Modeling study of the snow darkening effect by black carbon deposition over the Arctic during the melting period

Abstract. The rapid warming of the Arctic, accompanied by glacier and sea ice melt, has significant consequences for the Earth's climate, ecosystems, and economy. Recent evidence suggests that the snow-darkening effect (SDE) induced by light-absorbing particles, such as black carbon (BC) deposition, could greatly influence rapid warming in the Arctic. However, there is still a lack of ensemble simulations using high-resolution models for investigating the impacts of the SDE resulting from BC deposition on the Arctic surface energy balance. By integrating the physically based Snow, Ice, Aerosol, and Radiation (SNICAR) model with a polar-optimized version of the Weather Research and Forecasting model (Polar-WRF), this study aimed to quantify the impacts of the SDE due to BC deposition and analyze the relationship between BC aerosol mass in snow (represented by snow depth) and snow albedo reduction. The simulation results indicate that BC deposition can directly affect the surface energy balance by decreasing snow albedo and its corresponding radiative forcing (RF). On average, BC deposition at 50 ng g-1 causes a radiative forcing (RF) of 1.6 W m-2 in off-line simulations (without surface feedbacks) and 1.4 W m-2 in on-line simulations (with surface feedbacks). The high RF caused by BC deposition reached 1–4 W m-2 and mainly occurred in Greenland, Baffin Island and East Siberia, where areas with deep snow depths and large snow densities are prevalent. The changes in snow albedo are indeed strongly linked to the mass of BC aerosols. Notably, a clear linear relationship was established between snow depth and the reduction in snow albedo, with a correlation coefficient exceeding 0.9 and an R-squared value greater than 0.85 when the snow depth is shallow. However, as snow depth increases, the impact of BC on snow albedo gradually diminishes until it reaches its maximum value when the snowpack becomes sufficiently optically thick. Regions with deep snowpack, such as Greenland, tend to exhibit greater sensitivity to BC deposition due to the higher absolute mass of BC and the longer duration of the SDE. For a given column-mean BC concentration in snow, the impacts of the SDE are approximately 25–41 % greater in deep snow-covered areas than in shallow snow-covered areas, leading to a 19–40 % increase in snowmelt. A comparison between off-line and on-line coupled simulations using Polar-WRF/Noah-MP and SNICAR has provided valuable insights into the critical mechanisms and key factors influencing changes in surface heat transfer due to the impacts of the SDE induced by BC deposition in the Arctic. It has been observed that various processes, such as snow melting and land‒atmosphere interactions, play significant roles in assessing changes in the surface energy balance caused by BC deposition. Notably, off-line simulations tend to overestimate the impacts of the SDE, sometimes by more than 50 %, due to the lack of relevant processes. This study emphasized the importance of the impacts of snow conditions and land‒atmosphere interactions on evaluating the impacts of the SDE by BC deposition. It is therefore necessary to prioritize high-resolution modeling studies that incorporate detailed physical processes to enhance our understanding of the impacts of the SDE on Arctic climate change.