A spectral method for retrieving cloud optical thickness and effective radius from surface-based transmittance measurements

摘要

We introduce a new spectral method for the retrieval of optical thicknessand effective radius from cloud transmittance that relies on the spectralslope of the normalized transmittance between 1565 nm and 1634 nm, and oncloud transmittance at a visible wavelength. The standard dual-wavelengthtechnique, which is traditionally used in reflectance-based retrievals, isill-suited for transmittance because it lacks sensitivity to effectiveradius, especially for optically thin clouds. Using the spectral sloperather than the transmittance itself enhances the sensitivity oftransmittance observations with respect to the effective radius. This isdemonstrated by applying it to the moderate spectral resolution observationsfrom the Solar Spectral Flux Radiometer (SSFR) and ShortwaveSpectroradiometer (SWS), and by examining the retrieval uncertainties of thestandard and the spectral method for data from the DOE ARM Southern GreatPlains (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, andcompared 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 thestandard retrieval than for the spectral retrieval, particularly for thinclouds. When defining 2 μm as upper limit for the tolerable uncertaintyof the effective radius, the standard method returned only very few validretrievals for clouds with an optical thickness below 25. For the analyzedICEALOT data (mean optical thickness 23), the spectral method provided validretrievals for 84 % of the data (24 % for the standard method). For theSGP data (mean optical thickness 44), both methods provided a high return of90 % for the spectral method and 78 % for the standard method.