Kim Morris

The thermal conductivity of seasonal snow

Twenty-seven studies on the thermal conductivity of snow ( K eff ) have been published since 1886. Combined, they comprise 354 values of K eff , and have been used to derive over 13 regression equation and predicting K eff vs density. Due to large (and largely undocumented) differences in measurement methods and accuracy, sample temperature and snow type, it is not possible to know what part of the variability in this data set is the result of snow microstructure. We present a new data set containing 488 measurements for which the temperature, type and measurement accuracy are known. A quadratic equation, where ρ is in g cm −3 , and K eff is in W m −1 K −1 , can be fit to the new data ( R 2 = 0.79). A logarithmic expression, can also be used. The first regression is better when estimating values beyond the limits of the data; the second when estimating values for low-density snow. Within the data set, snow types resulting from kinetic growth show density-independent behavior. Rounded-grain and wind-blown snow show strong density dependence. The new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set. This shift is attributed to differences in snow types and sample temperatures in the sets. Using both data sets, we show that there are well-defined limits to the geometric configurations that natural seasonal snow can take.

Snow on Antarctic sea ice

Snow on Antarctic sea ice plays a complex and highly variable role in air-sea-ice interaction processes and the Earths climate system. Using data collected mostly during the past 10 years, this paper reviews the following topics: snow thickness and snow type and their geographical and seasonal variations; snow grain size, density, and salinity; frequency of occurrence of slush; thermal conductivity, snow surface temperature, and temperature gradients within snow; and the effect of snow thickness on albedo. Major findings include large regional and seasonal differences in snow properties and thicknesses; the consequences of thicker snow and thinner ice in the Antarctic relative to the Arctic (e.g., the importance of flooding and snow-ice formation); the potential impact of increasing snowfall resulting from global climate change; lower observed values of snow thermal conductivity than those typically used in models; periodic large-scale melt in winter; and the contrast in summer melt processes between the Arctic and the Antarctic. Both climate modeling and remote sensing would benefit by taking account of the differences between the two polar regions.

Crystal structure, stable isotopes (δ18O), and development of sea ice in the Ross, Amundsen, and Bellingshausen seas, Antarctica

The crystal structure and oxygen isotopic composition of ice cores obtained from floes at the end of summer in the eastern Ross Sea, the Amundsen Sea, and the western Bellingshausen Sea were investigated to determine the ice growth processes and conditions that contribute to sea ice development in the eastern Pacific sector of the southern ocean. The isotope data indicate that a moderate amount of snow contributes to the development of the sea ice. However, even the combined use of isotopes and crystal structure analysis does not unambiguously explain the means by which all of the snow is entrained in the ice. Nevertheless, it seems clear that much of the snow is contained in granular snow-ice that results from seawater flooding of floes and the base of the snow cover. The snow cover in the Ross-Amundsen region was as much as 2 m deep and supported by 7- to 8-m-thick floes primarily composed of frazil ice. In the Bellingshausen region the snow cover and the floes were thinner than in the Ross-Amundsen region. The Bellingshausen cores were composed primarily of multiple layers of frazil and congelation ice. In addition, in both regions there were numerous tipped or inclined blocks of congelation ice and layers of rafted nilas in the cores. The data indicate that the sea ice develops by multiple mechanisms in a turbulent environment.

Structural and stratigraphie features and ERS 1 synthetic aperture radar backscatter characteristics of ice growing on shallow lakes in NW Alaska, winter 1991–1992

Changes in ERS 1 C band synthetic aperture radar (SAR) backscatter intensity (σ°) from ice growing on shallow tundra lakes at three locations in NW Alaska are described. Ice core analysis shows that at all lakes on the coast at Barrow the ice, whether floating or frozen to the bottom, includes an inclusion-free layer overlying a layer of ice with tubular bubbles oriented parallel to the direction of growth. The clear ice may also be overlain by a discontinuous layer of bubbly snow ice. Backscatter is low (-16 to −22 dB) at the time of initial ice formation, probably due to the specular nature of the upper and lower ice surfaces causing the radar pulse to be reflected away from the radar. As the ice thickens during the autumn, backscatter rises steadily. Once the ice freezes to the lake bottom, regardless of the presence of forward scattering tubular bubbles, low backscatter values of-17 to −18 dB are caused by absorption of the radar signal in the lake bed. For ice that remains afloat all winter the ice-water interface and the tubular bubbles combine, presumably via an incoherent double-bounce mechanism, to cause maximum backscatter values of the order of −6 to −7 dB. The σ° saturates at −6 to −7 dB before maximum ice thickness and tubular bubble content are attained. A simple ice growth model suggests that the layer of ice with tubular bubbles need be only a few centimeters thick midway through the growth season to cause maximum backscatter from floating ice. During the spring thaw a previously unreported backscatter reversal is observed on the floating and grounded portions of the coastal lakes but not on the lakes farther inland. This reversal may be related to the ice surface topography and wetness plus the effects of a longer, cooler melt period by the coast. Time series of backscatter variations from shallow tundra lakes are a record of (1) the development of tubular bubbles in the ice and, by association, changes in the gas content of the underlying water and (2) the freezing of ice to the bottoms of the lakes and therefore lake bathymetry and water availability. SAR is also able to detect the onset of lake ice growth in autumn and the initiation of the spring thaw and thus has potential for monitoring high-latitude lake ice growth and decay processes in relation to climate variability.

