Absorption In Ice Research Articles

Summer snow on Arctic sea ice modulated by the Arctic Oscillation

Since the 1970s, Arctic sea ice has undergone unprecedented change, becoming thinner, less extensive and less resilient to summer melt. Snow’s high albedo greatly reduces solar absorption in sea ice and the upper ocean, which mitigates sea–ice melt and ocean warming. However, the drivers of summertime snow depth variability are unknown. The Arctic Oscillation is a mode of natural climate variability, influencing Arctic snowfall and air temperatures. Thus, it may affect summertime snow conditions on Arctic sea ice. Here we examine the role of the Arctic Oscillation in summer snow depth variability on Arctic sea ice in 1980–2020 using atmospheric reanalysis, snow modelling and satellite data. The positive phase leads to greater snow accumulation, ranging up to ~4.5 cm near the North Pole, and higher surface albedo in summer. There are more intense, frequent Arctic cyclones, cooler temperatures aloft and greater snowfall relative to negative and neutral phases; these conditions facilitate a more persistent summer snow cover, which may lessen sea-ice melt and ocean warming. The Arctic Oscillation influence on summertime snow weakens after 2007, which suggests that future warming and Arctic sea-ice loss might modify the relationship between the Arctic Oscillation and snow on Arctic sea ice.

The color of melt ponds on Arctic sea ice

Abstract. Pond color, which creates the visual appearance of melt ponds on Arctic sea ice in summer, is quantitatively investigated using a two-stream radiative transfer model for ponded sea ice. The upwelling irradiance from the pond surface is determined and then its spectrum is transformed into RGB (red, green, blue) color space using a colorimetric method. The dependence of pond color on various factors such as water and ice properties and incident solar radiation is investigated. The results reveal that increasing underlying ice thickness Hi enhances both the green and blue intensities of pond color, whereas the red intensity is mostly sensitive to Hi for thin ice (Hi < 1.5 m) and to pond depth Hp for thick ice (Hi > 1.5 m), similar to the behavior of melt-pond albedo. The distribution of the incident solar spectrum F0 with wavelength affects the pond color rather than its intensity. The pond color changes from dark blue to brighter blue with increasing scattering in ice, and the influence of absorption in ice on pond color is limited. The pond color reproduced by the model agrees with field observations for Arctic sea ice in summer, which supports the validity of this study. More importantly, the pond color has been confirmed to contain information about meltwater and underlying ice, and therefore it can be used as an index to retrieve Hi and Hp. Retrievals of Hi for thin ice (Hi < 1 m) agree better with field measurements than retrievals for thick ice, but those of Hp are not good. The analysis of pond color is a new potential method to obtain thin ice thickness in summer, although more validation data and improvements to the radiative transfer model will be needed in future.

Particle emission from polymer-doped water ice matrices induced by non-linear absorption of laser light at 1064 nm

Emission of PEG (polyethylene glycol) molecules and ions from an ice target induced by laser irradiation in the infrared (IR) regime at 1064 nm was studied. Matrices of 1% weight PEG flash-frozen solutions were used for polymer deposition with MAPLE (matrix-assisted pulsed laser evaporation). Even though linear absorption in defect-free water ice is two orders of magnitude larger at 1064 nm than 355 nm, the deposition rate and ion current density are much smaller for IR than for ultraviolet laser light. The similarity of results for both wavelengths indicates that non-linear absorption by electrons via optical breakdown is dominant.

Optical properties of deep glacial ice at the South Pole

We have remotely mapped optical scattering and absorption in glacial ice at the South Pole for wavelengths between 313 and 560 nm and depths between 1100 and 2350 m. We used pulsed and continuous light sources embedded with the AMANDA neutrino telescope, an array of more than six hundred photomultiplier tubes buried deep in the ice. At depths greater than 1300 m, both the scattering coefficient and absorptivity follow vertical variations in concentration of dust impurities, which are seen in ice cores from other Antarctic sites and which track climatological changes. The scattering coefficient varies by a factor of seven, and absorptivity (for wavelengths less than ∼450 nm) varies by a factor of three in the depth range between 1300 and 2300 m, where four dust peaks due to stadials in the late Pleistocene have been identified. In our absorption data, we also identify a broad peak due to the Last Glacial Maximum around 1300 m. In the scattering data, this peak is partially masked by scattering on residual air bubbles, whose contribution dominates the scattering coefficient in shallower ice but vanishes at ∼1350 m where all bubbles have converted to nonscattering air hydrates. The wavelength dependence of scattering by dust is described by a power law with exponent −0.90 ± 0.03, independent of depth. The wavelength dependence of absorptivity in the studied wavelength range is described by the sum of two components: a power law due to absorption by dust, with exponent −1.08 ± 0.01 and a normalization proportional to dust concentration that varies with depth; and a rising exponential due to intrinsic ice absorption which dominates at wavelengths greater than ∼500 nm.

