Description of task
Ice thickness will be mapped by an integrated system, whose near-real-time output will be an Arctic-wide grid of mean ice thickness, probability density functions of ice thickness, and the distribution of ridge depths.
Continuous monitoring will be undertaken by tiltmeter buoys, satellite altimetry and upward sonar from floats, while specific profiling experiments will be carried out using AUVs, helicopter EM and airborne laser in order to validate monitoring systems and obtain higher-resolution data on ridge shapes. The integrated mapping system consists of in situ and remote sensing components as follows:
- Tiltmeter ice thickness buoys (DAMTP, SAMS, NERSC).
- A network of floats, equipped with upward sonar (UPMC).
- Satellite altimetry, using ENVISAT (ESA) and possible input from the NASA IceSat satellite (DNSC, DAMTP).
- Data acquisition in the marginal ice zone from autonomous underwater vehicles (AUVs) and in the interior from submarines (DAMTP).
- Laser profiling, from airborne missions (DNSC).
- Airborne EM thickness profiling by helicopter, from ships and from land/ice bases (AWI).
Components (1) to (3) comprise a long-term monitoring system, designed to give near-real-time information on ice thickness properties for use in modelling. Components (4) to (6) comprise a validation and profiling system, designed to provide validation for the monitoring methods and including specific experiments to yield higher-resolution data than the monitoring system, to delineate the shape of the ice surface for applications such as understanding the role of pressure ridging.
Tiltmeter buoys use an orthogonal pair of tiltmeters with on-board processing and data/spectra transmission by low-orbit satellite. The buoys measure the power spectrum of flexural-gravity waves propagating through the ice, whose peak frequency is a function of area-averaged ice thickness. These buoys have been validated during the GreenICE and SITHOS EU projects. The existing rugged low-cost buoy will be used as the basis for an air-droppable version, deployed in concert with the annual International Arctic Buoy Programme (IABP) “White Trident” missions. Other buoys will be emplaced by hand at the Russian ice station and at ITP sites, although not connected to the ITP transmission system. The sensors will also deliver positional information.
Isobaric floats will measure ice draft, as well as temperature, salinity and currents (core theme 3), transferring data acoustically to the ice-tethered data systems when in range. The minimum dataset expected from this system will be a probability density function (pdf) of ice draft within the region of float operation, but if the acoustic tracking system for float velocity is accurate enough it is hoped to convert this into a true profile of ice draft.
Satellite radar altimetry will employ data from ENVISAT. In conjunction with the directly measured datasets listed above, the ENVISAT data will go on to provide a long-term basin-wide monitoring capability for mean thickness. If the NASA IceSat laser altimeter is still in operation, this will also be employed, with validation as for the airborne laser.
AUVs with swath-sounding sonar measure both ice thickness distribution and the actual shape of the under-ice surface in three dimensions, which includes the frequencies and underwater shapes of pressure ridges, an important parameter since recent submarine work has indicated a radical diminution in the occurrence of deep ridges in the Arctic, with the frequency of such ridges decreasing relatively faster than the thinning of the ice cover as a whole.. Plans involve the use of the Autosub AUV from CCGS “Amundsen” in the Beaufort Sea, and a smaller AUV (e.g. Canadian “MUN Explorer” and Icelandic Gavia) from this ship and from ice camp launching sites.
Ice bottom profiles from submarines will be an essential addition to the ice profile dataset, involving (a) analysis of data from the most recent UK cruise in April 2004, (b) its integration with earlier UK and US data to map changes of the thickness pdf in parts of the Arctic covered by submarines, (c) addition of new data from voyage(s) of opportunity during the DAMOCLES project, (d) integration with existing long-term datasets from moored upward sonar systems (NPI). The aim is to obtain a systematic 25-year+ dataset for modelling purposes (core theme 4) and to delineate the full shape of the ice underside to resolve the changing nature of the pressure ridge distribution.
Laser profiling will be carried out by DNSC using a swath sounding laser from a Twin Otter. This produces a map of freeboard heights, which requires validation in the same way as radar altimetry, with the additional need for a correction for snow depth effects. Snow depths will be measured by combination of radar and laser measurements on some flights.
Airborne EM sounding will be carried out by AWI using the EM31 helicopter EM system, focusing on the polar margins (e.g. the area north of Ellesmere Island and Greenland) which are accessible from land stations such as Nord and Alert with increased penetration into the Arctic Ocean achievable by setting out fuel dumps on the ice and making use of the North Pole and Russian drifting stations. This system of operation will also be used by the DNSC Twin Otter, and in both cases the aircraft can combine their remote sensing roles with supply and deployment operations (see logistic plans).
