Mesoscale aggregation of shallow marine cumulus convection
Shallow cumulus convection mixed with patchy stratocumulus is a climatically important cloud type over the oceans, both in the subtropics and in mid-latitude wintertime cold-air outbreaks. This cloud type typically exhibits clear mesoscale aggregation, a form of 'mesoscale cellular convection'. 10-20 km wide patches of deeper and more vigorous precipitating cumuli, embedded in thin stratocumulus layers, are surrounded by drier regions 100 km or more across supporting only very shallow cumuli. This aggregation may be important to the mean state, cloud cover, precipitation rate, and even cloud-climate feedbacks and cloud-aerosol interaction in trade cumulus boundary layers.
Large-eddy simulations (LES) covering domains 50 km or more across clearly exhibit mesoscale aggregation of shallow cumulus convection, but it is not fundamentally well understood. To further that understanding, we analyze the development of convective aggregation in a set of 36-hour long LES on a 108x108 km doubly periodic domain simulating mean summertime conditions at a location east of Hawaii. The simulated convection is initially horizontally homogeneous, but quickly aggregates after 12 hours. Vertically resolved heat and moisture budgets on subdomains elucidate the aggregation process. Shallow cumulus deepen preferentially in more humid regions of the boundary layer, heating the boundary layer and stimulating net moisture convergence into those regions, such that they further moisten. Sensitivity studies show that the aggregation does not require precipitation, is weakened but not prevented if radiative cooling and surface fluxes are horizontally homogenized, and is strengthened by feedbacks with inversion cloud.