Assessment of SAR Derived Wind Speed Accuracy with Numerical Model Generated Wind Speed

In this manuscript, a comparative study has been done to find the difference between Radar Imaging Satellite (RISAT-1) derived wind speeds with the winds simulated by Global Forecast System (GFS) model. Though the grid resolution of GFS is presently 12.5 km, the model outputs for the mentioned RISAT-1 period are available at 25 km. The RISAT-1 wind speed were retrieved using the wind retrieval algorithm developed at Space Applications Centre (SAC). The high resolution (similar to 1 km) wind speed were retrieved using an empirical C-band Geophysical Model Function (GMF), CMOD5.N. The GMF is tuned to provide the neutral wind speed valid at 10 m above the sea surface, and therefore GFS derived wind speeds are converted to equivalent neutral wind speed at 10 m. A separate comparative study has been made for different seasons like Pre-monsoon, Monsoon, and Post-monsoon over the Indian Ocean. The comparison of collocated RISAT-1 and GFS wind speed results in wind speed bias as - 0.7 m/s, - 1.8 m/s and - 0.5 m/s with corresponding Root Mean Square Difference (RMSD) 2.2 m/s, 3.0 m/s and 1.9 m/s, respectively, for pre-monsoon, monsoon, and post-monsoon seasons. The comparison results show a little bit more difference in comparison with the earlier study carried out for wind speed validation using satellite observation and buoy observations because GFS winds have their own limitation. This study shows the more difference in wind speed between RISAT-1 and GFS during monsoon season as one may find the rapid variability in winds during this season.