Black carbon-induced snow albedo reduction over the Tibetan Plateau: uncertainties from snow grain shape and aerosol–snow mixing state based on an updated SNICAR model

Abstract

Abstract. We implement a set of new parameterizations into the widely used Snow, Ice, and Aerosol Radiative (SNICAR) model to account for effects of snow grain shape (spherical vs. nonspherical) and black carbon (BC)–snow mixing state (external vs. internal). We find that nonspherical snow grains lead to higher pure albedo but weaker BC-induced albedo reductions relative to spherical snow grains, while BC–snow internal mixing significantly enhances albedo reductions relative to external mixing. The combination of snow nonsphericity and internal mixing suggests an important interactive effect on BC-induced albedo reduction. Comparisons with observations of clean and BC-contaminated snow albedo show that model simulations accounting for both snow nonsphericity and BC–snow internal mixing perform better than those using the common assumption of spherical snow grains and external mixing. We further apply the updated SNICAR model with comprehensive in situ measurements of BC concentrations in the Tibetan Plateau snowpack to quantify the present-day (2000–2015) BC-induced snow albedo effects from a regional and seasonal perspective. The BC concentrations show distinct and substantial sub-regional and seasonal variations, with higher values in the non-monsoon season and low altitudes. As a result, the BC-induced regional mean snow albedo reductions and surface radiative effects vary by up to an order of magnitude across different sub-regions and seasons, with values of 0.7–30.7 and 1.4–58.4 W m−2 for BC externally mixed with fresh and aged snow spheres, respectively. The BC radiative effects are further complicated by uncertainty in snow grain shape and BC–snow mixing state. BC–snow internal mixing enhances the mean albedo effects over the plateau by 30–60 % relative to external mixing, while nonspherical snow grains decrease the mean albedo effects by up to 31 % relative to spherical grains. Based on this study, extensive measurements and improved model characterization of snow grain shape and aerosol–snow mixing state are urgently needed in order to precisely evaluate BC–snow albedo effects.

Impact of light-absorbing particles on snow albedo darkening and associated radiative forcing over high-mountain Asia: high-resolution WRF-Chem modeling and new satellite observations

“…Aerosol optical properties are computed as a function of wavelength for each model grid cell. The Optical Properties of Aerosols and Clouds (OPAC) dataset (Hess et al, 1998) is used for the shortwave (SW) and longwave (LW) refractive indices of aerosols and a complex refractive index of aerosols (assuming internal mixture) is calculated by volume averaging for each chemical constituent of aerosols for each bin. A spectrally invariant value of 1.53 ± 0.003i is used for the SW complex refractive index of dust.…”

Radiative feedbacks of dust in snow over eastern Asia in CAM4-BAM

“…Dust cycles, including dust emissions, transport, and dry and wet depositions, are altered by DRF through affecting the atmospheric vertical thermal structures and surface wind speed. The mechanism of PBL (the planetary boundary layer) (Miller et al, 2004;Perez et al, 2006;Heinold et al, 2007) was initially proposed to explain the reduction of dust emissions induced by DRF (Perlwitz et al, 2001). It was described that the surface negative net DRF reduces the turbulent flux of surface sensible heat and reduces PBL mixing.…”

Intercomparison and improvement of two-stream shortwave radiative transfer schemes in Earth system models for a unified treatment of cryospheric surfaces

“…Third, adding light-absorbing particles in snow will not change our results qualitatively. Both dEdd-AD and SNICAR simulate the impact of light-absorbing particles (black carbon and dust) on snow and/or sea ice using selfconsistent particle SSPs that follow the SNICAR convention (e.g., Flanner et al, 2007;Holland et al, 2012). These particles are assumed to be either internally or externally mixed with snow crystals; the combined SSPs of mixtures (e.g., Appendix A of Dang et al, 2015) are then used as the inputs for radiative transfer calculation.…”