GCOS Ocean Surface ECV
Sea Ice

Definition: Sea Ice: 1. Specifically, ice formed by the freezing of seawater; as opposed, principally, to land ice. In brief, it forms first as lolly ice (frazil crystals), thickens into sludge, and coagulates into sheet ice, pancake ice, or into floes of various shapes and sizes. Thereafter, sea ice may develop into pack ice and/or become a form of pressure ice. 2. Generally, any ice floating in the sea. (from the AMS Glossary of Meteorology)

Introduction: Sea-ice variability is a key indicator of climate variability and change. A sea-ice component of the cryosphere research effort is ongoing, e.g., through the Climate and Cryosphere (CliC) project of the WCRP. Operational sea-ice products are being produced by JCOMM Services groups (JCOMM Sea Ice Page), such as the Expert Team on Sea Ice. Sea-ice extent, concentration, thickness and drift can be derived from the following observations:

1. Satellite passive microwave
2.  Satellite visible
3.  Satellite IR
4.  Satellite SAR
5.  Satellite altimetry
6.  Satellite scatterometer
7.  Sea-ice air-reconnaissance and ship observations
8.  Ice Profiling Sonar (upward-looking sonar (ULS), moored and submarine-based)
9.  Sea-ice in situ drilling
10.  Sea-ice buoys
11.  Observations of snow characteristics on sea ice
12.  Observations by coastal stations
13. Other sensors under development, e.g., electromagnetic, laser, retrieval of ice thickness from sea-ice vibrations caused by waves, etc. 
    

There are presently cryosphere-dedicated research satellites in orbit, e.g., the Ice, Cloud and Land Elevation Satellite (ICESat) and the Cryosphere Satellite (CryoSat-2). These satellites are critical to providing sea-ice thickness and drift, and such measurements need repeating at decadal intervals. Many other satellites are also retrieving sea-ice parameters, e.g., the European Remote Sensing Satellite (ERS-2), Envisat, and Radarsat. Most of the national sea-ice services base their operational output on satellite passive microwave, visual and IR data. 
 
Improved information about sea-ice extent, concentration, thickness, and drift requires:

1. Systematic efforts to improve the quality and coverage of sea-ice thickness observations. 
2.  Validation of algorithms, both for passive and active microwave sensors, particularly for the melt period when wet ice-snow surfaces and melt ponds strongly affect the retrievals.
3.  Improved and validated multi-year ice concentration algorithms. 
4.  Improved retrieval of sea-ice parameters from SAR (ice drift, shear and deformation, divergence, leads, ice ridging, etc.).
5.  Efficient use of SAR data, especially from the new wide-swath satellites (Envisat Advanced SAR (ASAR), Radarsat), requires a free-data policy from the space agencies and better coverage by SAR receiving stations for real-time use of the data.
6.  Exploiting ice motion fields from the Radarsat Geophysical Processor System, whose comparisons with other satellite-derived ice motion fields are promising.
7.  Effective use of Doppler sonar moored beneath the ice. This method may be especially attractive in marginal and seasonal sea-ice zones where the survival time of drifting buoys is very short.
8.  Improved techniques for assimilation of the whole range of sea-ice data into sea-ice/ocean models to provide consistent analyses of sea-ice concentration, thickness, motion and other parameters.

1. The validation of the spaceborne altimeter technique requires collocated observations such as ULS transects from autonomous underwater vehicles and/or submarines, moored ice profiling sonars, and airborne ice profiling sensors, and in situ snow on ice characteristics such as density and depth ULS from submarines have provided significant input to climate monitoring from historical observations to date. The future efficient use of submarine observations for the validation of altimetry depends on whether public release of data is granted and is prompt.
2.  The present ULS array is very sparse and inadequate. The problem, particularly for the Southern Hemisphere, is the destruction of instruments by icebergs – though less so if ULS themselves are deep. Deployment of sensors and data processing are difficult and labour-intensive. 
3.  The Radarsat Geophysical Processor System at the Alaska SAR facility can provide some quantitative ice-thickness information at the thin end of the distribution, but precision is low. High-resolution radar images are costly and the data access is not assured.
4.  Better seasonal and regional analyses of snow depth and density for climate are needed for ice-thickness retrieval from altimetry.

Issues impacting the observation of sea-ice drift include:

1.  Arctic Ocean ice buoys are located/deployed primarily on perennial ice so the seasonal ice pack is poorly sampled.
2.  The Antarctic buoy programme array is small with little engagement of operational agencies.  The large seasonal variability of Antarctic sea ice is a strong limitation to lifetime on the ice.
3.  The use of the passive microwave record for deducing ice motion in both hemispheres, starting in the 1970’s, is under active development. However, it will be necessary to identify and correct the source of occasional significant disparity of ice speeds measured by buoys and computed satellite imagery.
4.  Availability of a large number of SAR images, in particular from the large-swath satellite.

To address the issues raised above, it is proposed that space agencies, through CGMS and CEOS, and working with GCOS and the WMO Space Programme, continue the sustained satellite (microwave, SAR, visible and IR) operations addressing sea ice. The JCOMM will work with the OOPC and CliC to improve the in situ observations from sea-ice buoys, visual surveys, and ULS. 

