A comparison of ground-based and satellite-borne microwave radiometric observations in the Great Plains

We compare ground-based and the special sensor microwave/imager (SSM/I) bri ghtness temperatures at 19 and 37 GHz in the Northern and the Southern Grea t Plains. The comparison was conducted to examine season-related difference s in plot-scale and satellite footprint-scale brightness temperatures at th ese frequencies. The ground-based observations were from the three Radiobri ghtness Energy Balance Experiments (REBEXs), viz., REBEX-1, REBEX-4, and RE BEX-5. REBEX-1 and REBEX-4 were conducted near Sioux Falls, SD, in fall and winter 1992-93, and in summer 1996, respectively. REBEX-5 was conducted ne ar Lamont, OK, during summer 1997 as part of the Southern Great Plains Hydr ology Experiment-1997 (SGP'97). The instantaneous fields of view (FOV) of t he ground-based radiometers were only a few meters compared to those of the SSM/I, which were several tens of kilometers. The REBEX and the SSM/I brightness temperatures are moderately correlated a t both the 19 and 37 GHz. They match well during winter when there was unif orm snow cover over the SSM/I footprint. During spring, summer, and fall, R EBEX brightness temperatures at the grass-site were on average 18 K higher than the SSM/I brightness temperatures because the SSM/I footprint included nearby agricultural fields in summer and predominantly bare soil in fall a nd spring. During summer, REBEX-4 brightness temperatures at the bare soil site were on average 10 K cooler than the SSM/I brightness temperatures. In effect, the REBEX grass and bare soil brightness temperatures bracket the SSM/I observations with the SSM/I brightness temperatures lying closest to those of the bare soil.