Simultaneously retrieving cloud optical depth and effective radius for optically thin clouds

Abstract

[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,

Algorithms and uncertainties for the determination of multispectral irradiance components and aerosol optical depth from a shipborne rotating shadowband radiometer

“…Due to its geometry, the shadowband occults a solid angle of 15 • of the sky from the sensor in zenith position. The width of the BioSHADE shadowband is broader compared to the MFRSR (3.3 • ; Harrison et al, 1994) and the thin-cloud rotating shadowband radiometer (TCRSR; 2 and 5 • ; Bartholomew et al, 2011) and it is not feasible to measure the shape of the solar aureole for thin-cloud retrievals (Min and Duan, 2005). The uncertainty arising from the shadowband width on the calculation of the direct horizontal irradiance is discussed in Sects.…”

“…Aerosol size distributions can be obtained from the spectral dependence of the AOD (King et al, 1978). High-frequency sampling combined with a narrow shadowband can offer additional information about the distribution of circum solar radiation and can potentially be exploited to retrieve cloud optical depth and effective radius (Min and Duan, 2005;Bartholomew et al, 2011).…”

Retrievals and uncertainty analysis of aerosol single scattering albedo from MFRSR measurements

“…Under these constraints and assumptions, we solve the radiative transfer equation iteratively to derive the SSA by matching the measured DDR. To simulate both direct beam and diffuse irradiance measured by MFRSR, we first compute the radiative intensity field using our fast and accurate radiative transfer model [15], which combines the exact radiance of low orders of scattering with multiple scattering radiance. …”

Design of a Shadowband Spectral Radiometer for the Retrieval of Thin Cloud Optical Depth, Liquid Water Path, and the Effective Radius

“…The shortwave radiative fluxes of clouds are very sensitive to small changes in the LWP when the LWP is small (Sengupta et al 2003), which requires a particularly high degree of accuracy from observations and modeling. For example, an uncertainty of 620 g m 22 in LWP can translate to 6150 W m 22 in downwelling surface flux (Sengupta et al 2003;Min and Duan 2005;Turner et al 2007b). The common occurrence of thin clouds necessitates that we develop new methods, such as the one discussed here, that can observe their properties to within the accuracies needed for climate and numerical weather prediction applications.…”

“…Hence, any method that adequately retrieves two of the three factors provides the information needed to determine the third. For example, an uncertainty of 20 g m 22 in the LWP can lead to an uncertainty of 150 W m 22 in downwelling surface shortwave radiation when LWP is small (Min and Duan 2005). A number of instruments and techniques have been previously used to observe cloud properties and have also been applied to thin clouds; however, thin clouds require accuracies that strain the capabilities of traditional detectors (e.g., microwave receivers) and theoretical approaches such that no single device has proven to be capable to determine all three critical properties for LWP , 100 g m 22 (Turner et al 2007b).…”