Improving Shortwave Radiative Transfer in the Lake Component of the Common Land Model (CoLM-Lake) with a Spectral Scheme for the Full Snow-Ice-Water Column

Abstract. Shortwave radiation controls lake thermal structure and ice evolution, yet one-dimensional lake models typically use bulk Beer–Lambert attenuation in water and optically opaque snow and ice. Here, treating the full snow-ice-water column as a unified, optically active system, we couple the Snow, Ice, and Aerosol Radiation Model with Adding-Doubling version 5 (SNICAR-ADv5) with the lake component of the Common Land Model (CoLM-Lake) to represent spectral radiative transfer for lake columns. The coupled model accounts for wavelength-dependent absorption and scattering, direct and diffuse pathways, Fresnel interface effects, and radiative interactions among layers. We evaluate the coupled model across nine lakes spanning depth, clarity, climate, and ice cover. Its impacts are physically consistent but lake dependent. During open-water periods, the coupled model shifts absorbed shortwave energy toward near-surface, especially for diffuse radiation, and reduces heating below the mixed layer. This behavior improves temperature profiles and thermocline depths where the original model overheats below the mixed layer and places thermoclines too deep. In turbid lakes, observation-based water extinction coefficients provide first-order improvements, whereas the spectral scheme gives a more interpretable heating profile than single-band Beer–Lambert attenuation. During ice-covered periods, it partitions shortwave energy among snow, ice, and water, enabling representation of internal ice heating, under-ice warming, and basal melt. Limited improvements in some deep or clear lakes indicate that radiative transfer advances must be combined with improved mixing and heat storage. This implementation provides a process-based tool for diagnosing and improving lake thermal, ice, and lake–atmosphere interaction simulations in land, weather, and Earth system models.