北半球植被冬夏两季微波发射率特征分析

Study of the Microwave Emissivity Characteristics of Vegetation over the Northern Hemisphere

地表微波发射率是含水量、表面粗糙度、地物结构等的函数,这些因素具有明显的季节性。用晴空条件下高级微波扫描辐射计(AMSR-E)的瞬时发射率,辅助2003年IGBP(international geosphere-biosphere project labels)地表分类数据,获得了北半球各种植被覆盖地区2003年夏冬两季每半月平均的地表微波发射率。分析不同植被发射率随频率、极化、时间等变化的特性。结果表明,植被发射率在H和V极化均随频率的增加而增加,在有降雪的地区发射率随频率增加而迅速减小。植被区夏季发射率较高,89GHz的V极化发射率值大于0.944,且发射率时间序列上很稳定,极化差较低(<0.081),同一地区发射率时间序列RSME小于0.007 2。植被区冬季在积雪出现的地方发射率值降低,尤其表现在高频部分。随着植被密度的增加,植被地区发射率值增加而极化差降低。

The microwave emissivity is a function of structure, water content, and surface roughness, and all these factors have obvious seasonal variations. In the present study, the half-month averaged emissivities in summer and winter of 2003 over the vegetation of Northern Hemisphere were estimated using Advanced Microwave Scanning Radiometer-Earth Observing System （AMSR-E） combined with IGBP （International Geosphere-Biosphere Project labels） land classification data. Then the emissivi-ties of vegetation land covers at different frequencies, the polarization and their seasonal variations were analyzeo respecuve^y. The results show that the emissNities of vegetation increase with the increase in frequencies, and decline with the frequency in- creasing over snow region. In summer, the vegetation emissivity at V-polarization of 89 GHz is larger than 0. 944, and all emis- sivities are relatively stable and the RMSE of time series emissivity variation is less than 0. 007 2. In winter, emissivities decrease over snow covered area, especially for higher frequencies. Furthermore, with the increase in vegetation density, the emissivities increase and emissivity polarization difference decreases.