Environmental Science and Engineering Seminar

Cloud cover and relative humidity in the tropics are strongly influenced by organized atmospheric convection, which occurs across a range of spatial and temporal scales. One mode of organization that is found in idealized numerical simulations is "self-aggregation", a spontaneous transition from randomly distributed convection to organized convection despite homogeneous boundary conditions. We elucidate the physics of self-aggregation by applying a new diagnostic technique to the output of a cloud resolving model. Specifically, the System for Atmospheric Modeling is used to perform 3-d cloud system resolving simulations of radiative-convective equilibrium in a non-rotating framework, with interactive radiation and surface fluxes and fixed sea surface temperature (SST). We introduce a novel method to quantify the magnitudes of the various feedbacks that control self-aggregation within the framework of the budget for the spatial variance of column - integrated frozen moist static energy. The absorption of shortwave radiation by atmospheric water vapor is found to be a key positive feedback in the evolution of aggregation. In addition, we find a positive wind speed-surface flux feedback whose role is to counteract a negative feedback due to the effect of air-sea enthalpy disequilibrium on surface fluxes. The longwave radiation feedback can be either positive or negative in the early stages of aggregation; however, it is the dominant positive feedback that maintains the aggregated state once it develops. Importantly, we find that the mechanisms that maintain the aggregate state are distinct from those that instigate the evolution of self-aggregation. We also explore the influence of domain geometry on the mechanisms and temperature-dependence of self-aggregation of tropical convection. The results of simulations employing a highly elongated 3-d channel domain, in which self-aggregation takes the form of multiple moist and dry bands, are compared to that of a square domain, in which self-aggregation takes the form of a single moist cluster. In both cases, the physical mechanisms, growth rates, and spatial scales of the aggregation are characterized.