Numerical Simulation of Deep Tropical Convection Associated with Large-Scale Convergence

Abstract

A set of four-hour simulations has been carried out to study deep moist convection characteristic of the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE). The present model includes warm rain bulk cloud physics and effects associated with a large-scale, time-invariant convergence. The convection took approximately two hours to develop from a random moisture disturbance. The cloud efficiency, in terms of the total water vapor condensed, was near 40%. The heat and moisture budgets and the time?mean vertical fluxes of mass, heat, and moisture were calculated for the last 80 minutes of the simulations. In this study the primary emphasis was placed upon run A, the three-dimensional calculation. For this calculation, the layer centered near 4.0 km was a region of low mean cloudiness but of strong convection. The upward mass flux was strong and upward heat and moisture fluxes had maximum values in this layer. The strongest downward mass flux was due to weak downward velocities in the rainy area below cloud base. Time-mean data were also calculated for vertical velocity cores and compared with observed data. In run A, virtually all updraft cores are in-cloud and for a deep layer between 2.5 and 8.0 km the in-cloud up-ward mass flux is nearly all associated with cores. In this layer the upward mass flux due to cores is approximately twice the mass flux associated with the large-scale convergence. The fractional area of updraft cores is small, varying between 2.5% and 4.0% for vertical levels between 1 and 11 km. Calculated values of core diameter D? are in relatively good agreement with the observed data. For values of mean vertical velocity ??, however, the agreement is not nearly as good. For downdraft cores, values of ??, are significantly smaller than the observations. For updraft cores, values of ? at lower levels are small, whereas values in the upper levels are in reasonable agreement with observations. The weak updraft cores at lower levels may be related to the absence of strong gust fronts in the present simulations.