Toward a PDF representation of deep convection: Development and evaluation of a parameterization for convective transport of hydrometeors

 
Poster PDF

Authors

Mikhail Ovchinnikov — Pacific Northwest National Laboratory
May Wong — National Center for Atmospheric Research (NCAR)

Category

Deep convective clouds, including aerosol interactions

Description

Vertical advection fluxes for mass mixing ratios of four hydrometeor types during a simulated deep convection event in the ARM97 case. The benchmark flux is from a cloud-resolving model with a 250-m horizontal grid spacing. Eddy-diffusion flux often misrepresents the direction of transport, while fluxes computed with the proposed PDF decomposition generally match reference profile shapes. Scaling is needed to better match the magnitude of the flux, and adjustment allows the scheme to capture the net downward transport at lower altitudes.
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.