4D-Var 4D-Var data assimilation Earth system models GPM IMERG GPSRO GRACE GSI MCA analysis Mediterranean basin SMAP TRMM 3B42 WRF accuracy analog data assimilation atmospheric models bending angle data assimilation data-driven methods drought floods soil moisture fluorescence interpolation land data assimilation land data assimilation system land surface model land surface modeling land surface models merged CMORPH microwave remote sensing numerical weather prediction