Development of a soil moisture analysis for subseasonal forecasting: Soil moisture validation

The goal of this study is to produce a new soil moisture analysis for subseasonal forecasting. An accurate soil moisture initialization is today widely recognized as having a great potential to increase summertime subseasonal forecasting skill. In this study, soil moisture initial conditions are generated using the Precipitation Assimilation Reanalysis (PAR) technique. This technique consists mainly of nudging precipitation in a coupled land-atmosphere model by adjusting the vertical air humidity profile based on the difference between model and observed precipitation rates in a continuous assimilation period of a few months. The effect of the PAR technique on the model soil moisture estimates is evaluated using (1) a benchmark in global soil moisture analysis produced by the Global Soil Wetness Project Phase 2 (GSWP-2) and (2) in-situ observations from the state of Illinois. Using PAR, we find that the coupled land-atmosphere FSU/COAPS model reproduces with a reasonable accuracy the seasonal cycle and the temporal variability of monthly soil moisture anomalies produced by GSWP-2 across most of the globe. The temporal structure of monthly soil moisture anomalies match also fairly well with in-situ observations from the Illinois state deep into the soil. Finally, overall the PAR technique shows better results than the NCEP/DOE Reanalysis 2. Therefore, in this study we have developed a new soil moisture analysis product that (1) is physically consistent with the atmospheric physics of the coupled land-atmosphere FSU/COAPS model, (2) is globally comparable with GSWP-2 and (3) better fits in-situ observed soil moisture characteristics than R2 over Illinois.