Cloud fraction retrieval using data from Indian geostationary satellites and validation

Cloud fraction (CF) is known as the dominant modulator of Earth's radiation budget, thus regarded as Essential Climate Variable. CF is retrieved using Indian geostationary satellites Kalpana-1 and Indian National Satellite System (INSAT-3D) by calculating the fraction of area covered by the clouds in a given pixel divided by the total area of the pixel. The technique uses multi-channel thresholding for three channels in Kalpana-1, that is, thermal, visible, and water vapour, and four channels in INSAT-3D with mid-infrared channel in addition to the three mentioned for Kalpana-1. A 2-year record of CF at 30-min intervals was generated for the Indian region using the Kalpana-1 and INSAT-3D data. The retrieved CF data were compared against Moderate Resolution Imaging Spectroradiometer (MODIS) CF product in the near vicinity of simultaneous data availability (i.e., within +/- 15 min interval). This product agrees with MODIS (correlation coefficient 80%) with a root mean square error (RMSE) of 0.30, inspite of +/- 15 min of time difference between both the satellites. In addition, ground-based Total Sky Imager (TSI-440) retrieved data over Pune is used to validate the satellite retrieved CF over the same region. The probability of detection between retrieved CF and ground-based data is relatively more for range of CF between 0.00 and 0.25, that is, 90% and more than 20% for CF greater than 0.50. In view of the close agreement between retrieved CF from Kalpana-1 and INSAT-3D with MODIS and TSI-440, this product is operational and is being made available through National Information System for Climate and Environment Studies portal for use in better understanding of climate.