Evaluation of High Cloud Product of ECMWF Over South China Sea Using CALIOP

High clouds are globally widespread and are crucially important to global radiative processes and to the thermal balance of the Earth. Active remote sensing measurements can provide more reliable detection of optically thin cirrus clouds than passive remote sensing. The reanalysis datasets from European Centre for Medium-Range Weather Forecasts (ECMWF) have a variety of applications in different branches of atmospheric science. A careful evaluation of these datasets is a prerequisite for their successful use. In this paper, the high cloud product from ECMWF is compared with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) observations over the South China Sea from December 2006 to February 2019. The annual average of high cloud amount is analyzed and compared between CALIOP and ECMWF day and night, and the monthly comparison is also conducted. The results show that the distributions of high cloudiness in CALIOP and ECMWF are in fairly good agreement over the study area both day and night. The correlation coefficient between the two products is 0.79 during daytime, 0.75 nighttime. 50.8% of high cloud amount from ECMWF is 8.8% lower than 59.6% from CALIOP during daytime. A larger difference of 16.2% occurred at night with 53.9% from ECMWF, and 70.1% from CALIOP. Quantitative analysis of subvisible cirrus clouds (SVC, optical depth <0.03) is given. The results show that the discrepancies between the two data sets are reduced to 2.3% during daytime, and 5.2% at nighttime after screening out SVC. The correlation coefficients also rise to 0.82 both daytime and nighttime. The distribution and occurrence of SVC are presented over the study area.