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The turbulent transfer mechanisms in the atmospheric surface layer McBean, Gordon Almon

Abstract

The objective of this study was to investigate the turbulent transfer mechanisms near the surface. Direct measurements of the turbulent fluxes of momentum, heat, and moisture were made in the atmospheric surface layer: principally, 2 m above a grass surface at Ladner, Canada, and for comparison 8 m above the Atlantic Ocean near Barbados. The spectral correlation coefficients were considered to be a measure of the transfer efficiency as a function of scale size. For momentum transfer the efficiency decreased at all scales as instability increased. It was postulated that this was due to greater amounts of momentum being transferred in bursts of short duration, thus making the spectral correlation coefficient, averaged over sufficient time, smaller. The Ladner results for heat transfer showed that its transfer efficiency increased at all scales when instability increased. The ratios of the transfer efficiency of heat to that of momentum were greater than 1 for most scales, even for near neutral stratifications, and increased to between 2 and 3 for more unstable conditions. The efficiency of moisture transfer, when moisture is a passive scalar, was usually smaller than that for heat transfer and was found to depend on the correlation between moisture fluctuations and those of temperature, which is the active scalar. The results from Barbados pointed out two main differences between the subtropics and mid-latitudes: that the temperature spectrum is much narrower in bandwidth and that the humidity fluctuations make an equally important contribution to buoyancy. These features are reflected in the transfer mechanisms.

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