First spectral measurements of light attenuation in Greenland Ice Sheet bare ice suggest shallower subsurface radiative heating and ICESat-2 penetration depth in the ablation zone

Abstract

Abstract. Light transmission into bare glacial ice affects surface energy balance, bio-photochemical cycling, and light detection and ranging (LiDAR) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present in-ice solar irradiance measured over the spectral range 350–900 nm and 12–77 cm depth collected at a site in the western Greenland ablation zone. The acquired spectral irradiance measurements are used to calculate flux attenuation coefficients using an exponential decay Bouguer law model and are compared to values calculated from two-stream radiative transfer theory. Relative to asymptotic two-stream theory, our empirical attenuation coefficients are up to one order of magnitude larger in the range 350–530 nm, suggesting light absorbing particles embedded in ice enhance visible light absorption at our field site. The empirical coefficients accurately describe light attenuation in the ice interior but underestimate light attenuation near the ice surface. Consequently, Bouguer’s law overestimates transmitted flux by up to 50 % depending on wavelength. Refraction is unlikely to explain the discrepancy. Instead, vertical variation in the ice microstructure and the concentration of light absorbing particles appears to enhance near-surface attenuation at our field site. The magnitude of this near-surface attenuation implies that optical penetration depth is lower by up to 19 cm (28 %) at wavelengths relevant to visible-wavelength lidar altimetry of ice surface elevation (e.g. 532 nm for the Ice, Cloud, and Land Elevation Satellite-2) than is suggested by e-folding depths inferred from two stream theory for optically pure glacier ice. This enhanced near-surface attenuation implies shallower light transmission and therefore lower subsurface light availability for subsurface radiative heating and bio-photochemical cycling. We recommend radiative transfer models applied to bare ice in the Greenland Ice Sheet ablation zone account for vertical variation in light attenuation due to the vertical distribution of light absorbing particles and ice microstructure, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model validation and parameterization.

Impact of updated radiative transfer scheme in snow and ice in RACMO2.3p3 on the surface mass and energy budget of the Greenland ice sheet

“…Firstly, we determine the specific surface area (SSA) of a semi-infinite ice layer such that TARTES and SNOWBAL calculate a broadband albedo of 0.6, which represents the broadband albedo of clean blue ice (Reijmer et al, 2001;Dadic et al, 2013) for clear-sky conditions and a reference SZA of 60 • . For this, we find an SSA of 0.788 m 2 kg −1 (4.152 mm grain size).…”

Impact of updated radiative transfer scheme in RACMO2.3p3 on the surface mass and energy budget of the Greenland ice sheet

“…1b) for Rp3's spectral band 6 (778-1242 nm) and 4 (442-625 nm), respectively. As energy absorption depends on F (z) and τ , which in turn depends strongly on wavelength and snow structure (Meirold-Mautner and Lehning, 2004;Ackermann et al, 2006;Warren et al, 2006;He et al, 2017He et al, , 2018He and Flanner, 2020;Cooper et al, 2020), significantly more energy is absorbed within the SLED for fresh snow and near-IR radiation, as τ is small for this case, than for ice and visible light, for which τ is large, resulting in more internal energy absorption close to surface and a larger E SEB .…”

“…The vertical dashed line shows the 5 mm skin layer equilibration depth (SLED). Here, we assume typical net downward shortwave radiation at the surface and τ = 0.008 m for (a)(Meirold-Mautner and Lehning, 2004) and τ = 0.56 m for (b)(Cooper et al, 2020). All absorbed energy beyond z sled is internal.…”

Comment on tc-2020-378

“…7, dashed black line). This can be explained by additional processes that affect the local photon distribution but that we did not consider, such as the forward scattering in the atmosphere (Kurtz et al, 2008), the penetration of photons in the ice layer (Cooper et al, 2020), or simply the presence of outliers that passed the median absolute difference filter (Eq. ( 7)).…”