Ice Thickness Values Research Articles

Modelling of Ocean Shallow Convection under Fast Ice in the Arctic

Ocean convection in the Antarctic has been studied many times and has been revealed to be responsible for ice-cover reduction. In the Arctic, proof of that phenomenon has not been documented. It is believed that this phenomenon happens on a smaller scale in the Arctic when local circulation of deep warmer water melts and slows ice production. An example of this is the North Water (NOW) polynya in northern Baffin Bay. A polynya is an area of open water in an otherwise ice-covered area. As ice forms under the fast ice near the boundary of the polynya, ocean salts (brine) are ejected from the newly formed ice. This water, which has an increased concentration of salt, sinks and is replaced by warmer water from below, and this slows ice formation. In our study a coupled one-dimensional thermodynamic snow–fast ice model incorporating ocean heat flux input via a shallow convection model was used. Ice thickness was calculated using a thermodynamic model that included a current-induced entrainment model and a convection model to account for brine rejection during ice growth. Atmospheric observations from Grise Fiord and Thule and ocean profiles around the NOW polynya near these sites were used as input to the model. This purely thermodynamic study enables us to obtain ice thickness values that can be compared with qualitative observations. This modelling study compares two sites related to the NOW polynya. The results indicate that the shallow convection model simulates the reduction of fast ice near Thule but not near Grise Fiord.

Sea Ice Surface Temperature Estimation Using MODIS and AMSR-E Data Within a Guided Variational Model Along the Labrador Coast

In this study, a new method, entitled as the multi-modality guided variational (MGV) method, is proposed, in which the data from a passive microwave sensor is used jointly with the data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to estimate the sea ice surface temperature (IST). The method augments existing sea IST values from the MODIS IST map, while filling in areas in the MODIS image that may be sparsely sampled due to the cloud cover, or due to increased spacing between the pixels at the swath edges. The former issue is particularly problematic in the marginal ice zone, where the atmospheric conditions often lead to persistent cloud cover. The sea IST is of interest because it can be used to estimate the sea ice thickness, an important parameter for shipping, climate change, and weather forecasting applications. The impact of the MGV method is checked through a comparison between the sea ice thickness calculated using the swath surface temperature and that calculated using the surface temperature from MGV. Using the operational ice charts as a guideline, it is found that the sea ice thickness values calculated using the MGV surface temperature are realistic, and there is a 16% increase in the number of sea ice thickness data points available when the MGV method is used as compared to when the swath data are used.

An Assessment of Sea-Ice Thickness Along the Labrador Coast From AMSR-E and MODIS Data for Operational Data Assimilation

In this paper, sea-ice thickness values are calculated along the Labrador coast using data from two sensors representative of those available for operational data assimilation. Data from the first sensor, the Moderate Resolution Imaging Spectroradiometer (MODIS), are used to calculate the ice thickness using a heat balance equation. Relationships between the MODIS ice thickness and polarization ratio from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) are used to calculate the thickness of thin ice (less than 0.2 m) from the AMSR-E data. This is done for each frequency on the AMSR-E sensor in the range of 6.9-36.5 GHz. Through comparison with data from ice charts, it is found that the errors are lowest for thickness values calculated from low-frequency AMSR-E data. The accuracies of the ice thickness from MODIS, AMSR-E, operational ice charts, and two moored upward looking sonars are further assessed using the triple collocation method. It is found that the error associated with ice thickness from AMSR-E is the lowest and the error associated with ice thickness from MODIS is the highest. While the MODIS data represent the small-scale variability of the sea-ice thickness better than the AMSR-E data, the MODIS data can produce spurious values of ice thickness due to unmasked clouds. To use ice thickness from MODIS in an automated algorithm, quality control would need to be applied to the MODIS data to remove unmasked clouds which lead to spurious values of thick ice. The errors calculated for the ice thickness from AMSR-E, which are calculated based on a relationship calibrated with MODIS ice thickness from a clear-sky day, indicate that these data would be useful for operational data assimilation.

