C11B-0417
Comparison of ICESat Data With Airborne Laser Altimeter Measurements Over Arctic Sea Ice
The areal extent of sea ice has been monitored with satellites for nearly three decades, but much less is known about sea ice thickness. The Geoscience Laser Altimeter System (GLAS) on the NASA Ice, Cloud and land Elevation Satellite (ICESat) can monitor the third dimension of the earth, its height, with unprecedented accuracy. Recent studies suggest that the precision of ICESat is sufficient to provide useful information on the thickness of sea ice as well. In order to assess the accuracy of ICESat, we compare the surface elevation and roughness measurements with high-resolution airborne laser altimeter measurements over the Arctic sea ice north of Alaska taken during the March 2006 EOS Aqua Advanced Microwave Scanning Radiometer (AMSR-E) sea ice validation campaign. Comparison of the elevation measurements show that they agree quite well with correlations of around 0.9 for individual shots and a very small maximum bias of 0.5 cm. The differences are found to decrease quite rapidly when applying running means. Comparison of roughness measurements show that there are significant differences between the two data sets with ICESat generally having a higher value, but applying running means to the data significantly improves the correlations to as high as 0.9. Ocean surface elevation estimates are made with the high-resolution airborne laser altimeter data as well as several methods using lower resolution ICESat data for the conversion of elevation measurements into snow-ice freeboard. The relatively large size of the ICESat footprint is shown to introduce a small bias in the use of the lowest levels of the elevation data as sea level, but the average freeboard for the transect shows good agreement.
C11B-0418
Arctic sea ice Freeboard From ICESat and Envisat Altimetry
Knowledge of sea ice thickness, in combination with extent, is essential for determining the variability of sea ice volume and mass flux. Although the decreasing extent of sea ice cover in the Arctic Ocean has been well documented, any similar trends in ice thickness remain unclear. Recent studies have demonstrated the use of satellite radar altimeter data to estimate sea ice freeboard, and derive sea ice thickness, over basin scales. Using the latest data releases, which have benefited from improved ground processing and correction schemes, we present an analysis which compares satellite altimeter data gathered by the Geoscience Laser Altimeter System (GLAS) and the RA-2 instruments, flown onboard the ICESat and Envisat satellites respectively. Sea surface elevation is required as a reference to estimate sea ice freeboard from altimetric elevation measurements. Due to the complexities involved in identifying open water and leads in ICESat data over sea ice, a number of techniques exist to estimate sea surface elevation. We evaluate several of these techniques via comparisons with complimentary RA-2 sea surface elevation estimates. Also, we present freeboard estimates from both ICESat and Envisat. Comparisons of coincident laser and radar freeboard data offer the potential to retrieve snow accumulation on sea ice, and thereby to derive more precise thicknesses from freeboard estimates. We analyse seasonal and inter-annual differences in the freeboard signal and present snow depth estimates.
C11B-0419
Validating ICESat Freeboard Estimates of Arctic Sea Ice With Airborne Topographic Mapper (ATM) Observations
The Arctic Aircraft Altimeter (AAA) 2006 Campaign was carried out in Spring, 2006 as part of the effort to validate sea ice freeboard estimates derived from Envisat and ICESat altimeter data. During the AAA campaign, a NASA P- 3 aircraft underflew an ICESat track north of the Canadian Archipelago while collecting surface elevation measurements using the ATM. Here we employ airborne laser altimetry data from the AAA campaign, as well as visible imagery collected by both airplane and satellite, to validate ICESat freeboard estimates. We present a comparison of sea ice freeboard measurements gathered by the Geoscience Laser Altimeter System (GLAS) on ICESat with airborne observations from the ATM. Analysis of spatially and temporally coincident elevation profiles shows good agreement between these two independent estimates of sea ice freeboard. Comparisons with the finely detailed ATM surface mapping demonstrates the capability of ICESat to resolve small-scale sea ice features such as ridges, rubble fields, and leads. Also, a technique was developed to account for sea ice drift, enabling precise comparisons between individual ICESat echoes and topographic features captured by the airborne survey. Our analysis reveals that temporal and spatial coincidence between airborne and satellite measurements is critical for validation over sea ice. We suggest several strategies for future validation campaigns for current and planned satellite missions such as ICESat and Cryosat-II.
C11B-0420
Seasonal and Interannual Sea-Ice Freeboard and Thickness Variations in the Weddell Sea from ICESat (2003-2007)
Sea-ice freeboard heights for twelve ICESat campaign periods from March 2003 to April 2007 in the Weddell Sea are derived from ICESat laser altimeter measurements. Freeboard is combined with snow depth on the sea ice from AMSR-E passive microwave data and nominal densities of snow, water, and sea-ice, to estimate sea-ice thickness. Sea-ice freeboard and thickness distributions show clear seasonal variations which reflect the yearly cycle of the growth and decay of the Weddell Sea pack ice. During the Antarctic winter (Oct-Nov), sea ice grows to its seasonal maximum both in area and thickness, filling in the Weddell Sea. For the four winter periods, the mean freeboards are between 0.33 and 0.41 meters and the mean thicknesses are between 2.1 and 2.2 meters. During the Antarctic summer (Feb-Mar), thinner sea ice melts away, sea ice is mainly distributed in the west Weddell Sea near the Antarctic Peninsula. For the five summer periods, the mean freeboards are 0.34 to 0.45 meters and the mean thicknesses are 1.6 to 2.1 meters. During the Antarctic fall (May-Jun), large areas of new, thinner sea ice forms over the Weddell Sea. For the three fall periods, the mean freeboards and thicknesses vary from 0.26 to 0.28 and 1.3 to 1.5 meters, respectively. The interannual differences in freeboard and thickness in three different seasons, the area and volume for each period are also described. This study demonstrated the ability of satellite laser altimetry to monitor sea-ice mass balance, an important indicator of climate change.
C11B-0421
Arctic Sea-Ice Freeboard Heights and Estimated Ice Thicknesses from ICESat: Seasonal and Interannual Variations (2003-2007)
Sea ice freeboard heights (sea ice plus snow cover) are derived from ICESat's along-track elevation measurements made over 70 m footprints at 172 m spacings with a range precision of 2 cm. For each measurement location, an ocean reference level is selected by constructing distributions of the measured surface elevations within +- 50 km of the location and taking the average elevation of the lowest 1 percent of the elevations, which are assumed to be over open water and/or very thin ice. The method has also provided a new estimate of the ocean geoid, which has been iteratively used as the initial ocean reference level in the analysis. Snow cover estimates from both climatology and derived from ECMWF analysis are used to estimate sea-ice thicknesses from the derived sea-ice freeboards. Probability-density functions (PDF) of ice thicknesses, which are constructed on 50-km scales, show the classic patterns of thin ice, first year, multiyear ice, etc in various regions. ICESat measurements from 2003 to present have been made in the fall (Oct-Nov) and winter (Feb-Mar) and in some springs (May-Jun). The PDF's of the mean thickness in 50 km grids show typical mixtures of areas of new thin ice and areas of residual multiyear ice in fall (1.48 m mean in 2005), growth of thicker ice in winter (1.93 m mean in 2006), and continued growth and loss of thinner ice by spring (2.67 m in 2006). The mean thickness and ice volume in the fall appears to be decreasing, while the mean thickness in the winter has had significant interannual variability. In 2004, a significant decrease of thicker ice occurred in the classic region of thicker ice north of Canada and Greenland, followed by some regrowth in 2005, and then by decreases of the thicker ice in 2006 and 2007. Compared to the typical ice thickness distributions typical of the Arctic Ocean in 1980-1990's, there appears to have been a fundamental loss of much of the thicker 3 to 5 m ice in recent years.
