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Published December 15, 2004 | Published
Journal Article Open

Sensitivity Analysis of Cirrus Cloud Properties from High-Resolution Infrared Spectra. Part I: Methodology and Synthetic Cirrus


A set of simulated high-resolution infrared (IR) emission spectra of synthetic cirrus clouds is used to perform a sensitivity analysis of top-of-atmosphere (TOA) radiance to cloud parameters. Principal component analysis (PCA) is applied to assess the variability of radiance across the spectrum with respect to microphysical and bulk cloud quantities. These quantities include particle shape, effective radius (reff), ice water path (IWP), cloud height Zcld and thickness ΔZcld, and vertical profiles of temperature T(z) and water vapor mixing ratio w(z). It is shown that IWP variations in simulated cloud cover dominate TOA radiance variability. Cloud height and thickness, as well as T(z) variations, also contribute to considerable TOA radiance variability. The empirical orthogonal functions (EOFs) of radiance variability show both similarities and differences in spectral shape and magnitude of variability when one physical quantity or another is being modified. In certain cases, it is possible to identify the EOF that represents variability with respect to one or more physical quantities. In other instances, similar EOFs result from different sets of physical quantities, emphasizing the need for multiple, independent data sources to retrieve cloud parameters. When analyzing a set of simulated spectra that include joint variations of IWP, reff, and w(z) across a realistic range of values, the first two EOFs capture approximately 92%–97% and 2%–6% of the total variance, respectively; they reflect the combined effect of IWP and reff. The third EOF accounts for only 1%–2% of the variance and resembles the EOF from analysis of spectra where only w(z) changes. Sensitivity with respect to particle size increases significantly for reff several tens of microns or less. For small-particle reff, the sensitivity with respect to the joint variation of IWP, reff, and w(z) is well approximated by the sum of the sensitivities with respect to variations in each of three quantities separately.

Additional Information

© 2004 American Meteorological Society. (Manuscript received 16 September 2003, in final form 8 June 2004) The authors thank Kevin Bowman, Amy Braverman, Eric Fetzer, Robert Fovell, Xianglei Huang, Helen Steele, and John Worden for fruitful conversations. The authors are grateful to Doug Kinnison for providing the atmospheric profiles, and three anonymous reviewers for improvements to the manuscript. This work was supported by NASA-ESS Fellowship NGT-5-30372 (BHK), the Caltech President's Fund (AE and BHK), NASA New Investigator Grant NAG5-8812 (AE and BHK), and NSF Grant ATM00-82131 (MG). Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, under contract with NASA.

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