Effects of snow albedo change induced by aerosol deposition

Polluted snow on the Hallstätter Glacier, in the Dachstein Mountains.

Implementation and investigation of snow albedo change induced by aerosol deposition in ICON-ART

The fraction of sunlight reflected by the surface of the earth (surface albedo) plays a vital role in the radiation and energy budget of the earth. Snow and ice surfaces are characterized by exceptional high albedo and therefore reduce the absorption and heating of the earth. Wiscombe & Warren (1980) investigated the snow albedo and found a great impact of light absorbing particles on the surface albedo. They presented a simple analytic model for snow albedo calculations, which is useful for the entire solar spectrum, all zenith angles and different snow grain sizes. The model is based on the mie theory and the delta-Eddington approximation (Joseph et al., 1976). The computed albedo of pure snow is roughly 96-99% where a concentration of only 1 ppmw light absorbing particles can lower the albedo in the visible spectrum by 5-15% (Warren & Wiscombe, 1980).

The reduction of the surface albedo by light absorbing impurities can trigger further feedbacks. The additionally absorbed radiation energy can be utilized for additional snowmelt. Thin layers of snow can disappear and hereby reduce the albedo by revealing the darker underlying surface. An increased snow temperature accelerates the natural snow aging process, which involves the growth of snow grains. The snow grain size is a key feature, which defines the albedo of pure snow. Bigger snow grains imply a lower albedo and thus more absorption of solar radiation. Due to the changed radiation fluxes at the surface, the energy fluxes of the overlying atmosphere layer is altered and further feedbacks in the atmosphere can be triggered (Bond et al., 2013). As for instance, the surface temperature and the 2m air temperature are highly dependent on those fluxes.

The target of this study is the implementation of the impact of aerosol deposition on snow and ice surfaces and the investigation of the consequences in ICON-ART. At first, we improve the snow model by introducing an optical snow grain size featuring snow metamorphism to parameterize the snow type. The growth rate depends on the snow temperature and snow grain size whereas snowfall resets the snow grain radius. Furthermore, we implement calculations of the snow albedo according to Wiscombe and Warren, 1980. Combining the calculations of the snow albedo and the accumulation of aerosols on the snow surface, we can quantify the impact. We use the optical properties and the deposition of aerosols that are already present in the module ICON-ART.

The aerosols play a significant role for the ecosystem and the drinking water supply especially in mountainous regions as they can modify the time and duration of snowmelt and runoff.  The highly heterogeneous distribution of the aerosols and the difficult terrain establish a great challenge. ICON reaches a high spatial resolution and therefore has a great advantage in simulating complex terrain. We will apply the new implementations to simulate the Himalayan Region. This is part of the Task 3.2: Monsoon systems in a changing climate. This work carries forward the model investigation of such complicated areas, contributes to the development of the model ICON-ART and yields further insight in the repercussions of aerosols on snow and ice surfaces.