Assessment of ISCCP cloudiness over the Tibetan Plateau using CloudSat‐CALIPSO

Cloudiness over the Tibetan Plateau is difficult to estimate because ground‐based measurements are sparse. Satellite observations are thus the best tool and one of the longest climatologies available, the International Satellite Cloud Climatology Project (ISCCP), relies on passive remote sensing to characterize cloud cover and altitude. Active remote sensing from space is used to assess the accuracy of the ISCCP observations over the Tibetan Plateau. August 2006 is chosen to conduct the assessment and compared to February 2007. Cloud cover from ISCCP is underestimated by about 18%, in part because of misdetection of low‐level clouds at night. ISCCP cloud top pressures are overestimated by about 150–200 mb in August and 60–130 mb in February. However, the most accurate ISCCP cloud top pressures, with a maximum bias of about 50 mb, are obtained when there are thick single‐layer clouds. Within the region, there is no evidence that the differences are directly dependent on elevation. Problems identified in other regions, such as multilayer clouds and optically thin clouds, explain most of the discrepancies in our study region. These results indicate that ISCCP cloud retrievals can be used to compile a realistic climatology at the highest altitudes where single‐layer clouds dominate and that the retrievals are most accurate in winter.