Polar Firn Research Articles

Modeling heat, mass, and species transport in polar firn

A finite-element model for simulating multi-dimensional air flow with heat, mass and chemical species transport through firn is discussed. The model is applied to an investigation of near-surface layering effects on ventilation rates. Field measurements of permeability at Summit, Greenland, are presented that show that permeability varies by at least a factor of 10 over the top 3 m, with the surface windpack having much lower permeability, in general, than the underlying firn. The effect of a lower-permeability surface layer is to decrease the air flow in the underlying firn, yet there is still sufficient air flow in the top meters of the firn so that ventilation must be considered for species transport. Channeling, or increased air flow in a layer overlain by a less-permeable layer, can occur even if the microstructure of each layer is isotropic. Conventional estimates of chemical transport due to diffusion alone are likely to underestimate transport, while estimates of ventilation that consider the firn as a homogeneous half-space may overestimate ventilation effects at the near-surface. Effects of firn layering are important for ventilation and must be considered for accurate assessment of firn–air transport mechanisms.

Consistency in long‐term observations of oceans and ice from space

Satellite remote sensing has provided powerful tools for constructing long‐term records of observations of many of Earth's geophysical processes. Geophysicists studying climage change would like to construct time series of data which are as long, accurate, and consistent as possible. While technological advances continue to improve the quality of observations from space, they also present consistency problems. The scanning multichannel microwave radiometer (SMMR), which operated from October 1978 to August 1987, and the special sensor microwave imager (SSMI), which overlapped with the SMMR for nearly 2 months in 1987, have provided microwave radiances which have been used to monitor long‐term changes in sea ice concentration and snow accumulation, among other things. Inspection has shown, however, that brightness temperatures from the two sensors over the same polar firn‐covered scene can differ by as much as 14 K. Calibration corrections for polar firn have been derived; in this paper, we present calculations addressing the physics of the small frequency and viewing angle differences between the SMMR and the SSMI on microwave emission from polar scenes containing free water. We focus here only on surface emission. We use a surface scattering model and study the effects of surface roughness, snow wetness, snow density, and young sea ice concentration. We also compare data from Antarctica with our modeled results. We find that for most scenes, surface roughness dominates over the effects of frequency and viewing angle differences, but that for open ocean long‐term time series, data must be calibrated and constructed at the geophysical product level rather than at the level of measured radiances.

Air flow and dry deposition of non-sea salt sulfate in polar firn: Paleoclimatic implications

Non-sea salt sulfate aerosol (NSS) is an important factor for the Earth's albedo because it backscatters solar radiation and is the major cloud condensation nucleus over oceans. At Vostok, Antarctica, NSS concentration shows an increase in glacial period ice of 20–46% that cannot be attributed to changes in accumulation rate. The additional NSS may be due to enhanced dry deposition of NSS by topographic windpumping during the windy glacial periods. We model the volume flux of air into snow due to barometric pressure changes and air flow over surface microrelief. The Gormley-Kennedy equation approximately describes how aerosols advected into the snow pack are removed from the air by diffusion to the snow matrix. Barometric pressure and wind speed data from several polar sites have been used to quantify the vertical volume flux of air and mass flux of NSS. Model results indicate that air flow over small sastrugi, wind carved snow dunes commonly found on ice caps, is the dominant dry deposition mechanism for NSS. Paleo wind speed and surface roughness can significantly influence the aerosol record in ice cores.

Gravitational Separation in Polar Firn

Gravitational Separation in Polar Firn

Crystal growth rates in polar firn

Continuous crystal-size measurements made on the G6 Antarctic ice core (100m deep) show enhanced growth rates above a depth of 30 m (Zone 1) and in the interval between 70 and 80 m (Zone 2). Crystal growth in Zone 1 most probably takes place by a process of sublimation and condensation. The higher growth rate in Zone 2 is most probably related to the pore close-off transformation process in which a non-uniform strain field is created to form air bubbles by plastic deformation and “cannibalization” of individual ice crystals.

The depth of penetration in Antarctic firn at 10 GHz

Measurements taken by an X-band pulse radar system in East Antarctica in 1987 were analyzed to determine the depth of penetration of radar energy in polar firn. A minimum estimate of the penetration depth at 10 GHz for the cold and dry polar plateau region is approximately 4.7 m. Spatial variations in the amount of signal penetration were observed and are related to latitude, surface elevation, and mean annual temperature. The amount of signal penetration and its spatial variation are important factors that should be considered when processing datasets from microwave remote sensing systems that operate in the polar regions. >

Crystal growth rates in polar firn

Continuous crystal-size measurements made on the G6 Antarctic ice core (100m deep) show enhanced growth rates above a depth of 30 m (Zone 1) and in the interval between 70 and 80 m (Zone 2). Crystal growth in Zone 1 most probably takes place by a process of sublimation and condensation. The higher growth rate in Zone 2 is most probably related to the pore close-off transformation process in which a non-uniform strain field is created to form air bubbles by plastic deformation and “cannibalization” of individual ice crystals.

