Effects of neglecting polarization on the MODIS aerosol retrieval over land
Effects of neglecting polarization on the MODIS aerosol retrieval over land
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DOI：
10.1109/tgrs.2004.837336
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发表时间：
2004-11
影响因子：
8.2
通讯作者：
R. Levy;L. Remer;Y. Kaufman
中科院分区：
文献类型：
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作者：
R. Levy;L. Remer;Y. Kaufman
利用中分辨率成像光谱仪（MODIS）测量可见光和红外波长的反射率，分别得出海洋和陆地表面的气溶胶光学厚度（AOTs）和气溶胶特性。这两种算法都使用辐射传输（RT）代码来创建查找表，模拟卫星测量的大气层顶部（TOA）反射率。海洋上的算法使用包含大气极化效应的矢量RT代码，而陆地上的算法采用标量RT，因此忽略了极化效应。在红色（0.66 /spl mu/m）和红外（2.12 /spl mu/m） MODIS通道中，分子散射（瑞利散射）最小。在这些波段中，使用标量RT码具有足够的精度来模拟TOA反射率。然而，在蓝色（0.47 /spl mu/m）中，存在较大的瑞利散射（光学厚度接近0.2）导致不可忽略的极化。矢量计算的TOA反射率和标量计算的TOA反射率之间的绝对差异，即使在去极化气溶胶存在的情况下，也足以导致检索到的AOT产生很大的误差。使用允许矢量和标量计算的RT代码，我们检查了TOA处的反射率差异，假设大陆型气溶胶的离散载荷。我们发现蓝色通道TOA反射率（矢量-标量）的差异可能大于0.01，从而推导出的AOT误差可能大于0.1。误差可能是正的，也可能是负的，这取决于具体的几何形状，当对足够大的卫星几何形状样本进行平均时，误差往往会被抵消。因此，忽略极化给全球和长期平均带来的误差很小，但在较小尺度和个别检索上可能产生非常大的误差。这项研究的结果是，未来从陆地上的MODIS获取气溶胶的版本将包括大气中的极化。
Reflectance measurements in the visible and infrared wavelengths, from the Moderate Resolution Imaging Spectroradiometer (MODIS), are used to derive aerosol optical thicknesses (AOTs) and aerosol properties over ocean and land surfaces, separately. Both algorithms employ radiative transfer (RT) code to create lookup tables, simulating the top-of-atmosphere (TOA) reflectance measured by the satellite. Whereas the algorithm over ocean uses a vector RT code that includes the effects of atmospheric polarization, the algorithm over land assumes scalar RT, thus neglecting polarization effects. In the red (0.66 /spl mu/m) and infrared (2.12 /spl mu/m) MODIS channels, scattering by molecules (Rayleigh scattering) is minimal. In these bands, the use of a scalar RT code is of sufficient accuracy to model TOA reflectance. However, in the blue (0.47 /spl mu/m), the presence of larger Rayleigh scattering (optical thickness approaching 0.2) results in nonnegligible polarization. The absolute difference between vector- and scalar-calculated TOA reflectance, even in the presence of depolarizing aerosols, is large enough to lead to substantial errors in retrieved AOT. Using RT code that allows for both vector and scalar calculations, we examine the reflectance differences at the TOA, assuming discrete loadings of continental-type aerosol. We find that the differences in blue channel TOA reflectance (vector-scalar) may be greater than 0.01 such that errors in derived AOT may be greater than 0.1. Errors may be positive or negative, depending on the specific geometry, and tend to cancel out when averages over a large enough sample of satellite geometry. Thus, the neglect of polarization introduces little error into global and long-term averages, yet can produce very large errors on smaller scales and individual retrievals. As a result of this study, a future version of aerosol retrieval from MODIS over land will include polarization within the atmosphere.