Ice volume distribution and implications on runoff projections in a glacierized catchment

Abstract. A dense network of helicopter-based ground-penetrating radar (GPR) measurements was used to determine the ice-thickness distribution in the Mauvoisin region. The comprehensive set of ice-thickness measurements was combined with an ice-thickness estimation approach for an accurate determination of the bedrock. A total ice volume of 3.69 ± 0.31 km3 and a maximum ice thickness of 290 m were found. The ice-thickness values were then employed as input for a combined glacio-hydrological model forced by most recent regional climate scenarios. This model provided glacier evolution and runoff projections for the period 2010–2100. Runoff projections of the measured initial ice volume distribution show an increase in annual runoff of 4% in the next two decades, followed by a persistent runoff decrease until 2100. Finally, we checked the influence of the ice-thickness distribution on runoff projections. Our analyses revealed that reliable estimates of the ice volume are essential for modelling future glacier and runoff evolution. Wrong estimations of the total ice volume might even lead to deviations of the predicted general runoff trend.

Modeling of glacier bed topography from glacier outlines, central branch lines, and a DEM

Due to the expected future climate change, glacier ice as a resource will be further diminished and its sea-level rise contribution further increased. A key for a more accurate determination of future glacier evolution is to improve our currently sparse knowledge on glacier bedrock topography. Here, we present a simplified method implemented in a geographic information system to approximate subglacial topography at a regional scale from spatial interpolation of local ice thickness values. The latter were derived from an ice-dynamical approach, which relates glacier thickness to its local surface slope, total vertical extent, and basal shear stress. The only input data required are glacier outlines, a set of central branch lines, and a digital elevation model (DEM). The modeled glacier beds are in good agreement with other, more complex modeling approaches and direct measurements. The observed local deviations can be explained with the sensitive dependence of our approach on the surface slope and the different methods used for spatial interpolation. The mean glacier thickness derived here for a sample of 40 glaciers in the Swiss Alps is slightly smaller than that derived from a similar, but scalar approach. Further adjustments of the data processing are possible to achieve agreement with available validation data. However, despite its local inaccuracies, the resulting DEM without glaciers can facilitate several applications such as the detection of overdeepenings or the modeling of future glacier evolution.

Estimation of sea ice thickness distributions through the combination of snow depth and satellite laser altimetry data

Combinations of sea ice freeboard and snow depth measurements from satellite data have the potential to provide a means to derive global sea ice thickness values. However, large differences in spatial coverage and resolution between the measurements lead to uncertainties when combining the data. High‐resolution airborne laser altimeter retrievals of snow‐ice freeboard and passive microwave retrievals of snow depth taken in March 2006 provide insight into the spatial variability of these quantities as well as optimal methods for combining high‐resolution satellite altimeter measurements with low‐resolution snow depth data. The aircraft measurements show a relationship between freeboard and snow depth for thin ice allowing the development of a method for estimating sea ice thickness from satellite laser altimetry data at their full spatial resolution. This method is used to estimate snow and ice thicknesses for the Arctic basin through the combination of freeboard data from ICESat, snow depth data over first‐year ice from AMSR‐E, and snow depth over multiyear ice from climatological data. Due to the nonlinear dependence of heat flux on ice thickness, the impact on heat flux calculations when maintaining the full resolution of the ICESat data for ice thickness estimates is explored for typical winter conditions. Calculations of the basin‐wide mean heat flux and ice growth rate using snow and ice thickness values at the ∼70 m spatial resolution of ICESat are found to be approximately one‐third higher than those calculated from 25‐km mean ice thickness values.

Observations of sea ice thickness, surface roughness and ice motion in Amundsen Gulf

Ice thickness and surface roughness measurements of first‐year (FY) sea ice were collected with a fix‐mounted helicopter‐borne electromagnetic (HEM) ‐laser system in Amundsen Gulf in April to May 2004. The modal ice thickness values are in good qualitative agreement with different ice types identified in synthetic aperture radar (SAR) imagery and shown on ice charts produced by the Canadian Ice Service. Modal ice thickness values which generally represent level ice thicknesses were about 2.0 m over landfast ice. A large range of modal ice thicknesses was observed in the mobile ice region, with values of about 0.2 m (young ice) in leads (where there was high radar backscatter), 0.6 m (thin FY ice) in the polynya (where there was medium to high backscatter), and about 1.1–1.9 m (thick FY ice) elsewhere. High surface roughnesses are strongly associated with high radar backscatter in SAR imagery, and are observed in areas of large shear. The ratio of the standard deviations of ice draft and averaged roughness in an area of landfast ice is in good agreement with the ratio of the standard deviations of ice draft and ice‐equivalent roughness expected from isostasy, with constant level ice and snow thickness. However, the standard deviation of ice‐equivalent roughness may be significantly underestimated, due to differences in snow thickness between level and deformed ice, and limitations of the laser processing method. Modal ice (plus snow) thicknesses measured with the HEM system are within the range of historical values measured at Cape Parry.