Some numerical experiments on firn ventilation with heat transfer

Preliminary estimates of the thermal signature of ventilation in polar firn are obtained from two-dimensional numerical calculations. The simulations show that spatially varying surface pressure can induce airflow velocities of 10−5m s−1at 1.5 m depth in uniform firn, and higher velocities closer to the surface. The two-dimensional heat-transfer results generally agree with our earlier one-dimensional conclusions that the thermal effects of ventilation tend to decrease the temperature gradient in the top portions of the pack. Field observations of ventilation through temperature measurements are most likely to be observed when the firn temperature at depths on the order of 10 m is close to the air temperature, since steep temperature gradients can mask the thermal effects of ventilation. Preliminary indications are that, as long as surface-pressure amplitude is sufficient to move the air about in the top tens of centimeters in the snow, the resulting temperature profile during ventilation is fairly insensitive to the frequency of the surface-pressure forcing for pressure frequencies in the range 0.1–10.0 Hz.

Some numerical experiments on firn ventilation with heat transfer

Preliminary estimates of the thermal signature of ventilation in polar firn are obtained from two-dimensional numerical calculations. The simulations show that spatially varying surface pressure can induce airflow velocities of 10−5 m s−1 at 1.5 m depth in uniform firn, and higher velocities closer to the surface. The two-dimensional heat-transfer results generally agree with our earlier one-dimensional conclusions that the thermal effects of ventilation tend to decrease the temperature gradient in the top portions of the pack. Field observations of ventilation through temperature measurements are most likely to be observed when the firn temperature at depths on the order of 10 m is close to the air temperature, since steep temperature gradients can mask the thermal effects of ventilation. Preliminary indications are that, as long as surface-pressure amplitude is sufficient to move the air about in the top tens of centimeters in the snow, the resulting temperature profile during ventilation is fairly insensitive to the frequency of the surface-pressure forcing for pressure frequencies in the range 0.1–10.0 Hz.

Active and passive microwave signatures of Antarctic firn by means of field measurements and satellite data

The angular dependence and polarization behaviour of back-scattering and emission of polar firn at 5.2 GHz and 10.3 GHz were measured during an oversnow traverse in Dronning Maud Land, Antarctica. The signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements. Highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast. In permanently dry snow, areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities. Pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like-polarized backscattering coefficients. This behaviour is confirmed by the analysis of C-band scatterometer measurements of the Active Microwave Instrument aboard the European Space Agency's ERS-1.

Transformation process observations of polar firn to ice

The transformation of snow to ice under compressive stress is a very slow process unless meltwater is present. A qualitative measure of permeability variation with depth was continuously determined for various shallow-ice cores from the dry-snow zones in both Greenland and Antarctica to investigate this transformation process. Results of the study provide insight into snow densification mechanisms and generally characterize the pore close-off phenomena. Measurements show that bulk densities may be expressed as a function of overburden; pore close-off takes place when the overburden is approximately 3.0–4.5 bar.

Active and passive microwave signatures of Antarctic firn by means of field measurements and satellite data

The angular dependence and polarization behaviour of back-scattering and emission of polar firn at 5.2 GHz and 10.3 GHz were measured during an oversnow traverse in Dronning Maud Land, Antarctica. The signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements. Highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast. In permanently dry snow, areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities. Pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like-polarized backscattering coefficients. This behaviour is confirmed by the analysis of C-band scatterometer measurements of the Active Microwave Instrument aboard the European Space Agency's ERS-1.

Transformation process observations of polar firn to ice

The transformation of snow to ice under compressive stress is a very slow process unless meltwater is present. A qualitative measure of permeability variation with depth was continuously determined for various shallow-ice cores from the dry-snow zones in both Greenland and Antarctica to investigate this transformation process. Results of the study provide insight into snow densification mechanisms and generally characterize the pore close-off phenomena. Measurements show that bulk densities may be expressed as a function of overburden; pore close-off takes place when the overburden is approximately 3.0–4.5 bar.

