Skin temperature perturbations induced by surface layer turbulence above a grass surface

High-frequency (5 Hz) atmospheric surface layer (ASL) turbulent velocity (u9) and infrared skin temperature perturbations (T9s) were measured above a grass-covered forest clearing and analyzed for cloud free conditions. These measurements were used to investigate mechanisms responsible for the production of large short-lived T9s perturbations caused by rapid excursions in u9. To quantify the effects of u9 on rapid surface cooling, wavelet spectra of u9 and T9s and cospectra of u9T9s were computed. The u9 wavelet power spectra were then analyzed using Townsend’s [1961, 1976] hypothesis. Townsend’s hypothesis states that ASL eddy motion can be decomposed into an active component, which is a function of the ground shear stress (u*) and height ( z) above the zero plane displacement, and an inactive component, which is produced in the atmospheric boundary layer (ABL) outer region. A 21 power law in the u9 power spectrum was used as a signature for inactive eddy motion. Therefore the 21 power law was used to identify wavenumber ranges (about 1.5 decades) associated with inactive eddy motion. The measured T9s wavelet spectra and u9T9s cospectra identified with this wavenumber range demonstrate that much of the T9s energy and ^u9T9s& are due to inactive eddy motion, where the angle brackets indicate time averaging. Hence, in contrast to the laboratory experiments of Owen and Thomson [1963], it is argued that skin temperature perturbations at the canopy-atmosphere interface of a grass-covered surface (small thermal inertia) are strongly dependent on the inactive eddy motion produced in the outer layer of the ABL.