Self-Aggregation and Large-Scale Control of Tropical Deep Convection: A Modeling Study

Abstract

The Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model version 5 (MM5) is used to investigate whether superclusters represent a mode of self-aggregation of tropical deep convection that occurs spontaneously under horizontally uniform boundary conditions and large-scale forcings. The nonhydrostatic version of MM5 is run with prescribed domain-averaged vertical velocity and periodic boundary conditions in both east?west and north?south directions. Domain-averaged horizontal winds are relaxed to a specified reference wind profile. Two vertical profiles of mean ascent are used. One has an elevated maximum at the upper troposphere and near-zero vertical velocity in the lower troposphere. The other peaks at the midtroposphere, representing the vertical velocity distribution of convective towers only. Simulations with the same initial conditions but two different forcings show significant differences in convective organization. The run with elevated forcing develops larger cloud clusters than the run with midtropospheric forcing, suggesting some degree of self-aggregation under favorable large-scale forcings. A Fourier analysis of the precipitation organization in the elevated forcing run indicates considerable variance in propagating waves of wavelength 1000?2000 km in which convective heating is positively correlated with temperature and moisture anomalies. Sensitivity tests show that the long-wavelength organization does not require horizontal variability of surface fluxes and so cannot be explained by wind-induced surface heat exchange (WISHE)-type mechanisms. Sensitivity tests of model results to magnitude and vertical distribution of forcings, cloud?radiation feedbacks, reference wind profiles, and grid resolution are also conducted.