The Liquid Water Oscillation in Modeling Boundary Layer Cumuli with Third-Order Turbulence Closure Models

Abstract

A hierarchy of third-order turbulence closure models are used to simulate boundary layer cumuli in this study. An unrealistically strong liquid water oscillation (LWO) is found in the fully prognostic model, which predicts all third moments. The LWO propagates from cloud base to cloud top with a speed of 1 m s?1. The period of the oscillation is about 1000 s. Liquid water buoyancy (LWB) terms in the third-moment equations contribute to the LWO. The LWO mainly affects the vertical profiles of cloud fraction, mean liquid water mixing ratio, and the fluxes of liquid water potential temperature and total water, but has less impact on the vertical profiles of other second and third moments. In order to minimize the LWO, a moderately large diffusion coefficient and a large turbulent dissipation at its originating level are needed. However, this approach distorts the vertical distributions of cloud fraction and liquid water mixing ratio. A better approach is to parameterize LWB more reasonably. A minimally prognostic model, which diagnoses all third moments except for the vertical velocity, is shown to produce better results, compared to a fully prognostic model.