An Investigation of Terrain Effects on the Mesoscale Spectrum of Atmospheric Motions

G. D. Nastrom Meteorology Research, Control Data, Minneapolis, MN 55440

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D. C. Fritts Geophysical Institute, University of Alaska, Fairbanks, AK 99775

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K. S. Gage Aeronomy Laboratory, NOAA/ERL, Boulder, CO 80303

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Abstract

Wind and temperature data collected on commercial aircraft during the Global Atmospheric Sampling Program (GASP) are used to investigate the effects of underlying terrain on mesoscale variability, and the observational results are interpreted within the theories of gravity wave motions and quasi-two-dimensional turbulence. The data show the variances are up to six times larger over mountainous terrain than over oceans or plains, with the most striking differences at horizontal scales from 4 to 80 km. Results were subdivided between the stratosphere and troposphere, and between high- and low-background wind speed cases, and show basically the same response to topography in all cases. The linear theory of gravity waves is found to predict correctly the scaling of wave amplitude with background stability in the case of low-background wind speeds, while the two-dimensional turbulence theory correctly predicts the shape of the variance spectrum and the observed amplitude scales with ε as required by the theory. In other cases the theoretical predictions are less satisfactory. Some possible causes of the discrepancies and likely methods to resolve them are discussed.

Abstract

Wind and temperature data collected on commercial aircraft during the Global Atmospheric Sampling Program (GASP) are used to investigate the effects of underlying terrain on mesoscale variability, and the observational results are interpreted within the theories of gravity wave motions and quasi-two-dimensional turbulence. The data show the variances are up to six times larger over mountainous terrain than over oceans or plains, with the most striking differences at horizontal scales from 4 to 80 km. Results were subdivided between the stratosphere and troposphere, and between high- and low-background wind speed cases, and show basically the same response to topography in all cases. The linear theory of gravity waves is found to predict correctly the scaling of wave amplitude with background stability in the case of low-background wind speeds, while the two-dimensional turbulence theory correctly predicts the shape of the variance spectrum and the observed amplitude scales with ε as required by the theory. In other cases the theoretical predictions are less satisfactory. Some possible causes of the discrepancies and likely methods to resolve them are discussed.

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