Determining the Drag Coefficient from Vorticity, Momentum, and Mass Budget Analyses

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  • 1 Center for Experimental Design and Data Analysis, NOAA Environmental Data Service, Washington, D. C. 20235
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Abstract

Data from Period III (19 June–2 July, 1969) of the Barbados Oceanographic and Meteorological Experiment (BOMEX) are used to compute the vorticity budget in the planetary boundary layer. The computed residual, assumed to be the curl of the stress, is then used to obtain an estimate of the drag coefficient. This estimate compares well with results obtained by other BOMEX investigators and in other independent experiments. The top of the planetary boundary layer is assumed to be located at the level where both the stress and stress gradient, computed from the momentum conservation equations, vanish. This is at approximately 1300 m, but tests indicate that the results are rather insensitive to assumed values in the range of 1300 to approximately 1600 m, the base of the trade-wind inversion. Computations for a relatively undisturbed period show a near-balance between anticyclonic vorticity generation by boundary layer divergence and vorticity destruction by friction. However, during a mildly disturbed period the advection and local change terms no longer appear to be negligible.

Abstract

Data from Period III (19 June–2 July, 1969) of the Barbados Oceanographic and Meteorological Experiment (BOMEX) are used to compute the vorticity budget in the planetary boundary layer. The computed residual, assumed to be the curl of the stress, is then used to obtain an estimate of the drag coefficient. This estimate compares well with results obtained by other BOMEX investigators and in other independent experiments. The top of the planetary boundary layer is assumed to be located at the level where both the stress and stress gradient, computed from the momentum conservation equations, vanish. This is at approximately 1300 m, but tests indicate that the results are rather insensitive to assumed values in the range of 1300 to approximately 1600 m, the base of the trade-wind inversion. Computations for a relatively undisturbed period show a near-balance between anticyclonic vorticity generation by boundary layer divergence and vorticity destruction by friction. However, during a mildly disturbed period the advection and local change terms no longer appear to be negligible.

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