Keel depths of modern Antarctic icebergs and implications for sea-floor scouring in the geological record

Icebergs affect the geological record through the scouring or ploughing action of their keels, reworking sediments where they contact the sea floor. Satellite radar-altimeter measurements of the surface elevation of the marine margins of the Antarctic Ice Sheet are inverted to produce ice-thickness values. These data are assumed to represent the keel depths of newly-calved icebergs. There are three peaks in the frequency distribution of Antarctic iceberg–keel depths: (a) about 140–200 m in thickness, associated with smaller ice shelves fringing Antarctica (e.g. Wilkins, George VI and Brunt ice shelves); (b) about 250–300 m, from large ice shelves (e.g. Ross, Ronne, Amery); and (c) approximately 500–600 m, from fast-flowing ice-sheet outlet glaciers (e.g. Pine Island and Dibble glaciers), together with the Filchner Ice Shelf. Controls on iceberg keel depth are ice thickness at the grounding line, creep thinning and the rate of melting/freezing at the floating ice-shelf base with distance to the calving margin. The distribution of sea-floor scours produced by iceberg-grounding is dependent on the changing form and flow of the glaciers and ice sheets from which icebergs are derived and on berg drift tracks and melt rates. Sea-floor evidence for icebergs with particularly deep keels has sometimes been interpreted to indicate the presence of extensive former ice shelves. However, the largest modern Antarctic ice shelves do not produce large numbers of icebergs thicker than about 350 m, implying that sea-floor scours found in deeper water are unlikely to be an indicator of extensive past ice-shelf development. Iceberg scours found at water depths in excess of about 500 m in the geological record are, instead, probably indicative of either: (a) the former presence of fast-flowing ice-sheet outlet glaciers; (b) ice shelves fed from major interior basins where lateral spreading is constrained topographically; or (c) a pulse of large icebergs released during major deglaciation or ice-sheet collapse.

Atmospheric conditions and fast ice at Davis, East Antarctica: A case study

We present in situ observations of atmospheric and fast‐ice parameters at Davis (68°35′S, 77°58′E), East Antarctica. Surface measurements from 1969–2003 reveal negative trends for summer and autumn surface‐air temperatures, yet positive annual, winter and spring trends. Mean sea level pressure decreased over the observational record both annually (−0.10 hPa yr−1, significant at the 90% confidence level (90)) and seasonally. We mainly associate this with increased cyclonic activity. These trends reflect large‐scale circulation changes identified by others in the troposphere and lower stratosphere. Temporal changes in the Davis landfast‐ice characteristics are evaluated using in situ measurements from the late 1950s together with intermittent data from 1979 onward. There is large interannual variability in fast‐ice characteristics, which peaked during the 1990s. Mean values of annual maximum ice thickness taken over subintervals have changed little. However, dates of annual maximum ice thickness and final fast‐ice breakout are delayed (each by +0.43 d yr−1). Delays in ice breakout contribute to a prolonged persistence of the fast ice (+0.67 d yr−1, (90)). Linear regression was applied to relate changes in fast‐ice characteristics to those in the atmosphere. Significant at the 90% level or above are: Winter temperature and annual maximum ice thickness are anticorrelated (R2 = −0.69); the tendency toward later dates of annual maximum ice thickness appears influenced by winter (R2 = +0.47) and spring (R2 = +0.42) warming. Interannual changes in spectral density of the (annual and summer, respectively) mean sea level pressure within the cyclone‐active frequency band are crucial to annual maximum ice thickness (R2 = −0.54) and fast‐ice breakout (R2 = −0.59). Maximum ice thickness is furthermore influenced by winter wind speed (R2 = −0.52).

