The Applicability of a Mixed–Layer Model of the Planetary Boundary Layer to Real-Data Forecasting

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  • 1 Department of Meteorology, The Pennsylvania State University, University Park 16802
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Abstract

The mesoscale numerical model of the planetary boundary layer (PBL), which Lavoie applied to lake-effect snowstorm and to airflow over the Hawaiian Islands, is modified and utilized to assess the feasibility of producing short-range real-data forecasts of low-level flow patterns. The dry model atmosphere comprises three layers. A parameterized surface layer of fixed depth (50 m) follows the variable terrain and allows vertical fluxes of heat and momentum to affect the flow in the overlying PBL or “mixed layer.” The horizontal wind velocity and potential temperature, both prognostic variables, are assumed to be independent of height in the mixed layer. The height of the top of the mixed layer is an additional prognostic variable. A parameterized stable layer, characterized by a vertically constant potential temperature lapse rate, overlies the mixed layer. Synoptic-scale patterns of pressure and potential temperature are specified at the top of this uppermost layer as upper boundary conditions. Energy-conserving parameterizations for the entrainment of heat and momentum from the upper stable layer into the mixed layer and for convective adjustment are introduced. The simplifications in the atmospheric structure provide for considerable computational efficiency while preserving a high degree of physical realism under the assumed conditions of a well-mixed PBL.

Experiments with a cross-section version of the model are performed for a domain containing a smoothed Appalachian terrain profile and adjacent coastal waters in order to economically assess the model's response to variable terrain, differential heating and differential roughness at the coast. The terrain profile produces a perturbation in the quasi-steady-state westerly flow pattern that exhibits subsidence and higher wind speeds over a ridge in qualitative agreement with mountain-wave theory. While differential roughness causes subsidence at the coast, differential heating engenders a maximum of upward motion around 40 km inland that is considered to be a crude representation of a sea breeze superimposed on the westerly flow. The results of the cross-section experiments are used to aid in interpreting a real-data simulation of the daytime PBL over the Middle Atlantic States on 16 October 1973. The model resolves a lee trough in the flow east of the Appalachians, a surface pressure trough in eastern Virginia and eastern North Carolina, and realistic vertical motion patterns along the coastal regions and the Chesapeake Bay. Verification statistics are provided for the sea level pressure and surface potential temperature patterns.

Abstract

The mesoscale numerical model of the planetary boundary layer (PBL), which Lavoie applied to lake-effect snowstorm and to airflow over the Hawaiian Islands, is modified and utilized to assess the feasibility of producing short-range real-data forecasts of low-level flow patterns. The dry model atmosphere comprises three layers. A parameterized surface layer of fixed depth (50 m) follows the variable terrain and allows vertical fluxes of heat and momentum to affect the flow in the overlying PBL or “mixed layer.” The horizontal wind velocity and potential temperature, both prognostic variables, are assumed to be independent of height in the mixed layer. The height of the top of the mixed layer is an additional prognostic variable. A parameterized stable layer, characterized by a vertically constant potential temperature lapse rate, overlies the mixed layer. Synoptic-scale patterns of pressure and potential temperature are specified at the top of this uppermost layer as upper boundary conditions. Energy-conserving parameterizations for the entrainment of heat and momentum from the upper stable layer into the mixed layer and for convective adjustment are introduced. The simplifications in the atmospheric structure provide for considerable computational efficiency while preserving a high degree of physical realism under the assumed conditions of a well-mixed PBL.

Experiments with a cross-section version of the model are performed for a domain containing a smoothed Appalachian terrain profile and adjacent coastal waters in order to economically assess the model's response to variable terrain, differential heating and differential roughness at the coast. The terrain profile produces a perturbation in the quasi-steady-state westerly flow pattern that exhibits subsidence and higher wind speeds over a ridge in qualitative agreement with mountain-wave theory. While differential roughness causes subsidence at the coast, differential heating engenders a maximum of upward motion around 40 km inland that is considered to be a crude representation of a sea breeze superimposed on the westerly flow. The results of the cross-section experiments are used to aid in interpreting a real-data simulation of the daytime PBL over the Middle Atlantic States on 16 October 1973. The model resolves a lee trough in the flow east of the Appalachians, a surface pressure trough in eastern Virginia and eastern North Carolina, and realistic vertical motion patterns along the coastal regions and the Chesapeake Bay. Verification statistics are provided for the sea level pressure and surface potential temperature patterns.

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