中国比辐射率空间分布特征分析

Emissivity spatial distribution characteristics over China region

使用2003-2013年MOD/MYD11C3地表比辐射率光谱数据、MOD/MYD13C2植被指数光谱数据,合成全国各月地表比辐射率、NDVI(Normalized Difference Vegetation Index)。基于DEM数据分析比辐射率与NDVI随海拔、坡向的变化规律。结果表明:(1)比辐射率低值段(0.960~0.970)主要分布在我国西北荒漠地区,面积比例全年变化不显著,代表了干燥裸土下低比辐射率的特征;中值段(0.970~0.975)分布于我国大部分植被覆盖地区,面积比例夏高冬低,代表植被覆盖下混合像元的中比辐射率特征;高值段(0.975~0.980)位于我国部分高海拔和高纬度地区,面积比例冬高夏低,代表冰雪与植被混合像元的高比辐射率特征。(2)比辐射率与NDVI随坡向变化呈明显的"双峰双谷"分布。东南坡、西坡为峰值,最大值位于东南坡;南坡、北坡为谷值,最小值位于北坡。两者变化一致性很高。受不同坡向太阳方位角下的地形敏感性与植被覆盖综合影响,比辐射率表现出随坡向的峰谷变化规律。(3)随海拔升高,比辐射率呈垂直地带性变化。存在3个下降区:250 m^1 250 m、2 500 m^3 000 m和4 750 m^6 000 m;3个上升区:1 250 m^2 500 m、3 000 m^4 750 m和6 000 m^6 500 m。这与NDVI随海拔变化特征类似,反映垂直下垫面植被变化对比辐射率空间分布的影响。

MODIS monthly surface emissivity spectrum database（MOD11C3/MYD11C3）,monthly surface vegetation index spectrum database（MOD13C2/MYD13C2）were used to synthesize monthly emissivity and NDVI（Normalized Difference Vegetation Index）products from 2003 to 2013. Monthly emissivity and NDVI distribution characteristics with altitude and aspect were also discussed by using national basic geographical data（China terrain 1∶1 000 000 DEM,vector boundary data）. Results show as follows：（1）Area ratio of different segments indicates seasonal variation of emissivity. Low section（0.960-0.970）locates in the northwest desert region of China,representing the low emissivity of bare soil,proportion of that area does not change significantly throughout months. Middle section（0.970-0.975）covers most vegetated areas,representing the emissivity of vegetated mixed pixel. Proportion of the area is high in summer while low in winter. High section（0.975-0.980）covers parts of the high altitude mountainous or high latitude region,representing a high emissivity of mixed snow and vegetation. Proportion of the area is high in winter while low in summer.（2）A＂Twin Peaks double dip＂feature shows when emissivity and NDVI changes with azimuth. Peak values locate on southeast and west aspect,the maximum value locates on southeast. While dip values locate on south and north aspect,the minimum value locates on north. Both changes with high consistency.（3）An obvious vertical zonal distribution shows that emissivity changes with increasing altitude. Emissivity decreases during 250-1 250 m,2 500-3 000 m and 4 750-6 000 m,while increases during 1 250-2 500 m,3 000-4 750 m and 6 000-6 500 m. Emissivity shows a high consistency with NDVI below 6 000 m,which depends on the vegetation coverage. While above 6 000 m,it depends on bare soil covered with snow. Innovations of this research are summarized as follows ：（1）The latest remote sensed data were used to synthesize monthly mean emissivity over China region in 11 years（2003-2013）.Synthesis results clearly reflect the variation characteristics of surface physical properties.（2）Analysis of monthly emissivity distribution with the change of geographical terrain elements over China region has been conducted under both vertical characteristic：elevation,and horizontal characteristic：aspect conditions.