On the signature of the cirrus twilight zone
On the signature of the cirrus twilight zone
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发表时间：
2014
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通讯作者：
L. Remer
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作者：
U. Wollner;I. Koren;O. Altaratz;L. Remer
众所周知，卷云在气候系统中起着关键作用，但它们对地球辐射收支的总体影响尚未完全量化。不确定性部分是由于云的范围或覆盖范围不明确。在这里，我们表明，尽管仔细过滤多云像素，卷云有一个明确的统计特征。这种特征可以通过接近可探测的卷云来估计。这样的残余特征会影响依赖于无云大气的反演，如气溶胶光学深度（AOD）或海面温度。分析MODIS原始数据和产品，我们发现当接近可探测的卷云时，反射率明显增加。我们估计，在可探测到的卷云周围的第一公里内，AOD平均增加0.03±0.01，埃指数平均减少- 0.22±0.20。在5.5±1.8公里的典型距离上，效果衰减10倍。这种趋势证实了可能是冰晶的大颗粒对可探测到的卷云附近所谓的无云大气的贡献。
Cirrus clouds are known to play a key role in the climate system, but their overall effect on Earth’s radiation budget is not yet fully quantified. The uncertainties are, in part, due to ambiguities in cirrus extent or coverage. Here we show that despite careful filtering of cloudy pixels, cirrus clouds have a clear statistical signature. This signature can be estimated by the proximity to detectable cirrus clouds. Such a residual signature can affect retrievals that rely on a cloud-free atmosphere, such as aerosol optical depth (AOD) or sea surface temperature. Analyzing MODIS raw-data and products, we show a clear increase in the reflectance when approaching detectable cirrus clouds. We estimated a mean increase in AOD of 0.03 ± 0.01 and a decrease in the Angstrom-exponent of −0.22 ± 0.20 in the first kilometer around detectable cirrus. The effect decays tenfold at a typical distance of 5.5 ± 1.8 km. Such trends confirm the contribution of large particles that are likely to be ice crystals to the so-called cloud-free atmosphere near detectable cirrus clouds.