Shear, vertical moisture transport, and decoupling in the marine stratocumulus-topped boundary layer

Abstract:

Circulation in the marine stratocumulus-topped boundary layer, and vertical transport of moisture between the ocean surface and the inversion are primarily driven by buoyant production of turbulence kinetic energy (TKE). Buoyant production of TKE arises from cloud top longwave cooling, latent heat release in cloud updrafts, latent heat uptake in cloud downdrafts, and from the surface heat flux. TKE is, however, also produced by shear in the interaction of geostrophic wind with the ocean surface. The associated shear-driven circulation interferes with buoyancy-driven circulation in the sub-cloud layer, and thereby changes the efficiency of vertical moisture transport between the surface and cloud base. An inefficient vertical moisture transport in the sub-cloud layer manifests itself in boundary layer decoupling. We have conducted cloud-system resolving simulations of non-drizzling marine stratocumulus clouds to investigate the effect of shear due to geostrophic wind on decoupling. We find that shear-driven dynamics due to geostrophic wind modulates boundary layer decoupling in two ways: At nighttime, and at low wind speed during daytime, shear-driven circulation in the sub-cloud layer hampers vertical moisture transport in the sub-cloud layer and thereby increases decoupling. In contrast, during daytime, at high wind speed, and at strong decoupling, shear-driven circulation takes over from buoyancy-driven circulation in transporting moisture between the surface and cloud base, and thereby reduces decoupling and assists in maintaining cloud liquid water path.