Spatial-temporal Distribution Of Cloud Amount Over Yangtze River In Recent 21 Years Using NOAA/AVHRR Data

As one of the most important factors of climate, clouds can change the radiation budget of the earth-atmospheric system and so play a critical role in driving atmospheric circulation and regulating the earth climate and the water cycle process. Almost twenty years from June 1988 to December 2008 of advanced very high resolution radiometer (AVHRR) raw data were collected by China National Satellite Meteorological Center (NSMC), and then this dataset was used to generate a set of high spatial and temporal resolution cloud amount products. In this thesis, we try to analysis temporal and spatial distribution characteristics, annual trend, and seasonal change of cloudiness over Yangtze River. In addition, the relationship between cloud and precipitation, air temperature, relative humidity was studied.(1)Image processing such as radiation calibration, geometric correction is done to NOAA/AVHRR LIB dataset to get high level NOAA/AVHRR observation products for cloud distribution analysis, the result shows that there is regular annual change in radiation, and no obvious differences between radiations of each NOAA satellite. For cloud detection, an automated dynamic threshold cloud-masking algorithm is applied to get cloud mask result (NSMC-AVHRR-CLM). Having determined the presence or absence of clouds at each location and time, we could get 21yr NOAA/AVHRR cloud amount product (NSMC-AVHRR-CLA).(2)This paper describes the validation of NSMC-AVHRR-CLM by comparing with coincident surface weather observation, the results show that about 90.46% NOAA/AVHRR cloud detection samples agrees with surface weather observations, the miss detected rate is 5.31% and the false detected rate is 4.23%. Besides, CloudSat data are also used to investigate the impact of difference in observation time on the examine result of NSMC-AVHRR-CLM. Individual AVHRR results are lower than the individual surface observation results by about 2% in Summer, and more than the individual surface observation results by about 5% in Winner. Comparison with surface observation (SOB) monthly mean cloud amount, AVHRR monthly mean cloud amount prefer more when actual cloud amount low than 60%, and less when actual cloud amount more than 80%. In spring and summer, AVHRR has more cloud than ISCCP, in autumn and winter, AVHRR has less cloud than ISCCP. AVHRR cloud product has well correlation with other cloud climatologies, but not western of Sichuan Basin. The correlations between AVHRR and MODIS is well than the relation between AVHRR and ISCCP. (3)NSMC-AVHRR-CLA shows the consistent characteristics of total cloud amount distribution and climatology variation with surface observation, ISCCP, and MODIS. In addition, the annual and seasonal variation of cloud amount shows a declining tendency over the past decades over Yangtze River. In the boreal winner, more extensive cloudiness can be observed over the Sichuan Basin, Guizhou province, and Chongqing province. In the boreal summer, cloudiness decreases from southwest of the domain to northeast, corresponding to the summer monsoon. In coastal areas in southeast, less cloudiness can be observed.(4) The variations and interrelations of the cloudiness and some other climate factors of the studied domain are analyzed. The result shows that cloud amount is negative with mean temperature, highest temperature and diurnal temperature range (DTR), and is positive with precipitation and relative humidity, respectively.