地物反射光谱对MODIS近红外波段水汽反演影响的模拟分析

The Simulation Analyze of the Effect of Surface Reflective Spectrum on the Retrieval of Water Vapor with MODIS NIR Data

在近红外辐射传输方程的基础上,利用近红外波段水汽的不同吸收属性,在MODTRAN的模拟下,深入分析了基于MODIS近红外数据的可降水汽反演算法,并着重讨论了地物反射光谱非线性在可降水汽反演中的影响。研究结果显示,当波段间反射率之比不等于1时,MODIS近红外波段反演水汽将存在较大偏差。同时,在地物光谱库基础上,计算了不同地物反射率比值,其分布表明,大部分地物波段反射率比值不等于1。研究表明,应用现有MODIS近红外波段水汽反演算法,如果不考虑地表反射率光谱变化的影响,由地表反射光谱造成的误差最大约为反射率比值与1偏差的15倍,同时,这一误差还与大气波段透过率之比有关。

Based on the near infrared radiative transfer theory, the retrieval method of precipitable water vapor (PWV) with MODIS near infrared band data is analyzed under the simulation of radiative transfer model, MODTRAN. The effect of surface nonlinear reflective spectrum on PWV estimation is emphasized on. Both two bands ratio method and three bands ratio method are taken into account. The study indicates that the error due to the surface nonlinear reflective spectrum is very large if the reflectance ratio of water vapor band to atmospheric windows band is not equal to 1.The error is proportional to the ratio of transmittance ratio between these two bands. The ratio of band reflectance is calculated for hundreds kinds of surface material such as rocks, minerals, soils, vegetation and water in Johns Hopkins University spectral library. Most of the reflectance ratios have a considerable difference with 1.And the difference from 1 of two bands ratio method is smaller than three bands method. That indicates three bands method is predominant over two bands method. The coefficients used in three bands method is recalculated based on the spectral library. The result shows that their coefficients should be gained from the regression of whole spectral library and the sum of coefficients should be equal to 1.