Microphysical controls on the stratocumulus topped boundary-layer structure during VOCALS-REx

Abstract

Simulations at a range of resolutions are compared to observations from the South-East Pacific taken during VOCALS-REx. It is found that increased horizontal and vertical resolution make only small improvements to the bulk properties of the simulated cloud and drizzle, but the highest resolution simulation is able to realistically represent mesoscale features in the cloud field. We focus on the highest resolution simulation and demonstrate that a poor representation of the cloud microphysics results in excessive drizzle production. This promotes persistent drizzle induced decoupling of the boundary layer, giving a poor representation of the observed diurnal cycle of stratocumulus. Two simple changes to the microphysics scheme are implemented: a modified autoconversion parametrization and a new representation of the rain drop size distribution. This results in a more realistic simulation of boundary-layer diurnal decoupling, and improvements to the cloud liquid water path and surface drizzle rate.