Impact of INSAT-3DR Sounder and Imager Clear Sky Brightness Temperature assimilation in the NCMRWF global NWP model

The National Centre for Medium Range Weather Forecasting (NCMRWF) utilized INSAT-3D Sounder and Imager Clear-Sky Brightness Temperatures (CSBTs) until the satellite was declared non-operational. INSAT-3DR, launched by the Indian Space Research Organisation (ISRO) in September 2016, is the successor to INSAT-3D. This study investigates the impact of INSAT-3DR CSBT assimilation in the four-dimensional variational (4D-Var) data assimilation system of the NCMRWF Unified Model (NCUM) during January–February 2024 and during the development of Severe Cyclonic Storm (SCS) Remal in May 2024, using Observing SystemExperiments (OSEs). The control experiment (CNTL) assimilates all conventional and satellite observations operationally used in the NCUM system. Two additional OSEs were conducted: EXP1, which assimilates INSAT-3DR Sounder CSBTs, and EXP2, which assimilates INSAT-3DR Imager CSBTs, both in addition to the observations used in CNTL. Assimilation of INSAT-3DR Sounder and Imager CSBTs shows a positive impact on the analysis and background fits toother assimilated observations during January–February 2024. Temperature and humidity forecasts from EXP1 and EXP2 differ from those of CNTL, although no consistent spatial or temporal pattern is observed. These forecast impacts are primarily confined to oceanic regions at various vertical levels. Independent comparisons with ERA5 indicate a neutral impact ofSounder and Imager CSBT assimilation over land. Overall, the study suggests that INSAT-3DR Sounder and Imager CSBT assimilation leads to noticeable changes in the analysis fields, while the impact on forecast fields remains largely neutral, consistent with earlier findings for INSAT-3D CSBT assimilation. For the cyclone case, assimilation of INSAT-3DR Sounder and Imager CSBTs results in the simulation of a slower-moving cyclone, with a lag of approximately 12 hours, and a higher peak intensity compared to the India Meteorological Department (IMD) best-track estimates.