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

doi:10.1063/1.4975502

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 proceeding › Conference contribution

Abstract

The so-called bi-spectral method retrieves cloud optical thickness (τ) and cloud droplet effective radius (r_e) 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 r_e. 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 r_e 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 r_e 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 r_e retrieval and why it can sometimes outweigh the influence of variations in the SWIR band and dominate the error in r_e retrievals, leading to a potential contribution of positive bias to the r_e retrieval.

Original language	English (US)
Title of host publication	Radiation Processes in the Atmosphere and Ocean, IRS 2016
Subtitle of host publication	Proceedings of the International Radiation Symposium (IRC/IAMAS)
Editors	Werner Schmutz, Roger Davies, Luca Egli
Publisher	American Institute of Physics Inc.
ISBN (Electronic)	9780735414785
DOIs	https://doi.org/10.1063/1.4975502
State	Published - Feb 22 2017
Event	International Radiation Symposium 2016: Radiation Processes in the Atmosphere and Ocean, IRS 2016 - Auckland, New Zealand Duration: Apr 16 2016 → Apr 22 2016

Publication series

Name	AIP Conference Proceedings
Volume	1810
ISSN (Print)	0094-243X
ISSN (Electronic)	1551-7616

Other

Other	International Radiation Symposium 2016: Radiation Processes in the Atmosphere and Ocean, IRS 2016
Country/Territory	New Zealand
City	Auckland
Period	4/16/16 → 4/22/16

ASJC Scopus subject areas

General Physics and Astronomy

Online availability

10.1063/1.4975502

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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) Article 030002 (AIP Conference Proceedings; Vol. 1810). American Institute of Physics Inc.. https://doi.org/10.1063/1.4975502

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. / Zhang, Z.; Werner, F.; Cho, H. M. et al.
Radiation Processes in the Atmosphere and Ocean, IRS 2016: Proceedings of the International Radiation Symposium (IRC/IAMAS). ed. / Werner Schmutz; Roger Davies; Luca Egli. American Institute of Physics Inc., 2017. 030002 (AIP Conference Proceedings; Vol. 1810).

Research output: Chapter in Book/Report/Conference proceeding › Conference contribution

Zhang, Z, Werner, F, Cho, HM, Wind, G, Platnick, S, Ackerman, AS, 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., International Radiation Symposium 2016: Radiation Processes in the Atmosphere and Ocean, IRS 2016, Auckland, New Zealand, 4/16/16. https://doi.org/10.1063/1.4975502

Zhang Z, Werner F, Cho HM, Wind G, Platnick S, Ackerman AS et al. 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 Schmutz W, Davies R, Egli L, editors, Radiation Processes in the Atmosphere and Ocean, IRS 2016: Proceedings of the International Radiation Symposium (IRC/IAMAS). American Institute of Physics Inc. 2017. 030002. (AIP Conference Proceedings). doi: 10.1063/1.4975502

Zhang, Z. ; Werner, F. ; Cho, H. M. et al. / 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. Radiation Processes in the Atmosphere and Ocean, IRS 2016: Proceedings of the International Radiation Symposium (IRC/IAMAS). editor / Werner Schmutz ; Roger Davies ; Luca Egli. American Institute of Physics Inc., 2017. (AIP Conference Proceedings).

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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.",

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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

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