Fluctuations of Cloud, Humidity, and Thermal Structure near the Tropical Tropopause

Abstract

Thermal and humidity structures near the tropical tropopause are studied using microwave satellite retrievals of water vapor, along with contemporaneous dynamical structure in ECMWF analyses and cold clouds in high-resolution global cloud imagery. Examined during November 1991?February 1992, these fields all vary coherently with the outflow from convective centers?in the upper troposphere as well as in the lowermost stratosphere. The outbreak of deep convection is accompanied by diabatic heating below a level between 250 and 150 mb but by diabatic cooling at higher levels. The reversal from heating to cooling is broadly consistent with cumulus detrainment. Through irreversible mixing, that process serves as a heat source for the environment below the level of neutral buoyancy (LNB) but as a heat sink at higher levels. Calculations, inclusive of entrainment, place the LNB very near the observed reversal from heating to cooling. The outbreak of convection is also accompanied by humidification below 125 mb but by dehydration at higher levels. The reversal from humidification to dehydration coincides with levels where environmental conditions approach saturation. Those conditions suggest the efficient removal of total water from cumulus updrafts, leaving dessicated air to ventilate to higher levels. Cumulus detrainment then acts to humidify the environment beneath the zone of nearly saturated environmental conditions, while dehydrating it at higher levels. Dry air emerges from the region of the coldest cloud. It then extends into the winter hemisphere, along streamlines that characterize the Hadley circulation. Coinciding with diabatic cooling are stratospheric convergence and downwelling. These features of stratospheric motion amplify simultaneously with divergence at tropospheric levels, which represents the major outflow from deep convection. The deepest convection, found over the equatorial Pacific, coincides with the highest moist static energy. The latter yields an LNB that is some 3 km higher over the equatorial Pacific than elsewhere, in agreement with the observed reversal from heating to cooling. Observed brightness temperatures place the level at which cumulus anvils are most extensive very near the cold point over the equatorial Pacific. This, in turn, lies near the tropical tropopause throughout the Tropics. Collectively, these features suggest that the coldest cloud, found over the equatorial Pacific, plays a key role in maintaining temperature and humidity near the tropical tropopause.