C11B-0422
Antarctic sea ice thickness data archival and recovery at the Australian Antarctic Data Centre
A new effort is underway to establish a portal for Antarctic sea ice thickness data at the Australian Antarctic Data Centre (http://aadc-maps.aad.gov.au/aadc/sitd/). The intention is to provide a central online access point for a wide range of sea ice data sets, including sea ice and snow thickness data collected using a range of techniques, and sea ice core data. The recommendation to establish this facility came from the SCAR/CliC- sponsored International Workshop on Antarctic Sea Ice Thickness, held in Hobart in July 2006. It was recognised, in particular, that satellite altimetry retrievals of sea ice and snow cover thickness rely on large-scale assumptions of the sea ice and snow cover properties such as density, freeboard height, and snow stratigraphy. The synthesis of historical data is therefore particularly important for algorithm development. This will be closely coordinated with similar efforts in the Arctic. A small working group was formed to identify suitable data sets for inclusion in the archive. A series of standard proformas have been designed for converting old data, and to help standardize the collection of new data sets. These proformas are being trialled on two Antarctic sea ice research cruises in September - October 2007. The web-based portal allows data custodians to remotely upload and manage their data, and for all users to search the holdings and extract data relevant to their needs. This presentation will report on the establishment of the data portal, recent progress in identifying appropriate data sets and making them available online. http://aadc- maps.aad.gov.au/aadc/sitd/
C11B-0423
Antarctic fast-ice thickness
Fast ice makes up about 10 % by area (or 14 - 20 % by volume) of the total Antarctic pack. Its growth is largely determined by thermodynamics processes, which makes it a valuable tool to monitor changes in the polar climate system. Time series of fast-ice properties, especially when viewed with concurrent meteorological or oceanic data, contain clues on interactions and changes between climatic components. In contrast to the Arctic, there are few Antarctic locations, at which fast-ice conditions have been recorded. Antarctic data are often intermittent and generally limited to the few most recent decades. As part of SIMBA, the IPY Antarctic sea-ice mass balance investigation, the Antarctic Fast-Ice Network (AFIN) has been instigated. The aim is to establish a network of sites around the Antarctic coast at which regular and ongoing fast-ice measurements are carried out. AFIN seeks to utilize previous as well as existing fast-ice measurement sites, and to add new locations to collect baseline data on the seasonal evolution of the fast ice. The data collection is planned to continue beyond the IPY years. To enable the expansion of AFIN, autonomous instrumentation has been developed, including a fast-ice mass-balance station. This station is currently deployed at Davis, East Antarctica, and consists of a thermistor rod with 0.02 m vertical resolution, air-temperature and air-pressure sensors, and acoustic pingers above and below the fast ice. Initial results of measurements collected during austral winter 2007 will be presented here.
C11B-0424
Variability of sea ice production in the Weddell and Ross Seas polynyas from AMSR-E observations, 2003-2007
Using an algorithm to estimate heat loss from the 36GHz polarization ratios of AMSR-E, Martin et al. [2007] found that the combined ice production from three largest coastal polynyas in the Ross Sea was ~500 km3/yr. They also showed that the polynya ice production approximately equaled the ice export. Using the same approach, we investigate the ice production of polynyas in the southern Weddell Sea. The study shows that the Weddell polynyas are much smaller in area than those in the Ross Sea and that the ice production is only a small fraction of the net ice export from the southern Weddell. The reason for the difference between ice production in the Ross and Weddell seas is that in the Ross, persistent polynyas are maintained by the strong, cold off-shore winds blowing off the Ross ice shelf, consistently producing ice and consequently high salinity shelf water throughout the winter; while in the southern Weddell, there are no comparable wind patterns. Driven by a large offshore low-pressure system, the winds circulate around the Weddell in a clockwise pattern. This results in mostly weak sporadic polynyas distributed along the coastline, primarily along the Coats Land coast. This paper discusses the winter ice production of the Weddell coastal polynyas, compares them both in magnitude and variability with the Ross polynyas, and discusses the implications on the formation of high salinity shelf water in the Weddell and Ross Seas. (Reference: Martin, S., R. Drucker, and R. Kwok. The areas and ice production of the western and central Ross Sea polynyas, 1992-2002, and their relation to the B-15 and C-19 iceberg events of 2000 and 2002. J. Mar. Syst. (2007), doi:10.1016/j.jmarsys.2006.11.008.)
C11B-0425
Sea-Ice Thickness Derived from Ice Charts in the Southern Ocean
An evaluation of the weekly NIC (National Ice Center) ice chart dataset with in situ sea-ice thickness observations from the ASPeCt (Antarctic Sea Ice Processes and Climate) program during the 1995 to 1998 time period is being extended across the Southern Ocean. Sea-ice thickness calculations from both datasets are temporally joined with spatially averaged in situ observations matching their respective NIC ice chart using a Geographic Information System (GIS). The uncertainties of total ice thickness for both in situ observations and NIC ice charts are obtained through individual calculations and the GIS processing. Weekly ice charts thickness estimates correlate reasonably well with in situ observations during five voyages from May/June 1995, August 1995, May/June 1998, December 1998/January 1999, and December 1999/January 2000 in the Ross Sea. A temporal comparison of the NIC ice chart data along an individual ship track reveal insight into the differences, and these discrepancies are further investigated. Using the NIC charts, the sea-ice seasonal and interannual thickness distribution are derived for the Southern Ocean. The interannual and spatial variability, for example, is evident between years in the Ross Sea, with thinner sea-ice conditions in June 1998 as compared to June 1995, primarily because first-year thick and multi-year sea ice show a larger extent in June 1995 as compared to June 1998.
C11B-0426
Evaluating derived sea ice thickness estimates from two remote sensing data sets
Satellites that monitor the polar regions collect a wealth of information about sea ice. While elevation data (ice freeboard + snow) are obtainable from certain satellites (e.g. ICESat), this estimate only measures approximately 10 percent of the total ice thickness. Significant uncertainties exist when extrapolating from ice freeboard to total ice thickness. Direct ice thickness measurements taken from on the ice are the most accurate but difficult to obtain, but submarines provide an effective method to monitor basin-wide draft. There is less uncertainty in extrapolating total thickness from the ice draft. Satellite imagery, while not providing direct measurement of ice thickness, can be used to infer ice type and hence an ice thickness range estimate based on interpretation of the imagery and ancillary data. This study compares Arctic ice thickness estimated from submarine data to ice thickness from an interpretive product that relies heavily on satellite data where the two data sets overlap spatially and temporally during the period 1996 through 1998. The first source (submarine data) is available from the National Snow and Ice Data Center, while the second source of raw NIC charts is available from the National Ice Center (NIC). Both data sources are converted to ice thickness prior to the evaluation. A raw data analysis is performed and ice thickness distribution maps are produced in ArcMap based on the ordinary kriging spatial interpolation technique. Results from the raw data analysis indicate a low correlation between the two data sets with the least agreement in the multiyear ice zone (>200 cm). The geostatistical results portray a similar ice thickness pattern in the Arctic, even though the submarine data contain more ice thickness variability than the NIC charts.
