Observational assessment of the potential of satellite-based water vapor and thermal IR brightness temperatures in detecting semitransparent cirrus

Satellite observations of brightness temperature in the atmospheric window and water vapor bands are widely used for studying the regional/global distribution of semi‐transparent cirrus (STC). The present study provides the first quantitative assessment of the lower limit of cloud optical depth (COD) of STC that can be detected using this method and the conditions under which these observations are reliable, by comparing the STC information derived from the Very High Resolution Radiometer (VHRR) onboard the Indian geostationary satellite KALPANA‐1 over the tropical station Gadanki (13.5°N, 79.2°E) with cirrus COD obtained from collocated Lidar observations for 12 nights during different seasons under contrasting cloud conditions. Satellite‐derived STC amount is found to be highly reliable when COD > 0.02 and is an underestimate when it decreases below 0.02, especially when they appear as broken clusters. However, in most of the cases, satellite could detect very thin STC with COD < 0.02 when they are widespread.