2013 Volume 91 Issue 6 Pages 727-746
Numerical studies of stratocumulus clouds have revealed an interesting phenomenon: Clouds respond to decay monotonically through a stability parameter of cloud-top entrainment instability, k, which is defined by the ratio of jumps of equivalent potential temperature and total water mixing ratio across a cloud top. Further, clouds evaporate completely when k is sufficiently large. A nearly linear relationship between the liquid water path (LWP) and k eventually forms. This paper explores the underlying mechanisms of such a relationship by focusing on a transient phase of large-eddy simulations (LESs), which have not yet been studied in great detail.
As found in previous LES results, negative buoyancy due to evaporative cooling by mixing of air across a cloud top increases with an increase in k, and according to experimental observations, a larger k excites a stronger turbulent flow near the cloud top. As a result, during a transient phase, cloud water evaporates more rapidly.
An in-depth analysis of the boundary-layer processes reveal the following: mixing occurs between an interfacial layer, which forms with time near the inversion, and the underlying cloud layer. Because the evolving interfacial layer becomes colder and more humid compared with the original inversion air, the negative buoyancy due to mixing decreases (~O (10−3) m2 s−3) by 5 h. The system eventually reaches a near equilibrium and a notable reduction of LWP ceases thereafter. Budget analysis of turbulent kinetic energy also reveals that the reduced buoyancy production, along with other terms such as shear production, compensates energy dissipation. The result implies that the state in the stratification of the interfacial layer is important in controlling the resultant LWP at the eventual state, even if the k does not vary notably. This study also investigates the time scales in terms of the rapidly decaying clouds and boundary-layer turbulence.