Abstract. Accurate modeling of cryospheric surface albedo is essential for ourunderstanding of climate change as snow and ice surfaces regulate the globalradiative budget and sea-level through their albedo and massbalance. Although significant progress has been made using physicalprinciples to represent the dynamic albedo of snow, models of glacier icealbedo tend to be heavily parameterized and not explicitly connected withphysical properties that govern albedo, such as the number and size of airbubbles, specific surface area (SSA), presence of abiotic and biotic lightabsorbing constituents (LACs), and characteristics of any overlyingsnow. Here, we introduce SNICAR-ADv4, an extension of the multi-layertwo-stream delta-Eddington radiative transfer model with theadding–doubling solver that has been previously applied to represent snowand sea-ice spectral albedo. SNICAR-ADv4 treats spectrally resolved Fresnelreflectance and transmittance between overlying snow and higher-densityglacier ice, scattering by air bubbles of varying sizes, and numerous typesof LACs. SNICAR-ADv4 simulates a wide range of clean snow and ice broadbandalbedo (BBA), ranging from 0.88 for (30 µm) fine-grain snow to 0.03for bare and bubble-free ice under direct light. Our results indicate thatrepresenting ice with a density of 650 kg m−3 as snow with norefractive Fresnel layer, as done previously, generally overestimates theBBA by an average of 0.058. However, because mostnaturally occurring ice surfaces are roughened “white ice”, we recommendmodeling a thin snow layer over bare ice simulations. We find optimalagreement with measurements by representing cryospheric media with densitiesless than 650 kg m−3 as snow and larger-density media as bubbly icewith a Fresnel layer. SNICAR-ADv4 also simulates the non-linear albedoimpacts from LACs with changing ice SSA, with peak impact per unit mass ofLACs near SSAs of 0.1–0.01 m2 kg−1. For bare, bubble-free ice, LACsactually increase the albedo. SNICAR-ADv4 represents smooth transitionsbetween snow, firn, and ice surfaces and accurately reproduces measuredspectral albedos of a variety of glacier surfaces. This work paves the wayfor adapting SNICAR-ADv4 to be used in land ice model components of Earthsystem models.
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Spectral attenuation coefficients from measurements of light transmission in bare ice on the Greenland Ice Sheet
Abstract. Light transmission into bare glacial ice affects surfaceenergy balance, biophotochemistry, and light detection and ranging (lidar)laser elevation measurements but has not previously been reported for theGreenland Ice Sheet. We present measurements of spectral transmittance at350–900 nm in bare glacial ice collected at a field site in the westernGreenland ablation zone (67.15∘ N, 50.02∘ W). Empirical irradianceattenuation 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 transmissionmeasurements in Antarctica and optical mapping of deep ice at the SouthPole. From 350–530 nm, our empirical attenuation coefficients are nearly1 order of magnitude larger than theoretical values for optically pureice. The estimated absorption coefficient at 400 nm suggests the ice volumecontained a light-absorbing particle concentration equivalent to∼ 1–2 parts per billion (ppb) of black carbon, which is similar topre-industrial values found in remote polar snow. The equivalent mineraldust concentration is ∼ 300–600 ppb, which is similar to values forNorthern Hemisphere warm periods with low aeolian activity inferred from icecores. For a layer of quasi-granular white ice (weathering crust)extending from the surface to ∼ 10 cm depth, attenuationcoefficients are 1.5 to 4 times larger than for deeper bubbly ice. Owing tohigher attenuation in this layer of near-surface granular ice, opticalpenetration depth at 532 nm is 14 cm (20 %) lower than asymptoticattenuation lengths for optically pure bubbly ice. In addition to thetraditional concept of light scattering on air bubbles, our results implythat the granular near-surface ice microstructure of weathering crust isan important control on radiative transfer in bare ice on the Greenland IceSheet ablation zone, and we provide new values of flux attenuation,absorption, and scattering coefficients to support model development andvalidation.
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- Award ID(s):
- 1713072
- PAR ID:
- 10410899
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 15
- Issue:
- 4
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 1931 to 1953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/tc-15-1931-2021
