Simulation and Validation of INSAT-3D sounder data at NCMRWF S. Indira Rani and V. S. Prasad National Centre for Medium Range Weather Forecasting (NCMRWF) Earth System Science Organization (ESSO) Ministry of Earth Sciences (MoES) Government of India A-50, Sector-62, Noida, Uttar Pradesh-201309, India. Email: indira@ncmrwf.gov.in , vsprasad@ncmrwf.gov.in Abstract India's advanced weather satellite, INSAT-3D, the first geostationary sounder system over Indian Ocean, was launched (located at 83°E) on 26 July 2013, for the improved understanding of mesoscale systems. INSAT-3D carries a 6 channel imager and 19 channel sounder payload. Along with other polar satellite soundings, INSAT-3D provides fine resolution vertical profiles over India and surrounding region. National Centre for Medium Range Weather Forecasting (NCMRWF) routinely receives near-real time soundings from polar orbiting satellites, and recently started receiving INSAT- 3D sounder and imager data. Simulation of INSAT-3D sounder Brightness Temperature (BT) using Radiative Transfer models and Numerical Weather prediction models has been done at NCMRWF during the North Indian Ocean Cyclone (NIOC) period of 2013. BTs during four different cyclones, viz., Phailin, Helen, Lehar and Madi are simulated and validated against the observed BTs. The RT model used to simulate the BT is Radiative Transfer for TOVS (RTTOV) version-9 and the NWP model used is Direct Broadcast CIMSS Regional Assimilation System (dbCRAS). Clear sky condition is assumed during RTTOV simulation, though the cloudy pixels are not removed, since the cloud information is not available in the then dataset. RT model simulated sounder BT of three channels (12.66 μ, 12.02μ and 11.03 μ) and DbCRAS simulated 11 μ BTs are compared with the corresponding INSAT-3D sounder BTs. Bias in RT model simulated BT is less for the window channel 11μ compared to the other two 12 μ channels, and this is mainly because of the window channel is not conducive to moisture and can be used for moisture correction in other channels. Observed BT showed fast bias with respect to the RT simulated BT for the 12 μ channels, whereas slow bias for the window channel in all the cyclone cases. The biases and standard deviations were higher for the cyclone Phailin with respect to the RT simulated BTs. Validation of INSAT-3D window channel BT with respect to dbCRAS simulated 11 μ showed slow bias and the standard deviation was maximum for Phailin.