Convective Aggregation and the Size Distribution of Updrafts

Aggregated, or clustered, convection has demonstrably different properties than ordinary, isolated convective cells: vertical motion and precipitation tend to be stronger, and the surrounding non-convective region tends to be drier. Aggregation occurs in the natural world, and idealized convection permitting simulations often exhibit aggregation above a critical sea surface temperature threshold. It is not clear, however, how aggregation in these idealized simulations relates to aggregation in the natural world, or whether we should expect aggregation to become more common (as the idealized simulations might suggest) as the surface temperature warms.

A previous study has shown that the size distribution of updrafts in a high-resolution reanalysis closely follows a powerlaw (Donner, L. J., T. A. O’Brien, D. Rieger, B. Vogel, and W. F. Cooke, 2016, doi:10.5194/acp-16-12983-2016). This result could imply two main situations: (1) that normal convection and aggregated convection occur along a continuum, or (2) that convection falls along a continuum of updraft sizes, and aggregation alters that continuum. If situation (2) is the case, then the changes in the updraft size distribution might be used to diagnose changes in convective aggregation in the natural world. We investigate whether idealized, cloud resolving model simulations also exhibit such powerlaw scaling and whether the shape of the updraft distribution is affected by aggregation. As a step toward bridging these results from idealized simulations to the real world, we explore how the updraft size distribution changes along a hierarchy of simulation complexities.