Seasonal variations in the properties and structural composition of sea ice and snow cover in the Bellingshausen and Amundsen Seas, Antarctica

Sixty-three ice cores were collected in the Bellingshausen and Amundsen Seas in August and September 1993 during a cruise of the R.V. Nathaniel B. Palmer . The structure and stable-isotopic composition ( 18 O/ 16 O) of the cores were investigated in order to understand the growth conditions and to identify the key growth processes, particularly the contribution of snow to sea-ice formation. The structure and isotopic composition of a set of 12 cores that was collected for the same purpose in the Bellingshausen Sea in March 1992 are reassessed. Frazil ice and congelation ice contribute 44% and 26%, respectively, to the composition of both the winter and summer ice-core sets, evidence that the relatively calm conditions that favour congelation-ice formation are neither as common nor as prolonged as the more turbulent conditions that favour frazil-ice growth and pancake-ice formation. Both frazil- and congelation-ice layers have an av erage thickness of 0.12 m in winter, evidence that congelation ice and pancake ice thicken primarily by dynamic processes. The thermodynamic development of the ice cover relies heavily on the formation of snow ice at the surface of floes after sea water has flooded the snow cover. Snow-ice layers have a mean thickness of 0.20 and 0.28 m in the winter and summer cores, respectively, and the contribution of snow ice to the winter (24%) and summer (16%) core sets exceeds most quantities that have been reported previously in other Antarctic pack-ice zones. The thickness and quantity of snow ice may be due to a combination of high snow-accumulation rates and snow loads, environmental conditions that favour a warm ice cover in which brine convection between the bottom and top of the ice introduces sea water to the snow/ice interface, and bottom melting losses being compensated by snow-ice formation. Layers of superimposed ice at the top of each of the summer cores make up 4.6% of the ice that was examined and they increase by a factor of 3 the quantity of snow entrained in the ice. The accumulation of superimposed ice is evidence that melting in the snow cover on Antarctic sea-ice floes ran reach an advanced stage and contribute a significant amount of snow to the total ice mass.

The thickness distribution of sea ice and snow cover during late winter in the Bellingshausen and Amundsen Seas, Antarctica

Data collected from a voyage of RV Nathaniel B. Palmer to the Bellingshausen and Amundsen Seas during August–September 1993 are used to investigate the thickness distribution of sea ice and snow cover and the processes that influence the development of the first-year pack ice. The data are a combination of in situ and ship-based measurements and show that the process of floe thickening is highly dependent on ice deformation; in particular, rafting and ridging play important roles at different stages of floe development. Rafting is the major mechanism in the early stages of development, and core structure data show the mean thickness of individual layers of crystals to be only 0.12 m. Most ice 0.6 m having some surface deformation. Blocks within ridge sails are typically in the range 0.3–0.6 m thick, and ship-based observations estimate approximately 25% of the pack exhibits surface ridging. When corrected for biases in the observational methods, the data show that the dominant ice and snow thickness categories are >0.7 m and 0.2–0.5 m, respectively, and account for 40% and 36% of the surface area of the pack ice. Approximately 8% of the pack is open water. An estimate of the effects of ridging on the distribution of ice mass within the pack suggests that between 50 and 75% of the total mass is contained within the 25% of the pack that exhibits surface ridging.

Structural characteristics of congelation and platelet ice and their role in the development of Antarctic land-fast sea ice

