The effective thermodynamic temperature of the emitting surface at 6.6 GHz and consequences for soil moisture monitoring from space

An analysis of nine years of Nimbus/SMMR 6.6 GHz brightness temperatures over central Spain revealed a systematic discrepancy between daytime and nighttime H-polarization emissivities, calculated using independently derived surface temperatures. It was found that daytime emissivities were systematically lower than nighttime emissivities which is explained from the difference between the actual surface temperature (T_s) and the effective temperature of the microwave emitting surface layer (T_eff). In order to estimate the long-term mean difference between T_s and T_eff, average daytime and nighttime thermal correction factors were calculated. The criterion used for judging the magnitude of these thermal correction factors is the hypothesis that daytime emissivity, when averaged over a longer period, should not be lower than nighttime emissivity. From a combined analysis of all daytime and nighttime Nimbus/SMMR signatures over the nine-year period, and short-term field evidence of daytime and nighttime soil temperature profiles, the magnitude of the thermal correction factors could be estimated. Averaged over the nine-year period with 320 nighttime and 498 daytime observations the thermal correction factors amount to -12.5 K for the daytime and +5.0 K for the nighttime, with 90% of the cases satisfying the defined criterion