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首页> 外文期刊>Atmospheric chemistry and physics >Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals
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Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals

机译：在人工观测上测试遥感：毛毛雨和3-D云结构对有效半径检索的影响

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Remote sensing of cloud effective particle size with passive sensors likethe Moderate Resolution Imaging Spectroradiometer (MODIS) is an importanttool for cloud microphysical studies. As a measure of the radiativelyrelevant droplet size, effective radius can be retrieved with differentcombinations of visible through shortwave and midwave infrared channels. Inpractice, retrieved effective radii from these combinations can be quitedifferent. This difference is perhaps indicative of different penetrationdepths and path lengths for the spectral reflectances used. In addition,operational liquid water cloud retrievals are based on the assumption of arelatively narrow distribution of droplet sizes; the role of largerprecipitation particles in these distributions is neglected. Therefore,possible explanations for the discrepancy in some MODIS spectral sizeretrievals could include 3-D radiative transport effects, including sub-pixelcloud inhomogeneity, and/or the impact of drizzle formation.For three cloud cases the possible factors of influence are isolated andinvestigated in detail by the use of simulated cloud scenes and syntheticsatellite data: marine boundary layer cloud scenes from large eddysimulations (LES) with detailed microphysics are combined with Monte Carloradiative transfer calculations that explicitly account for the detaileddroplet size distributions as well as 3-D radiative transfer to simulateMODIS observations. The operational MODIS optical thickness and effectiveradius retrieval algorithm is applied to these and the results are comparedto the given LES microphysics.We investigate two types of marine cloud situations each with and withoutdrizzle from LES simulations: (1) a typical daytime stratocumulus deck attwo times in the diurnal cycle and (2) one scene with scattered cumulus.Only small impact of drizzle formation on the retrieved domain average andon the differences between the three effective radius retrievals is noticedfor both cloud scene types for different reasons. For our, presumablytypical, overcast stratocumulus scenes with an optical thickness of 8 to 9and rain rates at cloud bottom up to 0.05 mm/h clear drizzle impact on theretrievals can be excluded. The cumulus scene does not show much drizzlesensitivity either despite extended drizzle areas being directly visiblefrom above (locally >1 mm/h), which is mainly due to technicalcharacteristics of the standard retrieval approach. 3-D effects, on the otherhand, produce large discrepancies between the 1.6 and 2.1 μm channelobservations compared to 3.7 μm retrievals in the latter case. A generalsensitivity of MODIS particle size data to drizzle formation is notcorroborated by our case studies.
机译：使用无源传感器（如中分辨率成像分光光度计（MODIS））对云的有效粒径进行遥感是云微物理研究的重要工具。作为辐射相关液滴尺寸的度量，可以通过短波和中波红外通道的可见光的不同组合来获取有效半径。实际上，从这些组合中检索到的有效半径可能是完全不同的。这种差异可能表明所用光谱反射率的不同穿透深度和光程长度。另外，可操作的液态水云反演是基于相对较小的液滴尺寸分布的假设。忽略了较大的降水颗粒在这些分布中的作用。因此，对于某些MODIS光谱大小检索中的差异的可能解释可能包括3-D辐射传输效应，包括亚像素云不均匀性和/或毛毛雨形成的影响。 对于三种云情况，可能的影响因素是通过使用模拟云场景和合成卫星数据对影响进行隔离和详细研究：将具有详细微观物理的大型涡模拟（LES）产生的海洋边界层云场景与蒙特卡洛辐射传递计算相结合，从而明确考虑了详细的液滴尺寸分布以及3 -D辐射传递模拟MODIS观测。应用可操作的MODIS光学厚度和有效半径检索算法，并将结果与​​给定的LES微观物理学进行比较。 我们从LES模拟中研究了两种有无滴水的海洋云情况：（1）典型昼间平流层积层在昼夜周期中两次出现（2）一个散布积云的场景。由于两种原因，两种云场景类型都只注意到细雨形成对取回的区域平均值以及对三个有效半径取值之间差异的微小影响。对于我们可能是典型的，阴云密布的层状积云场景，光学厚度为8到9，云层底部的降雨率高达0.05 mm / h，可以排除细雨对植被的影响。尽管从上方可以直接看到扩大的毛毛雨区域（局部> 1 mm / h），但积云场景也没有表现出任何毛毛雨敏感性，这主要是由于标准检索方法的技术特性。另一方面，3-D效应在1.6和2.1μm的通道观测值之间产生较大的差异，而在后一种情况下，则为3.7μm。我们的案例研究无法确定MODIS粒度数据对毛毛雨形成的一般敏感性。

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