A hydrodynamical model of shear flow over semi-infinite barriers with application to density currents
- Univ. of Oklahoma, Norman (United States)
Vertically sheared airflow over semi-infinite barriers is investigated with a simple hydrodynamical model. The idealized flow is steady, two-dimensional, neutrally buoyant, and inviscid, bounded on the bottom by a semi-infinite impermeable barrier and on the top by a rigid tropopause lid. With attention further restricted to an exponentially decreasing wind shear, the equations of motion (Euler's equations) reduce, without approximation, to a modified Poisson equation for a pseudo streamfunction and a formula for the Exner function. The free parameters characterizing the model's environment are the tropopause height, the density scale height, the wind speed at ground level, and the wind speed at tropopause level. Additional parameters characterize the barrier geometry. Exact solutions of the equations of motion are obtained for semi-infinite plateau barriers and for a barrier qualitatively resembling the shallow density current associated with some thunderstorm outflows. These solutions are noteworthy in that the reduction of a certain nondimensional shear parameter (through negative values) results in greater vertical parcel displacements over the barrier despite a corresponding reduction in the vertical velocity. This steepening tendency culminates in overturning motions associated with both upstream and down-stream steering levels. In this latter case the low-level inflow impinging on the barrier participates in a mixed jump and overturning updraft reminiscent of updrafts simulated in numerical convective models. Conversely, for large values of the nondimensional shear parameter, parcels undergo small vertical parcel displacements over the barrier despite large vertical velocities. This latter behavior may account for the finding that strong convergence along the leading edge of storm outflows does not always trigger deep convection even in unstable environments.
- OSTI ID:
- 6507190
- Journal Information:
- Journal of the Atmospheric Sciences; (United States), Vol. 49:23; ISSN 0022-4928
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ATMOSPHERIC CIRCULATION
EQUATIONS OF MOTION
WIND
SHEAR
CONVECTION
FLUID FLOW
GROUND LEVEL
HYDRODYNAMICS
POISSON EQUATION
SCALE HEIGHT
TROPOPAUSE
DIFFERENTIAL EQUATIONS
DIMENSIONS
EARTH ATMOSPHERE
ENERGY TRANSFER
EQUATIONS
FLUID MECHANICS
HEAT TRANSFER
HEIGHT
LEVELS
MASS TRANSFER
MECHANICS
PARTIAL DIFFERENTIAL EQUATIONS
TROPOSPHERE
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