Physical mechanisms controlling self‐aggregation of convection in idealized numerical modeling simulations

• Published in: Journal of advances in modeling earth systems 2014-03, Vol.6 (1), p.59-74
• Main Authors: Wing, Allison A., Emanuel, Kerry A.
• Format: Article
• Language: English
• Subjects: Aggregation; Atmospheric water; Atmospheric water vapor; Cloud systems; Convection; Cooling; Cyclones; Evolution; Feedback; Information systems; Laboratories; Long wave radiation; Mesoscale convective complexes; Modelling; Moist static energy; Negative feedback; Physics; Positive feedback; Radiation; Radiation absorption; Sea surface; Sea surface temperature; self‐aggregation; Surface fluxes; Surface temperature; Water vapor; Water vapour; Wind speed
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1002/2013MS000269

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 nonrotating framework, with interactive radiation and surface fluxes and fixed sea surface temperature (SST). We note that self‐aggregation begins as a dry patch that expands, eventually forcing all the convection into a single clump. Thus, when examining the initiation of self‐aggregation, we focus on processes that can amplify this initial dry patch. 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 and intermediate stages of aggregation; however, it is the dominant positive feedback that maintains the aggregated state once it develops. Importantly, the mechanisms that maintain the aggregate state are distinct from those that instigate the evolution of self‐aggregation. Key Points A new analysis method is used to quantify feedbacks governing self‐aggregation Feedbacks that establish the cluster are different than those that maintain it Positive WISHE feedback opposes a negative surface flux disequilibrium feedback