Characteristics of UV-B irradiance at Syowa Station, Antarctica: Analyses of the measurements and comparison with numerical simulations

The characteristics of UV-B (λ = 290-315 nm) irradiance at Syowa Station (69°00′S, 39°35′E) in Antarctica were investigated by the numerical simulations with the multiple scattering radiative transfer model for the atmosphere-snow system, and the analyses of measured UV-B irradiance in 1996. By the numerical simulations a study has been made on the sensitivity of UV-B irradiance to the snow grain size, surface condition, cloud cover, Antarctic aerosol, solar zenith angle and total ozone amount. On the snow surface it was found that the effect of multiple reflection between atmosphere (or cloud) and snow surface of high albedo is important for the characteristics of UV-B irradiance. By the analyses of measured UV-B irradiance it was found that the occurrence of the maximum UV-B irradiance was not in December at maximum solar elevation, but in November when the low total ozone amount was observed. This shows that the time in a year reaching the minimum total ozone amount is important for determining the annual trend of UV-B irradiance. It was also found that the values of UV-B irradiance would be enhanced by the occurrence of cloud for the cloud amount of 9/10-10^-/10 due to the effect of multiple reflection between cloud layers and snow surface. As a whole, theoretically calculated values of spectral and spectrally integrated UV-B irradiance agree well with the measured ones, however, at the low solar elevation or low energy level of UV-B irradiance, the measured values of UV-B irradiance were larger than the calculated ones due to the less sensitivity of the instrument to the weak irradiance.