Atmos. Chem. Phys. Discuss., 10, 14557-14581, 2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Spectral invariant behavior of zenith radiance around cloud edges simulated by radiative transfer
J. C. Chiu1, A. Marshak2, Y. Knyazikhin3, and W. J. Wiscombe2,4
1University of Maryland Baltimore County, Baltimore, MD, USA
2NASA/Goddard Space Flight Center, Greenbelt, MD, USA
3Boston University, Boston, MA, USA
4Brookhaven National Laboratory, New York, NY, USA

Abstract. A previous paper discovered a surprising spectral-invariant relationship in shortwave spectrometer observations taken by the Atmospheric Radiation Measurement (ARM) program. Here, using radiative transfer simulations, we study the sensitivity of this relationship to the properties of aerosols and clouds, to the underlying surface type, and to the finite field-of-view (FOV) of the spectrometer. Overall, the relationship is mostly sensitive to cloud properties and has little sensitivity to the other factors. At visible wavelengths, the relationship primarily depends on cloud optical depth regardless of cloud thermodynamic phase and drop size. At water-absorbing wavelengths, the slope of the spectral-invariant relationship depends primarily on cloud optical depth; the intercept, by contrast, depends primarily on cloud absorption properties, suggesting a new retrieval method for cloud drop effective radius. These results suggest that the spectral-invariant relationship can be used to infer cloud properties even with insufficient or no knowledge about spectral surface albedo and aerosol properties.

Citation: Chiu, J. C., Marshak, A., Knyazikhin, Y., and Wiscombe, W. J.: Spectral invariant behavior of zenith radiance around cloud edges simulated by radiative transfer, Atmos. Chem. Phys. Discuss., 10, 14557-14581, doi:10.5194/acpd-10-14557-2010, 2010.
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