A spectral method for retrieving cloud optical thickness and effective radius from surface-based transmittance measurements 1Laboratory for Atmospheric and Space Physics, University of Colorado, Campus Box 392, Boulder, Colorado 80309-0392, USA
25 Jul 2011
2Department of Atmospheric and Oceanic Sciences, University of Colorado, Campus Box 311, Boulder, CO 80309-0311, USA
3NOAA Earth Systems Research Laboratory, Physical Science Division, Weather and Climate Physics Branch, 325 Broadway, Boulder, CO 80305, USA
Received: 06 October 2010 – Published in Atmos. Chem. Phys. Discuss.: 17 January 2011 Abstract. We introduce a new spectral method for the retrieval of optical thickness
and effective radius from cloud transmittance that relies on the spectral
slope of the normalized transmittance between 1565 nm and 1634 nm, and on
cloud transmittance at a visible wavelength. The standard dual-wavelength
technique, which is traditionally used in reflectance-based retrievals, is
ill-suited for transmittance because it lacks sensitivity to effective
radius, especially for optically thin clouds. Using the spectral slope
rather than the transmittance itself enhances the sensitivity of
transmittance observations with respect to the effective radius. This is
demonstrated by applying it to the moderate spectral resolution observations
from the Solar Spectral Flux Radiometer (SSFR) and Shortwave
Spectroradiometer (SWS), and by examining the retrieval uncertainties of the
standard and the spectral method for data from the DOE ARM Southern Great
Plains (SGP) site and a NOAA ship cruise (ICEALOT). The liquid water path
(LWP) is derived from the retrieved optical thickness and effective radius,
based on two different assumptions about the cloud vertical profile, and
compared to the simultaneous observations from a microwave radiometer.
Optical thickness and effective radius is also compared to MODIS retrievals.
In general, the effective radius uncertainties were much larger for the
standard retrieval than for the spectral retrieval, particularly for thin
clouds. When defining 2 μm as upper limit for the tolerable uncertainty
of the effective radius, the standard method returned only very few valid
retrievals for clouds with an optical thickness below 25. For the analyzed
ICEALOT data (mean optical thickness 23), the spectral method provided valid
retrievals for 84 % of the data (24 % for the standard method). For the
SGP data (mean optical thickness 44), both methods provided a high return of
90 % for the spectral method and 78 % for the standard method.
Revised: 07 June 2011 – Accepted: 07 July 2011 – Published: 25 July 2011
Citation: McBride, P. J., Schmidt, K. S., Pilewskie, P., Kittelman, A. S., and Wolfe, D. E.: A spectral method for retrieving cloud optical thickness and effective radius from surface-based transmittance measurements, Atmos. Chem. Phys., 11, 7235-7252, doi:10.5194/acp-11-7235-2011, 2011.