Modeling study of black carbon in snow over Arctic and Northern China: sensitivity to meltwater scavenging efficiency and snow aging scaling factor

Light absorbing aerosols (LAA, e.g. black carbon, dust) deposited on snow or ice could reduce snow reflectance (surface darkening) and accelerate the snow or glacier melting. Climate modeling studies suggest this mechanism has greater warming and snow-melting efficacy than any other anthropogenic agent. This large impact results from a series of positive feedback mechanisms: (1) as melt commences some of the LAA are washed away with meltwater but some remains on the surface of the snow, the LAA could accumulate on the surface or inside of snowpack; (2) additional warming to snow with reduced albedo increases snow grain sizes, which further lower the albedo and absorb more solar radiation; (3) With sufficient melt more of the darker underlying surface is exposed, leading to the well-known “snow albedo feedback”. The magnitude of these positive feedbacks depends on the scavenging efficiency of LAA by snowmelt water and snow aging factor. To better understand the impact of LAA on snow or ice, it is critical to exploit the uncertainty associated with the snow aging and melting water scavenging that has large uncertainties in their treatment in current earth system models.

In this study, we conduct a series of sensitivity experiments using the Community Atmosphere Model version 5 (CAM5), coupled with the Snow, Ice, and Aerosol Radiative (SNICAR) model that provides online calculation of snow albedo as well as solar absorption within each snow layer, to examine the impact of the snow aging factor and meltwater scavenging efficiency on the snow melting and radiative forcing of BC. We first evaluate the simulated BC concentration in top snow layer against the measurements collected from multiple field campaigns over the Arctic and Northern China. The comparison to observations show that, with the improvement of the representation of aerosol transport and wet removal, the deposition of BC to snow surface are better simulated. The observed gradient in BC concentration in snow between the lower latitudes (Northern China) and higher latitudes (Arctic) is well captured. The maximum concentration and forcing of BC in snow occurs during late spring to early summer, indicating a possible post-depositional enrichment of BC in snowpack. The uncertain BC scavenging coefficient in melting water and snow aging scaling factor can significantly change the concentration and vertical profile of BC in snow and its radiative forcing. A reasonably large BC scavenging coefficient could decrease the BC concentration and forcing in snow by a factor of 2-4. The impacts of snow aging scaling factor are more complex, having region and season dependence. This study highlights the need of more comprehensive study on the processes in snow aging and melting water scavenging from both measurement and modeling in order to reduce the uncertainty in quantifying the climatic and hydrological effect of LAA in snow/ice.