Implementation of a Stable PBL Turbulence Parameterization for the Mesoscale Model MM5: Nocturnal Flow in Complex Terrain |
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Authors: | Sang-Mi Lee W Giori M Princevac H J S Fernando |
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Institution: | (1) Department of Civil and Environmental Engineering, Environmental Fluid Dynamics Program, Arizona State University, Tempe, AZ, 85287-9809, U.S.A;(2) Department of Mechanical and Aerospace Engineering, Environmental Fluid Dynamics Program, Arizona State University, Tempe, AZ, 85287-9809, U.S.A;(3) Present address: Department of Mechanical Engineering, University of Calfornia, 92521 Riverside, CA, U.S.A |
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Abstract: | The difficulties associated with the parameterization of turbulence in the stable nocturnal planetary boundary layer (PBL)
have been a great challenge for the nighttime predictions from mesoscale meteorological models such as MM5. As such, there
is a general consensus on the need for better stable boundary-layer parameterizations. To this end, two new turbulence parameterizations
based on the measurements of the Vertical Transport and Mixing (VTMX) field campaign were implemented and evaluated in MM5.
A unique aspect of this parameterization is the use of a stability-dependent turbulent Prandtl number that allows momentum
to be transported by the internal waves, while heat diffusion is impeded by the stratification. This improvement alleviates
the problem of over-prediction of heat diffusion under stable conditions, which is a characteristic of conventional atmospheric
boundary-layer schemes, such as the Medium Range Forecast (MRF) and Blackadar schemes employed in MM5. The predictions made
with the new PBL scheme for the complex terrain airshed of Salt Lake City were compared with those made with a default scheme
of MM5, and with observations made during the VTMX campaign. The new schemes showed an improvement in predictions, particularly
for the nocturnal near-surface temperature. Surface wind predictions also improved slightly, but not to the extent of temperature
predictions. The default MRF scheme showed a significantly higher surface temperature than observed, which could be attributed
to the enhanced vertical heat exchange brought about by its turbulence parameterization. The modified parameterizations reduced
the surface sensible heat flux, thus enhancing the strength of the near-surface inversion and lowering the temperature towards
the observed values. |
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Keywords: | Eddy diffusivities Heat and momentum fluxes Mesoscale models Stability-dependent turbulent Prandtl number Turbulence parameterizations |
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