We present experiments using the MM5 mesoscale model in an idealized configuration to study the extent to which deep tropical convection forced by horizontally homogeneous boundary conditions can 'self-aggregate' into tropical circulations of length scales 1000 km or more. This issue is fundamental to the understanding and forecasting of transient variability of synoptic and large-scale tropical circulations. The model domain is nonrotating, and either doubly periodic or a periodic channel with walls at the meridional boundaries. Sea surface temperature is constant, and a large-scale vertical velocity profile is imposed uniformly to represent the effects of large-scale convergence. Control runs are done at 15 km horizontal resolution and use the Kain-Fritsch convective parameterization in addition to a simple explicit moisture scheme. Previous work with this model configuration has shown that, under certain favorable conditions, convection in the model tends to organize on scales up to and including the domain scale, which in the present experiments is 2250 km in the longitudinal dimension and 600 km in the meridional. The present study extends this previous work by attempting to diagnose in more detail the mechanisms controlling the model's behavior. New experiments are performed which are identical to the control runs, except for changes in horizontal resolution, convective parameterization, or both. These runs include runs with no convective parameterization at horizontal resolutions of 5 km and 15 km, and runs at 15 km horizontal resolution in which we vary the trigger function in the Kain-Fritsch scheme in order to investigate the extent to which the convective organization may be under "activation control". One noteworthy preliminary result is that the run with 5 km horizontal resolution and no convective parameterization shows much less large-scale organization than do any of the 15 km runs. Another is that the 15 km runs with and without the convective parameterization are much more similar than might have been expected in a number of respects, including the degree and type of convective organization.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics