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THE VARIATION WITH HEIGHT OF THE VERTICAL FLUX OF HEAT AND MOMENTUM

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  • 1 Massachusetts Institute of Technology
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Abstract

Direct measurements have been made of the vertical flux of heat and momentum in the layer from 2 to 12 meters. Eddy velocities were obtained from hot-wire anemometers and light bivanes, mounted at four levels; temperature fluctuations were measured with fast-response thermocouples, mounted at three levels. Data were recorded by taking photographs of indicating dials, at the rate of one exposure per second. Six sets of data have been analyzed, each set corresponding to one period of observation approximately 10 minutes in length. In four sets of data, the flow was over a rough land surface; in one set, the flow came directly from a water (ocean) surface; in the remaining set, the flow was principally over water except for a short land trajectory immediately upwind from the point of observation.

The flux data show a maximum variation from two- to four-fold within the layer. Over land, the shearing stress tends to decrease with height during the day and to increase with height at night; over water, both the heat flux and the momentum flux tend to decrease with height and also are significantly smaller at all levels than over a land surface. Values are presented for the coefficients of eddy viscosity Km, eddy conductivity Kh, surface drag Cd, and for von Kámán's constant K. For a land surface, the eddy coefficients are approximately equal near the ground; Km increases with height at a slower rate than Kh during the day, while at night the reverse is true. Over a water surface, Km is considerably larger than Kh at all levels. The values for cd are in good agreement with previous estimates, based on less direct measurements. The rather complicated variation of k with stability and height is discussed; the average of all determinations is approximately 0.4.

Abstract

Direct measurements have been made of the vertical flux of heat and momentum in the layer from 2 to 12 meters. Eddy velocities were obtained from hot-wire anemometers and light bivanes, mounted at four levels; temperature fluctuations were measured with fast-response thermocouples, mounted at three levels. Data were recorded by taking photographs of indicating dials, at the rate of one exposure per second. Six sets of data have been analyzed, each set corresponding to one period of observation approximately 10 minutes in length. In four sets of data, the flow was over a rough land surface; in one set, the flow came directly from a water (ocean) surface; in the remaining set, the flow was principally over water except for a short land trajectory immediately upwind from the point of observation.

The flux data show a maximum variation from two- to four-fold within the layer. Over land, the shearing stress tends to decrease with height during the day and to increase with height at night; over water, both the heat flux and the momentum flux tend to decrease with height and also are significantly smaller at all levels than over a land surface. Values are presented for the coefficients of eddy viscosity Km, eddy conductivity Kh, surface drag Cd, and for von Kámán's constant K. For a land surface, the eddy coefficients are approximately equal near the ground; Km increases with height at a slower rate than Kh during the day, while at night the reverse is true. Over a water surface, Km is considerably larger than Kh at all levels. The values for cd are in good agreement with previous estimates, based on less direct measurements. The rather complicated variation of k with stability and height is discussed; the average of all determinations is approximately 0.4.

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