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Use of Representative Pixels of Motion for Wind Vector Height Assignment of Semi-transparent Clouds
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摘要: 半透明云风矢量高度指定是卫星风矢量算法的重要部分，需要使用来自半透明云体的辐射和云下背景辐射两个变量。为了更精确地获得来自半透明云体的辐射，采用像元对追踪相关系数的贡献和红外亮温两个变量进行统计分析，将追踪图像块中的像元分为3个部分：小贡献像元、暖区段大贡献像元和冷区段大贡献像元；对暖区段、冷区段内大贡献像元的特征进行考察表明，冷区段的大贡献像元更能代表追踪图像块运动，称为代表运动像元。选择代表运动像元参与计算来自半透明云体的辐射，对半透明云风矢量高度指定算法进行改进，在该算法中分别使用和不使用代表运动像元，计算FY-2气象卫星风矢量，将结果与欧洲中期天气预报中心（ECMWF）分析场进行对比表明，在半透明云高度指定算法中使用代表运动像元，FY-2气象卫星风矢量误差明显降低。
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关键词:
- 风矢量;
- 半透明云风矢量高度指定;
- 代表运动像元
Abstract: Height assignment of semi-transparent clouds needs two variables:Radiation from semitransparent clouds themselves and radiation from the background underneath semitransparent clouds. Generally, the former radiation is calculated using algorithms based on all pixels in tracer boxes. A new algorithm, however, is proposed considering only pixels with great contribution to correlation coefficients during wind vector tracing stage, and only allow these pixels to join the calculation. Scatter diagrams are made with infrared brightness temperature against their contribution to the feature tracking correlation coefficients. Scatter diagrams show a "C" shape pattern and can be used to recognize representative pixels of motion. Pixels with relative less contribution to correlation coefficients are at the left side of the "C" shape with infrared brightness temperature in the middle range. The other pixels with relative more contributions are naturally separated into two branches with warmer and colder infrared brightness temperature respectively. When tracer boxes are partly covered by clouds, the warm branch pixel brightness temperature is with typical ground feature at both the scope and the variability. And when tracer boxes are totally covered by clouds, warm branch pixels are in the slot of clouds with typical cloud features among cloud cells. Thus, no matter tracer boxes are partly covered or totally covered by clouds, cold branch pixels with greater contribution to correlation coefficients are better at representing the cloud motion and can be named as pixels with contribution to the motion. The algorithm using different pixel's contribution to the feature tracking improves height assignment quality by effectively removing rapid changing pixels in tracer boxes when estimating the cloud height. Using representative pixels of motion at the estimation of radiation from semi-transparent clouds themselves, FY-2 wind vector height assignment quality is improved.-
Key words:
- wind vector;
- height assignment;
- pixels representative of motion
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图 1 半透明云风矢量高度指定算法示意图
Fig. 1 Schematic diagram of the algorithm of semi-transparent cloud height estimation
图 2 追踪区部分被云覆盖案例 (a) 追踪图像块位于30.94°N, 120.44°E, (b) 追踪图像块位于15.86°N, 118.81°E, (c) 追踪图像块位于33.41°N, 123.83°E
Fig. 2 Typical examples for the tracer partly covered by clouds (a) tracer box located at 30.94°N, 120.44°E, (b) tracer box located at 15.86°N, 118.81°E, (c) tracer box located at 33.41°N, 123.83°E
图 3 追踪区全部被云覆盖典型案例 (a) 追踪图像块位于34.56°N, 112.82°E, (b) 追踪图像块位于6.27°N, 139.42°E, (c) 追踪图像块位于5.33°N, 93.30°E，(d) 追踪图像块位于2.42°N, 90.35°E, (e) 追踪图像块位于29.27°S, 97.89°E
Fig. 3 Typical examples for the tracer totally covered by cloud (a) tracer box located at 34.56°N, 112.82°E, (b) tracer box located at 6.27°N, 139.42°E, (c) tracer box located at 5.33°N, 93.30°E, (d) tracer box located at 2.42°N, 90.35°E, (e) tracer box located at 29.27°S, 97.89°E
图 4 暖区段和冷区段大贡献像元数及像元平均贡献量关系
Fig. 4 Diagram of pixel number ratio and pixel mean contribution ratio between warm and cold segments
图 5 暖区段和冷区段所有像元对相关系数总贡献之比直方图
Fig. 5 Histogram of total contribution ratio between warm and cold segments
图 6 暖区段和冷区段像元数之比直方图
(a) 暖区段和冷区段总贡献之比小于0.3，(b) 暖区段和冷区段总贡献之比为0.8~1.2，(c) 暖区段和冷区段总贡献之比大于2
Fig. 6 Histogram of pixel number ratio between warm and cold segments
(a) total contribution ratio between warm and cold segments less than 0.3, (b) total contribution ratio between warm and cold segments in 0.8-1.2, (c) total contribution ratio between warm and cold segments greater than 2
图 7 追踪图像块中最高红外亮温的直方图
(a) 暖区段和冷区段总贡献之比小于0.3，(b) 暖区段和冷区段总贡献之比为0.8~1.2，(c) 暖区段和冷区段总贡献之比大于2
Fig. 7 Histogram of IR maximum temperature in tracer boxes
(a) total contribution ratio between warm and cold segments less than 0.3, (b) total contribution ratio between warm and cold segments in 0.8-1.2, (c) total contribution ratio between warm and cold segments greater than 2
表 1 半透明云高度指定算法中是否使用代表运动像元FY-2E卫星红外高层风风速与ECMWF分析场比较
Table 1 Bias and standard deviations between FY-2E wind vectors and ECMWF analyses for height assignment algorithms whether using contribution to the motion
统计量 未使用代表运动像元 使用代表运动像元 偏差/(m·s-1) -4.239 -2.103 标准差/(m·s-1) 7.787 7.436 风矢量数 13895 13536 
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