Cloud fraction biases in CALIPSO simulators of CMIP5 models over India

Abstract

Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) is one of the longest-serving observation platforms which provides the cloud fraction data with higher spatial and vertical resolutions. Their simulators are also incorporated in the General Circulation Models (GCMs) under the Coupled Model Intercomparison Project Phase 5 (CMIP5). Here, we have examined the spatial and vertical cloud fraction (CF) distributions and corresponding biases (CFmodel−CFobservations; CF bias) from CALIPSO observations and their simulators for the period 2006–2015 over Indian land and oceanic regions for Indian summer monsoon season of June to September. It is reported that the CF biases over the whole Indian land and oceanic regions are largely negative (> 55%) except the northeastern land regions and southern Bay of Bengal (BOB) which show positive biases (~ 10–60%). Such CF biases are prominent and consistent in all CMIP5 models. The CF biases are further analyzed in the vertical dimension using contoured frequency by altitude diagrams (CFADs). The large CF biases exist at each vertical level especially at higher altitudes (> 12 km). CF biases over BOB based on spatial distributions give the false impression of better performance of simulators which are exhibited by the co-existence of both positive and negative biases at the same level, especially at higher altitude levels. As a result, both positive and negative CF biases cancel each other which lead to the minimum values of CF biases at respective altitude levels. This mutual ambiguity between spatial and vertical CF biases is found to be a permanent feature of the CALIPSO simulator.