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The Subgrid Velocity Scale in the Bulk Aerodynamic Relationship for Spatially Averaged Scalar Fluxes Public Deposited

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  • The exchange coefficients for area-averaged surface fluxes can become anomalously large when the large-scale flow is weak and significant fluxes of heat and moisture are driven by mesoscale motions within the averaging or subgrid area. To prevent this erratic behavior of the exchange coefficient, the “subgrid velocity scale” must be included to account for generation of turbulent fluxes by subgrid mesoscale motions. This velocity scale is obtained by spatially averaging the local time-averaged velocity used in the bulk aerodynamic relationship. The subgrid velocity scale is distinct from the free convection velocity scale included in the bulk aerodynamic relationship to represent transport induced by convectively driven boundary-layer-scale eddies (Godfrey and Beljaars; Beljaars).The formulation of Godfrey and Beljaars is derived by time averaging the velocity scale of the bulk aerodynamic relationship. The behavior of the subgrid velocity scale is explored using data from five different field programs. Ubiquitous “nameless” mesoscale motions of unknown origin are found in all of the datasets. The addition of the subgrid velocity scale reduces the dependence of the exchange coefficients on grid size. Based on the data analysis, the subgrid velocity scale increases with grid size and contains a contribution due to surface heterogeneity.
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  • Mahrt, L. T., Jielun Sun, 1995: The Subgrid Velocity Scale in the Bulk Aerodynamic Relationship for Spatially Averaged Scalar Fluxes. Monthly Weather Review, 123(10), 3032–3041.
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  • 123
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  • 10
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  • This material is based upon work supported by Grant DAAH04-93-G-0019 from the Army Research Office, Grant ATM-9310576 from the Physical Meteorology Program of the National Science Foundation, and Contract F19628-94-K-0001 from the Phillips Laboratory.
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