The Projection of Turbulent Mixing Variations on an Extratropical Cyclone

Sub-grid scale turbulence in current NWP forecast models is simulated via a planetary boundary layer (PBL) parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid, and act upon temperature, moisture, and momentum within and above the boundary layer in order to produce a more realistic representation of the atmospheric vertical structure. On the column scale, differences in PBL mixing strength may vary thermal and moisture profiles that can have a substantial impact on forecasted sensible weather, particularly in mixed-precipitation environments. However, less is known about how differences in parameterized mixing may project onto larger-scale circulations such as extratropical cyclones. It is hypothesized thatchanges in PBL temperature and moisture may alter the upper-level synoptic-scale environment through the vertical redistribution of potential vorticity (PV) via latent heat release as low-level parcels are lifted past their lifted condensation levels.
This study uses the Weather Research and Forecasting (WRF) model, run at convection-permitting resolution, to examine the effect of PBL mixing on the evolution of an extratropical cyclone. Boundary layer tendencies of potential temperature, moisture, and wind suggest the various parameterized mixing profiles applied by PBL schemes can have a substantial impact on precipitation location through altering the track of the surface cyclone. The lack of direct information regarding which PBL scheme produces the most realistic mixing in the diverse stability regimes surrounding extratropical cyclones motivates using stochastic perturbations within the PBL scheme to curtail model error stemming from mixing parameters that are not well constrained.