Abstract
Abstract. The Snow, Ice, and Aerosol Radiative (SNICAR) model has been used in various capacities over the last 15 years to model the spectral albedo of snow with light-absorbing constituents (LACs). Recent studies have extended the model to include an adding-doubling two-stream solver and representations of non-spherical ice particles; carbon dioxide snow; snow algae; and new types of mineral dust, volcanic ash, and brown carbon. New options also exist for ice refractive indices and solar-zenith-angle-dependent surface spectral irradiances used to derive broadband albedo. The model spectral range was also extended deeper into the ultraviolet for studies of extraterrestrial and high-altitude cryospheric surfaces. Until now, however, these improvements and capabilities have not been merged into a unified code base. Here, we document the formulation and evaluation of the publicly available SNICAR-ADv3 source code, web-based model, and accompanying library of constituent optical properties. The use of non-spherical ice grains, which scatter less strongly into the forward direction, reduces the simulated albedo perturbations from LACs by ∼9 %–31 %, depending on which of the three available non-spherical shapes are applied. The model compares very well against measurements of snow albedo from seven studies, though key properties affecting snow albedo are not fully constrained with measurements, including ice effective grain size of the top sub-millimeter of the snowpack, mixing state of LACs with respect to ice grains, and site-specific LAC optical properties. The new default ice refractive indices produce extremely high pure snow albedo (>0.99) in the blue and ultraviolet part of the spectrum, with such values only measured in Antarctica so far. More work is needed particularly in the representation of snow algae, including experimental verification of how different pigment expressions and algal cell concentrations affect snow albedo. Representations and measurements of the influence of liquid water on spectral snow albedo are also needed.
Highlights
Snow is among the most reflective natural surfaces on Earth and plays an important role in determining its climate state
The “Aquablack162” soot applied in this study had a smaller and narrower size distribution than the standard Black carbon (BC) used in SNICAR, so for this comparison we created a separate species of BC with rm = 65 nm, σg = 1.3, and ka = 6.0 m2 g−1 at λ = 550 nm, matching the reported specifications by Brandt et al (2011)
We have presented the formulation of a publicly available model and accompanying library of optical properties that are used to simulate the spectral albedo of snow that is dependent on many variables, including the content of lightabsorbing constituents (LAC)
Summary
Snow is among the most reflective natural surfaces on Earth and plays an important role in determining its climate state. The purpose of this document is to describe the technical formulation, accompanying library of constituent optical properties, and evaluation of a spectral snow albedo model – The Snow, Ice, and Aerosol Radiative model with AddingDoubling solver, version 3.0 (SNICAR-ADv3). Wiscombe and Warren (1980) and Warren and Wiscombe (1980) combined a two-stream radiative transfer solution with the delta-Eddington approximation and Mie solutions to simulate hemispheric albedo of a single snow layer of any thickness and grain size, overlying a surface with any albedo, and including the influence of light-absorbing particles. Libois et al (2013) developed a multi-layer twostream snow albedo model utilizing formulations of single scatter properties from Kokhanovsky and Zege (2004) that explicitly account for non-spherical ice particles, thereby improving simulation of the vertical profile of light extinction in snow.
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