A regime-dependent retrieval algorithm for near-surface air temperature and specific humidity from multi-microwave sensors

Abstract Near-surface specific humidity ( q a ) and air temperature ( T a ) over the global ocean are important meteorological variables, but they cannot be retrieved directly from remote sensing. Many efforts have been made to develop algorithms that derive q a and T a from multisensor microwave and/or infrared observations using in situ measurements as training datasets. However, uncertainty remains large in the resultant q a and T a retrievals. In this study, 147 moored surface buoys are used to examine how buoy measured q a and T a are related to satellite microwave brightness temperature (Tb) on the spatial scale from the warm/humid tropics to the cold/dry high latitudes. It is found that the Tb – q a and Tb – T a relations are structured along two distinct, near-linear bands, with the primary band in the warm/humid regime and a secondary (weaker) band in the cold/dry regime. The step-like transition (or separation) between the two regimes occurs at 8–10 g kg −1 for q a and 14–17 °C for T a . The evidence suggests that one algorithm may not be sufficient to extract q a and T a from Tb in all regimes. Therefore, a high-latitude enhancement is added to the global algorithm so that the q a and T a retrievals in the dry/cold regime can be specifically addressed. The new algorithms are applied to 11 microwave sensors, including SSM/I, SSMIS, and AMSU-A, from 1988 to 2016. Based on the 475,717 buoy collocations during the 29-year period, the retrieved q a and T a have root-mean-square differences of 0.82 g kg −1 and 0.51 °C, respectively.