Impact of the assimilation of INSAT-3D radiances on short-range weather forecasts

The present study concentrates on the assimilation of clear-sky radiances from the recently launched INSAT-3D satellite. The imager and sounder are two primary meteorological instruments aboard the INSAT-3D satellite. Pre-assimilation monitoring of the radiances has been carried out from April to July 2014. A double-difference technique using the High Resolution Infrared Sounder (HIRS) on MetOp-A is employed to remove the effect of deficiencies in the model-analysed profiles which can contribute to the biases. Observed radiances of both instruments appear to be colder (∼1.5–2 K) with respect to radiative transfer model (RTM) simulated radiances, except water vapour channel radiances, which are warmer (0.5–1 K). The standard deviations of observed minus RTM-simulated radiances are between 0.5 and 1.5 K. Prior to data assimilation, these biases are corrected using a variational bias correction scheme. The largest impacts on the analyses, when assimilating imager radiances, are found in the mid- and upper-tropospheric moisture, while assimilation of sounder radiances impacted both moisture and temperature throughout the troposphere. The more accurate analyses with the INSAT-3D radiance assimilation lead to improved moisture, wind, temperature and precipitation forecasts compared to the control case in which only conventional observations were assimilated. The results demonstrate the ability of temperature- and water vapour-sensitive radiances to improve not only the temperature and moisture fields, but also the wind fields, enhancing their importance, particularly over tropical regions where wind observations are more important.