Quantifying the Impacts of Subpixel Reﬂectance Variability on Cloud Optical Thickness and Eﬀective Radius Retrievals Based On High-Resolution ASTER Observations
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Type of Work20 pages
Citation of Original PublicationWerner, F., Zhang, Z., Wind, G., Miller, D. J., & Platnick, S. (2018). Quantifying the impacts of subpixel reﬂectance variability on cloud optical thickness and eﬀective radius retrievals based on high-resolution ASTER observations. Journal of Geophysical Research: Atmospheres, 123, 4239–4258. https://doi.org/10.1002/2017JD027916
RightsThis 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.
Subjectsplane-parallel homogeneous bias (PPHB)
Advanced Spaceborne Thermal Emission and Reﬂection Radiometer (ASTER)
subpixel reflectance variability
UMBC High Performance Computing Facility (HPCF)
Bias in cloud optical thickness and effective droplet radius
second-order Taylor series expansion
bispectral solar reﬂective method
high-resolution reﬂectance observations
marine boundary layer cloud scenes
Recently, Zhang et al. (2016, https://doi.org/10.1002/2016JD024837) presented a mathematical framework based on a second-order Taylor series expansion in order to quantify the plane-parallel homogeneous bias (PPHB) in cloud optical thickness (τ) and eﬀective droplet radius (r ₑ𝒻𝒻) retrieved from the bispectral solar reﬂective method. This study provides observational validation of the aforementioned framework, using high-resolution reﬂectance observations from the Advanced Spaceborne Thermal Emission and Reﬂection Radiometer (ASTER) over 48 marine boundary layer cloud scenes. ASTER reﬂectances at a horizontal resolution of 30 m are aggregated up to a scale of 1,920 m, providing retrievals of τ and r ₑ𝒻𝒻 at diﬀerent spatial resolutions. A comparison between the PPHB derived from these retrievals and the predicted PPHB from the mathematical framework reveals a good agreement with correlation coeﬃcients of r > 0.97 (for Δτ) and r > 0.79 (for Δr ₑ𝒻𝒻 ). To test the feasibility of PPHB predictions for present and future satellite missions, a scale analysis with varying horizontal resolutions of the subpixel and pixel-level observations is performed, followed by tests of corrections with only limited observational high-resolution data. It is shown that for reasonably thick clouds with a mean subpixel τ larger than 5, correlations between observed and predicted PPHB remain high, even if the number of available subpixels decreases or just a single band provides the information about subpixel reﬂectance variability. Only for thin clouds the predicted Δr ₑ𝒻𝒻 become less reliable, which can be attributed primarily to an increased retrieval uncertainty for r ₑ𝒻𝒻 .
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