An Efficient Parameterization of Convective and Nonconvective Planetary Boundary Layers for Use in Numerical Models

Simon Wei-Jen Chang JAYCOR, 205 S. Whiting Street, Alexandria, VA 22304

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Abstract

An efficient, multilayer model for predicting the diurnal variations in the thermal and momentum fields in the planetary boundary layer (PBL) is proposed for incorporating into mesoscale or large-scale dynamical models. The ground temperature is given by a soil slab heated (or cooled) by net radiation and sensible heat from the atmospheric surface layer and a ground thermal reservoir. The surface heat flux can be generated by two mechanisms: 1) the convective mixing depending on the temperature difference between the ground and the screen level and 2) the mechanical mixing depending on the wind stress. Following Blackadar (1976), a prediction equation is employed for the screen-level temperature. In the PBL, the heat and momentum exchanges are computed by a Richardson number adjustment scheme. Heat and momentum exchanges occur mainly due to thermal instability under convectively unstable conditions and due to shear instability under convectively stable conditions. A case study shows good agreement between model results and observation. Additional experiments are performed to test the scheme under calm and stronger wind situations. Since no explicit diffusion coefficient is needed in the adjustment scheme, the model time step is not restricted by computational stability requirements of the diffusion term. This PBL parameterization scheme is therefore very appealing for use in numerical models that use large time steps yet have good vertical resolutions in the PBL.

Abstract

An efficient, multilayer model for predicting the diurnal variations in the thermal and momentum fields in the planetary boundary layer (PBL) is proposed for incorporating into mesoscale or large-scale dynamical models. The ground temperature is given by a soil slab heated (or cooled) by net radiation and sensible heat from the atmospheric surface layer and a ground thermal reservoir. The surface heat flux can be generated by two mechanisms: 1) the convective mixing depending on the temperature difference between the ground and the screen level and 2) the mechanical mixing depending on the wind stress. Following Blackadar (1976), a prediction equation is employed for the screen-level temperature. In the PBL, the heat and momentum exchanges are computed by a Richardson number adjustment scheme. Heat and momentum exchanges occur mainly due to thermal instability under convectively unstable conditions and due to shear instability under convectively stable conditions. A case study shows good agreement between model results and observation. Additional experiments are performed to test the scheme under calm and stronger wind situations. Since no explicit diffusion coefficient is needed in the adjustment scheme, the model time step is not restricted by computational stability requirements of the diffusion term. This PBL parameterization scheme is therefore very appealing for use in numerical models that use large time steps yet have good vertical resolutions in the PBL.

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