Origins and variations of nitrate in south polar precipitation

South polar firn cores spanning the last millennium have been analyzed to determine the nitrate background level of high‐latitude precipitation and its temporal variations. The resulting data reveal no evidence of a positive correlation between solar activity (11‐year solar cycle, low solar activity time periods, and solar proton events) and the NO3 content of south polar snow. These data therefore suggest that NOx production in the upper stratosphere, mesosphere, and thermosphere does not contribute significantly to the antarctic NO3 budget. This study of the NO3 content of high latitude precipitation suggests a major contribution by lightning (from a third to a half of the total) and by NOx produced in the lower stratosphere (approximately a third from N2O oxidation and to a lesser extent galactic cosmic rays) to the NO3 budget of this background atmosphere, the remaining portion being related to the present NOx surface sources of the southern hemisphere. For the first time, our data point to a decrease of NO3 content when very large amounts of sulfuric acid are present in South Pole snow layers. This observation suggests a possible interaction (via the hydroxyl radical) between S‐ and N‐derived species when large quantities of SO2 are injected by volcanic eruptions. Although the deposition mechanism of HNO3 in South Pole snow layers is far from well understood, our data suggest that only a minor amount of this compound is deposited directly on surface snow layers. Finally, the role played by particles (volcanic ash, terrestrial impurities, or ice particles) to enhance the uptake of odd nitrogen from the atmosphere by heteregeneous processes is discussed. In particular, it is suggested that in late winter, under certain meteorological conditions leading to ice particle formation, a significant uptake of HNO3 from the lower stratosphere can occur.

Glacier and Permafrost Signals of 20th-Century Warming

Currently-available evidence of 20th-century warming from glaciers and permafrost is briefly reviewed. The signals are clear and strong: warming of polar firn and permafrost, and mass losses of glaciers at lower latitudes, were most striking towards the middle of the century. The easily observable length-reduction of mountain glaciers confirms the global character of the evolution. A probably intermittent reversal of the trend was observed in places after about 1950.

Glacier and Permafrost Signals of 20th-Century Warming

Currently-available evidence of 20th-century warming from glaciers and permafrost is briefly reviewed. The signals are clear and strong: warming of polar firn and permafrost, and mass losses of glaciers at lower latitudes, were most striking towards the middle of the century. The easily observable length-reduction of mountain glaciers confirms the global character of the evolution. A probably intermittent reversal of the trend was observed in places after about 1950.

Concerning the Deposition and Diagenesis of Strata in Polar Firn

Abstract Depth hoar in polar firn forms when large temperature gradients act on low-density firn, but high-density firn does not develop into depth hoar. Low densities in firn may be depositional (burial of surface hoar or still-air snowfall) or diagenetic (mass loss to the free atmosphere, typically in autumn); however, diagenesis is sufficiently strong to cause significant mass loss only in the top 50–100 mm of firn. Between about 50–100 mm and 2 m depth, grain growth and densification are accelerated strongly by temperature-gradient effects; from 2 to 5 m, temperature gradients have a small but measurable effect on rates of processes in firn, and below 5 m rates essentially have isothermal values. Diagenetic depth-hoar layers typically develop in the autumn, are relatively thick, and have smooth bases. Depositional depth-hoar layers may develop at any season, are relatively thin, and have irregular bases. In low-accumulation regions, visual stratification may preserve only an annual signal, but in high-accumulation regions individual storms or other features may be recognizable.

Concerning the Deposition and Diagenesis of Strata in Polar Firn

AbstractDepth hoar in polar firn forms when large temperature gradients act on low-density firn, but high-density firn does not develop into depth hoar. Low densities in firn may be depositional (burial of surface hoar or still-air snowfall) or diagenetic (mass loss to the free atmosphere, typically in autumn); however, diagenesis is sufficiently strong to cause significant mass loss only in the top 50–100 mm of firn. Between about 50–100 mm and 2 m depth, grain growth and densification are accelerated strongly by temperature-gradient effects; from 2 to 5 m, temperature gradients have a small but measurable effect on rates of processes in firn, and below 5 m rates essentially have isothermal values.Diagenetic depth-hoar layers typically develop in the autumn, are relatively thick, and have smooth bases. Depositional depth-hoar layers may develop at any season, are relatively thin, and have irregular bases. In low-accumulation regions, visual stratification may preserve only an annual signal, but in high-accumulation regions individual storms or other features may be recognizable.

A Pressure-sintering Model for the Densification of Polar Firn and Glacier Ice

AbstractA comprehensive multi-mechanism theory of pressure sintering has been applied to the densification of two polar ice sheets. The comparison, which is made using pressure-sintering mechanism maps, indicates that power-law creep is the controlling mechanism between 50% and 98% theoretical density. Lattice diffusion becomes dominant at low porosities. The densification rates predicted by the theory are in good agreement with the data, and suggest that a reasonable estimation of the densification behaviour of a polar ice sheet can be made using the theory, based on information obtained from a relatively shallow core.

A Pressure-sintering Model for the Densification of Polar Firn and Glacier Ice

Abstract A comprehensive multi-mechanism theory of pressure sintering has been applied to the densification of two polar ice sheets. The comparison, which is made using pressure-sintering mechanism maps, indicates that power-law creep is the controlling mechanism between 50% and 98% theoretical density. Lattice diffusion becomes dominant at low porosities. The densification rates predicted by the theory are in good agreement with the data, and suggest that a reasonable estimation of the densification behaviour of a polar ice sheet can be made using the theory, based on information obtained from a relatively shallow core.