C11B-0427
A Unified Sea Ice Thickness Data Set for Model Validation
Can we, as a community, do better at using existing ice thickness measurements to more effectively evaluate the changing nature of the Arctic ice pack and to better evaluate the performance of our models? We think we can if we work together. We are trying to create a unified ice thickness data set by combining observations from various ice thickness measurement systems. It is designed to facilitate the intercomparison of different measurements, the evaluation of the state of the ice pack, and the validation of sea ice models. Datasets that might be included are ice draft estimates from various submarine and moored upward looking sonar instruments, ice thickness estimates from airborne electromagnetic instruments, and satellite altimeter freeboard measurements. Three principles for the proposed data set are: 1) Full documentation of data sources and characteristics, 2) Spatial and temporal averaging to approximately common scales, and 3) Common data formats. We would not mix data types and we would not interpolate to locations or times not represented in the observations. The target spatial and temporal scale for the measurements would be 50 lineal km of ice and/or one month. Point measurements are not so useful in this context. Data from both hemispheres and any body of ocean water would be included. Documentation would include locations, times, measurement methods, processing, snow depth assumptions, averaging distance and time, error characteristics, data provider, and more. The cooperation and collaboration of the various data providers is essential to the success of this project and so far we have had a very gratifying response to our overtures. We would like to hear from any who have not heard from us and who have collected sea ice thickness data at the approximate target scales. With potentially thousands of individual samples, much could be learned about the measurement systems, about the changing state of the ice cover, and about ice model performance and errors.
C11B-0428
The Climate and Cryosphere Project (CliC): Helping bring sea ice Models and Observations together.
The Climate and Cryosphere Project is sponsored by the World Climate Research Program (WCRP) and the Scientific Committee for Antarctic Research (SCAR). One of the four themes within the CliC project is the Marine Cryosphere Theme (MarC). This paper will review the recent projects and workshops held within this Theme and how they relate to other, international initiatives. Recent recommendations on sea ice thickness are being implemented, and groups have been formed to work towards improvements in models, particularly in their representation of the Southern Ocean. SOPHOCLES (Southern Ocean Physical Oceanography and Cryosphere Processes and Climate) will work with other modeling groups to improve the representation of the Southern Ocean in climate models. This will include cooperation with other modeling and observational groups to develop metrics to help evaluate models. In the Arctic, we are working to help develop, standardize, and implement observation and measurement protocols for Arctic sea ice in coastal, seasonal, and perennial ice zones.
C11B-0429
Three Dimensional Mapping of the Sea Ice Underside From AUVs
In August 2004 the Autosub-II AUV, operating off NE Greenland, carried out the first multibeam digital terrain mapping of the sea ice underside, using a Kongsberg EM2000 sonar. We show some of the high-quality imagery from the experiment, and discuss its implications for ice thickness mapping and other applications. In April 2007 the second such mapping experiment took place, but this time using a small ice-launched Gavia AUV, equipped with a GeoSwath 500 kHz interferometric sonar system. Gavia could be launched and recovered manually through 3 x 1 m holes while Autosub required a ship and a crane. The greater range of Autosub is contrasted with the greater flexibility of Gavia in multisensor programs. The Gavia imagery shows the morphological distinctions between first-year (FY) and multi-year (MY) ice undersides, the contrast between the shapes of FY and MY ridges, and the appearance of refrozen leads. Ice drafts were validated by drilling, and it was found that ridge slope statistics and local probability density functions (PDFs) of draft could be derived with high precision.
C11B-0430
Mining Existing Radar Altimetry for Sea Ice Freeboard and Thickness Estimates
Although satellites can easily monitor ice extent and a variety of ice attributes, they cannot directly measure ice thickness. As a result, very few ice thickness measurements exist to constrain models of Arctic climate change. We estimated sea ice freeboard and thickness from X-band radar altimeter measurements collected over seven field seasons between 1992 and 1999 as part of a Naval Research Lab (NRL)-sponsored airborne geophysical survey of gravity and magnetics over the Arctic Ocean. These freeboard and thickness estimates were compared with the SCICEX ice draft record and the observed thinning of the Arctic Ocean ice cover during the 1990's. Our initial calculations (shown here) suggest that retrieved profiles from this radar altimeter (with uncertainty of about 5 cm) are sensitive to openings in the ice cover. Thus, conversion of these profiles to ice thickness adds an invaluable dataset for assessment of recent and future changes of Arctic climate. And, snow loading is a minor issue here as all the airborne surveys were conducted during mid- to late-summer when the ice cover is mostly bare. The strengths of this dataset are its small antenna footprint of ~50 m and density of spatial coverage allows for detailed characterization of the field of ice thickness, and it provides surveys of regions not covered by SCICEX cruises. The entire survey covers more than half the Arctic Ocean. We find that the Canadian Basin sea ice behavior differs from that in the Eurasian Basin and ultimately affects mean sea ice thickness for each basin.
C11B-0431
Determining Deformed Sea Ice Properties Using Multisensor Observations
Deformed ice (ridges) is a major component of the sea ice volume, yet this property has proven difficult to quantify from satellite sensors in terms of frequency, orientation, extent and height. One problem is detectability, where it is known, for example, that imaging radars with L-band SARs (1.2 GHz) and even lower in frequency may identify ridges and deformed ice more clearly than the more commonly flown C-band SAR (5.6 GHz) sensors. Another problem is that nearly all fine-resolution sensors generally have a coarser resolution than the ridges themselves, also making clear detection difficult. In this study we examine fine resolution L-band SAR imagery from both JERS-1 (18 m) and recently obtained ALOS PALSAR (6-10m resolution) over the Arctic with coincident in situ measurements of ice draft from upward looking sonar placed on moorings (JERS-1) and of thickness from electromagnetic induction sensors flown on a helicopter (ALOS) during the Aplis"07/SEDNA field campaign. Geometrical corrections were applied to the SAR imagery in order to closely align the satellite imagery in space with the in situ measurements. Temporal adjustments related to ice motion were made through feature detection and additional SAR imagery from Radarsat and ERS-2. We will discuss the correlation of radar backscatter with the extent, orientation, and height of ridges and the possibility of quantifying deformed ice properties from satellite SAR imagery.
C11B-0432
Decrease of Sea Ice Thickness at Hopen, Barents Sea, During 1966-2007
Seasonal shore-fast sea ice thickness at the island of Hopen in the northwestern Barents Sea was monitored over 40 years from a permanently manned meteorological station. Sea ice thickness variability is an important climate indicator, providing more quantitative information on the state of the ice cover than solely sea ice extent data series. Compared to North America and Siberia, few longer time series of seasonal ice exist in the European Arctic. Fast ice formation at Hopen starts on average just before December, and maximum ice thicknesses are reached in May, before the ice starts to melt and breaks up. Being more exposed to swell, currents, and variable wind regimes than in sheltered bays or fjords, the development of fast ice at Hopen is interrupted various times during several of the seasons observed. Then, the ice is removed and new ice forms. Annual maximum ice thicknesses at Hopen average 0.99 m, with thickest ice measured 2.0 m. Since 2000, no ice thicker than 1.0 m was observed. Over the entire span of the time series (1966-2007), we find a negative trend of the ice thickness anomaly and seasonal maximum ice thickness. This decrease coincides with an increase in both local surface air temperature and surface water temperature at Hopen. The observed sea ice thickness changes are consistent with reductions of sea ice extent in the Barents Sea and the entire Arctic.
