Surface albedo measurements over sea ice in the Baltic Sea during the spring snowmelt period Roberta PIRAZZINI, 1,2 Timo VIHMA, 3 Mats A. GRANSKOG, 4* Bin CHENG 2 1 Department of Physical Sciences, PO Box 64, University of Helsinki, FIN-00014 Helsinki, Finland 2 Finnish Institute of Marine Research, PO Box 2, FIN-00561 Helsinki, Finland E-mail: pirazzini@fimr.fi 3 Finnish Meteorological Institute, PO Box 503, FIN-00101 Helsinki, Finland 4 Arctic Centre, University of Lapland, PO Box 122, FIN-96101 Rovaniemi, Finland ABSTRACT. The snow/ice albedo was studied during a 4week field experiment over first-year sea ice in the Gulf of Bothnia, Baltic Sea, in spring 2004. Observations were made on radiative fluxes, cloud cover, wind, air temperature and humidity, as well as snow/ice temperature, thickness, density and grain size. The albedo variation during the observation period was large: the daily mean albedo ranged from 0.79 over a new snow cover to 0.30 over bare, melting ice. The evolution of the albedo was related to the surface properties, but existing parameterizations based on Arctic data did not explain the observations well. The snow thickness was found to be the most critical factor affecting the albedo. A new parameterization was derived for the albedo dependence on snow thickness, to be applied over the Baltic Sea in spring, when periods of melting and freezing alternate but the ice is still relatively thick (about 0.6m). The diurnal cycle of solar radiation was large, and the snow/ice metamorphism due to the melting during daylight and refreezing during the night caused a diurnal albedo cycle with a maximum in the early morning and a minimum in the afternoon, with an albedo difference up to 0.14 between the two. INTRODUCTION Surface albedo is a critical factor for the growth and melt of sea ice and its snow cover. Snow surface albedo depends on both surface properties and atmospheric conditions (Warren and Wiscombe, 1980). Albedo decreases when snow ages and the grains become more rounded and increase in size. Snow metamorphism depends on the temperature: during melting, the snow grains grow quickly and the presence of melted water between the grains further decreases the albedo. Fresh snow is usually highly faceted and reflective, but also its albedo may vary greatly depending on the wetness of the grains. As the snow albedo decreases, the penetration depth of light increases, and the surface albedo is increasingly affected by the reflectivity of the deeper layers. Thus, the surface albedo depends very much on the snow thickness, especially when it is <0.1 m (Grenfell and Perovich, 2004). Snow albedo increases with increasing solar zenith angle, especially when the grains at the surface are faceted, as the light incident at lower angles penetrates deeper into the snowpack and is more likely trapped. Albedo also increases with increasing cloud cover, as the ratio of diffuse to global radiation increases and the incoming radiation flux becomes richer in the visible spectrum, for which snow albedo is higher. Over sea ice, the range of variability of the surface albedo depends on the thickness of the underlying ice layer: the lower limit is given by the albedo of the melting bare ice, which decreases with decreasing ice thickness, and the upper limit is given by the albedo of fresh snow. In the Arctic Ocean the onset of melt occurs in spring or summer (Anderson and Drobot, 2001), while in the Baltic Sea snowmelt and even the total disappearance of the snowpack can occur even in mid-winter during periods of warm-air advection. The intensity of the melting and the time required to completely melt the snowpack depends on the snow thickness and the turbulent and radiative surface fluxes. Since snowmelt may occur frequently during winter and spring, a detailed characterization of the evolution of albedo during the melting process is fundamental for a correct representation of the ice/snow mass and energy budgets in the Baltic Sea. An accurate representation of the albedo of snow-covered sea ice in weather-prediction and climate models is a chal- lenge. This is due to (a) the numerous factors that affect the albedo and (b) the feedback effects related to the albedo. For example, snow- and ice melt decrease the albedo, which favors further melt, while an increase in the albedo causes a decrease in the surface temperature, which favors a higher albedo. In spring, when the amount of solar radiation increases rapidly, the snow/ice albedo can change a lot in a short time. Several albedo parameterizations have been developed, with various degrees of complexity (Curry and others, 2001). The simplest schemes apply two or more constant values of albedo for different surface types; other schemes add a temperature dependence when the surface approaches the melting point (Ross and Walsh, 1987; Ingram and others, 1989). More sophisticated schemes also include the albedo dependence on snow/ice thickness (Flato and Brown, 1996) and cloud fraction (Shine and Henderson- Sellers, 1985). The use of snow/ice thickness to parameterize albedo has the purpose of capturing the drastic albedo changes during the melt season. On the other hand, surface temperature is the most commonly available quantity that directly affects snow metamorphism and melting, and it is therefore used to parameterize the effects of snow aging on albedo. Due to the positive feedback effect, a strong Annals of Glaciology 44 2006 * Present address: Centre for Earth Observation Science, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada. 7