Properties of spatial temperature derivatives in the atmospheric surface layer

Abstract

A pair of parallel cold wires separated in either the vertical or lateral direction was used to obtain the three components θx, θy, θz of the temperature derivative in the streamwise, lateral and vertical directions, respectively. The average absolute skewness values of θx and θz are nonzero and approximately equal, while the skewness of θy is approximately zero. These results appear to be consistent with the presence of a large, three-dimensional organised structure in the surface layer. There is an apparent low-frequency contamination in the spectral density of θy and θz due mainly to small errors in estimating the sensitivity of the cold wires. The temperature derivatives were high-pass filtered, the filter being set to remove possible contributions from the large structure and to minimise low-frequency sensitivity contamination. The filtered rms ratios \~θx/\~θy and \~θx/\~θz were in the range 0.7 to 0.9, a result in qualitative agreement with that obtained in the laboratory boundary layer by Sreenivasan et al. (1977). The skewness of filtered θx or θz is negligible, consistent with local isotropy of small-scale temperature fluctuations and in support of the high wavenumber spectral isotropy discussed in Antonia and Chambers (1978).