The SMAP Level 4 Surface and Root Zone Soil Moisture (L4_SM) data assimilation product
The presentation provides an overview of the SMAP L4_SM algorithm development, testing, and pre-launch validation using observations from the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission launched in November 2009. Specifically, multi-year, multi-angular L-band (1.4 GHz) brightness temperature observations from SMOS were used to (i) calibrate the parameters of the microwave radiative transfer model used in the L4_SM system and (ii) derive brightness temperature observations at 40 degrees incidence angle that mimic the SMAP Radiometer data. The latter observations were then assimilated into the L4_SM system and a prototype L4_SM data product was derived. This prototype product was validated using in situ measurements from four densely instrumented watersheds in the United States and more than 100 single-profile sensors scattered across the United States. The validation results indicate that the prototype soil moisture product satisfies the formal RMSE requirement for the L4_SM product of 0.04 m3/m3 (after removal of the long-term mean bias). An examination of the observation-minus-forecast residuals from the L4_SM system suggests where the system could be improved further.