A Composite Life Cycle of Nonsquall Mesoscale Convective Systems over the Tropical Ocean. Part II: Heat and Moisture Budgets

Abstract

The heat and moisture budgets associated with five large nonsquall cloud clusters observed during Phase 3 of the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) are investigated. The input data for the budget computations are objectively analyzed fields of wind, temperature and relative humidity that were based on conventional upper-air soundings. Estimates of the radiative heating rate were obtained from Cox and Griffith. A compositing technique is used to summarize the budget results for the growing, mature and dissipating stages of the clusters. The budgets in the growing stage are characterized by a very large low-level, apparent moisture sink separated in height from the region where the apparent heating is realized. In the mature stage, the apparent heating maximum shifts upward, accompanied by the development of a corresponding secondary maximum of apparent drying. A composite of radiative heating estimates from Cox and Griffith shows that the horizontal radiative heating gradients reach their maximum strength during the mature stage. In the dissipating stage, the apparent heat source is approximately balanced by the apparent moisture sink above the freezing level; below the freezing level, the implied vertical convective flux of sensible and latent heat is approximately constant with height. The time-dependent behavior of the budgets gives support to the hypothesis of Leary and Houze that the widespread upper-level cloud decks associated with cloud clusters play an active and important role in determining large-scale beat and moisture budgets in the tropics.