Validation of INSAT-3D temperature and moisture sounding retrievals using matched radiosonde measurements

An evaluation of temperature and moisture profiles retrieved from a geostationary Indian National Satellite (INSAT-3D) sounder, launched in 2013, is performed against collocated radiosonde (RAOB) observation measurements of more than 1 year. This evaluation is carried out in terms of bias and root mean square error (RMSE) in temperature and relative humidity. An error analysis is carried out for different surface types, different seasons and day/night cases. The key finding of this study is that INSAT-3D retrievals show good agreement with RAOB measurements with overall RMSE accuracies similar to 1-2K and 10-20%, respectively, for temperature and relative humidity in the troposphere. However, the temperature and relative humidity retrievals over land or in dry atmosphere show degraded performance. This degradation might be related to uncertainty in surface emissivity over land and possibility of undetected cloud in dry atmospheric condition. In addition to it, a similar analysis is carried out to assess the relative performance of INSAT-3D-retrieved profiles, Atmospheric Infrared Sounder (AIRS) L2 Standard Physical Retrieval (AIRS-only) version 6 (AIRS2RET) profiles and European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim) reanalysis with respect to spatially and temporally collocated RAOBs. In this analysis, temperature and moisture profiles from RAOBs serve as reference measurements and all retrievals and ERA-interim are compared with RAOBs. AIRS and INSAT-3D temperature retrievals gave comparable accuracies in upper and lower troposphere where as the quality degrades in middle troposphere resulting in larger errors. This may be due to improper bias correction coefficients used for brightness temperature of clear sky pixels before physical retrievals. In case of relative humidity, INSAT-3D profiles have comparable accuracies as AIRS in troposphere.