Assimilation of SMOS Brightness Temperature to Improve Surface and Root-Zone Soil Moisture

L-band (1.4 GHz) microwave brightness temperature observations provide information about the near-surface soil moisture. The Soil Moisture Ocean Salinity (SMOS) mission was launched in 2009 and has since collected a useful record of microwave data to project the expected benefit of brightness temperature measurements from the planned Soil Moisture Active Passive (SMAP) mission on soil moisture assimilation analyses.

We evaluate the assimilation of multi-angular, horizontally and vertically polarized observations of L-band (1.4 GHz) microwave brightness temperature from SMOS mission in a land-only data assimilation system. The assimilation system consists of an ensemble Kalman filter (EnKF) and the NASA Catchment land surface model, supplemented with a tau-omega radiative transfer model. The Catchment model is driven with surface meteorological forcing data from the NASA GEOS-5 atmospheric analysis system, with precipitation corrected towards gauge-based observations. The radiative transfer model is calibrated to deal with long-term biases, whereas shorter-term biases are addressed inside the assimilation system. The assimilation system includes downscaling of coarse-scale (~ 36 km) observations and provides improved surface and root zone soil moisture estimates at a horizontal resolution of 9 km every three hours.

We assess the performance of the assimilation system by validating the SMOS-based soil moisture assimilation results against independent in situ measurements for select watersheds and for sparse networks. Our results indicate that the SMAP L4_SM root zone soil moisture data product will meet its accuracy requirement of 0.04 m3/m3 (after removal of the long-term mean bias).