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Numerical Study of Terrain-Induced Mesoscale Motions in a Mixed Layer

Y-J. HanDepartment of Atmospheric Sciences, Oregon State University, Corvallis 97331

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K. UeyoshiDepartment of Atmospheric Sciences, Oregon State University, Corvallis 97331

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J. W. DeardorffDepartment of Atmospheric Sciences, Oregon State University, Corvallis 97331

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Abstract

Numerical integrations using a potential enstrophy conserving scheme are presented for the flow within a mixed layer over hilly terrain using the hydrostatic shallow-water equations with a quadratic drag law. The mesoscale area treated is 150 km on a side; cyclic lateral boundary conditions are used. It is found that for the idealized conditions treated (no surface heating, no entrainment and no pressure adjustments aloft), the topography quickly induces a steady state flow pattern by means of surface friction. Unsteadiness does not occur unless a surface-friction Reynolds number, R = /(CDL), exceeds ∼100, where h̄h is the mean mixed-layer thickness, CD is the surface drag coefficient and L is a representative horizontal terrain length scale. Effects of varying the Rossby number, Froude number and terrain-height parameter are examined.

Abstract

Numerical integrations using a potential enstrophy conserving scheme are presented for the flow within a mixed layer over hilly terrain using the hydrostatic shallow-water equations with a quadratic drag law. The mesoscale area treated is 150 km on a side; cyclic lateral boundary conditions are used. It is found that for the idealized conditions treated (no surface heating, no entrainment and no pressure adjustments aloft), the topography quickly induces a steady state flow pattern by means of surface friction. Unsteadiness does not occur unless a surface-friction Reynolds number, R = /(CDL), exceeds ∼100, where h̄h is the mean mixed-layer thickness, CD is the surface drag coefficient and L is a representative horizontal terrain length scale. Effects of varying the Rossby number, Froude number and terrain-height parameter are examined.

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