Wen Guoyong, Marshak Alexander, Várnai Tamás, Levy Robert
NASA Goddard Space Flight Center, Greenbelt, Maryland.
GESTAR/Morgan State University, Maryland.
J Geophys Res Atmos. 2016 Aug 24;121(16):9661-9674. doi: 10.1002/2016JD025021. Epub 2016 Aug 9.
A transition zone exists between cloudy skies and clear sky, such that clouds scatter solar radiation into clear sky regions. From a satellite perspective, it appears that clouds enhance the radiation nearby. We seek a simple method to estimate this enhancement, since it is so computationally expensive to account for all 3-dimensional (3D) scattering processes. In previous studies, we developed a simple two-layer model (2LM) that estimated the radiation scattered via cloud-molecular interactions. Here we have developed a new model to accounts for cloud-surface interaction (CSI). We test the models by comparing to calculations provided by full 3D radiative transfer simulations of realistic cloud scenes. For these scenes, the MODIS-like radiance fields were computed from the Spherical Harmonic Discrete Ordinate Method (SHDOM), based on a large number of cumulus fields simulated by the UCLA Large Eddy Simulation (LES) model. We find that the original 2LM model that estimates cloud-air molecule interactions accounts for 64% of the total reflectance enhancement, and the new model (2LM+CSI) that also includes cloud-surface interactions accounts for nearly 80%. We discuss the possibility of accounting for cloud-aerosol radiative interactions in 3D cloud induced reflectance enhancement, which may explain the remaining 20% of enhancements. Because these are simple models, these corrections can be applied to global satellite observations (e.g. MODIS) and help to reduce biases in aerosol and other clear-sky retrievals.
多云天空和晴朗天空之间存在一个过渡区域，云层会将太阳辐射散射到晴朗天空区域。从卫星观测的角度来看，云层似乎增强了附近的辐射。由于考虑所有三维（3D）散射过程的计算成本过高，我们寻求一种简单的方法来估算这种增强效应。在之前的研究中，我们开发了一个简单的两层模型（2LM）来估算通过云 - 分子相互作用散射的辐射。在此，我们开发了一个新模型来考虑云 - 表面相互作用（CSI）。我们通过与逼真云场景的全3D辐射传输模拟所提供的计算结果进行比较来测试这些模型。对于这些场景，类似MODIS的辐射场是根据球谐离散坐标法（SHDOM）计算得出的，该方法基于加州大学洛杉矶分校大涡模拟（LES）模型模拟的大量积云场。我们发现，估算云 - 空气分子相互作用的原始2LM模型占总反射率增强的64%，而同时考虑云 - 表面相互作用的新模型（2LM + CSI）占近80%。我们讨论了在3D云诱导反射率增强中考虑云 - 气溶胶辐射相互作用的可能性，这可能解释了其余20%的增强效应。由于这些是简单模型，这些校正可应用于全球卫星观测（例如MODIS），并有助于减少气溶胶和其他晴空反演中的偏差。