Retrieval of thin-ice thickness using the L-band polarization ratio measured by the helicopter-borne scatterometer Heliscat

AbstractClimate warming makes an increasing thin-ice fraction likely to occur in the Arctic, underpinning the need for its regular observation. synchronous helicopter-borne measurements of the sea-ice thickness and like-polarized L-band radar backscatter carried out along identical flight tracks north of svalbard during winter are combined to develop an algorithm to estimate the thin-ice thickness solely from the L-band backscatter co-polarization ratio (LCPR). Airborne ice-thickness and LCPR data are smoothed along track (to reduce noise), co-located and compared. A linear and a logarithmic fit are applied using thickness values between 0.0 and 0.6m and 0.0 and 1.0 m, respectively. The thin-ice thickness is derived from the LCPR data using these fits, first for dependent data (used to obtain the fits) and subsequently for independent data. The results are compared to airborne ice-thickness measurements for ice-thickness values between 0.0 and 0.6m using linear regression. The logarithmic fit gives the most reliable results, with a correlation of 0.72 and a rms difference of 8 cm. It permits us to derive the thickness of thin ice (below 50–60cm thickness) from airborne LCPR data with an uncertainty of about 10 cm.

Simulation of the Martian Atmosphere Using a CCSR/NIES AGCM

The new simulation of Martian general circulation based on CCSR/NIES AGCM has been performed and the numerical results analyzed. We attempt to reproduce the atmospheric states of Mars which havebeen observed by the Mars Global Surveyor, and Viking by introducing three kinds of time- and latitude- dependent dust opacity scenarios which are made to be consistent with the past observational results, as the observed dust distribution during spring and summer in south hemisphere varies largely at eachyear and so does observed atmospheric conditions. The model with TES2 dust scenario, which is the dustdistribution based on the observation by the Mars Global Surveyor in 1999, generally reproduces distributions of temperature and zonal wind observed by the Mars Global Surveyor in 1999, though the temperature tends to be lower at high altitude. The maximum value of CO2 ice thickness at polar regions relatively reproduces the observation, but the edge of the northern and southern seasonal caps are outside of the observational results during spring, which results in lower surface pressure especially in northern spring. Annual variations of the amplitudes of the diurnal and semidiurnal tide are qualitatively comparable between our model with VIK1 dust scenario, which is the dust distribution based on the observation by Viking Orbiter in 1977, and observation by Viking Lander 1 in 1977. The baroclinic waves with consistent periods, wavenumbers and phase speeds with observational results by Viking Lander 2 in 1977 are reproduced in the autumn, though in the winter, during the second dust storm, and in the spring, the phase speeds are faster than observation.

Characteristics of Ice Storms in the United States

Freezing rainstorms in the United States during 1949–2000 resulted in 87 catastrophic events, storms causing property losses of more than $1 million, with resulting losses totaling $16.3 billion. Catastrophes and their losses were greatest in the northeast, southeast, and central United States, and only 3% occurred in the western United States. A greater percentage of the freezing-rain occurrences in the deep South produced catastrophes than did freezing-rain occurrences in the northern United States, a result of differences in storm-producing conditions. A dense network of observers who measured ice storms during a 9-yr period provided definitive data on damaging storm areas and ice thicknesses on wires. The geographic distribution of the 368 damaging–ice storm areas matched well with that of catastrophes; both were greatest in a belt from the Southwest, across the Midwest, and into New England. Storm-area sizes ranged from 205 to 796 000 km2, with 50% being less than 21 840 km2. Most storm-area shapes were elongated with length–width ratios from 2:1 up to 15:1, although 18% were oblate shaped, particularly those in the northern High Plains and upper Midwest. Two-thirds of the ice-storm areas and catastrophe loss patterns assumed one of seven spatial types. The most prevalent type was a storm restricted to the Northeast, the second had losses in the Midwest and Northeast, and third was a storm in both the Northeast and Southeast regions. Five large-sized catastrophes extended from the Southwest, through the Midwest, into the Northeast, and these caused $3.5 billion in losses, 21% of the 52-yr total. The radial thickness of damaging ice on telegraph wires was sampled at 1689 sites, and the greatest average (>1.3 cm) and maximum (>5 cm) sizes occurred in the deep South and southern plains where storms have long durations and ample moisture aloft. Maximum and average ice thickness were relatively large in New England and the Northeast. Ice-thickness values were least in the upper Midwest and Pacific Northwest. The results collectively show that the greatest risk of ice-storm damages (based on event frequency, ice thickness, storm size and shape, and financial loss) is in the northeastern United States, followed by the risk in the lower Midwest and that in the southern United States.

