Spectral attenuation coefficients from measurements of light transmission in bare ice on the Greenland Ice Sheet

Abstract

Abstract. Light transmission into bare glacial ice affects surface energy balance, biophotochemistry, and light detection and ranging (lidar) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present measurements of spectral transmittance at 350–900 nm in bare glacial ice collected at a field site in the western Greenland ablation zone (67.15∘ N, 50.02∘ W). Empirical irradiance attenuation coefficients at 350–750 nm are ∼ 0.9–8.0 m−1 for ice at 12–124 cm depth. The absorption minimum is at ∼ 390–397 nm, in agreement with snow transmission measurements in Antarctica and optical mapping of deep ice at the South Pole. From 350–530 nm, our empirical attenuation coefficients are nearly 1 order of magnitude larger than theoretical values for optically pure ice. The estimated absorption coefficient at 400 nm suggests the ice volume contained a light-absorbing particle concentration equivalent to ∼ 1–2 parts per billion (ppb) of black carbon, which is similar to pre-industrial values found in remote polar snow. The equivalent mineral dust concentration is ∼ 300–600 ppb, which is similar to values for Northern Hemisphere warm periods with low aeolian activity inferred from ice cores. For a layer of quasi-granular white ice (weathering crust) extending from the surface to ∼ 10 cm depth, attenuation coefficients are 1.5 to 4 times larger than for deeper bubbly ice. Owing to higher attenuation in this layer of near-surface granular ice, optical penetration depth at 532 nm is 14 cm (20 %) lower than asymptotic attenuation lengths for optically pure bubbly ice. In addition to the traditional concept of light scattering on air bubbles, our results imply that the granular near-surface ice microstructure of weathering crust is an important control on radiative transfer in bare ice on the Greenland Ice Sheet ablation zone, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model development and validation.

Mapping the aerodynamic roughness of the Greenland Ice Sheet surface using ICESat-2: evaluation over the K-transect

“…Runoff occurs mostly in the low-lying ablation area of the GrIS, where bare ice is exposed to on-average positive air temperatures throughout summer (Smeets et al, 2018;Fausto et al, 2021). As a consequence, the downward turbulent mixing of warmer air towards the bare ice, the sensible heat flux, is an important driver of GrIS mass loss next to radiative fluxes (Fausto et al, 2016;Kuipers Munneke et al, 2018;Van Tiggelen et al, 2020).…”

Direct measurement of optical properties of glacier ice using a photon-counting diffuse LiDAR

“…Fig. 12 of Cooper and others (2021)), suggesting that the ice we measured at Collier Glacier contained substantial concentrations of BC or other LAPs. Such large coefficients and the fact that the absorption is almost identical for the green and blue part of the spectrum agree with observations made on icebergs and glacier ice contaminated with algae and organic LAPs (Warren and others, 2019b).…”

“…For example, differences between the scattering and absorption coefficients of deep ice melting out near the terminus of large glaciers and ice sheets relative to younger ice near the equilibrium line altitude may be explainable in terms of a common age or ice provenance. Therefore, the ToF histogram approach presented here could provide additional context for optical coefficients measured by others in Greenland (Cooper and others, 2021) and Antarctica (Ackermann, 2006), and could provide the basis for a systematic investigation of underlying structural controls on ice-sheet albedo, which has been identified as a measurement priority needed to improve albedo parameterizations in large-scale models (Dadic and others, 2013).…”

“…So far, in situ measurements of scattering and absorption coefficients of glacier ice have remained challenging. Transmission measurements rely on assumptions about the wavelength-dependence of the scattering coefficient at visible wavelengths as well as the qualitative influence of LAPs, for which reference values are needed (Warren and others, 2006; Cooper and others, 2021). The same is true for scattering intensity measurements, where merely an effective constant describing both scattering and absorption can be retrieved (Allgaier and Smith, 2021).…”

Breakdown of light transport models in photonic scattering slabs with strong absorption and anisotropy

“…The most popular method to solve the RTE for light is the Monte Carlo simulation of light transport [25][26][27][28][29][30][31][32][33], a statistical method that converges to the exact solution of the RTE. To obtain a high accuracy, however, this method comes with the cost of extremely long computation times [34], high computational power requirements with concomitant high energy consumption.…”