Self-aggregation and large-scale control of tropical deep convection: A modeling study

The Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model version 5 (MM5) is used to investigate whether supercluster s represent a mode of self-aggregation of tropical deep convection that occ urs spontaneously under horizontally uniform boundary conditions and large- scale forcings. The nonhydrostatic version of MM5 is run with prescribed domain-averaged ve rtical velocity and periodic boundary conditions in both east-west and nort h-south directions. Domain-averaged horizontal winds are relaxed to a speci fied reference wind profile. Two vertical profiles of mean ascent are used. One has an elevated maximum at the upper troposphere and near-zero vertica l 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 sho w significant differences in convective organization. The run with elevated forcing develops larger cloud clusters than the run with midtropospheric f orcing, suggesting some degree of self-aggregation under favorable large-sc ale forcings. A Fourier analysis of the precipitation organization in the e levated forcing run indicates considerable variance in propagating waves of wavelength 1000-2000 km in which convective heating is positively correlat ed with temperature and moisture anomalies. Sensitivity tests show that the long-wavelength organization does not require horizontal variability of su rface fluxes and so cannot be explained by wind-induced surface heat exchan ge (WISHE)-type mechanisms. Sensitivity tests of model results to magnitude and vertical distribution of forcings, cloud-radiation feedbacks, referenc e wind profiles, and grid resolution are also conducted.