Zhang, Z.Werner, F.Wind, G.Platnick, S.Ackerman, A. S.Girolamo, L. DiMarshak, A.Meyer, KerryCho, H. M.2018-09-192018-09-192017-02-22Z. Zhang, F. Werner, H. M. Cho, G. Wind, S. Platnick, A. S. Ackerman, L. Di Girolamo, A. Marshak, and Kerry Meyer, A framework for quantifying the impacts of sub-pixel reflectance variance and covariance on cloud optical thickness and effective radius retrievals based on the bispectral method, 2017, https://doi.org/10.1063/1.4975502https://doi.org/10.1063/1.4975502http://hdl.handle.net/11603/11324The so-called bi-spectral method retrieves cloud optical thickness (τ) and cloud droplet effective radius (rₑ) simultaneously from a pair of cloud reflectance observations, one in a visible or near infrared (VIS/NIR) band and the other in a shortwave-infrared (SWIR) band. A cloudy pixel is usually assumed to be horizontally homogeneous in the retrieval. Ignoring sub-pixel variations of cloud reflectances can lead to a significant bias in the retrieved τ and rₑ. In this study, we use the Taylor expansion of a two-variable function to understand and quantify the impacts of sub-pixel variances of VIS/NIR and SWIR cloud reflectances and their covariance on the τ and rₑ retrievals. This framework takes into account the fact that the retrievals are determined by both VIS/NIR and SWIR band observations in a mutually dependent way. In comparison with previous studies, it provides a more comprehensive understanding of how sub-pixel cloud reflectance variations impact the τ and rₑ retrievals based on the bi-spectral method. In particular, our framework provides a mathematical explanation of how the sub-pixel variation in VIS/NIR band influences the rₑ retrieval and why it can sometimes outweigh the influence of variations in the SWIR band and dominate the error in rₑ retrievals, leading to a potential contribution of positive bias to the rₑ retrieval.7 pagesen-USThis item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please contact the author.bi-spectral methodUMBC High Performance Computing Facility (HPCF)cloud optical thicknessloud droplet effective radiuscloud reflectance observations in a visible or near infrared (VIS/NIR) bandcloud reflectance observations, in a shortwave-infrared (SWIR) bandsub-pixel variations of cloud reflectancesText