a b s t r a c tIn this paper seven vector radiative transfer codes are inter-compared for the case of underlying black surface. They include three techniques based on the discrete ordinate method (DOM), two Monte-Carlo methods, the successive orders scattering method, and a modified doubling-adding technique. It was found that all codes give very similar results. Therefore, we were able to produce benchmark results for the Stokes parameters both for reflected and transmitted light in the cases of molecular, aerosol and cloudy multiply scattering media. It was assumed that the single scattering albedo is equal to one. Benchmark results have been provided by several studies before, including Coulson et al. [22], Garcia and Siewert [7,8], Wauben and Hovenier [10], and Natraj et al. [11] among others. However, the case of the elongated phase functions such as for a cloud and with a high angular resolution is presented here for the first time. Also in difference with other studies, we make inter-comparisons using several codes for the same input dataset, which enables us to quantify the corresponding errors more accurately.
[1] Cloud optical properties inferred from a multifilter rotating shadowband radiometer have been validated against in situ measurements during the second ARM Enhanced Shortwave Experiment (ARESE II) field campaign at the ARM South Great Plains (SGP) site. On the basis of eight aircraft in situ vertical profiles (constructed from measurements), Forward Spectra Scattering Probe (FSSP), we find that our retrieved cloud effective radii for single-layer warm water clouds agree well with in situ measurements, within 5.5%. A sensitivity study also illustrates that (for this case) a 13% uncertainty in observed liquid water path (LWP, 20 g/m 2 ) results in 1.5% difference in retrieved cloud optical depth and 12.7% difference in inferred cloud effective radius, on average. The uncertainty of the LWP measured by the microwave radiometer (MWR) is the major contributor to the uncertainty of retrieved cloud effective radius. Further, we conclude that the uncertainty of our inferred cloud optical properties is better than 5% for warm water clouds based on a surface closure study, in which cloud optical properties inferred from narrowband irradiances are applied to a shortwave model and the modeled broadband fluxes are compared to a surface pyranometer.
[1] A method for estimating fractional sky cover from spectral measurements has been developed. The spectral characteristics of clouds and clear-sky aerosols are utilized to partition sky fraction. As illustrated in our sensitivity study and demonstrated in real measurements, the transmittance ratio at selected wavelengths is insensitive to solar zenith angle and major atmospheric gaseous absorption. With a localized baseline procedure, retrievals of this ratio method are independent of absolute calibration and weakly sensitive to changes in cloud and aerosol optical properties. Therefore this method substantially reduces the retrieval uncertainty. The uncertainty of this method, estimated through the sensitivity study and intercomparison, is less than 10%. With globally deployed narrowband radiometers, this simple ratio method can substantially enhance the current capability for monitoring fractional sky cover.Citation: Min, Q., T. Wang, C. N. Long, and M. Duan (2008), Estimating fractional sky cover from spectral measurements,
[1] A new technique for simultaneously retrieving cloud optical depth and effective radius has been proposed. This approach is based on the angular distribution of scattered light in the forward scattering lobe of cloud drops. The angular distributions can be observed by multiple shadowband scans. Radiative transfer modeling simulations demonstrate that accuracies for cloud optical depth, effective radius, and liquid water path are 2%, 10%, and 2 gm À2 , respectively, for given possible instrument noise and uncertainties. Further, we have tested different measurement strategies and achieved consistent accuracies. This technique will provide an approach to deal with the issue of ''CLOWD (cloud with low optical (water) depth).'' Citation: Min, Q., and M. Duan (2005), Simultaneously retrieving cloud optical depth and effective radius for optically thin clouds,
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