LOCAL ISOTROPY AND ANISOTROPY IN THE SHEARED AND HEATED ATMOSPHERIC SURFACE-LAYER

Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of eddy-motion anisotropy wi thin the inertial subrange. Rather than simply test the adequacy of lo cally isotropic relations, we investigated directly the sources of ani sotropy, These sources, in a daytime desert-like climate, include: (1) direct interaction between the large-scale and small-scale eddy motio n, and (2) thermal effects on the small-scale eddy motion. In order to explore these two anisotropy sources, we developed statistical measur es that are sensitive to such interactions. It was found that the larg e-scale/small-scale interaction was significant in the inertial subran ge up to 3 decades below the production scale, thus reducing the valid ity of the local isotropy assumption. The anisotropy generated by ther mal effects was also significant and comparable in magnitude to the fo rmer anisotropy source. However, this thermal anisotropy was opposite in sign and tended to counteract the anisotropy generated by the large -scale/small-scale interaction, The thermal anisotropy was attributed to organized ramp-like patterns in the temperature measurements, The i mpact of this anisotropy cancellation on the dynamics of inertial subr ange eddy motion was also considered. For that purpose, the Kolmogorov -Obukhov structure function equation, as derived from the Navier-Stoke s equations for locally isotropic turbulence, was employed. The Kolmog orov-Obukhov structure function equation in conjunction with Obukhov's constant skewness closure hypothesis reproduced the measured second- and third-order structure functions. Obukhov's constant skewness closu re scheme, which is also based on the local isotropy assumption, was v erified and was found to be in good agreement with the measurements. T he accepted 0.4 constant skewness value derived from grid turbulence e xperiments overestimated our measurements. A suggested 0.26 constant s kewness value, which we derived from Kolmogorov's constant, was found to be adequate.