Numerical Simulations of a Stratocumulus-Capped Boundary Layer Observed over Land

Abstract

Detailed observations of both mean and turbulence fields of an anticyclonic, quasi-steady state, stratocumulus-capped boundary layer obtained with ground-based and balloonborne equipment during the night of 19/20 November 1976 at Cardington, Bedford, UK, are simulated in relation to large-scale subsidence, longwave radiative model cooling, and large-scale moisture supply from sea to land, using a simplified second-order turbulence-closure radiative model. Using a one-dimensional version of the model, most of the observed features are well simulated, including the large temperature ?jump? in a thin layer at cloud top, thermodynamic profiles within the boundary layer, cloud depth and cloud liquid water content, turbulence in the cloud layer, and radiative fluxes and their associated cooling (heating) rates. The results also show that in order to reproduce the observed features, the large-scale subsidence rate and horizontal moisture input should be properly incorporated. In addition to the one-dimensional simulations for the observed balloon profiles, we used a three-dimensional version of the model to investigate the mechanisms which resulted in a cloudless band embedded in this large sheet of stratocumulus, observed during the same night around the north shore of the English Channel. The physics derived from the one-dimensional simulations applies well in the three-dimensional model. The sensitivity tests show that the terrain effects, which induce larger downward vertical motion, are primarily responsible for this clear band.