Climate response of fossil fuel and biofuel soot, accounting for soot's feedback to snow and sea ice albedo and emissivity

The first three‐dimensional global model in which time‐dependent spectral albedos and emissivities over snow and sea ice are predicted with a radiative transfer solution, rather than prescribed, is applied to study the climate response of fossil fuel plus biofuel black carbon plus organic matter (ff+bf BC+OM) when BC absorption in snow and sea ice is accounted for. The model treats the cycling of size‐resolved BC+OM between emission and removal by dry deposition and precipitation from first principles. Particles produce and enter size‐resolved clouds and precipitation by nucleation scavenging and aerosol‐hydrometeor coagulation. Removal brings BC to the surface, where internally and externally mixed BC in snow and sea ice affects albedo and emissivity through radiative transfer. Climate response simulations were run with a ff+bf BC+OC emission inventory lower than that used in a previous study. The 10‐year, globally averaged ff+bf BC+OM near‐surface temperature response due to all feedbacks was about +0.27 K (+0.32 in the last 3 years), close to those from the previous study (5‐year average of +0.3 K and fifth‐year warming of +0.35 K) and its modeled range (+0.15 to +0.5 K) because warming due to soot absorption in snow and sea ice here (10‐year average of +0.06 K with a modeled range of +0.03 to +0.11 K) offset reduced warming due to lower emission. BC was calculated to reduce snow and sea ice albedo by ∼0.4% in the global average and 1% in the Northern Hemisphere. The globally averaged modeled BC concentration in snow and sea ice was ∼5 ng/g; that in rainfall was ∼22 ng/g. About 98% of BC removal from the atmosphere was due to precipitation; the rest was due to dry deposition. The results here support previous findings that controlling ff+bf BC+OM and CO2 emission may slow global warming.

Temperature dependence of absorption in ice at 532 nm

A YAG laser was used to emit nanosecond pulses of light at 532 nm at depths from 1185 to 2200 m in Antarctic ice, corresponding to temperatures increasing from 229 to 249 K. From the timing distributions of photons arriving at phototubes at distances up to 100 m and at similar depths, the scattering and absorption coefficients were measured, and the temperature dependence of absorptivity at 532 nm was determined. Despite the absorptivity being many orders of magnitude lower at 532 nm than in the near ultraviolet and near infrared, the fractional increase of absorptivity, a(-1)da/dT = 0.01 K(-1), was the same in the visible, ultraviolet, and infrared. Analysis of published data at other wavelengths shows that a(-1)da/dT is ~0.01 K(-1) from 175 nm to ~1 cm, above which it increases strongly from 1 cm to 10 m. That temperature dependence applies only in regions not close to absorption bands.

Optical properties of deep ice at the South Pole: absorption

We discuss recent measurements of the wavelength-dependent absorption coefficients in deep South Pole ice. The method uses transit-time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At depths of 800-1000 m scattering is dominated by residual air bubbles, whereas absorption occurs both in ice itself and in insoluble impurities. The absorption coefficient increases approximately exponentially with wavelength in the measured interval 410-610 nm. At the shortest wavelength our value is approximately a factor 20 below previous values obtained for laboratory ice and lake ice; with increasing wavelength the discrepancy with previous measurements decreases. At ~415 to ~500 nm the experimental uncertainties are small enough for us to resolve an extrinsic contribution to absorption in ice: submicrometer dust particles contribute by an amount that increases with depth and corresponds well with the expected increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The laser pulse method allows remote mapping of gross structure in dust concentration as a function of depth in glacial ice.

Numerical modeling of long‐range low‐frequency propagation in irregular Arctic waveguide

An improved model of the Arctic ice cover and the mode coupling approach have been used for numerical calculations of low‐frequency sound propagation over the irregular trans‐Arctic paths proposed for acoustic monitoring of Arctic climate change. The ice cover was represented by a rough elastic layer with the roughness statistically determined by its spatial spectrum and correlation between the upper and lower ice surfaces. Horizontal change of the ice statistics along the supposed paths was approximated by a step function in accordance with existing data. The bottom relief and horizontal variation of the typical sound speed profile were approximated by segment‐linear functions. In the ice modeling particular attention has been given to the influence of the ice cracks on low‐frequency propagation loss. This can be regarded as a shear wave attenuation in the broken ice plate which differs strongly from shear wave absorption in normal ice and mainly depends on a mean size of the ice fields. Calculated acoustic propagation loss over the trans‐Arctic paths differ from the empirical to experimental data after a 1000‐km distance, with the calculated loss significantly less than the empirical prediction. This is caused by a relative increase of the deep‐water mode contribution to the total sound field at such long distances.

