Cloud three‐dimensional effects evidenced in Landsat spatial power spectra and autocorrelation functions
Cloud three‐dimensional effects evidenced in Landsat spatial power spectra and autocorrelation functions
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Landsat 空间功率谱和自相关函数证明云三维效应
DOI：
10.1029/2000jd900153
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发表时间：
2000
影响因子：
--
通讯作者：
G. Wen
中科院分区：
文献类型：
--
作者：
L. Oreopoulos;A. Marshak;Robert F. Cahalan;G. Wen
对太阳波长和多云条件下的天底反射率、空间傅里叶功率谱和自相关函数进行了分析。数据来自陆地卫星专题成像仪（TM）的观测，而蒙特卡洛（MC）模拟被用来帮助解释的观测和检查各种因素的敏感性。我们表明，短波辐射过程产生一致的签名的功率谱和自相关函数。功率谱具有各种形状，在以前的观测研究中没有显示或解释。我们表明，TM光谱可能会受到影响的辐射“粗糙化”在中等尺度（101 -5公里）和辐射“平滑”在小尺度（<1公里）。这些过程与波长有关，在保守的（云滴）TM波段4（0.8 μm）和吸收波段7（0.22 μm）之间存在系统差异。频带7比频带4呈现更多的粗糙化和更少的平滑化，并且自相关下降更快。由于光学和/或几何侧面照明和阴影，粗糙化在大太阳天顶角处更普遍。MC光谱说明，尺度不变的光学深度场可以产生复杂的功率谱，在不同的条件下采取各种形状。辐射粗糙度随单次散射散射角的减小和太阳天顶角的增大而增大（与观测结果相同）。对于低太阳天顶角，有一个明确的转移，在辐射平滑尺度较小的值作为液滴吸收的增加。功率谱也表现出更强的光学深度和反射率之间的去相关云顶变化时，更明显。最后，它表明，功率谱分析是一个有用的工具，用于评估新的光学深度反演技术在消除三维辐射效应的技能。新技术使用逆非局部独立像素近似和归一化的最低点反射率差产生的光学深度场，更好地匹配真实光学深度场的尺度变化。
An analysis of nadir reflectivity spatial Fourier power spectra and autocorrelation functions for solar wavelengths and cloudy conditions is presented. The data come from Landsat thematic mapper (TM) observations, while Monte Carlo (MC) simulations are used to aid the interpretation of the observations and to examine sensitivity to various factors. We show that shortwave radiative processes produce consistent signatures in power spectra and autocorrelation functions. Power spectra take a variety of shapes not shown or explained in previous observational studies. We demonstrate that TM spectra can potentially be affected by radiative “roughening” at intermediate scales (∼1–5 km) and radiative “smoothing” at small scales (<1 km). These processes are wavelength-dependent, with systematic differences between conservative (for cloud droplets) TM band 4 (∼0.8 μm) and absorbing band 7 (∼2.2 μm). Band 7 exhibits more roughening and less smoothing than band 4 and faster decrease in autocorrelation. Roughening is more prevalent at large solar zenith angles due to optical and/or geometrical side illumination and shadowing. MC spectra illustrate that scale invariant optical depth fields can produce complex power spectra that take a variety of shapes under different conditions. Radiative roughening increases with decreasing single scattering albedo and increasing solar zenith angle (as in the observations). For low solar zenith angles, there is a clear shift in the radiative smoothing scale to smaller values as droplet absorption increases. Power spectra also show stronger decorrelations between optical depth and reflectivity when cloud top variations are more pronounced. Finally, it is shown that power spectral analysis is a useful tool for evaluating the skill of novel optical depth retrieval techniques in removing three-dimensional radiative effects. New techniques using inverse nonlocal independent pixel approximation and normalized difference of nadir reflectivity yield optical depth fields which better match the scale-by-scale variability of the true optical depth field.