Mikhail Ovchinnikov — Pacific Northwest National Laboratory
May Wong — National Center for Atmospheric Research (NCAR)
Category
Deep convective clouds, including aerosol interactions
Description
An assumed PDF scheme called Cloud Layers Unified By Binormals (CLUBB) is employed in global climate models, including ACME and CAM5, to treat turbulence, shallow convection, and stratiform clouds. When the scheme is extended to treat all cloud types, including deep convection, the connection between CLUBB’s dynamics and precipitation microphysics needs to be strengthened. The counter gradient vertical transport of precipitation species by infrequent strong updrafts, for example, cannot be modeled under a traditional eddy-diffusivity approximation in CLUBB. Earlier, we have shown that realistic hydrometeor vertical transport fluxes can be obtained by sampling joint distributions of vertical velocity and hydrometeor mass mixing ratios into quadrants separated by their respective mean values, and then scaling the quadrant mean fluxes to account for within-quadrant correlations between vertical velocity and the microphysics (Wong et al., 2015). The proposed partitioning of the flux into contributions from the four quadrants, however, requires more information about the bivariate distribution than CLUBB currently is able to provide. In this presentation, we describe how the proposed algorithm is tailored toward CLUBB, which predicts a marginal distribution of vertical velocity, but not its joint distributions with microphysics variables. An adjustment to the flux by strong updrafts is also introduced to enforce the dominant role of downdrafts in hydrometeor transport at lower altitudes. We evaluate the newly developed scheme diagnostically, using output from high-resolution simulations of continental and oceanic convection, and in an interactive setting of a single column model.
Reference:
Wong M.W.S., M. Ovchinnikov, and M. Wang, 2015: Evaluation of subgrid-scale hydrometeor transport schemes using a high-resolution cloud-resolving model, J. Atmos. Sci., 72, 3715–3731, doi: 10.1175/JAS-D-15-0060.1.