Numerical simulation of the atmospheric effects on snow albedo with a multiple scattering radiative transfer model for the atmosphere-snow system

The atmospheric effects on the spectral and spectrally integrated snow albedos at the snow surface and top of the atmosphere (TOA) are investigated. A multiple scattering radiative transfer model based on the "doubling and adding" method, combined with the Mie theory is applied to estimate the effects of absorption and scattering by atmospheric molecules, absorptive gases, aerosols and clouds. It is shown that the spectral surface albedo is reduced by the atmospheric absorptive gases at large salar zenith angles. The solar zenith angle dependence is weakened in the wavelength region shorter than 0.5 mu m by the Rayleigh scattering, and at almost all wavelengths by the atmospheric aerosols and cloud cover. H2O rich atmosphere decreases the spectral surface albedo at large solar zenith angles in the H2O bands, while the additional reduction of downward solar flux in the near infrared region by H2O absorption causes the spectrally integrated surface albedo to increase by several percent. Aerosols increase the spectrally integrated surface albedo at small solar zenith angles and reduce it at large solar zenith angles, however they reduce the spectrally integrated planetary albedo, except at large solar zenith angles. Optically-thick cloud cover increases both the spectrally integrated surface and planetary albedos at any solar zenith angle. In the visible region at small solar zenith angles, the downward solar flux on the snow surface under cloudy sky can exceed that for the clear case, and both further exceed the extraterrestrial solar flux, resulting from the multiple reflection between snow surface and the atmosphere (cloud cover). The global solar radiation on the snow surface under cloudy sky, however, never exceeds that for the clear case and that at TOA.