A reappraisal of global soil effective temperature schemes

Traditionally, effective soil temperature (Teff) has been considered to be a secondary intermediate variable in microwave radiative transfer theory. However, its impact on microwave radiometry is comparable to that of vegetation cover, soil surface roughness and dielectric constant. Teff is defined as the weighted temperature of the emitting layers, where the weighting involved depends on the soil moisture profile. In this study, we evaluate the suitability of various models for estimating Teff using temperature and moisture profiles obtained from a land surface model. MERRA-Land (the Modern-Era Retrospective analysis for Research and Applications-Land) soil moisture profiles and temperature profiles were used to reproduce global Teff data sets with single parameter schemes (e.g. the Beta Soil Moisture Active Passive, SMAP scheme), two-parameter Choudhury's schemes (e.g. the current SMAP's scheme), two-parameter Wigneron schemes (e.g. the current Soil Moisture and Ocean Salinity scheme, SMOS), and multilayer Teff schemes (e.g. Lv′s scheme). The results show that differences in Teff between these schemes are usually < 5 K. The comparison between Wigneron's and Lv′s schemes indicates that, the difference is small (RMSD, root mean squared difference < 1 K). In exceptional cases (< 1%), the RMSD between Choudhury's Teff scheme and Lv′s scheme can reach around 5 K. The Beta SMAP Teff has a difference of around 5 K, compared to Lv′s scheme. Such a change in Teff could lead to an emissivity difference of around 0.015. The most extreme emissivity difference is only found in desert areas, at 42.5° (the incidence angle used by the SMAP mission is about 40°).