C11B-0433
Community-based sea ice thickness observatories in the Arctic
The thickness of sea ice is a fundamental diagnostic variable for assessing the state of the ice cover. At the scale of the Arctic Basin, the ice thickness distribution determines the volume of the ice pack and its susceptibility to a warming climate as well as affecting the exchange of heat between the ocean and atmosphere. At the local scale, it dictates where and when it is safe to travel on the ice or through the water. Measuring the thickness of sea ice is challenging both technically and logistically and any measurement program strikes a balance between cost and coverage accordingly. Accurately measuring the thickness of large areas of sea ice generally requires airplanes, ice breakers or submarines and electromagnetic or acoustic devices. In this study, we use one of the least technical methods combined with support from remote communities to establish a set of sea ice observation stations in Barrow (Alaska), Clyde River (Baffin Island, Nunavut) and Qaanaaq (northwest Greenland). We employ hunters from these communities, who are experts in traveling and working on the ice, and train them to deploy ice observation stations and take measurements. Each station consists of snow stakes and hot-wire ice thickness gauges and the local observers take measurements on a weekly basis. Involvement of the community is fundamental to the success of these measurement programs and ensures the data collected are relevant to the local use of the sea ice. Community elders and hunters chose the station locations according to where they hunt and travel and to be representative of local variability. As partners in research, the scientists and local hunters are able to share and synthesize their knowledge; the scientific community gains a better understanding of the extraordinary depth of traditional knowledge and the communities improve their understanding of global changes and ability to adapt. Here we present data from observation stations near Clyde River and Qaanaaq. At Clyde River, in comparison with measurements taken by the Canadian Ice Service during the period 1959-93, the sea ice in 2006-07 was below but within one standard deviation of the mean thickness. Combined with local air temperature measurements from nearby meteorological stations, we calculated approximate surface energy balances that indicate the ocean heat flux is significantly greater at Qaanaaq than Clyde River, despite otherwise being similar environments for sea ice. Findings such as this are important in understanding the specific ways in which sea ice is changing in different locales and are vital for community planning for the near future.
C11B-0434
Making sea ice Motion Data From RGPS More Accessible
The Radarsat Geophysical Processing System (RGPS) was designed to generate sea ice products providing information about sea ice motion, deformation and sea ice thickness. Radarsat-1 ScanSAR Wide B (SWB) imagery has been acquired over more than a decade for the Arctic Ocean with a spatial resolution of 100 m. At the beginning of each winter season a regular grid is initialized and the grid points are tracked over the season to monitor the sea ice motion. With the changing ice conditions the regular grid becomes distorted in shape and location. The distorted Lagrangian grid is used to generate the RGPS data products which reflect the ice condition for a three-day snapshot. These products are currently distributed in a custom designed binary format. They are only used for the long-term monitoring of sea ice on the Arctic basin scale, hence the data is vastly underutilized. The resolution also allows long-term monitoring studies on the regional scale as well as on a local scale. The goal of this prototype development is to make the RGPS data more accessible to allow the data to be used at the regional and local scale, e.g. to develop lead typologies or verify ice charting forecasts. A prototype has been developed that makes the RGPS data more accessible to the research community. A number of raster and vector products are generated for the nominal three-day snapshot. The image mosaics for the part of the Arctic basin that is covered in the snapshot have a 500 m spatial resolution. Basic metadata are provided that allow the user to identify features of interest in the mosaics and their corresponding image within an image data coverage layer. With this metadata the imagery of interest can be directly ordered. Additionally, a weather data layer is derived from model data. For RGPS data that has already been processed the sea ice monitoring information a sea ice layer is created that include all the relevant information from the RGPS database. These vector layers are now available in the commonly used ArcGIS shape file format. This allows for a more streamlined and user friendly access to RGPS data. Image mosaics from more than a decade can now be used for long-term monitoring of the Arctic basin. Based on this prototype the study of sea ice phenomena at regional and local scale becomes more feasible.
C11B-0435
Atmospheric and Oceanic forcing of sea ice drift and deformation during the SEDNA field campaign
This paper provides a synopsis of the surface atmospheric and oceanic forcing of sea ice drift and deformation during the Sea ice Experiment: Dynamical Nature of the Arctic (SEDNA) Beaufort Sea campaign of April 2007. During the campaign two hexagonal arrays of GPS buoys were deployed with maximum diameters of 10km and 70km, respectively, providing an excellent record of sea ice drift and deformation. This record is analyzed in light of in situ surface wind and current measurements, high resolution Weather Research and Forecast Model (WRF) simulations focussed on the Beaufort Sea, and pan-arctic synoptic weather events represented in NCEP II atmospheric analyses. Analysis of the drift track and wind-forcing in the time domain suggests little trace of inertial or tidally induced motion during the field camp. A broader understanding of the relative contribution of semi-diurnal motion to Beaufort Sea ice drift and deformation is gained by comparing barotropic ice-tide model output with a 12-month Beaufort Sea ice-drift time series of buoys deployed in August 2006. Spectral comparisons of the buoy-drift with our model's output reveal that semi-diurnal deformation spectra does not always result from strong local surface winds, regardless of sea ice concentration. This result is particularly significant because the ice camp was located close to the latitude at which the M2 tide oscillates at the resonant inertial frequency, potentially making excitation of the oceanic boundary layer by storms more likely. One interpretation of this result is that specific ice-ocean boundary layer treatments in ocean models may have some baring on their estimates of the ice mass balance of the Arctic Ocean.
C11B-0436
Empirical Sea Ice Thickness Estimation in the Arctic Ocean
This study evaluates methods to improve a recently developed neural network (NN) algorithm that estimates sea ice thickness with spatial resolution nearby 100 km at monthly intervals during 1982 - 2003 (Belchansky et al., accepted, J. Climate). For any grid cell, at each position along its drift trajectory, sea ice thickness changes are controlled by geophysical inputs that include dynamic and thermodynamic forcing parameters such as short- and long-wave radiation, cumulative freeze-degree days, ice drift velocity, and an ice-drift derived divergence/convergence index. Improvements to the original method included: 1) expanding the learning data with updated submarine draft data from NSIDC; 2) partitioning all learning data into non-overlapping categories of ice thickness; 3) learning the NN independently for each ice thickness category, and then combining fractions of ice categories to derive a sea ice thickness distribution for each grid cell; 4) replacing and expanding the original NCEP-NCAR Reanalysis radiation inputs with their analogs from the NCEP-DOE Reanalysis-2 data sets; 5) reconstructing the ice divergence-convergence index; and 6) separating the learning data into level ice and ridged ice categories. The contributions of dynamical and thermodynamical components to sea ice volume change in the central Arctic were examined. The influence of ice thickness to the sea ice volume balance is predominant for high latitudes, while for low latitudes, ice volume is related to ice extent.
C11B-0437
SAR observations and modelling of dynamics and sea-ice export in the Southern West New Siberian polynya
Polynya processes in the south-eastern Laptev Sea between January and June 2004 have been studied by means of satellite Synthetic Aperture Radar (SAR) imagery and a simple polynya ice flux model based on the Haarpaintner approach. The polynya model was forced by meteorological data recorded at a nearby weather station, and calibrated using satellite observations of the southern West New Siberian (WNS) polynya. The good correspondence between the modelled polynya evolution and SAR observations as well as between the calculated salinity increase of the water column and long-term salinity records suggests that the Haarpaintner model is a suitable tool to investigate dynamics and export rates of flaw polynyas. A total of 66 km3 of ice in both thin-ice and open-water zones was produced from January to June 2004 in the southern 195-km wide segment of the WNS polynya. Due to generally calm wind conditions in the Laptev region, the development of large open-water zones is restricted, and ice growth takes mainly place under the large areas of new thin-ice. Therefore, the resulting salt flux is not high enough to destabilize the generally strongly stratified water column, which supports observations by Dmitrenko et al. [2005]. To evaluate the importance of the observed ice and salt fluxes in the entire WSN polynya for the local circulation system and the interaction with the hydrography of the Arctic Ocean, coincident in-situ observations of ice and water properties should be performed. In particular, we will obtain continuous observations of water temperature and salinity and ice thickness by means of seafloor-moored instruments in the polynya region in coming years.
