Effect of inhomogeneous surface albedo on UV radiation in the Antarctic environment

Each year in Antarctica, the period of greatest biological production in the coastal waters coincides with the onset of the ozone hole [Davidson and van der Heijden, 2000]. UV irradiance, enhanced by stratospheric ozone depletion and by highly reflective snow, may impact on the biota blooming in sea ice and the marginal sea ice zone. This thesis examines how surface reflectivity influences the downwelling UV irradiance at the ground in the Antarctic environment. The work is based on numerical model simulations and on field observations. A 3-D Monte Carlo model of the transmission of UV radiation through the Antarctic atmosphere was developed for both clear and overcast sky conditions. It incorporated inhomogeneity of surface reflectance. Simulations were performed over a surface of 100 x 100 km\\(^2\\), one half of which was snow-covered ice and the other half was ocean. Multiple reflections of photons between the high albedo snow-covered surface and the atmosphere enhance both spectral (296 - 400 nm) and erythemal irradiance at the ground. In clear sky conditions, the enhancement is found to be a function of wavelength, distance from the ice edge, total ozone column (TOC) and surface reflectivity, but is independent of solar zenith angle. The enhancement increases with decreasing TOC and increasing surface albedo, and reaches limiting values within 20 - 25 km of the ice edge on both the water side and the snow side. It is strongest at wavelengths between 315 and 320 nm. For surface albedos of 0.90 (snow) and 0.05 (water), and with TOC = 350 DU, the total irradiance enhancement (ratio of limiting irradiance over snow to that over water) at 315 nm can reach 54%. The enhancement of erythemal irradiance behaves in a very similar fashion to the spectral irradiance at 306 nm. In overcast conditions the enhancement follows a similar pattern, except that it reaches its limiting values within 3 - 8 km either side of the ice edge. The enhancement increases with wavelength, surface reflectivity and cloud albedo, but decreases with increasing cloud base height. The enhancement of erythemal irradiance is found to be independent of TOC, in spite of a decrease in enhancement with increase in TOC at wavelengths up to 320 nm. For a snow albedo of 0.90, a water albedo of 0.05, TOC = 350 DU, cloud optical depth of 15, cloud albedo of 0.60 and a cloud base height of 1 km, the total enhancement can reach 156% at a wavelength of 400 nm. The enhancement of erythemal irradiance in overcast sky conditions behaves in an analogous fashion to the spectral irradiance at 315 nm. Direct experimental measurements of erythemal irradiance were obtained near an ice/water boundary at Davis Station, Antarctica. Clear sky observations showed a total enhancement of 10%, with limiting values reached at 2.5 km either side of the ice edge. In overcast conditions the enhancement was 30% at 2.5 km from the ice edge. When the surface albedo distribution from NOAA/AVHRR is incorporated into the Monte Carlo model, there is a good agreement with the observations. It is demonstrated that the UV irradiance at a given point is strongly controlled by the albedos of the far surroundings, with important contributions from regions tens of kilometres away. The implication is that, if the albedo of an area is not properly represented, radiative transfer models may greatly over- or under- estimate the UV irradiance, especially in areas with highly reflective snow cover. These results will need to be taken into account when using satellite imagery to map the UV irradiance in the Antarctic sea ice zone, as well as when studying the effect of the UV-B enhancement on the Antarctic biota living in sea ice and the marginal sea ice zone.