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

Abstract

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 Correspondence to: P. J. McBride (patrick.mcbride@colorado.edu) 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.

Characterizing a New Surface-Based Shortwave Cloud Retrieval Technique, Based on Transmitted Radiance for Soil and Vegetated Surface Types

“…With surface measurements of zenith cloud radiance, a one-to-one mapping of cloud optical thickness to transmittance does not exist [3]. Downward diffuse radiance increases with optical thickness up to an optical thickness of about 5, beyond which it decreases.…”

“…Cloud optical thickness and particle size are not entirely separable in the inverse solution because there is a dependency on cloud optical thickness even at wavelengths where water absorbs. However, as clouds become increasingly optically thick, reflectance at non-absorbing wavelengths depends almost entirely on optical thickness while that at absorbing wavelengths almost entirely on droplet radius (see McBride et al, Figure 3(a) of [3]). …”

“…In contrast, transmitted cloud radiation, which can only be measured from below the cloud, interacts with all layers of a cloud [2], providing a unique perspective not available from space. McBride et al [3] highlight advantages to ground-based retrievals of cloud properties from optical transmittance compared to satellite retrievals based on reflected cloud radiance: more direct retrievals of cloud properties for deriving the surface radiative forcing, and the improved capability for studying cloud and aerosol interactions that comes with the increased temporal resolution of ground-based transmittance measurements.…”

“…They showed that the spectral slope method had smaller uncertainties over a broader range in cloud optical thickness values. However, McBride et al [3] emphasized the need for a more formal error analysis treatment to investigate the impact of errors in the forward model inputs such as prescribed vertical cloud profile, thermodynamic phase, and underlying surface albedo on the retrieved cloud properties.…”

“…To improve the sensitivity of the retrieval to droplet effective radius, McBride et al [3] developed a new algorithm for the retrieval of cloud optical properties based on the magnitude and shape of transmitted spectral radiance. Using error propagation methods, they investigated the impacts of measurement uncertainty on the retrieved cloud properties from the new spectral slope algorithm compared to the standard technique applied to cloud reflectance.…”