基于亮温通道变率的FY-3C微波湿度计陆地云检测新方法

云检测是卫星资料同化的重要前处理步骤，无论是晴空资料同化还是有云资料同化，都需要准确地区分有云和晴空资料。由于陆地地表发射率的多变性和微波能穿透部分云类的特点，微波湿度计资料在陆地上空的云检测研究一直是难点。利用快速辐射传输模式（CRTM）分析了不同云类条件下FY-3C微波湿度计（MWHS-Ⅱ）各通道亮温的通道间变率特征，根据MWHS-Ⅱ亮温通道间变率随云高以及云中液态水含量的增大而减小的特点，提出了一个基于亮温通道变率的MWHS-Ⅱ陆地资料云检测方法。与已有的云产品比较结果表明:新的云检测算法能有效地剔除大部分受云影响的资料，剔除后的晴空资料观测和模拟偏差更好地符合高斯分布。新方法对过冷水云、冰云、重叠云的检测能力较强，正确检测率可以达到80%，对卷云以及高度较低的水云的检测能力相对较弱。新方法能有效利用MWHS-Ⅱ观测资料自身完成云检测，在MWHS-Ⅱ资料同化中有很好的应用前景。 

A new cloud detection algorithm based on brightness temperature variation for FY-3C Microwave Humidity Sounder over land

Cloud detection is an important pre-processing procedure in satellite data assimilation. It is necessary to distinguish cloudy and clear-sky data for both clear-sky and all-sky assimilation. However, cloud detection for microwave humidity sounders over land is still challenging due to the variety of surface emissivity and microwave sounders' ability to penetrate clouds. Therefore, a new cloud detection algorithm is proposed for FY-3C microwave humidity sounder Ⅱ (MWHS Ⅱ) data assimilation over land in this study. By using the Community Radiative Transfer Model (CRTM), brightness temperatures of all sounding channels are simulated under different cloud parameters, based on which observations of 7 out of 15 channels are then selected to develop a new land index for cloud detection over land regions. Simulations show that the variety of brightness temperature observations among MWHS-Ⅱ reduces with the increase in cloud height and cloud water content. Compared with AHI cloud products, this new cloud detection method can remove most of the radiances contaminated by clouds, the probability distribution of O-B is more consistent with Gaussian distribution for data on clear sky. This new method has a better capability for identifying supercooled water clouds, opaque ice clouds and overlapping than for cirrus and water clouds with low cloud heights. The detectable rates of supercooled water clouds, opaque ice clouds and overlapping are up to 80%. This algorithm can detect cloudy radiances using MWHS-Ⅱ itself and has a good application prospect.