Spectrally-invariant behavior of zenith radiance around cloud edges simulated by radiative transfer 1Department of Meteorology, University of Reading, Reading, UK
2Climate and Radiation Branch, NASA/Goddard Space Flight Center, Greenbelt, MD, USA
3Department of Geography, Boston University, Boston, MA, USA
4Atmospheric Sciences Division, Brookhaven National Laboratory, New York, NY, USA
Received: 28 Apr 2010 – Published in Atmos. Chem. Phys. Discuss.: 11 Jun 2010 Abstract. In a previous paper, we discovered a surprising spectrally-invariant
relationship in shortwave spectrometer observations taken by the Atmospheric
Radiation Measurement (ARM) program. The relationship suggests that the
shortwave spectrum near cloud edges can be determined by a linear
combination of zenith radiance spectra of the cloudy and clear regions.
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 other factors. At visible wavelengths, the
relationship primarily depends on cloud optical depth regardless of cloud
phase function, thermodynamic phase and drop size. At water-absorbing
wavelengths, the slope of the relationship depends primarily on cloud
optical depth; the intercept, by contrast, depends primarily on cloud
absorbing and scattering properties, suggesting a new retrieval method for
cloud drop effective radius. These results suggest that the
spectrally-invariant relationship can be used to infer cloud properties near
cloud edges even with insufficient or no knowledge about spectral surface
albedo and aerosol properties.
Revised: 03 Nov 2010 – Accepted: 19 Nov 2010 – Published: 30 Nov 2010
Citation: Chiu, J. C., Marshak, A., Knyazikhin, Y., and Wiscombe, W. J.: Spectrally-invariant behavior of zenith radiance around cloud edges simulated by radiative transfer, Atmos. Chem. Phys., 10, 11295-11303, doi:10.5194/acp-10-11295-2010, 2010.