Implementation of the Oceanic Domain:

• In-situ Data:
  • Component Networks:
    • Sea-Ice Buoys
  • Coordinating Bodies:
    • International Arctic/Antarttic Buoy Programme (JCOMM, DBCP)
  • International Data Centres and Archives:
    • RNODC/DB: ISDM
• Satellite Data:
  • Component Networks:
    • Satellite IR (polar orbit and geostationary)
    • AMSR-class microwave SST satellite
    • Surface vestor wind satellite (two wide-swath scatterometers are highly desired)
    • Satellite SAR
  • Coordinating Bodies:
    • CEOS, CGMS
  • International Data Centres and Archives:
    • Individual space agencies

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Observations: Sea ice variability is a key indicator of climate variability and change which is characterised by a number of parameters. Systematic global observation of sea ice extent and concentration, inferred from passive microwave radiometry, has produced a 30 year record. The length and consistency of this record has made it the most often cited data source for sea ice climate research. Sea ice observations from newer instruments have relatively short records, but offer complementary characteristics such as greater accuracy for determining ice concentration and improved resolution. In addition to monitoring ice extent (the total area covered by ice at any concentration) and concentration (the area covered by ice per unit area of ocean), it is necessary to know ice thickness in order to estimate sea ice volume or mass balance. In the past, only scarce in situ data from boreholes, or upward-looking sonar from moored instruments or submarines, were available for this purpose.

Now, satellite borne altimeters are emerging as an important new data source. Early work with radar altimeters demonstrated the utility of altimetry for ice thickness. The Geoscience Laser Altimeter System (GLAS) on board ICESat, launched in 2003, has provided high resolution ice thickness maps. CryoSat-2, due for launch in 2010, has a radar altimeter that will provide precise ice thickness maps. All-weather, day and night active radar, including the low resolution QuikSCAT scatterometer and high resolution RADARSAT synthetic aperture radar, is sensitive to the unique electromagnetic signature of multiyear ice. This ice has survived a summer’s melt and is generally thicker than younger ice. Active radar and other new sensors played an important part in attributing the surprisingly low Arctic ice extent of September 2007 to various causes. Summer ice extent has had a downwards trend since the 1990s, as determined by the passive microwave record. The active microwave sensors provided data that showed that the Arctic Ocean had lost a considerable amount of multiyear sea ice over the past few years as a result of the prevailing circulation pattern, suggesting that the ice cover was unusually thin as summer began and predisposed to melting back further. Wide area sea ice motion and deformation products from visible band sensors, as well as higher resolution AMSR data, provided corroborating evidence. Finally, investigators using ICESat confirmed that the ice thickness at the beginning of summer was well below its typical average value. Operational ice services place a higher priority on timeliness and accuracy than on consistency over a long data record, and accordingly use a wide variety of near-real-time remote sensing data to construct ice charts. These charts are used by shipping to avoid damage and delay, and to reduce fuel costs; offshore drilling companies; maritime insurance companies; and government environmental regulatory bodies. High resolution synthetic aperture radars, such as those on Envisat and RADARSAT, offer the best source of data for operational services. Data from these instruments provide information on the nature, extent and drift of ice cover and are used not only for status reports, but also for ice forecasting and as an input for meteorological and ice drift models. JAXA’s PALSAR radar provides polarimetric data, which will improve the accuracy of sea ice classification. Low resolution scatterometer observations, such as those from ASCAT on MetOp, can also be used to retrieve information on sea ice extent and concentration in all weather conditions, day or night. Looking to the future, continuation of RADARSAT/ Envisat class radar-equipped missions is important in providing complementary high resolution data to further elucidate sea ice processes. JAXA’s AMSR-E radiometer on Aqua and operational sensors such as the DMSP SSM/I will ensure continuity of the passive microwave global sea ice concentration data source in the near term. The MIS sensor, currently planned for the second NPOESS flight, will be the follow-on sensor for SSM/I. It will offer improved capabilities, including a baseline aperture size of 1.8 m compared to SSMIS’ 0.6 m. The baseline channel selection for MIS includes the SSM/I channel set with minor modifications, with channels at 6 and 10 GHz as well.

In 2006, CEOS defined a series of actions to better meet the GCOS-defined needs for the sea ice Essential Climate Variable: CEOS agencies will examine their respective plans to maintain provision of microwave brightness temperatures and visible/infrared radiances for the sea ice ECV; CEOS space agencies will consult with the science community on appropriate retrieval algorithms of passive microwave observation for reprocessing sea ice products; New space-based measurements and products, including ice thickness and ice drift, will be considered by CEOS agencies as part of their future research missions (Satellite Missions) (Source:CEOS EO Handbook - Earth Observation Plans by Measurement)

Additional Information:

• National Activities Summaries of Operational & Planned Observation Programs (Moorings, ARGO, Sea Level, XCTD/XBT/TSG, TS Hydrography, VOS, Sea Ice, Satellites, Black Sea, BOOS, NEAR-GOOS, Bio/Chem, Carbon, Coastal)

Referrences:

• Sea-Ice Information Services in the World (Third Edition) WMO-No. 574
• Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523
• The Second Report on the Adequacy of the Global Observing Systems for Climate in Support of the UNFCCC - April 2003 (GCOS-82/WMO/TD Noc 1143)
• CEOS EO Handbook - Earth Observation Plans by Measurement
• NOAA/NCDC - Climate Indicators
• BAMS State of the Climate Reports

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