The deterioration of freshwater ice due to radiation decay

This paper reports on the processes governing the transfer of solar radiation energy to river-ice covers, the effects of snow-cover properties on transmittance and reflectance and the nature of shortwave radiation absorption in ice leading to intergranular melt. The influence of this melt on ice strength is illustrated using previously obtained borehole jack indentation data and concomitant determinations of porosity. The indentation data are analyzed using a plastic yield approach to estimate the uniaxial compression strength for ice idealized as a Tresca material. The analysis reveals that the ultimate platen-indentation-pressure to uniaxial-compressive-strength ratio is approximately 5.7; agreeing closely with available borehole jack and uniaxial compression data for columnar grained ice at 0°C. Available data describing the strength reductions associated with intergranular melt are reviewed. They show good correspondence with those resulting from existing theoretical strength-porosity models. The util...

Reflectivity of sea ice

The reflections from the face of a finite block of sea ice are usually predicted as the combination of two phenomena, as allowed under the Kirchhoff approximation. One is the reflection loss due to the change in acoustic impedance at an infinite water‐ice boundary. The other is the interference pattern produced by the coherent return from a finite block face. Measurements of the reflections from a block of natural sea ice, described in a companion paper, showed that the reflection is further reduced by the skeletal layer. More information on the acoustic properties of this layer is needed in order to model its effect. Reflection losses are described by an amplitude reflection coefficient which would be 0.34 due to the impedance mismatch at the water‐ice boundary. Reflection coefficients measured for 1‐ to 2‐m‐thick ice in the Arctic at frequencies 20–80 kHz are compared with measurements by Stanton and Jezek for thin ice. During the Arctic measurements, the return from the back face of the block was also detected, providing information on the absorption of sound in the ice. Field experiments have been designed to separate the absorption in the skeletal and transition layer from the absorption in solid ice. [Work supported by the Office of Navy Technology with technical management by NORDA.]

Spectrophotometers for the Measurement of Light in Polar Ice and Snow

Two portable spectrophotometers have been designed to record light scattering and absorption in polar ice and snow. In the first instrument optical fibers are used to transmit light from the interior of the ice to the spectrophotometer. Such an arrangement allows light measurements up to 2 m away from the instrumentation with minimal disturbance of the natural environment. A miniaturized, submersible spectrophotometer was also built for in situ measurements under floating sea ice. This version, except for the recording apparatus, is entirely self-contained and is housed in a cylindrical tube 9 cm in diameter and 60 cm in length. The unit can be lowered into the ocean through a small borehole in the ice; position and orientation are controlled from the surface. Both spectrophotometers are designed to measure light intensities in the visible spectrum (400-1000 nm). Wavelength resolution is adjustable down to 8 nm at a wavelength of 400 nm, with a field of view of less than 3 degrees . Sensitivities in the pr sent versions are sufficient for measurements through several meters of sea ice with a relative accuracy of 1%. Instrument operation has been tested in the Arctic down to temperatures of -25 degrees C.

Field Measurements of Dielectric Absorption in Antarctic Ice and Snow at Very High Frequencies

AbstractField measurements are presented of dielectric absorption in Antarctic snow and ice at frequencies of a few hundred megahertz. They are compared with measurements by other authors at very high frequencies. The dielectric absorption in ice at these frequencies is accounted for in terms of absorption bands both at radio frequencies and in the infrared. Bands at radio frequencies are caused by a relaxation mechanism which depends upon the temperature and the impurity content of the ice. These two factors are therefore included in an account of the dielectric absorption in ice at very high frequencies.

Field Measurements of Dielectric Absorption in Antarctic Ice and Snow at Very High Frequencies

AbstractField measurements are presented of dielectric absorption in Antarctic snow and ice at frequencies of a few hundred megahertz. They are compared with measurements by other authors at very high frequencies. The dielectric absorption in ice at these frequencies is accounted for in terms of absorption bands both at radio frequencies and in the infrared. Bands at radio frequencies are caused by a relaxation mechanism which depends upon the temperature and the impurity content of the ice. These two factors are therefore included in an account of the dielectric absorption in ice at very high frequencies.