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.
Highlights
IntroductionUnderstanding the transmission, absorption, and scattering of light in ice is important for snow and ice energy balance modelling (Brandt and Warren, 1993), lidar remote sensing of snow surface elevation and grain size (Deems et al, 2013; Yang et al, 2017), primary productivity beneath sea ice (Frey et al, 2011; Grenfell, 1979), biophotochemistry in ice and snowpack (France et al, 2011), and theoretical predictions of “Snowball Earth” palaeoclimates (Dadic et al, 2013; Warren et al, 2002)
This study reports on the irradiance attenuation coefficient katt of bare glacier ice in the Greenland Ice Sheet ablation zone, a critical parameter needed to calculate subsurface absorption and scattering of transmitted radiation that to our knowledge has received no direct field study
For wavelengths > 500 nm, T rapidly decreases both with wavelength and with depth; beyond ∼ 800 nm most incident light is attenuated within 36 cm of the ice surface, substantial attenuation is apparent in the 12–36 cm depth region (Fig. 4b)
Summary
Understanding the transmission, absorption, and scattering of light in ice is important for snow and ice energy balance modelling (Brandt and Warren, 1993), lidar remote sensing of snow surface elevation and grain size (Deems et al, 2013; Yang et al, 2017), primary productivity beneath sea ice (Frey et al, 2011; Grenfell, 1979), biophotochemistry in ice and snowpack (France et al, 2011), and theoretical predictions of “Snowball Earth” palaeoclimates (Dadic et al, 2013; Warren et al, 2002). Cooper et al.: Light transmission in bare ice on the Greenland Ice Sheet where kabs (m−1) is the absorption coefficient, ksca (m−1) is the scattering coefficient, and all are functions of wavelength λ. This study reports on the irradiance attenuation coefficient katt of bare glacier ice in the Greenland Ice Sheet ablation zone, a critical parameter needed to calculate subsurface absorption and scattering of transmitted radiation that to our knowledge has received no direct field study
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