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- Title
Skin temperature perturbations induced by surface layer turbulence above a grass surface.
- Authors
Katul, Gabriel G.; Schieldge, John; Hsieh, Cheng-I; Vidakovic, Brani
- 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.
- Publication
Water Resources Research, 1998, Vol 34, Issue 5, p1265
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/98WR00293