Calculation of Turbulent Fluxes by Integral Methods

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  • 1 University of Washington, Seattle
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Abstract

Aircraft and dropsonde measurements of humidity and wind velocity over the Indian Ocean during the winter monsoon season have been used to calculate the horizontal flux of water vapor and the rate of evaporation from the water surface. Calculations of evaporation over areas several hundred kilometers on a side are compared with calculations made at a point from buoy measurements of the vertical profiles of wind velocity, humidity and temperature. Using direct measurements of differences in horizontal flux, large errors occurred in three of nine cases; these discrepancies may be traced to changes in the wind field which occurred during the flights of the observing aircraft. Using values of horizontal flux divergence estimated from the heat flux equation, the two independent methods agree in sign and in approximate magnitude. The results are also compared with a small sample of aircraft measurements of the covariance of vertical velocity and absolute humidity. Individual values of covariance taken over 1-min periods vary widely in sign and magnitude, but average values agree in sign and approximate magnitude with results using the other methods.

Further comparisons of the integral, covariance and profile methods for calculating vertical fluxes in the boundary layer are proposed, and suggestions are made for coping with more complex situations.

Abstract

Aircraft and dropsonde measurements of humidity and wind velocity over the Indian Ocean during the winter monsoon season have been used to calculate the horizontal flux of water vapor and the rate of evaporation from the water surface. Calculations of evaporation over areas several hundred kilometers on a side are compared with calculations made at a point from buoy measurements of the vertical profiles of wind velocity, humidity and temperature. Using direct measurements of differences in horizontal flux, large errors occurred in three of nine cases; these discrepancies may be traced to changes in the wind field which occurred during the flights of the observing aircraft. Using values of horizontal flux divergence estimated from the heat flux equation, the two independent methods agree in sign and in approximate magnitude. The results are also compared with a small sample of aircraft measurements of the covariance of vertical velocity and absolute humidity. Individual values of covariance taken over 1-min periods vary widely in sign and magnitude, but average values agree in sign and approximate magnitude with results using the other methods.

Further comparisons of the integral, covariance and profile methods for calculating vertical fluxes in the boundary layer are proposed, and suggestions are made for coping with more complex situations.

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