increased role of horizontal transport. Ventilation of the upper troposphere by convection is also evidenced by thermal structure: Inside regions of deep convection the vertical separation of θ surfaces is expanded, stability being weakened by convective mixing inside a deep layer centered near 200 hPa. Supported by a cloud detrainment, convection humidifies the upper troposphere, elevating mixing ratio surfaces over their surroundings. The reverse is observed at subtropical latitudes, where air is anomalously dry. Jointly, these features characterize vertical transport by the Hadley and Walker circulations. Copyright 2001 by the American Geophysical Union.