WCD-11. Process-level differences between two PBL schemes used in NOAA’s GFS model

The behavior of two eddy-diffusivity mass-flux (EDMF) planetary boundary layer (PBL) parameterizations used in NOAA’s Global Forecast System (GFS) is examined at the level of mixing processes. This examination is performed by comparing the schemes in Single-Column Model simulations of convective PBL growth. The currently operational TKE-EDMF scheme mixes more and leads to less convective instability than the Hybrid-EDMF scheme. The excessive mixing of the TKE-EDMF scheme is consistent with results from three-dimensional GFS forecasts compared with radiosonde data. Diagnosis of contributing terms indicates that nonlocal mixing via the mass-flux term is stronger in the TKE-EDMF scheme compared to the Hybrid-EDMF scheme. Quantitative aspects of the local eddy diffusivity are also different between the two schemes, pointing to uncertainty in the physical partitioning of local and nonlocal mixing in the formulation of the two schemes. Sensitivity experiments show essential parameters that can be optimized according to observations and/or large-eddy-simulation results that provide a more realistic partition of local and non-local mixing.