几种地表微波比辐射率变化特征的地面观测

Field Measurements of the surface microwave emissivity for different surface types

基于地表微波比辐射率观测试验,我们探讨了不同下垫面的地表微波比辐射率的变化特征以及降雨前后地表比辐射率的昼夜变化特征.同时,通过比较红外扫描仪和温度计同步测量的地表温度,发现将温度计浅埋土里比半埋土里测量的更为合理,后者测量的地表温度在中午时不合理偏高15～20℃.对于四种地表,草地比辐射率最高(～0.94),其次是裸土地比辐射率(～0.86),然后是沙地比辐射率(～0.82),水面比辐射率最小(～0.4).在微波辐射计观测入射角>60°时,土地和沙地比辐射率都随入射角度增加而减小,尤其前者更为敏感;草地和水面比辐射率随入射角度变化较小.不同地表比辐射率都呈现出昼夜差异,尤其土地、沙地和水面比辐射率在降雨之后的昼夜差异较为显著,夜里普遍偏高白天0.02～0.04;草地比辐射率昼夜差异较小,基本是白天略微高于晚上.降水后,草地微波比辐射率变化较小,裸土地和沙地比辐射率则显著降低.沙地和草地比辐射率随频率变化较小,裸土地比辐射率在降雨之后随频率明显增加.

We investigated the variation characteristics of the land emissivity for different surface types as well as their diurnal variations before and after precipitation based on field measurements of the surface microwave emissivity. Meanwhile, the comparisons of surface temperature measured from the infrared scanning sensor and thermometer show that the method inserting the thermometer into shallow soil surface is more reasonable than the method putting thermometer half in air and half in soil, and surface temperature measured from the latter method is overestimated 15 ~20~C at noon. For the four surface types, the grass land shows highest emissivity （～0.94）, secondly the bare soil （～0.86）, then sand soil （～0.82）, and water shows the lowest emissivity （～0.4）. When the incident angle is large than 60°, the emissivities of bare soil and sand soil decrease with increasing incident angle, especially the former is more sensitive. Relatively, for grass land and water, their emissivities slightly change with the incident angle. For different surface types the emissivity shows the diurnal variation, especially for bare soil, sand soil, and water. The emissivity in the night is higher 0.02~0.04 than that in the day after precipitation. The diurnal emissivity difference for grass land is slight, and its emissivity in the day is little higher than that in the night. After precipitation the emissivity of grass land changes little while the emissivities of bare soil and sand obviously decrease. The emissivities of grass land and sand soil vary less with frequency, while the emissivity of bare soil increases clearly with increasing frequency after precipitation.