The Influence of Boundary Layer Mixing Strength on the Evolution of a Baroclinic Cyclone

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  • 1 Department of Atmospheric and Environmental Sciences, University at Albany - State University of New York, Albany, New York
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Abstract

Sub-grid scale turbulence in numerical weather prediction models is typically handled by a PBL parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid in the vertical direction, and act upon temperature, moisture, and momentum within the boundary layer. This study varies the PBL mixing strength within 4-km WRF simulations of the 26–29 January 2015 snowstorm to assess the sensitivity of baroclinic cyclones to eddy diffusivity intensity. The bulk critical Richardson number for unstable regimes is varied between 0.0–0.25 within the YSU PBL scheme, as a way of directly altering the depth and magnitude of sub-grid scale turbulent mixing. Results suggest varying the bulk critical Richardson number is similar to selecting a different PBL parameterization. Differences in boundary layer moisture availability, arising from reduced entrainment of dry, free tropospheric air, lead to variations in the magnitude of latent heat release above the warm frontal region, producing stronger upper-tropospheric downstream ridging in simulations with less PBL mixing. The more amplified ow pattern impedes the northeastward propagation of the surface cyclone and results in a westward shift of precipitation. Additionally, trajectory analysis indicates ascending parcels in the less-mixing simulations condense more water vapor and terminate at a higher potential temperature level than ascending parcels in the more-mixing simulations, suggesting stronger latent heat release when PBL mixing is reduced. These results suggest spread within ensemble forecast systems may be improved by perturbing PBL mixing parameters that are not well constrained.

Corresponding author: Matthew T. Vaughan, mvaughan@albany.edu

Abstract

Sub-grid scale turbulence in numerical weather prediction models is typically handled by a PBL parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid in the vertical direction, and act upon temperature, moisture, and momentum within the boundary layer. This study varies the PBL mixing strength within 4-km WRF simulations of the 26–29 January 2015 snowstorm to assess the sensitivity of baroclinic cyclones to eddy diffusivity intensity. The bulk critical Richardson number for unstable regimes is varied between 0.0–0.25 within the YSU PBL scheme, as a way of directly altering the depth and magnitude of sub-grid scale turbulent mixing. Results suggest varying the bulk critical Richardson number is similar to selecting a different PBL parameterization. Differences in boundary layer moisture availability, arising from reduced entrainment of dry, free tropospheric air, lead to variations in the magnitude of latent heat release above the warm frontal region, producing stronger upper-tropospheric downstream ridging in simulations with less PBL mixing. The more amplified ow pattern impedes the northeastward propagation of the surface cyclone and results in a westward shift of precipitation. Additionally, trajectory analysis indicates ascending parcels in the less-mixing simulations condense more water vapor and terminate at a higher potential temperature level than ascending parcels in the more-mixing simulations, suggesting stronger latent heat release when PBL mixing is reduced. These results suggest spread within ensemble forecast systems may be improved by perturbing PBL mixing parameters that are not well constrained.

Corresponding author: Matthew T. Vaughan, mvaughan@albany.edu
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