Observations and modeling of ice cloud shortwave spectral albedo during the Tropical Composition, Cloud and Climate Coupling Experiment (TC4)

摘要

Ice cloud optical thickness and effective radius have been retrieved from hyperspectral irradiance and discrete spectral radiance measurements for four ice cloud cases during the Tropical Composition, Cloud and Climate Coupling Experiment (TC4) over a range of solar zenith angle (23°-53°) and high (46-90) and low (5-15) optical thicknesses. The retrieved optical thickness and effective radius using measurements at only two wavelengths from the Solar Spectral Flux Radiometer (SSFR) irradiance and the Moderate Resolution Imaging Spectroradiometer Airborne Simulator (MAS) were input to a radiative transfer model using two libraries of ice crystal single-scattering optical properties to reproduce spectral albedo over the spectral range from 400 to 2130 nm. The two commonly used ice single-scattering models were evaluated by examining the residuals between observed spectral and predicted spectral albedo. The SSFR and MAS retrieved optical thickness and effective radius were found to be in close agreement for the low to moderately optically thick clouds with a mean difference of 3.42 in optical thickness (SSFR lower relative to MAS) and 3.79 μmin effective radius (MAS smaller relative to S SFR). The higher optical thicknesscase exhibited a larger difference in optical thickness (40.5) but nearly identical results for effective radius. The single-scattering libraries were capable of reproducing the spectral albedo in most cases examined to better than 0.05 for all wavelengths. Systematic differences between the model and measurements increased with increasing optical thickness and approached 0.10 between 400 and 600 nm and selected wavelengths between 1200 and 1300 nm. Differences between radiance and irradiance based retrievals of optical thickness and effective radius error sources in the modeling of ice single-scattering properties are examined.