NTRS - NASA Technical Reports Server

Back to Results
A Framework for Quantifying the Impacts of Sub-Pixel Reflectance Variance and Covariance on Cloud Optical Thickness and Effective Radius Retrievals Based on the Bi-Spectral Method.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.
Document ID
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
Zhang, Z
(Maryland Univ. Baltimore, MD, United States)
Werner, F.
(Maryland Univ. Baltimore, MD, United States)
Cho, H. -M.
(Maryland Univ. Baltimore, MD, United States)
Wind, Galina
(Science Systems and Applications, Inc. Greenbelt, MD, United States)
Platnick, S.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Ackerman, A. S.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Di Girolamo, L.
(Illinois Univ. Urbana-Champaign, IL, United States)
Marshak, A.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Meyer, Kerry
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
March 14, 2017
Publication Date
February 1, 2017
Publication Information
Publication: AIP Conference Proceedings
Publisher: AIP Publishing LLC
Volume: 1810
Issue: 1
Subject Category
Earth Resources And Remote Sensing
Mathematical And Computer Sciences (General)
Report/Patent Number
Funding Number(s)
Distribution Limits
optical thickness
clouds (meteorology)

Available Downloads

There are no available downloads for this record.
No Preview Available