Spectral attenuation coefficients from measurements of light transmission in bare ice on the Greenland Ice Sheet
Spectral attenuation coefficients from measurements of light transmission in bare ice on the Greenland Ice Sheet
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DOI：
10.5194/tc-15-1931-2021
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发表时间：
2021-04
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通讯作者：
M. Cooper;L. Smith;Å. Rennermalm;M. Tedesco;R. Muthyala;S. Leidman;S. Moustafa;J. Fayne
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文献类型：
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作者：
M. Cooper;L. Smith;Å. Rennermalm;M. Tedesco;R. Muthyala;S. Leidman;S. Moustafa;J. Fayne
摘要。光透射到裸露的冰川冰面会影响表面能量平衡、生物光化学以及光探测和测距（激光雷达）激光高程测量，但以前没有关于格陵兰冰盖的报道。我们在格陵兰岛西部消融带的一个野外地点采集了350-900 nm的裸冰的光谱透射率（67.15°N, 50.02°W）。对于12-124 cm深度的冰，350-750 nm的经验辐照衰减系数为~ 0.9-8.0 m−1。吸收最小值在~ 390-397 nm，与南极的雪透射测量和南极深冰的光学测绘一致。在350-530 nm范围内，我们的经验衰减系数比光纯冰的理论值大了近一个数量级。在400纳米处估计的吸收系数表明，冰体积中含有的吸收光的颗粒浓度相当于10亿分之1-2 （ppb）的黑碳，这与在遥远的极地雪中发现的工业化前的值相似。等效矿物粉尘浓度为~ 300-600 ppb，这与从冰芯推断的低风成活动的北半球暖期的值相似。对于从表面延伸至~ 10cm深度的准颗粒状白冰（风化壳）层，衰减系数是深层气泡冰的1.5 ~ 4倍。由于近表面颗粒冰层的衰减较大，在532 nm处的光学穿透深度比光学纯气泡冰的渐近衰减长度低14 cm（20%）。除了传统的气泡光散射概念外，我们的研究结果表明，风化壳的颗粒状近地表冰微观结构是格陵兰冰盖消融带裸冰辐射传输的重要控制因素，我们提供了新的通量衰减、吸收和散射系数值，以支持模型的开发和验证。
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.