Venezuelan Rain Systems and the General Circulation of the Summer Tropics I: Rain Systems

Abstract

It was a main objective of the Venezuela experiments in 1969 and 1972 to examine the links between low and high atmosphere furnished by mesoscale rain areas and embedded undilute towers and, as far as possible, to attain a quantitative description of the role of cumulonimbi in tropical weather systems. A mesoscale model could be defined with precipitation yield of 2.5 cm in 2 h over an area of 2000 km2. The ratio of cumulonimbus precipitation to the condensate in undilute updrafts is 0.5 compared to zero in the trades and 1.0 in hurricane cores. Heat is eroded from cumulonimbus towers to the environment in the upper troposphere. Freezing processes, previously not considered, contribute substantially to heating the upper troposphere. These two mechanisms compensate for about half the radiation cooling in the tropical belt. The other half must be derived from sinking motion with downward mass transport attaining one-quarter of the ascent in the towers. The synoptic-scale rain areas and their mesosystems are subject to external forcing in the Caribbean area, mainly extratropical troughs intruding into the tropics from the north and large cloud lines progapating northward from southern South America. The inflow into the mesosystems usually is at their forward edge in low levels. But in the middle troposphere the air overtakes the radar echoes, suggesting a mechanism for limiting their life. The thermal structure of rain areas is stable, confirming that the mesosystem rain areas cold-cored with respect to their environment at least to the middle troposphere. Only the concentration of ascent in undilute towers permits the tropical rain system to do swork maintaining the general circulation.