Late surge glacial conditions on Bakaninbreen, Svalbard, and implications for surge termination

Bakaninbreen is a polythermal glacier in southern Spitsbergen, Svalbard, that last surged between 1985 and 1995. Seismic reflection data were acquired during early quiescence in spring 1998, just upstream of the surge front. The results were combined with complementary ground‐penetrating radar data to investigate the glacial structure and basal conditions. We find no difference between the ice thickness values determined from the seismic and radar methods, suggesting that any layer of basal ice cannot be greater than ∼5 m thick. Interpretation of the amplitude of the seismic reflections indicates the presence of permafrost close to the glacier base. A thin layer of thawed deforming sediment separates the glacier from this underlying permafrost. In an area just upstream of the surge front the permafrost becomes discontinuous and may even be absent, the ice being underlain by 10–15 m of thawed sediments overlying deeper bedrock. High‐pressure water is believed to have been required to maintain the propagation of the surge, and this area of thawed sediment is interpreted as a route for that water to escape from the basal system. When the surge front passed over this thawed bed, the escaping water reduced the pressure in the subglacial hydraulic system, initiating the termination of the surge. Surge termination was therefore primarily controlled by the presurge permafrost distribution beneath the glacier, rather than any feature of the surge itself. This termination mechanism is probably limited to surges in polythermal glaciers, but the techniques used may have wider glaciological applications.

Video Monitoring of Nearshore Ice in Southern Lake Michigan

We set up a time-lapse video camera system on the southwestern shore of Lake Michigan for 16 days in January and February 1991 to monitor ice conditions. The system was used to determine longshore and cross-shore ice-drift rates in a zone of mobile brash and slush ice on 11 days. Daily mean longshore ice drift was to the southeast on 10 of these days at speeds ranging between 0.047 and 0.377 m s −1 Mean cross-shore ice drift was always offshore and varied between 0.009 and 0.126 m s −1. Shear and strain were rare in the brash/slush zone. On 6 of the days when drift rates were determined, we were also able to determine the width of the brash/slush zone. Using these data along with published values of ice thickness, ice concentration, and sediment concentration in ice, we were able to estimate longshore and cross-shore sand fluxes and longshore sand transport rates. The longshore and cross-shore ice-induced sand fluxes ranged from 0.008 to 0.760 t hr −1 and 0.0016 to 0.096 t hr −1, respectively. Net ice-induced longshore sand transport was to the south at rates of 1.8 to 83.7 t hr −1. Measured cross-shore sand flux was consistently offshore. The calculated fluxes indicate that large amounts of sand are moved alongshore and offshore by ice in southernwestern Lake Michigan during the winter months.

Interannual Variability of Landfast Ice Thickness in the Canadian High Arctic, 1950-89

A physical one-dimensional heat transfer model of fast ice growth was used to investigate the interannual variability of maximum fast ice thickness at four sites in the High Arctic over the period 1950-89. The insulating role of snow cover was found to be the most important factor, explaining 30-60% of the variance in maximum ice thickness values. Other snow-related processes such as slushing and density variations were estimated to explain a further 15-30% of the variance. In contrast, annual variation in air temperatures explained less than 4% of the variance in maximum ice thickness. No evidence was found for the systematic ice thinning trend anticipated from greenhouse gas-induced global warming. However, recent ice thinning and thickening trends at two sites (Alert and Resolute) are consistent with changes in the average depth of snow covering the ice and may be explained by changes in cyclone frequencies. A response surface sensitivity analysis following Fowler and de Freitas (1900) indicated the High Arctic landfast ice regime would be more sensitive to air temperature variations under a warmer, snowier environment.Key words: landfast ice, snow, interannual variability, climate change, Canadian High Arctic

A Survey of the Vanderford and Adams Glaciers in East Antarctica (Abstract)

