Saharan dust in nighttime thermal imagery: Detection and reduction of related biases in retrieved sea surface temperature
Saharan dust in nighttime thermal imagery: Detection and reduction of related biases in retrieved sea surface temperature
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DOI：
10.1016/j.rse.2006.03.007
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发表时间：
2006-09
影响因子：
13.5
通讯作者：
C. Merchant;Owen Embury;P. Borgne;B. Bellec
中科院分区：
文献类型：
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作者：
C. Merchant;Owen Embury;P. Borgne;B. Bellec
通过辐射传输模型的撒哈拉沙尘（气溶胶）的影响，在3.9，8.7，11和12 μm的亮度温度测量的旋转增强可见光和红外成像仪（SEVIRI）的指导下，我们提出了一个指标的撒哈拉沙尘夜间（仅红外）SEVIRI图像的存在。辐射传输建模进行使用快速大气透射率模型和三角洲爱丁顿近似散射。使用气溶胶单散射特性适合模拟撒哈拉沙漠的灰尘给质量好的模拟灰尘影响的亮度温度。从这些模拟中，我们发现，无气溶胶的亮度温度是紧密聚集在主轴周围（解释99%的方差）在三维亮度-温度差空间（轴为3.9 μm-8.7 μm、3.9 μm-12 μm和11 μm-12 μm）;与此相反，当转换到这个空间时，模拟的受气溶胶影响的亮温明显离轴。虽然我们的模拟没有定量再现气溶胶对亮度温度的详细影响，这种离轴特性的经验发现也是真实的撒哈拉沙漠沙尘影响的亮度温度的观测。第二个主成分的亮度温度差空间被确定为一个有用的指标撒哈拉尘埃。与独立卫星测量的气溶胶光学厚度（0.55 μm）的比较表明，新的撒哈拉沙尘指数（SDI）与气溶胶光学厚度的相关性较弱（r=0.34）。由于SDI对气溶胶的高度分布也很敏感，从而影响气溶胶的热发射，因此相关性不强。SDI是用来制定一个经验的修正方案，SST反演受撒哈拉沙漠尘埃。SDI校正方案的应用消除了SEVIRI SST对来自所有纬度的浮标验证的0.2 K的独立误差。
Guided by radiative transfer modelling of the effects of Saharan dust (aerosol) on brightness temperatures at 3.9, 8.7, 11 and 12 μm measured by the Spinning Enhanced Visible and Infrared Imager (SEVIRI), we propose an indicator of the presence of Saharan dust in nighttime (infrared-only) SEVIRI imagery. Radiative transfer modelling is performed using a fast atmospheric transmittance model and a delta-Eddington approximation for scattering. Using aerosol single scattering properties appropriate to simulation of Saharan dust gives qualitatively good simulations of dust-affected brightness temperatures. From these simulations, we show that aerosol-free brightness temperatures are tightly clustered around the principal axis (explaining 99% of variance) in a 3-dimensional brightness-temperature difference space (the axes being 3.9 μm minus 8.7 μm, 3.9 μm minus 12 μm, and 11 μm minus 12 μm); in contrast, brightness temperatures affected by aerosol are simulated to be significantly off-axis when transformed into this space. Although the detailed effects of aerosol on brightness temperatures are not quantitatively reproduced by our simulations, this off-axis characteristic is empirically found to be also true of Saharan-dust-affected brightness temperatures in observations. The second principal component of the brightness-temperature difference space is identified as a useful index for Saharan dust. Comparisons with independent satellite measurements of aerosol optical depth (AOD, at 0.55 μm) show that the new Saharan dust index (SDI) is loosely correlated with AOD (r=0.34). The correlation is loose because the SDI is also sensitive to the height distribution of aerosol, which affects the aerosol's thermal emission. The SDI is used to develop an empirical correction scheme for SST retrievals affected by Saharan dust. Application of the SDI correction scheme removes an independent error of 0.2 K from SEVIRI SSTs validated against buoys from all latitudes.