City microwave emissivity extraction and characteristic analysis in China

城市是陆表重要的覆盖类型之一,精确提取城市微波发射率、研究其辐射特征和影响因素有利于提高城市密集区被动微波遥感反演土壤水分的精度。利用2008年AMSR-E(Advanced Microwave Scanning Radiometer for Earth Observing System)6.925GHz和10.65GHz双极化的时间序列亮温数据和其他多种辅助数据,在分析并去除无线电干扰的前提下,提取中国区域相对纯净城市像元的发射率,并分析其辐射特征的季节变化和影响因素。结果表明,根据本文方法提取得到的中国区域纯净城市像元有两个,分别位于北京和上海。城市亮温受到无线电干扰影响,一年内不同城市、波段、过境时间的卫星观测亮温受到的干扰强度不同。去除无线电干扰后,各通道的城市发射率在一年中变化不大,且卫星降轨观测时刻的发射率比升轨时刻更加稳定。降雨是影响城市发射率的重要因子,当降雨量达到一定程度后,城市发射率与降雨量呈现明显的负线性相关。

City is a very important land cover type on the earth, making understanding city microwave emissivity essential in improving soil moisture retrieval accuracy using passive microwave remote sensing data, especially in urbanized regions. In this paper, time series Advanced Microwave Scanning Radiometer for Earth Observing System （AMSR-E） data in 2008 and other auxiliary data were used to extract and analyze the characteristics of city emissivity in China. During the process, radio frequency interference （RFI） in each pixel was evaluated first and the effects were removed before emissivity calculation. Also we studied the characteristics and influencing factors of city emissivity during the year. The results show that with the extraction method, only two pure city pixels in China are found, which are Beijing and Shanghai, respectively. Satellite observed city brightness temperature can be affected by RFI, but its effects varied between cities, bands and sensor crossing time. After eliminating RFI, the city emissivity fluctuated slightly in a year and was more stable in descending time than in ascending time. Rainfall played an important role in city emissivity variation. There was an obvious negative correlation relationship between city emissivity and rainfall when rainfall reached a certain value.