Articles | Volume 16, issue 2
Research article
21 Jan 2016
Research article |  | 21 Jan 2016

How large-scale subsidence affects stratocumulus transitions

J. J. van der Dussen, S. R. de Roode, and A. P. Siebesma

Abstract. Some climate modeling results suggest that the Hadley circulation might weaken in a future climate, causing a subsequent reduction in the large-scale subsidence velocity in the subtropics. In this study we analyze the cloud liquid water path (LWP) budget from large-eddy simulation (LES) results of three idealized stratocumulus transition cases, each with a different subsidence rate. As shown in previous studies a reduced subsidence is found to lead to a deeper stratocumulus-topped boundary layer, an enhanced cloud-top entrainment rate and a delay in the transition of stratocumulus clouds into shallow cumulus clouds during its equatorwards advection by the prevailing trade winds. The effect of a reduction of the subsidence rate can be summarized as follows. The initial deepening of the stratocumulus layer is partly counteracted by an enhanced absorption of solar radiation. After some hours the deepening of the boundary layer is accelerated by an enhancement of the entrainment rate. Because this is accompanied by a change in the cloud-base turbulent fluxes of moisture and heat, the net change in the LWP due to changes in the turbulent flux profiles is negligibly small.

Short summary
A large-eddy simulation model is used to show that a weakening of the large-scale subsidence, which is associated with a future warmer climate, leads to a delay of the moment of break up of stratocumulus clouds during subtropical stratocumulus transitions. To understand what causes this delay, a novel analysis method is used to distil the contributions of individual physical processes to the evolution of the stratocumulus cloud thickness.
Final-revised paper