半透明云风矢量高度算法中云下背景辐射的估计

半透明云风矢量高度指定是卫星风矢量算法的重要部分，需要来自半透明云体的辐射和云下背景辐射两个变量。云下背景辐射发生在云层下面，未被卫星直接观测到，为了在半透明云风矢量高度指定算法中更精确地获得云下背景辐射，使用风矢量附近无云区的红外/水汽散点图，估计云下背景辐射。分析表明:在追踪区域里存在无云区的情况下，追踪区的最高红外亮温可代表红外通道的背景辐射；而水汽通道的背景辐射，却在红外亮温高值区段内水汽亮温相对较低区段。追踪区内找不到无云区时应扩大搜索范围，找到无云区后可估计云下背景辐射。在半透明云风矢量高度指定算法中使用云下背景辐射估计的改进算法前后，计算FY-2气象卫星进行风矢量，并将结果与欧洲中期天气预报中心（ECMWF）分析场进行对比表明，在半透明风矢量高度指定算法中使用云下背景辐射估计，FY-2气象卫星风矢量误差明显降低。

Estimation of Background Radiation Underneath Clouds for Wind Vector Height Assignment of Semi-transparent Clouds

Height assignment of semi-transparent clouds needs two variables:Radiation from the semi-transparent clouds themselves and radiation from the background underneath semi-transparent clouds. The background radiation is not directly observed by the satellite, but can be estimated with infrared/water vapor scatter diagrams of satellite measurements around tracer image boxes. According to the cloud amount and type observed, tracer image boxes can be divided into 4 types:With different levels of clouds inside, near the cloud edge, with only high or middle-high clouds inside and with only middle-low clouds or ground in tracer box. By studying characteristics of infrared/water vapor scatter diagrams of satellite measurements in 8 typical cases of 4 types which are variety, different estimation methods are discussed and a new estimation method for only high or middle-high clouds in tracer box is proposed. When there are cloud-free pixels in tracer boxes, infrared maximum brightness temperature may represent the background radiation for infrared channel; on the other hands, observation pixels with the highest infrared but relative lower water vapor brightness temperatures are used to estimate the background radiation for water vapor channel. Observation pixels with high infrared, but relative lower water vapor brightness temperatures are located in the clear sky region near the cloud edge, where atmospheric stratification conditions are relatively wetter than the region far away from clouds. The clear sky region near the cloud edge relatively better represent the condition underneath clouds better. Due to absorption and retransition processes, the water vapor brightness temperatures with wetter atmospheric stratification conditions in the clear sky region near the cloud edge are relative lower. When there are no cloud-free pixels in the tracer image box, the searching area should be expanded in the east-west direction to find cloud free pixels, until the background radiation can be estimated.Based on the new estimation method, FY-2 wind vector height assignment algorithm is improved, and effectiveness is analyzed by comparing with ECMWF reanalysis data, showing that FY-2 wind vector quality including biases and standard deviations is improved obviously.