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
Loading...
Links to Files
Author/Creator ORCID
Date
2017-02-22
Type of Work
Department
Program
Citation of Original Publication
Z. 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/10.1063/1.4975502
Rights
This 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.
Subjects
bi-spectral method
UMBC High Performance Computing Facility (HPCF)
cloud optical thickness
loud droplet effective radius
cloud reflectance observations in a visible or near infrared (VIS/NIR) band
cloud reflectance observations, in a shortwave-infrared (SWIR) band
sub-pixel variations of cloud reflectances
UMBC High Performance Computing Facility (HPCF)
cloud optical thickness
loud droplet effective radius
cloud reflectance observations in a visible or near infrared (VIS/NIR) band
cloud reflectance observations, in a shortwave-infrared (SWIR) band
sub-pixel variations of cloud reflectances
Abstract
The 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.