Abstract
Abstract. A new retrieval scheme for cloud optical thickness, effective radius, and thermodynamic phase was developed for ground-based measurements of cloud shortwave solar spectral transmittance. Fifteen parameters were derived to quantify spectral variations in shortwave transmittance due to absorption and scattering of liquid water and ice clouds, manifested by shifts in spectral slopes, curvatures, maxima, and minima. To retrieve cloud optical thickness and effective particle radius, a weighted least square fit that matched the modeled parameters was applied. The measurements for this analysis were made with the ground-based Solar Spectral Flux Radiometer in Boulder, Colorado, between May 2012 and January 2013. We compared the cloud optical thickness and effective radius from the new retrieval to two other retrieval methods. By using multiple spectral features, we find a closer fit (with a root mean square difference over the entire spectra of 3.1% for a liquid water cloud and 5.9% for an ice cloud) between measured and modeled spectra compared to two other retrieval methods which diverge by a root mean square of up to 6.4% for a liquid water cloud and 22.5% for an ice cloud. The new retrieval introduced here has an average uncertainty in effective radius (± 1.2 μm) smaller by factor of at least 2.5 than two other methods when applied to an ice cloud.
Highlights
Clouds strongly influence Earth’s radiative energy balance by modulating the transfer of solar radiation through the atmosphere
29 % of the time series when retrieved with the 15-parameter method. These large uncertainties obtained from the slope and two-wavelength method are likely due to a lower signalto-noise ratio of ice cloud transmitted radiance compared to a liquid water cloud transmitted radiance near 1600 nm
The 15 parameters generalize cloud retrieval techniques based on spectral radiance transmitted through clouds and were inspired by the spectral feature quantified by McBride et al (2011)
Summary
Clouds strongly influence Earth’s radiative energy balance by modulating the transfer of solar radiation through the atmosphere. Large uncertainties arise in transmittance retrievals because the information content from inverse methods optimized for reflectance is reduced when applied to cloud transmittance, for particle size This demands new methods of extracting information based on the unique physics of cloud transmittance, which is revealed in the observed spectral signatures. These problems have motivated the development of novel retrieval approaches specific to cloud transmittance Such approaches include the transmittance at two wavelengths for a cloud overlying a vegetated surface (Chiu et al, 2010; Marshak et al, 2004), the transmittance at two wavelengths where condensed water absorption varies (Kikuchi et al, 2006; Rawlins and Foot, 1990), differential optical absorption spectroscopy (DOAS) (Daniel, 2002; Daniel et al, 2003, 2006; Schofield et al, 2007), and the slope of transmittance in selected spectral bands (McBride et al, 2012, 2011). Extra details of the case studies are presented in Appendix A
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