Use of Representative Pixels of Motion for Wind Vector Height Assignment of Semi-transparent Clouds

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.