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Assessment of the Accuracy of the Conventional Ray-Tracing Technique: Implications in Remote Sensing and Radiative Transfer Involving Ice Clouds.A fundamental problem in remote sensing and radiative transfer simulations involving ice clouds is the ability to compute accurate optical properties for individual ice particles. While relatively simple and intuitively appealing, the conventional geometric-optics method (CGOM) is used frequently for the solution of light scattering by ice crystals. Due to the approximations in the ray-tracing technique, the CGOM accuracy is not well quantified. The result is that the uncertainties are introduced that can impact many applications. Improvements in the Invariant Imbedding T-matrix method (II-TM) and the Improved Geometric-Optics Method (IGOM) provide a mechanism to assess the aforementioned uncertainties. The results computed by the II-TMþIGOM are considered as a benchmark because the IITM solves Maxwell's equations from first principles and is applicable to particle size parameters ranging into the domain at which the IGOM has reasonable accuracy. To assess the uncertainties with the CGOM in remote sensing and radiative transfer simulations, two independent optical property datasets of hexagonal columns are developed for sensitivity studies by using the CGOM and the II-TMþIGOM, respectively. Ice cloud bulk optical properties obtained from the two datasets are compared and subsequently applied to retrieve the optical thickness and effective diameter from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. Additionally, the bulk optical properties are tested in broadband radiative transfer (RT) simulations using the general circulation model (GCM) version of the Rapid Radiative Transfer Model (RRTMG) that is adopted in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM, version 5.1). For MODIS retrievals, the mean bias of uncertainties of applying the CGOM in shortwave bands (0.86 and 2.13 micrometers) can be up to 5% in the optical thickness and as high as 20% in the effective diameter, depending on cloud optical thickness and effective diameter. In the MODIS infrared window bands centered at 8.5, 11, and 12 micrometers biases in the optical thickness and effective diameter are up to 12% and 10%, respectively. The CGOM-based simulation errors in ice cloud radiative forcing calculations are on the order of 10Wm(exp 2).
Document ID
20140017197
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
doi:10.1016/j.jqsrt.2014.03.017
Authors
Bi, Lei (Texas A&M Univ. College Station, TX, United States)
Yang, Ping (Texas A&M Univ. College Station, TX, United States)
Liu, Chao (Texas A&M Univ. College Station, TX, United States)
Yi, Bingqi (Texas A&M Univ. College Station, TX, United States)
Baum, Bryan A. (Wisconsin Univ. Madison, WI, United States)
Van Diedenhoven, Bastiaan (Columbia Univ. New York, NY, United States)
Iwabuchi, Hironobu (Tohoku Gakuin Univ. Sendai, Japan)
Date Acquired
December 9, 2014
Publication Date
March 26, 2014
Publication Information
Publication: Journal of Quantitative Spectroscopy and Radiative Transfer
Publisher: Elsevier
Volume: 146
Subject Category
Earth Resources And Remote Sensing
Optics
Report/Patent Number
GSFC-E-DAA-TN16358
Funding Number(s)
CONTRACT_GRANT: NSF AGS-1338440
CONTRACT_GRANT: NNX10AU63A
CONTRACT_GRANT: NNX11AK37G
CONTRACT_GRANT: NNX11AF40G
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
remote sensing
light scattering
Ice clouds

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