A comprehensive, airborne survey of the Vanderford and Adams glaciers was started in January 1983, continued through the austral summer season 1984/5, and completed in February 1985. Ice-thickness and surface-elevation data were collected over some 4500 square kilometres, on a grid spacing of approximately 5 kilometres. The measurement system was based on a Bell 206 helicopter, fitted with ANARE 100 MHz ice radar, Motorola Mini-Ranger navigation equipment, and a digital, pressure altimeter. A JMR, satellite, doppler receiver was used to position the navigation ground stations precisely. Gravity measurements were used to fill in ice-thickness coverage, where the ice radar failed to produce an echo and also to help determine where the glacier was floating. Ice-movement profiles were measured across the front sections of the glaciers and additional spot values were obtained further upstream by utilizing the 3 m accuracy of the navigation equipment to locate markers quickly at both the beginning and end of the season’s work. A data logger in the helicopter recorded time, navigation distances, aircraft to ground clearance, and air pressure, at 10 second intervals. These data were later merged with manually-scaled, ice-thickness values, for computer processing. The results show that the Vanderford glacier dominates the system and drains about 5 cubic kilometres of ice per annum, mainly from the inland ice sheet to the south. Ice flowing into the Adams Glacier tends to come from nearer the coast and to the south and west of the glacier. Bedrock topography beneath the Vanderford shows that the deep, inland trench, similar to that found below other outlet glaciers, drops to 2500 m below sea level, 60 kilometres from the front. The trench has steep sides to the east and gives a clearly-defined edge to the fast glacier flow. The western side, however, is much more complicated, particularly further inland, where the flow is not clearly separate from that of the Adams glacier.

A Survey of the Vanderford and Adams Glaciers in East Antarctica (Abstract)

A comprehensive, airborne survey of the Vanderford and Adams glaciers was started in January 1983, continued through the austral summer season 1984/5, and completed in February 1985.Ice-thickness and surface-elevation data were collected over some 4500 square kilometres, on a grid spacing of approximately 5 kilometres.The measurement system was based on a Bell 206 helicopter, fitted with ANARE 100 MHz ice radar, Motorola Mini-Ranger navigation equipment, and a digital, pressure altimeter. A JMR, satellite, doppler receiver was used to position the navigation ground stations precisely. Gravity measurements were used to fill in ice-thickness coverage, where the ice radar failed to produce an echo and also to help determine where the glacier was floating.Ice-movement profiles were measured across the front sections of the glaciers and additional spot values were obtained further upstream by utilizing the 3 m accuracy of the navigation equipment to locate markers quickly at both the beginning and end of the season’s work.A data logger in the helicopter recorded time, navigation distances, aircraft to ground clearance, and air pressure, at 10 second intervals. These data were later merged with manually-scaled, ice-thickness values, for computer processing.The results show that the Vanderford glacier dominates the system and drains about 5 cubic kilometres of ice per annum, mainly from the inland ice sheet to the south. Ice flowing into the Adams Glacier tends to come from nearer the coast and to the south and west of the glacier. Bedrock topography beneath the Vanderford shows that the deep, inland trench, similar to that found below other outlet glaciers, drops to 2500 m below sea level, 60 kilometres from the front. The trench has steep sides to the east and gives a clearly-defined edge to the fast glacier flow. The western side, however, is much more complicated, particularly further inland, where the flow is not clearly separate from that of the Adams glacier.

Tabular Icebergs: Implications from Geophysical Studies of Ice Shelves

AbstractRecent geophysical and glaciological investigations of the Ross Ice Shelf have revealed many complexities in the ice shelf that can be important factors in iceberg structure. The presence of rift zones, surface and bottom crevasses, corrugations, ridge/troughs, and other features could substantially modify the hydraulics of iceberg towing and lead to disintegration in the course of transport.The relationships between the elevation above sea-level and total ice thickness for three ice shelves (Ross, Brunt, and McMurdo) are given; from them, expressions for the thickness/freeboard ratios of tabular icebergs calved from these ice shelves are obtained. The relationships obtained from the measured values of surface elevation and ice thickness are in agreement with models derived assuming hydrostatic equilibrium.Areas of brine infiltration into the Ross Ice Shelf have been mapped. Examples of radar profiles in these zones are shown. Absorption from the brine layers results in a poor or absent bottom echo. It is probable that little saline ice exists at the bottom of the Ross Ice Shelf front due to a rapid bottom melting near the ice front, and that the thickness of the saline ice at the bottom of icebergs calving from the Ross Ice Shelf is no more than a few meters, if there is any at all.We have observed many rift zones on the ice shelf by airborne radar techniques, and at one site the bottom and surface topographies of (buried) rift zones have been delineated. These rift zones play an obvious role in iceberg formation and may also affect the dynamics of iceberg transport. Bottom crevasses with different shapes, sizes, and spacings are abundant in ice shelves; probably some are filled with saline ice and others with unfrozen sea-water. Existence of these bottom crevasses could lead to a rapid disintegration of icebergs in the course of transport, as well as increasing the frictional drag at the ice-water boundary.Radar profiles of the ice shelf front at four sites in flow bands of very different characteristics are shown. In some places rifting up-stream from the front shows regular spacings, suggesting a periodic calving. Differential bottom melting near the front causes the icebergs to have an uneven surface and bottom (i.e. dome shaped).Electrical resistivity soundings on the ice shelf can be applied to estimate the temperature-depth function, and from that the basal mass-balance rate. With some modifications, the technique may also be applied to estimating the basal mass balance rates of tabular icebergs.

