Towards development of a P- and L-band Tb assimilation framework in the JULES Land Surface Model

The L-band (1.4 GHz) microwave radiometer provides soil moisture (SM) information limited to around 5 cm soil depth. Deeper information (up to 10 cm) can be obtained using low frequency sensors (P-band: 0.3-1 GHz) with reduced effects of surface roughness and vegetation. The present study explored the capability of P-band and/or L-band singly or in combination, via direct assimilation of brightness temperature (Tb) into the Joint UK Land Environment Simulator (JULES) land surface model. JULES was driven by ERA-5 (ECMWF Reanalysis v5) meteorological forcing data and calibrated model parameters for bare soil. The assimilation framework consists of a radiative transfer model to convert simulated SM to Tb and an Ensemble Kalman Filter to generate an observation corrected SM trajectory. This framework was first validated with an open loop experiment in a synthetic environment over Cora Lynn, Victoria, Australia for the period of 9th May to 14th June, 2019. Assimilation experiments with synthetic observations were then set-up to investigate the sensitivity of i) number of ensembles, ii) observation error, iii) incidence angle, iv) assimilation interval, and iv) frequency bands. The diagnostics (Kalman gain and Jacobians) showed that P band was more sensitive to the deeper layers as compared to L-band. The results also showed substantial improvement in the soil moisture analysis state in both the dry and wet period of the study when both L- and P-band Tbs were assimilated. Further study will include investigating improvement in soil moisture estimates when using real field observations and assimilating Tb with multiple incidence angles.Keywords: Ensemble Kalman filter, Tb assimilation, P-band, JULES Land surface model, Radiative Transfer Model