不同地物类型微波发射率特征分析

Study of the Microwave Emissivity Characteristics over Different Land Cover Types

地表微波发射率是表征地表特征的重要参数,同时也是反演地表、大气参数的重要条件。不同地物类型由于其结构,含水量,粗糙度等的差异而具有各自的辐射特征。利用2003年6～8月,2003年12月～2004年2月两个时段针对AMSR-E(advanced microwave scanning radiometer-EOS)传感器的晴空条件下全球地表微波发射率数据,结合IGBP分类标准及覆盖度数据,选择相对较纯像元,分析了不同地表类型的发射率特征以及不同地物类型的发射率在不同波段随季节的变化规律。结果表明,地表发射率跟频率与极化相关,几乎所有植被和裸土类型双极化下的发射率变化趋势是随频率的增加而增大,而冰雪覆盖类型V极化发射率随频率增加而减小。此外,发射率跟地表类型密切相关。植被类型的发射率要高于裸土的发射率。因此,随着植被密度的降低,裸土的贡献增大,导致总体发射率的降低。对于植被类型,一般在夏季处于生长旺盛期,发射率比较稳定,可作为特征发射率。积雪覆盖是一个重要因素,尤其在冬季影响着地表的发射率变化。

The microwave emissivity over land is very important for describing the characteristics of the lands,and it is also a key factor for retrieving the parameters of land and atmosphere.Different land covers have their emission behavior as a function of structure,water content,and surface roughness.In the present study the global land surface emissivities were calculated using six month （June,2003-August,2003,Dec,2003-Feb,2004） AMSR-E L2A brightness temperature,MODIS land surface temperature and the layered atmosphere temperature,and humidity and pressure profiles data retrieved from MODIS/Aqua under clear sky conditions.With the information of IGBP land cover types,＂pure＂ pixels were used,which are defined when the fraction cover of each land type is larger than 85%.Then,the emissivity of sixteen land covers at different frequencies,polarization and their seasonal variation were analyzed respectively.The results show that the emissivity of vegetation including forests,grasslands and croplands is higher than that over bare soil,and the polarization difference of vegetation is smaller than that of bare soil.In summer,the emissivity of vegetation is relatively stable because it is in bloom,therefore the authors can use it as its emissivity in our microwave emissivity database over different land cover types.Furthermore,snow cover can heavily impact the change in land cover emissivity,especially in winter.