C11B-0438
Evidence for Significant Acceleration of Arctic Sea Ice Drift over the last 25 Years
The Arctic sea ice cover is undergoing significant climate-induced changes, affecting both its extent and thickness. This arctic sea ice decline is generally attributed to a thermodynamic effect of warmer surface air temperatures enhanced by a positive feedback loop involving albedo. In this context, sea ice shrinkage is associated with a climate change particularly intense in the Arctic. Here we show that this change in the sea ice cover is also accompanied by an acceleration of the average sea ice drift over the last 25 years. We performed an analysis of the Arctic sea ice drift from the buoys trajectories of the IABP dataset between 1979 and 2006. A strong seasonal variability of the sea ice mean drift is revealed, out of phase with respect to the sea ice extent seasonal variability, i.e. with a maximum average velocity occurring in October and a minimum in April. Averaging sea ice drift over the summer months, we observed a significant increase of the mean velocity over the whole Arctic basin at a rate of +10 percent per decade. Such acceleration is also observed in winter (+15 percent per decade). The associated standard deviations of the drift velocities, which are proxies of sea ice dispersion at large spatial and time scales and consequently of sea ice deformation (Rampal et al., 2007), are also increasing significantly over the period. This acceleration of sea ice drift and deformation might suggest that sea ice dynamics intervenes in the albedo feedback loop: larger deformation implies an increasing sea ice fracturing, i.e. more open water and therefore a decreasing albedo. This will accelerate sea ice thinning in summer and delay refreezing in early winter, therefore decreasing the mechanical strength of the cover and allowing more fracturing and larger drift and deformation.
C11B-0439
Scaling properties of the Arctic sea ice Deformation from Buoy Dispersion Analysis
A temporal and spatial scaling analysis of Arctic sea ice deformation is performed over time scales from 3 hours to 3 months and over spatial scales from 300 m to 300 km. The deformation is derived from the dispersion of pairs of drifting buoys, using the IABP (International Arctic Buoy Program) buoy data sets. This study characterizes the deformation of a very large solid plate -the Arctic sea ice cover- stressed by heterogeneous forcing terms like winds and ocean currents. It shows that the sea ice deformation rate depends on the scales of observation following specific space and time scaling laws. These scaling properties share similarities with those observed for turbulent fluids, especially for the ocean and the atmosphere. However, in our case, the time scaling exponent depends on the spatial scale, and the spatial exponent on the temporal scale, which implies a time/space coupling. An analysis of the exponent values shows that Arctic sea ice deformation is very heterogeneous and intermittent whatever the scales, i.e. it cannot be considered as viscous-like, even at very large time and/or spatial scales. Instead, it suggests a deformation accommodated by a multi-scale fracturing/faulting processes.
C11B-0440
Sea-Ice Roughness, Morphogenesis and Kinematics --- Approaches to Learn from the Complexity of Sea Ice
Recent studies of the alarming retreat of the Arctic sea ice have been largely based on observations of sea-ice coverage. This is not sufficient to capture changes in the sea-ice's mass, hence there is an increasing interest in measuring the thickness of sea-ice. However, the complexity of sea ice renders the latter a difficult task: (1) at any time, the sea ice has a complex form and appearance in remote-sensing observations, (2) due to ridging and rubbling, the mass of sea ice is not directly related to its thickness, (3) sea ice forms in a series of morphogenetic processes, and (4) sea ice moves. Here we present mathematical approaches to analyze spatial roughness of the surface of sea ice and of its snow-layer thickness, morphogenetic processes and deformation characteristics as a means to quantify and characterize sea-ice properties, processes and provinces. Applications include analyses of passive microwave data, SAR data, laser and radar elevation data and multispectral image data, from satellite, unmanned aerial vehicle and aircraft platforms, and field data.
C11B-0441
Short Term Variability of Sea Ice Thickness in the Beaufort Sea
To investigate the relationship between variability of sea ice thickness and ice dynamics, helicopter borne electromagnetic sea ice thickness sounding was performed at the APLIS ice camp in the Beaufort sea in April 2007. The field campaign includes sea ice thickness observations close to the camp with repeated flight tracks of different length scales and a transect ranging from 75°N to the coast of Barrow, Alaska at 71.2°N. In total sea ice thickness with a point spacing of 3-4 meters was obtained on profiles with a total length of more than 2000 km. The data comprises changes in the ice thickness distribution in spatial scales up to 100 km at a time period of nearly two weeks close to the camp. The deformation of the sea ice field in the vicinity of the ice camp was covered by two arrays of GPS drifting buoys yielding the drift and rotation of the ice field. The most recent positions of these buoys were used as waypoints for the ice thickness observations, which guarantees the comparability of the successive profiles. The modal ice thickness around the ice camp did not change significantly in the time range of the field campaign. The modal ice thickness including the snow depth of the first year ice amounts to 1.7 m, while the multiyear ice was about 2.6 m thick. Changes due to the dynamics of the ice pack can be observed in the mean ice thickness which decreased by roughly 10 cm in a time span of 9 days (2.70 m to 2.59 m). This decrease can be explained by a higher fraction of newly formed ice in the thickness distribution due to strong winds, which exceeded the similar increased contingent of deformed ice in the ice thickness distribution. At the regional scale no significant trend in the sea ice thickness could be observed between the coast and the northernmost point, showing that the surveyed sea ice was of the same age in the whole measurement area. Differences could only be observed in the fraction of open water and the deformation rate between the coastal zone and the central Beaufort Sea.
C11B-0442
A mechanism for increasing ice mass, pushing cold ice into the upper mixed layer of the ocean
We are exploring a process which involves the rapid growth of ice when ice blocks are submerged into the upper ocean mixed layer, such as occurs during ice deformation events that involve the ridging or rafting of blocks of sea ice. Initial observations of this mechanism were obtained during the 2007 ice camp in the Beaufort Sea. Field observations indicate that between 10 to 15 percent new ice growth by mass occurs rapidly after the ice blocks are submerged. This ice growth is focused along the long linear features and leads to a focused line source of brine being input into the uppermost layer of the ocean. This line source of brine is enhanced by brine draining out of the lead ice that is rapidly warmed by being submerged in seawater. The rates and magnitudes of these two processes, new ice growth and brine drainage from the recently submerged ice blocks, are not equal, and understanding how they vary is important. We hypothesize that these fluxes are large enough to have dynamic implications and yield an enhanced oceanic heat flux. The increase in heat flux would be due to the circulation caused by the negative surface buoyancy flux related to the injection of dense brine at the ocean surface. We will explore parameterization of these processes for ice models which explicitly model the initiation and evolution leads and ridges.
