Representation of boundary layer moisture transport in cloud-resolving models

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 (SOS) 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 SOS 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 SOS updraft-downdraft properties, the statistical-closure model needs a scheme to predict both SOS vertical-velocity and moisture variances, while the local-gradient model requires estimation of the SOS vertical-velocity variance.