Meyer K, Platnick S, Arnold G T, Holz R E, Veglio P, Yorks J, Wang C
Goddard Earth Sciences Technology and Research (GESTAR) Universities Space Research Association, Columbia, Maryland, USA.
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
Atmos Meas Tech. 2016;9(4):1743-1753. doi: 10.5194/amt-9-1743-2016. Epub 2016 Apr 20.
Previous bi-spectral imager retrievals of cloud optical thickness (COT) and effective particle radius (CER) based on the Nakajima and King (1990) approach, such as those of the operational MODIS cloud optical property retrieval product (MOD06), have typically paired a non-absorbing visible or near-infrared wavelength, sensitive to COT, with an absorbing shortwave or midwave infrared wavelength sensitive to CER. However, in practice it is only necessary to select two spectral channels that exhibit a strong contrast in cloud particle absorption. Here it is shown, using eMAS observations obtained during NASA's SEACRS field campaign, that selecting two absorbing wavelength channels within the broader 1.88 μm water vapor absorption band, namely the 1.83 and 1.93 μm channels that have sufficient differences in ice crystal single scattering albedo, can yield COT and CER retrievals for thin to moderately thick single-layer cirrus that are reasonably consistent with other solar and IR imager-based and lidar-based retrievals. A distinct advantage of this channel selection for cirrus cloud retrievals is that the below-cloud water vapor absorption minimizes the surface contribution to measured cloudy TOA reflectance, in particular compared to the solar window channels used in heritage retrievals such as MOD06. This reduces retrieval uncertainty resulting from errors in the surface reflectance assumption, as well as reduces the frequency of retrieval failures for thin cirrus clouds.
以往基于中岛和金（1990年）方法的双光谱成像仪对云光学厚度（COT）和有效粒子半径（CER）的反演，如业务中分辨率成像光谱仪（MODIS）云光学特性反演产品（MOD06）的反演，通常会将一个对COT敏感的非吸收性可见光或近红外波长，与一个对CER敏感的吸收性短波或中波红外波长配对。然而，在实际操作中，只需要选择两个在云粒子吸收方面呈现强烈对比的光谱通道即可。在此展示，利用美国国家航空航天局（NASA）平流层对流层环境化学研究（SEACRS）野外考察期间获得的地球静止业务环境卫星（eMAS）观测数据，在更宽的1.88μm水汽吸收带内选择两个吸收波长通道，即冰晶单次散射反照率有足够差异的1.83μm和1.93μm通道，能够对薄至中度厚度的单层卷云进行COT和CER反演，且与其他基于太阳和红外成像仪以及基于激光雷达的反演结果相当一致。这种用于卷云反演的通道选择的一个显著优势在于，云底以下的水汽吸收使地表对测量的多云顶反照率的贡献最小化，特别是与传统反演（如MOD06）中使用的太阳窗通道相比。这降低了因地表反射率假设误差导致的反演不确定性，同时也减少了薄卷云反演失败的频率。
