Analyzing the Grell-Freitas Convection Scheme from Hydrostatic to Non-Hydrostatic Scales Within a Global Modeling Framework

Abstract:

Parameterizations of moist convection were originally developed for atmospheric modeling systems which horizontal resolutions were too coarse to explicitly simulate subgrid-scale convective motions. As global numerical models move towards non-hydrostatic scales and convective updrafts occupy a larger fraction of the model grid-box, these parameterizations must be modified to include the resolution dependence of the vertical convective eddy transport.

We implemented and tested the scale-aware Grell-Freitas (Grell and Freitas 2014) parameterization of convection in the Model for Prediction Across Scales (MPAS) using a variable-resolution mesh spanning between 50 km and 3 km centered over South America. Using three four-day forecasts initialized on 10^thJanuary 2014 with GFS data, we analyzed the scale dependence of the partitioning between convective and grid-scale precipitation as a function of the convective updraft fraction.

Results show that grid-scale precipitation contributes a major part to the total precipitation over the most refined region of the mesh in the tropics and that, conversely, convective precipitation contributes a major part of total precipitation over the coarsest region of the mesh. Furthermore, results show that there exists a smooth transition in the partitioning between convective and grid-scale precipitation as resolution varies between hydrostatic and non-hydrostatic scales. Comparisons of simulated precipitation against TRMM- Precipitation Analysis data are also presented.

Reference:

Grell, G.A., and S.R. Freitas, 2014: A scale and aerosol aware stochastic convection parameterization for weather and air quality modeling. Atmos. Chem. Phys., 14, 5233-5250.