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

Abstract

High‐frequency (5 Hz) atmospheric surface layer (ASL) turbulent velocity (u′) and infrared skin temperature perturbations (T′s) 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 T′s perturbations caused by rapid excursions in u′. To quantify the effects of u′ on rapid surface cooling, wavelet spectra of u′ and T′s and cospectra of u′T′s were computed. The u′ 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 −1 power law in the u′ power spectrum was used as a signature for inactive eddy motion. Therefore the −1 power law was used to identify wavenumber ranges (about 1.5 decades) associated with inactive eddy motion. The measured T′s wavelet spectra and u′T′s cospectra identified with this wavenumber range demonstrate that much of the T′s energy and 〈u′T′s〉 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.