Diurnal variation of vertical temperature gradients within a field of maize: implications for Satellite microwave radiometry

We present the diurnal variation of vertical temperature differences measured within and beneath a maize canopy over the course of a growing season, and we analyze the implied temperature gradients in the context of microwave radiometry and soil moisture retrieval in particular. We find that the temperature differences can be as large as 9 K in magnitude within the vegetation canopy and as large as 10 K between the soil surface and a depth of 4.5 cm. Satellite overpass times at 1:30 A.M. and 1:30 P.M. occur close to when the magnitude of the temperature differences are largest. For 6 A.M. and 6 P.M. overpass times, temperature differences were smaller in magnitude at 6 P.M. This contradicts the widely held assumption that surface temperature gradients are more uniform at 6 A.M. than at 6 P.M.