Impact of Snow Grain Shape and Internal Mixing with Black Carbon Aerosol on Snow Albedo and Radiative Effect Analysis

We develop a set of parameterizations for spectral albedos of clean and black carbon (BC)-contaminated snow by explicitly resolving various snow grain shapes and BC-snow internal mixing. The albedo of pure snow is parameterized as a function of snow effective size with an accuracy of >99%. For the same effective size, Koch snowflakes have the highest albedo across the solar spectrum, whereas spheres and spheroids have the lowest albedo. The difference enlarges as snow grain size increases, with up to 0.01 at visible wavelengths and 0.05 at near-infrared (NIR) wavelengths for an effective radius of 1000 mm. The BC-induced snow albedo reduction is parameterized as a function of snow effective size and BC concentration in snow, with an accuracy of >99%. For the same snow effective size and BC concentration, snow albedo decreases most for spheres and spheroids at near-UV and visible wavelengths, with a reduction of 0.1 for 1000 ppb BC in fresh snow (effective radius = 100 mm) and a factor of 2-3 stronger for old snow (effective radius =1000 mm). The Koch snowflakes show 30–50% weaker albedo reduction compared with spheres and spheroids. The BC-induced albedo reduction at NIR wavelengths is more complex for different snow shapes but much smaller than that at visible wavelengths. Compared with external mixing, BC-snow internal mixing enhances snow albedo reduction by a factor of 1.3-1.9 for spheres and 1.7-2.5 for Koch snowflakes, depending on BC concentration. Based on observed BC concentration in snow, we estimate that the surface radiative effect caused by BC-induced snow albedo reduction during winter and spring is up to 5 W/m2 for fresh snow and 12 W/m2 for old snow over mid-latitude snowpack and decreases northward to ~0.4 W/m2 in the Arctic, with an uncertainty (1-sigma) of 30–40% due to different snow shapes and BC-snow mixing states. Overall, snow grain shape and size along with BC-snow internal mixing play critical roles in quantifying snow albedo and associated radiative effects due to impurity contamination.