C11B-0443
A one-dimensional, thermodynamic model for the consolidation of multiply rafted sea- ice
Rafting is an important process in the deformation of sea ice that occurs when two ice sheets collide, such that one sheet is overridden by another, resulting in a local doubling of the ice thickness. This phenomenon is particularly common in the Caspian Sea where multiple rafting can produce thick sea ice features that are a hazard to offshore operations. In this study a one-dimensional, numerical computer model is used to investigate the consolidation process of multiply rafted sea ice. We consider the consolidation of a saline liquid layer located between two identical ice sheets of uniform thickness, embedded within a multiply rafted section. Understanding this is of importance because the degree of consolidation will affect the strength of a rafted structure, which is of interest for mechanical sea ice models and engineers, who may need to incorporate the various strengths associated with rafted ice into the design loads for offshore structures and vessels.
C11B-0444
Sea ice rheology and the sub-grid scale
The sea ice component of global climate models (GCMs) must contain representations of sea ice stress appropriate to their spatial grid resolution. We explore the relationship between the sea ice stress and sub-grid scale floe and lead interaction using a clearly defined methodology. The calculation of GCM sea ice stress depends upon the assumed mechanical behaviour of sea ice, the geometry of floes and leads, and the manner in which a collection of floes and leads deform in response to large-scale atmospheric and oceanic forcing. We discuss the sensitivity of GCM sea ice stress to the sub-grid scale sea ice state, calculations using Radarsat SAR data coincident with the SHEBA site, and implications for sea ice modelling.
C11B-0445
Modelling the rheology of sea ice as a collection of diamond-shaped floes
In polar oceans, seawater freezes to form a layer of sea ice of several metres thickness that can cover up to 8% of the Earth's surface. The modelled sea ice cover state is described by thickness and the orientational distribution of interlocking, anisotropic diamond-shaped ice floes delineated by slip lines, as supported by observation. The purpose of this study is to develop a set of equations describing the mean-field sea ice stresses that result from interactions between the ice floes and the evolution of the ice floe orientation, which are simple enough to be incorporated into a climate model. The sea ice stress caused by a deformation of the ice cover is determined by employing an existing kinematic model of ice floe motion, which enables us to calculate the forces acting on the ice floes due to crushing into and sliding past each other, and then by averaging over all possible floe orientations. We describe the orientational floe distribution with a structure tensor and propose an evolution equation for this tensor that accounts for rigid body rotation of the floes, their apparent re-orientation due to new slip line formation, and change of shape of the floes due to freezing and melting. The form of the evolution equation proposed is motivated by laboratory observations of sea ice failure under controlled conditions. Finally, we present simulations of the evolution of sea ice stress and floe orientation for several imposed flow types. Although evidence to test the simulations against is lacking, the simulations seem physically reasonable.
C11B-0446
Experimental Investigation of the Flooding of Snow-Laden Sea Ice
The surfaces of sea ice floes can become flooded with seawater. This is usually a result of the floe sinking under the weight of snow on its surface, and the subsequent infiltration of seawater into the snow layer by overflowing the sides of a floe, or by percolating upwards through the permeable sea ice. This process, and the subsequent formation of snow ice, impacts on ice and snow thermodynamics, influences the physical and compositional properties of sea ice, and plays a role in sea ice ecosystems. Tank experiments have been designed to investigate the dynamics and thermodynamics of these processes. The experiments take place in a cold room and involve the use of thermocouples, hypodermic needles, thin- sectioning techniques, and a digital refractometer. Measurements of temperature and salinity, and inferred local solid fractions, reveal the changing distribution of salt during flooding and refreezing.
C11B-0447
Early Springtime Snowcover on East Antarctic Sea Ice, ARISE 2003: Variability and Satellite Validation.
This work provides an analysis of snow thickness and properties data collected during the "Antarctic Remote Ice Sensing Experiment" (ARISE) cruise to the region of the East Antarctic sea ice zone bounded by 64-65°S and 112- 119°E (September/October 2003). Detailed sea ice and snowcover data were acquired within a 100 x 50 km region defined by an array of 9 GPS beacons deployed onto ice floes, and divided into eight 25 x 25 km sub- regions equivalent to the dimension of EOS AMSR-E sea ice product pixels. The location of the beacons through time was used to determine sampling sites so that the ice being sampled was always that which was within the original area, even though it had drifted westwards and deformed. Measurements made within and around the experimental area included digital aerial photography, snow thickness sampling of 40 floes by helicopter, and detailed in situ measurements of snow and ice thickness and properties on floes at 13 "ice stations". These were typically occupied for 8-10 hours, although one station lasted for 4 days and was later revisited after a storm. This enabled the measurement of changes in snow thickness due to snowfall. In this study, we examine the impact of an ephemeral melt event on snow and ice physical properties and on AMSR-E snow thickness retrieval accuracy, and show first results from a validation (using the ARISE data) of important new regional-scale ice thickness estimates derived from coincident ICESat laser altimeter data.
C11B-0448
Seasonal Variability of Snow Stratigraphy and Spectral Optical Properties on Sea Ice
The optical properties of snow strongly influence the surface energy balance within the coupled atmosphere-ice- ocean system. They control the amount of solar short-wave radiation, reflected at the surface (surface albedo), scattered and absorbed within snow, and transmitted into the sea ice underneath. Snow stratigraphy and spectral transmission are crucial for biological studies on sea ice related communities and bio-chemical processes. Furthermore, the increasing importance of remote sensors for studying snow and sea ice in both Polar Regions raises the need of ground truth data of physical and. spectral optical snow and sea ice properties. We perform simultaneous spectral radiation and detailed snow stratigraphy measurements in various regions in the Arctic (Arctic Basin, Svalbard, Fram Strait) during different seasons. Time series of spectral albedo and transmission through snow and sea ice together with snow observations were gathered during drift stations and long term monitoring programs. These data sets give valuable information about the seasonal cycle of surface characteristics. For the optical measurements we are using high resolution ASD FieldSpec and TriOS Ramses radiometers. Snow studies are mainly based on systematic snow pit measurements of various properties, e.g. grain size, grain type, wetness, temperature complemented by photo documentary, thickness profiles, and physical properties of sea ice. Additionally, data analyses benefits from the application of the high resolution numerical snow model SNTHERM, which performs well in simulating snow properties on sea ice under various boundary conditions. Simulations are conducted for regions and times where in situ snow data are not available, and for performing detailed snow process studies.
C11B-0449
Constraining the Time-Scale of Interaction of Sea Ice Sediments and Surface Sea Water in the Arctic Ocean Using Short-Lived Radionuclide Tracers
We measured the activities of short-lived radionuclides (Th-234, Be-7, Po-210, Pb-210, Cs-137, Th-234, Ra-226 and Ra-228) and concentrations of several elements (Be, Pb, Fe, Al, Co, Ni, Cu and Zn) on a suite of ice-rafted sediments (IRS) collected during BERINGIA-2005 in the Western Arctic Ocean. A suite of water samples were also collected and analyzed for particulate and dissolved Be-7, Po-210, Pb-210, Th-234, Ra-226 and Ra-228. The activities of Be-7 and Pb-210 in the IRS are 1-2 orders of magnitude higher than those reported in the source sediments. Presence of excess Th-234 in the IRS indicates that the removal of Th-234 from surface seawater took place on time scales comparable to the mean-life of Th-234. While the Po-210/Pb-210 activity ratios in the source sediments (1.0) and the atmospheric depositional input (~0.1) are known, varying ratios of 0.78 to 1.0 were found in the IRS. This ratio can be utilized to obtain the residence time of the IRS in sea ice. The activity of Ra-226 and Ra-228 in all the IRS is nearly constant (within a factor of 1.6) and are comparable to the benthic sediments in the source region. The activities of atmospherically-delivered radionuclides, Be-7 and Pb-210, in IRS varied by factors of ~4.5 and 9, respectively, and this variation is attributed to differences in the extent of interaction of surface water with IRS and differences in the mean-lives of these nuclides. While significant enrichment of Be-7 and Pb-210 has been found, there is no enrichment of stable Pb or Be. The Al-normalized enrichment factor for elements measured (Co, Ni, Cu, Zn, Pb and Be) indicate that there is no significant enrichment of these elements, with Al-normalized enrichment factors less than 1.3.
