Diurnal albedo of seasonally melting Arctic snow at wavelenghts of UV and visible

Seasonally melting Arctic snow at Sodankylä (67°22‘ N, 26°39’ E, 179 m a.s.l.), Finland, allows us to study the diurnal and long-term changes in snow albedo and other snow properties, combined with AWS measured meteorological parameters. Albedo of snow is wavelength dependent, and here we concentrate on wavelenghts of UV and visible. As the cloud effect for UV wavelengths is smaller than for the whole solar spectrum, UV albedo remains less affected, too. We have found the midday erythemally weighted UV albedo to range from 0.6 to 0.8 in the accumulation period, and from 0.5 to 0.7 during melting. Moreover, we have detected (i) albedo cases with an unexpected diurnal decrease of 0.05 in albedo soon after midday, and recovery thereafter, as well as (ii) albedo cases with SZA asymmetry up to 10 %. The diurnal decrease with the recovery (i) was obvious during the snow melt period, under cases of an almost clear sky and variable cloudiness. Using ancillary data, we suspect this diurnal albedo change to be due to the daily metamorphosis of the surface of the snowpack, in which the temperature of the surface increases, melting some of the snow to liquid water, after which the surface freezes again. In the SZA asymmetry (ii), a diurnal decrease in UV snow albedo as a function of time was found (higher albedo in the morning and lower in the evening). These included cases with either new snow on the previous night, or daytime snow melt combined with refreezing during night, or high relative humidity and low surface temperature during the previous night favorable to frost and higher albedo on the next morning. Further studies on the importance of these diurnal and spectral dependencies to climate-albedo feedback, and to parametrizations of snow albedo in various models, are needed.