Observed effects of horizontal surface temperature variations on the atmosphere over a Mid-west watershed during CASES 97
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The association between 10-km scale horizontal variation of radiometric surface temperature (Ts) and aircraft-derived fluxes of sensible heat (H) and moisture (LE) is the focus of this work. We use aircraft, surface, and satellite data from a Cooperative Atmospheric-Surface Exchange Studies (CASES) field program, which took place in the southern part of the 60 100 km Walnut River (Kansas) watershed from 22 April to 22 May 1997, when winter wheat matured and prairie grass greened up. Aircraft Ts observed along repeated flight tracks above the surface layer showed a persistent pattern: maxima over ridges characterized by shallow soil and rocky outcroppings and minima over riparian zones. H and Ts reached maxima in the same longitude zone on two flight tracks 40 km apart. Satellite Ts data from March to June reveal similar persistent patterns with minima more persistent than maxima. Two mechanisms are suggested to explain the association of H and Ts maxima: (1) for winds between 6 and 8 ms 1, modulation of the surface energy budget by vegetation effects; or (2) for winds equal to or below 4 ms 1,a thermally driven circulation centered on Ts maxima. Both mechanisms were possibly enhanced by increased static instability over the Ts maxima. Owing to the small sample available, these results are suggestive rather than conclusive. Effects of rainfall and vegetation on watershed-scale Ts gradients are also explored.