6.4 Determining Albedo Reduction by Light-absorbing Particles in Snow over Large Areas - What Can We Learn from Field Observations?

Tuesday, 24 January 2017: 4:45 PM
Conference Center: Skagit 3 (Washington State Convention Center )
Cheng Dang, University of Washington, Seattle, WA; and S. G. Warren, Q. Fu, S. J. Doherty, and M. Sturm

Large-area surveys for black carbon (BC) and other light-absorbing particles (LAPs) in snow have been carried out in the Arctic, China, and North America (Doherty et al., 2010, 2014; Wang et al., 2013). Based on the LAP mixing ratios and the snow conditions observed in the field expeditions, we calculate the snow albedo and the albedo reduction caused by LAPs using radiative transfer models, and examine sources of uncertainty (e.g. snow depth, snow-depth distribution, vertical profile of LAPs, albedo of underlying surface).

For single-layer snowpacks consisting of new snow with grain radius 100 micrometers, the average albedo reductions caused by all LAPs in the Arctic, North America, and China are 0.005, 0.011, and 0.042, respectively, of which the albedo reductions caused by BC alone are 0.003, 0.005, and 0.014. For typical solar fluxes at the surface (clear sky) in April at the corresponding latitudes, the albedo reduction by BC will increase the solar energy absorbed by snowpacks by 0.2, 1.1 and 3.1 watts per square meter.

The BC-induced albedo reduction would be larger if snowpacks were optically thick or did not contain non-BC LAPs. For example, if all snowpacks were optically thick, the albedo reduction would be 10 - 15% larger in the Arctic and North America. If the snowpacks in China did not contain non-BC LAPs, the albedo reduction by BC would increase by 55%. At 22 sites in subarctic Canada where BC was analyzed, the frequency distribution of snow depth was also reported (Sturm et al., 2008). The BC-induced albedo reduction for the mean depth at each site is larger than the albedo reduction for a depth-distribution, but only by 3% on average. For some sampling sites, the albedo is calculated using a multi-layer radiative transfer model with observed vertical profiles of LAPs as input. For a specific site, the resulting albedo can be greater or less than the albedo calculated from a single-layer model with the same optical thickness, but the regional-average albedo is approximately the same as that computed using a single-layer model. The albedo of the underlying ground has negligible impact on the system albedo at all sampling sites.


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