Representation of Boundary Layer Moisture Transport in Cloud-Resolving Models

• Published in: Monthly weather review 2012-11, Vol.140 (11), p.3682-3698
• Main Authors: MOENG, Chin-Hoh, ARAKAWA, Akio
• Format: Article
• Language: English
• Subjects: Boundary layers; Clouds; Cold; Convection; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Spatial distribution; Studies; Transport processes
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/MWR-D-12-00046.1

One of the important roles of the PBL is to transport moisture from the surface to the cloud layer. However, how this transport process can be accounted for in cloud-resolving models (CRMs) is not sufficiently clear and has rarely been examined. A typical CRM can resolve the bulk feature of large convection systems but not the small-scale convection and turbulence motions that carry a large portion of the moisture fluxes. This study uses a large-eddy simulation of a tropical deep-convection system as a benchmark to examine the subgrid-scale (SGS) moisture transport into a cloud system. It is shown that most of the PBL moisture transport to the cloud layer occurs in the areas under low-level updrafts, with rain, or under cloudy skies, although these PBL regimes may cover only a small fraction of the entire cloud-system domain. To develop SGS parameterizations to represent the spatial distribution of this moisture transport in CRMs, three models are proposed and tested. An updraft–downdraft model performs exceptionally well, while a statistical-closure model and a local-gradient model are less satisfactory but still perform adequately. Each of these models, however, has its own closure issues to be addressed. The updraft–downdraft model requires a scheme to estimate the mean SGS updraft–downdraft properties, the statistical-closure model needs a scheme to predict both SGS vertical-velocity and moisture variances, while the local-gradient model requires estimation of the SGS vertical-velocity variance.