Mesoscale organization, entrainment, and the properties of a closed-cell stratocumulus cloud Journal of Advances in Modeling Earth Systems

Closed-cell mesoscale organization and its relationship to entrainment and the properties of a low, nonprecipitating stratocumulus cloud is investigated. Large eddy simulations were run over 10 periodic diurnal cycles during which mesoscale organization could fully develop and approach a quasi-steady state on five domains sized from 2.4 km x 2.4 km to 38.4 km x 38.4 km. The four smaller domains hosted a single cell with an aspect ratio that increased with domain size. On the largest domain, mesoscale organization consisted of a cell population that evolved over the course of the diurnal cycle. It is found that with increasing cell aspect ratio, entrainment weakens and the boundary layer becomes shallower, cooler, moister, and more decoupled. This causes an increase in cloud water path and cloud radiative effect up to a cell aspect ratio of 16. With further increase in cell aspect ratio, circulation on the cell scale becomes less effective in supplying moisture to the cloud and in producing turbulent kinetic energy (TKE). This mechanism can explain scale saturation in closed-cell mesoscale organization. The simulations support a maximum stable aspect ratio of closed-cell mesoscale organization between 32 and 64, consistent with the observational limit of approximate to 40. The simulations show furthermore that entrainment does not, in general, scale with buoyant production of TKE. Instead, entrainment correlates with the vertical component of TKE. This implies vertical motion as a driver of entrainment, and a convective velocity scale based on the vertical component of TKE rather than on buoyant production of TKE.