C11B-0450
Snow density from Bulk and Pit Samples during APLIS07 Ice Camp
Snow density is a critical parameter for hydrostatic calculations and remote sensing calibration. A basic set of in situ snow measurements were incorporated into the APLIS07 surveys conducted in the Beaufort Sea from April 1- 15 at the start of IPY. The measurements were taken to ensure that bulk density, stratigraphy, and basic snow characteristics were recorded as part of an instrumental intercalibration study. Results show that the snow cover variability on sea ice ranged from a dusting to 1m drifts on the multiyear floes and an average of 20 cm on level ice surfaces. Depth hoar accounted for up to half of the snow pack depth and was half the density of the wind slab snow. Several of the depth hoar samples include very large cup crystals (1-2 cm) with broken capped bullet crystals in the wind slab and rime deposition on the fresh snow crystals. The largest depth hoar crystals were located over level refrozen leads where the ocean heat flux and moisture could still reach the bottom of the snow pack beneath the wind slab. Error analysis and an overview of these findings will be presented.
C11B-0451
A preliminary study of AMSR-E sea ice temperature products for snow-ice flooding detection
In this study, we examined AMSR-E Sea Ice Temperature product as compared with ice mass balance buoy data from the Southern GLOBEC (Global Ocean Ecosystems Dynamics) project in the Marguerite Bay area on the west side of the Antarctic peninsula (Perovich et al. 2004). This area was noted by Perovich et al. (2004) to have extensive surface flooding of ice floes during the 2001 and 2002 austral spring seasons. Co-located and concurrent AMSR-E Daily Averaged Brightness Temperatures and Sea Ice Temperature (L3, 25 km) data were obtained from a pixel or pixels corresponding to the daily position of the buoy. Several results have been found. (1) the AMSR-E product is generally significantly colder than the actual snow-ice temperature as measured by the buoys, which brings into question its value as a representative of snow-ice temperature, as we have documented previously (Lewis and Xie, 2006; Lewis et al., 2006). (2) when the buoy interface temperature rises to ~~271 K, indicative of surface flooding (Sept. 16), the AMSR-E temperature, instead of a corresponding rise, shows a drop almost instantaneously (within the daily frequency of measurement) of 6?0 K below the buoy interface temperature, although nearly 7 days prior to the buoy interface temperature~{!/~}s indication of surface flooding. As another flooding event (Oct. 29) is indicated by the buoy temperature rise at the end of the record, the AMSR-E again undergoes a sharp decline of 6?0 K, this time nearly at the same time as the buoy temperature indication of flooding. We explain that the good match of the later event, Oct. 29, may be representative of a regional large flooding event, while the Sept. 9 event was actually the first representative flooding event for the region. But locally, at the buoy position, this event did not happen until Sept. 16 as snow depth continued to increase. This result suggests that snow-ice interface flooding events may be detected by rapid brightness temperature decreases from AMSR-E, in contrast to actual interface temperature increases resulting from the sea water intrusion. This analysis projects a great potential of using the AMSR-E temperature to detect flooding events of regional scale. More previous in-situ measurements and their relative AMSR-E data will be analyzed to further test this hypothesis. ht http://www.utsa.edu/LRSG/
C11B-0452
Evolution of Snow Over Sea-Ice during SHEBA using the SNTHERM Snow Model
Snow processes over sea-ice are currently represented with a simple one-layer snow model in the Meteorological Service of Canada (MSC) operational forecasting systems. Although fully coupled with a multi- layer sea-ice model, the snow scheme does not represent detailed processes like snow densification, vertical profiles of density and temperature, liquid water in the snow, among others. Previous studies have shown that the widely accepted SNTHERM snow model predicts reasonably well the behavior of snow during cold periods. But this scheme has not been implemented yet in operational systems. To more realistically describe snowpack and sea ice dynamics, while preserving reasonable computing efficiency, SNTHERM has been included in MSC's physics package and coupled with the multi-layer sea-ice model. This coupled system is evaluated with forcings and validated data obtained from the Surface Heat Budget of the Arctic Ocean (SHEBA) field campaign conducted in 1997. Results with SNTHERM are compared with those obtained using MSC's current operational system; they provide an interesting framework for exploring the ice-snow interactions in the Arctic.
C11B-0453
A Study of Snow Thickness on First- and Multiyear Sea Ice Using Laser and Radar Altimetry
Verifying the accurate thickness of sea ice is of major importance. The snow depth on sea ice is a dominating source of error when detecting the baseline thickness of the polar sea ice using airborne or satellite laser altimetry. By combining laser and radar altimetry new information is obtained and the boundary between the snow pack and the ice surface can be distinguished. This technique will be used on the CryoSat-2 mission, due for launch in 2009. In spring 2006 a major validation campaign of the CryoSat-2 was carried out in the Arctic Ocean North of Greenland and Canada. The aim was to validate the airborne radar and laser instrumentation planned for the CryoSat-2 mission. The radar ESA coherent radar ASIRAS with 13 GHz center frequency and 1 GHz band width resembling the radar on CryoSat-2. In situ snow depths, snow physics, sea ice thickness and freeboard were measured at two sites on the sea ice north of the Canadian Forces Station Alert, Ellesmere Island (82°30' N 62°19' W). Site 1 was on multiyear fast ice (MYI), approximately 5 km from the coast line. Site 2 was approximately 10 km north of Alert, on first-year ice (FYI). These two sites were marked with radar corner reflectors that ensure truly coincident observations. On FYI ASIRAS data show good agreement between in situ and airborne measured data. On MYI the layered snow pack generate uncertainties due to the complex return signal from ice layers of refrozen melt water within the snow pack. The MYI site poses several difficulties due to the disturbed ice field consisting of new ice, old porous ice, refrozen melt water and ridges. These disturbances generally raise the noise level and introduce false reflectors caused by ridges nearby.
C11B-0454
Impact of a Changing Snow Cover on the Development of Arctic Sea Ice Albedo in Spring and Summer
The energy budget and climate of the Arctic region are significantly affected by the variable surface albedo in spring and summer. The albedo of Arctic sea ice decreases dramatically throughout spring as the snow cover on sea ice melts and melt ponds develop at the sea ice surface. This process is implicit in climate model parameterizations of sea ice albedo, based on observations under present-day snow and ice conditions. However, the validity of such parameterizations is questionable if the snow cover on sea ice changes. In order to assess the sensitivity of sea ice albedo on the snow cover, we examine surface melt, meltwater percolation, and melt pond development in the light of mechanical, fluid-dynamical, and thermodynamic processes. Specifically, we consider how melt pond development is affected by a snow cover of inhomogeneous thickness. Preliminary results indicate that mechanical deflection of the ice surface due to snow loading is capable of inducing a lateral meltwater flow at the ice--snow interface if snow patches are at least approximately 30 m in lateral extent. A model of heat and fluid transfer through ice suggests that different ice warming rates under snow cover of variable depth results in lateral variations in the permeability structure of sea ice. Lateral meltwater redistribution is enhanced once the sea ice warms and the lateral and vertical permeability of the sea ice increase. While field observations of these processes appear to be lacking to date, we postulate that these processes work in concert to control the early melt evolution of ponding and hence ice albedo. Eventually, the melt pond distribution is fixed in place by preferential ablation of sea ice that is covered by meltwater.