Tabular Icebergs: Implications from Geophysical Studies of Ice Shelves

AbstractRecent geophysical and glaciological investigations of the Ross Ice Shelf have revealed many complexities in the ice shelf that can be important factors in iceberg structure. The presence of rift zones, surface and bottom crevasses, corrugations, ridge/troughs, and other features could substantially modify the hydraulics of iceberg towing and lead to disintegration in the course of transport.The relationships between the elevation above sea-level and total ice thickness for three ice shelves (Ross, Brunt, and McMurdo) are given; from them, expressions for the thickness/freeboard ratios of tabular icebergs calved from these ice shelves are obtained. The relationships obtained from the measured values of surface elevation and ice thickness are in agreement with models derived assuming hydrostatic equilibrium.Areas of brine infiltration into the Ross Ice Shelf have been mapped. Examples of radar profiles in these zones are shown. Absorption from the brine layers results in a poor or absent bottom echo. It is probable that little saline ice exists at the bottom of the Ross Ice Shelf front due to a rapid bottom melting near the ice front, and that the thickness of the saline ice at the bottom of icebergs calving from the Ross Ice Shelf is no more than a few meters, if there is any at all.We have observed many rift zones on the ice shelf by airborne radar techniques, and at one site the bottom and surface topographies of (buried) rift zones have been delineated. These rift zones play an obvious role in iceberg formation and may also affect the dynamics of iceberg transport. Bottom crevasses with different shapes, sizes, and spacings are abundant in ice shelves; probably some are filled with saline ice and others with unfrozen sea-water. Existence of these bottom crevasses could lead to a rapid disintegration of icebergs in the course of transport, as well as increasing the frictional drag at the ice-water boundary.Radar profiles of the ice shelf front at four sites in flow bands of very different characteristics are shown. In some places rifting up-stream from the front shows regular spacings, suggesting a periodic calving. Differential bottom melting near the front causes the icebergs to have an uneven surface and bottom (i.e. dome shaped).Electrical resistivity soundings on the ice shelf can be applied to estimate the temperature-depth function, and from that the basal mass-balance rate. With some modifications, the technique may also be applied to estimating the basal mass balance rates of tabular icebergs.

Freshwater Ice Thickness Observations Using Passive Microwave Sensors

Walden Reservoir, a freshwater lake in north-central Colorado, was overflown six times by a NASA C-130 aircraft between January 1977 and April 1980. The aircraft was equipped with four microwave radiometers operating between 0.81 and 6.0 cm in wavelength (37.0 to 5.0 GHz). The 6.0-cm radiometer data showed a good relationship (R2 = 0.98) with ice thickness based on a sample of four ice thickness values. The 1.67- and 1.35-cm radiometer data showed weaker relationships with ice thickness. The 0.81-cm sensor data showed no positive relationship with ice thickness. None of the relationships was statistically significant because of the small sample size. The 6.0-cm sensor data in the nadir-viewing mode was found to have the most potential of all the wavelengths studied, for use in remotely determining ice thickness. The 6.0-cm radiometer probably sensed the entire thickness of the ice on the reservoir (ranging from 25.4 to 67.3 cm in thickness) and was apparently not significantly affected by the snow overlying the ice. The shorter wavelengths are scattered by the snow overlying the ice and are more suitable for snow studies than for ice thickness studies.