Stratocumulus updrafts as drivers of boundary layer growth and entrainment

Abstract:

Circulation in the stratocumulus-topped boundary layer (STBL) is thought to be primarily driven by the emission of longwave radiation at cloud top, and enhanced by latent heat release in updrafts and latent heat uptake in downdrafts. However, for optically thick clouds, with a liquid water path (LPW) ≳ 50 g m^-2, long wave emissions increase only negligibly with LWP. Yet, additional moistening of the boundary layer, e.g. due to wind speed driving a higher surface moisture flux, leads to faster boundary layer growth and entrainment. In this presentation we investigate cloud layer updrafts as drivers of boundary layer growth and entrainment in optically thick stratocumulus clouds. We conduct cloud-system resolving simulations of stratocumulus clouds in which additional moistening of the boundary layer by wind speed leads to a stronger production of turbulence kinetic energy (TKE) due to latent heat release in cloud updrafts, and thereby to enhanced boundary layer growth and entrainment. We find that LWP increases as well, but without an effect on longwave cooling and TKE production. Furthermore, we find that even in the strongly decoupled daytime boundary layer, wind speed drives additional TKE production in cloud layer updrafts and thereby boundary layer growth and entrainment. This mechanism could, potentially, result in a runaway breakup of the cloud, or initiate or assist in the transition from stratocumulus to shallow cumulus, especially at higher wind speeds.