C11B-0455
Estimating Melt Pond Fraction and Surface Albedo During the Melt Season Using Passive Microwave Data
New techniques are presented for estimating and mapping melt pond fraction and all-sky broadband surface albedo using passive microwave brightness temperatures and estimates of sea ice concentration, age, and thickness. The method is based on non-linear parameterization of NSIDC AVHRR Polar Pathfinder broadband albedo and melt pond fraction. The latter was implicitly investigated using 2D look-up tables for NSIDC AVHRR Polar Pathfinder broadband albedo and surface temperature. Methods were verified with SHEBA field data and high-resolution optical satellite imagery. Derived daily maps (1979-2006) of melt pond fraction and broadband albedo (in the NSIDC Polar stereo 25km grid) clearly illustrate the negative relationship between these two parameters. Melt pond fraction has greater interannual variability in the seasonal ice zone. During the 1990s, surface albedo (melt pond fraction) decreased (increased) in the Beaufort and Chukchi Seas and increased (decreased) in the East Siberian Sea. Duration of the melt pond period was limited by melt onset date in the spring, and freeze onset date or ice disappearance date in the end. There were indications of earlier melt onset dates in the central Arctic. The earliest melt onset dates were observed in 2005 and 2006, while freeze onset dates did not show significant trends during 1979-2006. Mean July melt pond fraction did not show a significant trend, however, surface albedo in July slightly decreased south of 80-degrees north latitude.
C11B-0456
A continuum model of melt pond evolution on Arctic sea ice
During the northern hemisphere summer, absorbed solar radiation melts snow and the upper surface of Arctic sea ice to generate meltwater that accumulates in ponds. Melt ponds are poorly represented in GCMs despite their significant influence on the albedo of sea ice during the melting season. Melt ponds cover up to 50% of the sea ice surface during the melting season, decreasing the albedo of the surface by up to 10%. In our work we developed a stand-alone melt pond evolution model suitable for inclusion in existing GCM sea ice models: considering that these do not describe the real ice topography, various assumptions have been made. Other related issues such as lateral drainage and enhanced melting rates depending on the pond depth are taken into account. We will present the results of a stand-alone melt pond model. Furthermore we will present some preliminary results of the incorporation of this new parameterisation into the stand-alone version of the CICE sea ice model.
C11B-0457
The Iceberg Calving Problem: Can we Model Iceberg Calving with an Empirical Calving Law?
Iceberg calving provides an efficient mechanism to transfer large amounts of ice to the ocean in a near instantaneous fashion, thereby not only drastically changing the mass balance of a glacier/ice sheet, but the geometry of the glacial system. These changes can have a profound effect on the flow of inland ice through a variety of feedbacks. For example, removal of sections of ice can (i) reduce the back-pressure or "buttressing" of inland ice or (ii) perturb basal hydrology and alter the basal sliding conditions. Furthermore, because iceberg calving tends to be important in regions with high longitudinal stresses, the non-local nature of longitudinal stresses allows the possibility that perturbations are transmitted far into the interior of the glacier or ice sheet. Despite this important role iceberg calving is rarely implemented in large scale numerical models. As a first step, we have developed a depth-integrated flowline model that includes vertical shear, longitudinal stresses and (a parameterization of) lateral shear. The simplicity and computational efficiency of this model allows us to explore the time-dependent behavior of a variety of iceberg calving laws in combination with different basal sliding laws (for inland ice) and lateral stress regimes. We present preliminary results, using the model to evaluate the interplay between ice dynamics and a calving law in which calving occurs when the ratio of crevasse penetration depth to ice thickness exceeds a critical ratio. We find that this model exhibits a wide range of behavior ranging from cycling detachment of large tabular icebergs to steady a calving front to sudden meltwater triggered disintegration episodes.
C11B-0458
Theoretical Impact of the Inverse Barometer Effect on Giant Icebergs of the Ancient Arctic
Erosional plowmarks or flutes (furrows) 700 to 1000 m below the sea surface on the floor of the Lomonosov and Northwind Ridges, and the Chukchi Borderland and Yermak Plateau in the Arctic Ocean provide convincing evidence that icebergs carved this region over a period of time ranging from 600 to 20 kybp. [Reed, 2001], [Polyak et al., 2001], and [Kristoffersen et al., 2004] However, taking into account the fact that sea level was lower by 100 to 150 m during glacial periods than during interglacials, and that modern icebergs calving off Greenland and Antarctica are never more than 550 m thick, leads us to the conclusion that these ancient icebergs must have been about 800 to 900 m thick to be able to reach the sea floor. [Reed, 2001] and [Kristoffersen et al., 2004] Here, I evaluate the effect the IBE may have had on an iceberg with the horizontal dimensions of the Antarctic iceberg B15A through experimentation in my theoretical model. In comparison to experiments involving modern iceberg drift, an adjustment I needed to make in the model configuration besides increasing the iceberg thickness to 800 m was to replace ocean current drag across the bottom surface with the basal friction force of ice sliding on till sediment of the ocean floor. I also assume that the sea surface gradient gravitational force should have no impact on grounded icebergs, so in reality I am only testing the impact of the pressure gradient force in determining the theoretical trajectories of the ancient mega-bergs. I also assume that the iceberg is barely grounded or buoyant, so that the freeboard is still approximately \frac{1}{10th} of the iceberg's total height.
C11B-0459
20th Century Russian ice variability: results from a new digital dataset
As the Arctic sea ice pack retreats to record-breaking minimum extents, it is increasingly important to be able to set these changes in a longer-term context. Here, we present a recently digitized set of sea ice charts provided by the Arctic and Antarctic Research Institute (AARI), St Petersburg, Russia. The earliest chart dates back to July 1933 making the AARI ice charts perhaps the longest-lived systematic sea ice record in existance. Converting the ice charts to equal area scaleable Earth (EASE) grid format, we compare the sea ice concentrations reported in the AARI ice charts with those from two other datasets: the National Ice Center (NIC) gridded sea ice charts; and the Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST) dataset. Where the data overlap spatially and temporally, we are able to identify long term and seasonal differences between the datasets. By identifying the underlying causes of these differences, we present the basis for assimilating the concentration data with the view of creating an optimal hybrid sea ice data set for the Arctic. We also present an analysis of sea ice and air temperature variability in the Russian Arctic, derived from the AARI ice charts and meteorological station data, respectively. These results reveal three distinct periods of variability: a period of warm winters and decreasing summer and fall sea ice extent (period A), followed by a cool period of stable or slightly increasing extent (period B) before a period of year-round warm temperatures and ice loss (period C). In magnitude and seasonality, the warming and ice loss during period C are more significant that those during period A. However, the Russian Arctic ice pack did not fully recover during period B, suggesting that the early 20th Century warming during period A may have preconditioned the Arctic for greater change in recent decades. At the end of period B, there is a rapid expansion of both first year and multi year ice extent, which may also have been a catalyst for the subsequent rapid changes in recent years.