Cloud optical properties using ground-based measurements of zenith radiance in Xianghe
WANG Jing, XU Xiao-feng, XU Dan
Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China
A new method for retrieving cloud optical depth (COD) using AERONET cloud mode is introduced. Using statistical analysis for temporal variation characteristics of cloud optical properties at Xianghe site based on the COD data obtained by this algorithm from January 2011 to June 2012, we find that there was significant daily variation COD in zenith direction and the number of occurrence of non-precipitating clouds (Num). Comparing the COD data retrieved by ground-based remote sensing and MODIS satellite under different sky conditions (broken clouds and overcast cases), the two data sets fit very well with high correlation. In these two cases, the average values of MODIS COD are 10.8 and 9.4, smaller than those of ground observation. COD had a decreasing trend in the morning and dusk, and a increasing trend at noon and in the afternoon. Num day curve was "bimodal" and peaked in the morning and afternoon, respectively, with a minimum around noon. The seasonal average COD was in the order of autumn> spring> winter> summer, each season had more than 65% of the COD concentrated in the range of 10~40; summer observations showed the maximum number of days, minimum change. At least a few days in the winter observations, it showed the maximum change. The observed days or change were consistent in spring and autumn. A linear regression analysis for COD and AOD time series was used at Xianghe site, and a significance test was carried out, showing that observed frequency of COD and fine mode AOD had a strong positive correlation.
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