The PSU/NCAR mesoscale model MM5 is used to investigate the mechanisms responsible for the 'superclustering' of tropical deep convection. In particular, we are interested in whether mesoscale cloud clusters and super cloud clusters represent a mode of 'self-aggregation' of convection, or they result from the interaction of large-scale circulation with convective systems. Here, 'self-aggregation' refers to convective organization that occurs spontaneously under horizontally uniform boundary conditions and large-scale forcing.
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 types of average vertical velocity profiles are used. One is associated with observed mature clusters (MC) profile, which is characteristic of elevated maximum at upper troposphere and near zero net vertical velocity in the lower troposphere. The other (referred to as CP) represents vertical velocity profile of convective towers only, which peaks at middle-troposphere.
Simulations with the same initial conditions but two different forcings show significant difference in convective organization. The run with MC forcing develops much larger cloud clusters than the run with CP forcing. The model reaches quasi-equilibrium after 40 hours integration. The equivalent radius of ensuing clusters in the MC run can be as large as 400 km, while no clusters bigger than 300 km in equivalent radius form in the CP run after the model reaches quasi-equilibrium. This result suggests that there exists a certain degree of 'self-aggregation' under favorable large-scale forcings. The cloud organization in the MC run includes propagating waves of wave length 1000-2000 km in which convective heating is positively correlated with temperature and moisture anomalies. The vertical thermal structure is similar to wave-CISK modes. Sensitivity tests show that such modes do not require horizontal variability of surface fluxes and so cannot be explained by WISHE-type mechanisms. Further analysis and sensitivity studies are
in progress.