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

Z. Zhang, F. Werner, H. M. Cho, G. Wind, S. Platnick, A. S. Ackerman, L. Di Girolamo, A. Marshak, Kerry Meyer

Research output: Chapter in Book/Report/Conference proceedingConference contribution

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.

Original languageEnglish (US)
Title of host publicationRadiation Processes in the Atmosphere and Ocean, IRS 2016
Subtitle of host publicationProceedings of the International Radiation Symposium (IRC/IAMAS)
EditorsWerner Schmutz, Roger Davies, Luca Egli
PublisherAmerican Institute of Physics Inc.
ISBN (Electronic)9780735414785
DOIs
StatePublished - Feb 22 2017
EventInternational Radiation Symposium 2016: Radiation Processes in the Atmosphere and Ocean, IRS 2016 - Auckland, New Zealand
Duration: Apr 16 2016Apr 22 2016

Publication series

NameAIP Conference Proceedings
Volume1810
ISSN (Print)0094-243X
ISSN (Electronic)1551-7616

Other

OtherInternational Radiation Symposium 2016: Radiation Processes in the Atmosphere and Ocean, IRS 2016
CountryNew Zealand
CityAuckland
Period4/16/164/22/16

ASJC Scopus subject areas

  • Physics and Astronomy(all)

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    Zhang, Z., Werner, F., Cho, H. M., Wind, G., Platnick, S., Ackerman, A. S., Di Girolamo, L., Marshak, A., & Meyer, K. (2017). 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. In W. Schmutz, R. Davies, & L. Egli (Eds.), Radiation Processes in the Atmosphere and Ocean, IRS 2016: Proceedings of the International Radiation Symposium (IRC/IAMAS) [030002] (AIP Conference Proceedings; Vol. 1810). American Institute of Physics Inc.. https://doi.org/10.1063/1.4975502