Ice cores were obtained in January 1990 from the land-fast ice in McMurdo Sound for a study of variations in texture, fabric, sub-structure, composition and development. Two primary ice types were observed, congelation and platelet, with a minor amount of frazil ice. Congelation ice growth precedes platelet-ice accretion. Congelation-ice fabrics show frequent moderate to strong alignments, a phenomenon believed to be due to water-current control of selective ice-crystal growth. Platelet ice originates at the base of the congelation ice, initially as a porous latticework of tabular ice crystals which subsequently consolidate by congelation of the interstitial water. Interstitial congelation-ice fabrics generally have little or no alignment, indicating the reduced effect of currents within the platelet latticework prior to solidification. Platelet-crystal textures range from small, wavy-edged forms to large, blade-like forms. Platelet-crystal fabrics indicate that, in addition to being randomly oriented, the platelet latticeworks commonly include many crystals with their flat (0001) faces oriented both parallel and normal to the base of the overlying ice. Plate-width data suggest that the interstitial congelation ice-growth rates remain similar to those of the overlying congelation ice. This effective increase in growth rates probably happens because the latticework of accumulating platelets ahead of the freezing interface ensures that the water within the platelet layer is at the freezing point and less heat has to be removed from platelet-rich water than from platelet-free water for a given thickness of congelation ice to grow. The negative oceanic heat flux associated with platelet-ice formation in McMurdo Sound explains why McMurdo Sound fast ice is thicker than Ross Sea pack ice, and also why it reaches a greater thickness than Arctic fast ice grown in a similar polar marine climate. Plate widths in the McMurdo Sound congelation ice suggest, however, that it grows no faster than Arctic congelation ice.

Ice characteristics and processes, and remote sensing of frozen rivers and lakes

This chapter is a broad overview of ice characteristics and processes on northern rivers and lakes in the time between initial ice formation in autumn and final melt-out in spring. Using primarily synthetic aperture radar (SAR: ERS-1, ERS-2, JERS-1, RADARSAT-1) images supplemented by aerial photographs, passive microwave and Landsat images, and examples from the literature, the aim is to illustrate the potential for remote sensing to contribute to a better knowledge and understanding of river ice and lake ice in the context of their role in northern hydrology and their value as sensitive indicators of northern environmental variability and change. The following seasonal ice characteristics and processes are described: (1) autumn freeze-up; (2) growth, thickening, and grounding; (3) fracturing and motion, including the effect on Mentasta Lake, Alaska, of the magnitude 7.9 earthquake of 3 November 2002; (4) aufeis; and (5) spring break-up. They are followed by a description of perennial ice characteristics, processes, and recent (1997-2002) change on lakes and epishelf lakes in northernmost Ellesmere Island, Nunavut, Canada. The merits of different current and future remote sensing instruments are briefly discussed. We conclude that, as the constellation of increasingly sophisticated spaceborne instruments grows, multisensor remote sensing of frozen rivers and lakes has the potential to contribute more to the study of the role of bodies of water in northern hydrology and of the effects of environmental variability and change on the ice and hydrological system.

Evidence for platelet ice accretion in Arctic sea ice development

Moderate quantities of platelet ice are reported in ice cores obtained from floes in the central and western Beaufort Sea, Arctic Ocean. The crystal structure characteristics include a fabric of long, thin, ragged-edged crystals with randomly oriented c axes. The cellular substructure that is characteristic of congelation ice is not evident in individual platelet ice crystals, but they do occur together with congelation ice in layers ranging from 0.05 to 0.6 m thick. Platelet ice crystals are observed primarily in second-year and multiyear floes. The structural characteristics of Arctic platelet ice are similar to those of Antarctic platelet ice. The platelet ice layers have a minimum δ 18O value of −5.3‰, and in some cases the platelet ice layers have more negative δ 18O values than the ice layers above and below them. The salinity of platelet ice layers is not significantly different from other ice layers. A number of platelet ice growth mechanisms and sources are discussed. Processes associated with the Ellesmere ice shelves, nearshore processes such as anchor ice growth in shallow waters off the north coast of Alaska, and ice growth in freshwater impounded behind grounded ice dams might be locally important but are unlikely to contribute significant quantities of platelet ice to pack ice floes. Two processes probably account for the presence of platelet ice. The first is growth in underice melt ponds which form in inverted depressions at the base of floes when meltwater floes off the ice surface. The second is the operation of ice pumps in which ice that is melted off the deeper parts of floes and ridges is deposited as platelet ice at a higher level. In either case, initially loose accumulations of platelet crystals are entrained in the floes by the growth of congelation ice. The widespread occurrence of platelet ice in the central and western Beaufort Sea suggests that ice growth in both underice melt ponds and associated with the operation of ice pumps might be more common than was previously realized.

Lake ice growth and decay in central Alaska, USA: observations and computer simulations compared

Abstract The Canadian Lake Ice Model (CLIMo), a one-dimensional, thermodynamic model with unsteady heat conduction and penetrating solar radiation, is used to simulate ice growth and decay on shallow ponds in and near Fairbanks, central Alaska, USA. Simulations are compared with observations of ice thickness and composition (snow ice, congelation ice), freeze-up, break-up and duration. Simulations run using US National Weather Service meteorological data as input variables do not agree well with ice-thickness measurements. The simulations improve significantly when the model is run with more representative, locally measured data for air temperature and depth of snow on the ice. The causes of some discrepancies between simulations and observations are discussed and some suggestions for model improvements are made.