Observed effects of horizontal radiative surface temperature variations on the atmosphere over a midwest watershed during CASES 97

The association between ∼10-km scale horizontal variation of radiometric surface temperature (T_S) 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 T_S 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 T_S reached maxima in the same longitude zone on two flight tracks 40 km apart. Satellite T_S 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 T_S 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 T_S maxima. Both mechanisms were possibly enhanced by increased static instability over the T_S maxima. Owing to the small sample available, these results are suggestive rather than conclusive. Effects of rainfall and vegetation on watershed-scale T_S gradients are also explored.