SKIN TEMPERATURE PERTURBATIONS INDUCED BY SURFACE-LAYER TURBULENCE ABOVE A GRASS SURFACE

High-frequency (5 Hz) atmospheric surface layer (ASL) turbulent veloci ty (u') and infrared skin temperature perturbations (T-s') were measur ed above a grass-covered forest clearing and analyzed for cloud free c onditions. These measurements were used to investigate mechanisms resp onsible for the production of large short-lived T-s' perturbations cau sed 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 usin g Townsend's [1961, 1976] hypothesis. Townsend's hypothesis states tha t 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 z ero plane displacement, and an inactive component, which is produced i n the atmospheric boundary layer (ABL) outer region. A -1 power law in the u' power spectrum was used as a signature for inactive eddy motio n. 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 waven umber 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 av eraging. Hence, in contrast to the laboratory experiments of Owen and Thomson [1963], it is argued that skin temperature perturbations at th e canopy-atmosphere interface of a grass-covered surface (small therma l inertia) are strongly dependent on the inactive eddy motion produced in the outer layer of the ABL.