| The macro and micro physical properties of the cloud vary widely in space and time. The satellite is the primary means cloud monitoring. By the way of retrieval,we can get many cloud paraments such as the cloud effective radius. The response capability of the cloud parameters on the structure of real clouds is related to the application accuracy of these parameters. Understanding the cloud parameters carefully.we can research the global climate change clearly, judge the weather modification’s extending area and cloud seeding effect effectively,perceive the impact of human activities on the environment distinctly.The satellite’s3.7μm channel is widely used in the retrieval of cloud droplet effective radius. In this paper,the effects of cloud paraments’vertical inhomogeneity(such as the effective radius and the cloud optical thickness’s vertical inhomogeneity) on FY-3A satellite’s3.7μm channel is analysised.Based on the SBDART radiative transfer mode,the3.7μm reflectance is calculated separately for two particle profiles, the droplet effective radius vertical stratified homogeneous distribution and droplet effective radius vertical continuous changing distribution.To the different effective vertical distribution of particle radius, the trend of3.7μm reflectivity with the change of optical thickness is analysised and the error is calculated.For the first type, the results shows that when the optical thickness is large, the3.7μm reflectance only can be effected by the cloud parameters near the cloud top’s "shallow"(effective radiation layer,whose optical thickness is about3to6). There exist negative correlation between the optical thickness of effective radiation layer and the droplet effective radius near cloud top, solar zenith angle and satellite zenith angle, not obvious for the relative azimuth angle. To the second type, for the thin cloud(optical thickness less than10),there will exist obvious error when the effective radius of vertical inhomogeneous cloud is retrieved as the vertical homogeneous cloud and the error decreases with the increase of the optical thickness. If the cloud is thick enough(optical thickness greater than10),the error is less than10%. In this case, we can regard that the changes of cloud droplets’ vertical structure don’t influent the reflectance of3.7μm channel, in addition,if the absolute value of the difference between cloud top and cloud base’s effective radius is the same.the error of cloud whose effective radius of base is greater than the top is greater than the error of cloud whose effective radius of base is less than the top.Because the3.7μm channel can only get the information of the cloud top’s droplet,the other channels of FY-3A is also used to retrieve the effective radius. Based on the principle that the reflectivity at a water(or ice) absorbing band primarily depends on cloud particle size, channel3(3.55-3.93μm)of FY-3A Visible and Infrared Radiometer(VIRR)data and channel6(1.64μm),channel7(2.13μm)of FY-3A Medium Resolution Spectral Imager(MERSI)data are used to retrieve the effective radius based on the SBDART radiative transfer mode. It is found, cloud reflectivity of the three channels of1.64μm,2.13μm and3.7μm reduces in turn. The reflectivity of1.64μm and2.13μm is more sensitive to the larger particles, but3.7μm shows better sensitivity when optical thickness is smaller. Finally, using reflectivity of2.13μm/3.7μm and effective particle radius retrieved from it, it is analyzed that pollution has much effect on microphysical structure of marine cumulostratus clouds. Compared with effective particle radius product of MODIS, the correlation coefficient reaches0.843. |
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