A Comparison of Numerical Simulations of Hydrostatic Flow over Mountains with Observations

Klaus P. Hoinka Institute of Atmospheric Physics, German Aerospace Research Establishment (DFVLR), D‐8031 Oberpfaffenhofen, Federal Republic Germany

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Abstract

Numerical solutions of a hydrostatic mesoscale model for stratified flow over topography are compared with analytical results and observational data. The numerical model was found to reproduce well phase and amplitude of linear and nonlinear waves provided by analytical models. The model was effective in simulating observed chinook associated with an intense mountain wave. In a nonchinook case the model was successful in reproducing main features of the observed velocity distribution associated with an elevated region of blocking. The observed and simulated momentum flux profiles are almost identical. The comparisons of the model results with observations and analytical results demonstrate the overall ability of the model to realistically simulate mountain wave flow. In a series of numerical experiments we have investigated how far the steepness of the waves depends on stratospheric wind structure and on orography. Our simulations suggest that there is a bifurcation line between the linear and nonlinear regime of flow depending on the magnitude of the stratospheric wind. Tests with an asymmetric orography show significant increase in wave amplitude up to the magnitude which was observed. Finally, two bora‐type flows, a cyclonic and one anticyclonic, are simulated and show the expected structure in the wind and temperature fields.

Abstract

Numerical solutions of a hydrostatic mesoscale model for stratified flow over topography are compared with analytical results and observational data. The numerical model was found to reproduce well phase and amplitude of linear and nonlinear waves provided by analytical models. The model was effective in simulating observed chinook associated with an intense mountain wave. In a nonchinook case the model was successful in reproducing main features of the observed velocity distribution associated with an elevated region of blocking. The observed and simulated momentum flux profiles are almost identical. The comparisons of the model results with observations and analytical results demonstrate the overall ability of the model to realistically simulate mountain wave flow. In a series of numerical experiments we have investigated how far the steepness of the waves depends on stratospheric wind structure and on orography. Our simulations suggest that there is a bifurcation line between the linear and nonlinear regime of flow depending on the magnitude of the stratospheric wind. Tests with an asymmetric orography show significant increase in wave amplitude up to the magnitude which was observed. Finally, two bora‐type flows, a cyclonic and one anticyclonic, are simulated and show the expected structure in the wind and temperature fields.

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