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Catalogue of Satellite Missions
Catalogue of Satellite Instruments
Snow and Ice

Ice-Sheet Topography

Essential Climate Variables: Glaciers, Ice Caps and Ice Sheets

The state of the polar ice sheets and their volumes are both indicators and important parts of climate change processes and feedbacks. Consequently, it is important to monitor and study them in order to investigate the impact of global warming and to forecast future trends. The IPCC expects that, globally, ice sheets will continue to react to climate warming and contribute to sea-level rise for thousands of years after the global climate has been stabilised. They note that:

— Contraction of the Greenland ice sheet is projected to continue to contribute to sea-level rise after 2100. Current models suggest virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m if global average warming in excess of 1.9oC to 4.69oC relative to pre-industrial values were sustained for millennia;

— Ice dynamic models suggest that melting of the West Antarctic ice sheet could contribute up to 3 m of sea level rise over the next 1000 years, but such results are strongly dependent on model assumptions regarding climate change scenarios, ice dynamics and other factors.

Satellite remote sensing allows observations of the changes in the shape of ice sheets, and identification of the shape and size of large icebergs that have detached from the ice sheet. SAR instruments are one source of data on the polar ice sheets. RADARSAT-2 provides routine surveillance of polar regions and has created the first
high-resolution radar images of Antarctica, enabling detection of changes in the polar ice sheet and improved understanding of the behaviour of the Antarctic glaciers.
spacer ASAR on the Envisat mission continued the observations of polar ice topography started by the ERS-1 and ERS-2 satellites.

Interferometric measurements by PALSAR, together with observations by the AVNIR-2 instrument on JAXA’s ALOS mission (to be continued with ALOS-2 from 2014) contributed to understanding the ice-sheet mass balance and glacier variation near the South Pole and in Greenland.

Altimeters provide useful data on ice-sheet topography. While many have high vertical resolution, their limited horizontal resolution means that their observations over smoother, near-horizontal portions of ice sheets are of greatest value. The RA-2 instrument on Envisat has provided improved mapping of ice caps.

Given the significance of information on changes in the continental ice sheets, two missions dedicated to their study have been developed: NASA’s ICESat (launched January 2003 and concluded in August 2010) and ESA’s CryoSat-2 (launched 2010, following the loss on launch of CryoSat in 2005). CryoSat-2 provides an instrument for the ice sheet interiors and margins, for sea ice and other topography, with three-mode operation:

— Conventional pulse-limited operation for the ice-sheet interiors (and oceans if desired);

— Synthetic aperture operation for sea ice;

— Dual-channel synthetic aperture/interferometric
operation for ice-sheet margins.

Click to view the Ice Sheet Topography mission timeline.


Snow Cover, Edge and Depth

Essential Climate Variables: Snow Cover

Regular measurements of terrestrial snow cover are important because snow dramatically influences surface albedo, thereby a significant impact on the global climate, as well as influencing hydrological properties and the regulation of ecosystem biological activity. The IPCC has found that – on the evidence of satellite data – there is likely to have been a decrease of about 10% in the extent of snow cover since the late 1960’s.

Snow forms a vital component of the water cycle. In order to make efficient use of meltwater runoff, resource agencies must be able to make early predictions of the amount of water stored in the form of snow. Coverage area, snow water equivalent and snow pack wetness are the key parameters to be determined in this process.

Snow cover information has a range of additional applications such as detecting areas of winterkill in agriculture that result from lack of snow cover to insulate plants from freezing temperatures. Locally, monitoring of snow parameters is important for meteorology and for enabling warnings of when melting is about to occur – which is crucial for hydrological research and for forecasting the risk of flooding.

A range of different instrument types can contribute to measurements of snow. Visible/near-infrared satellite imagery provides information of good horizontal and temporal resolution and accuracy on snow cover in the daytime in cloud-free areas. AVHRR provides snow cover information and this will be continued in the future by VIIRS on Suomi NPP and the planned JPSS series. MODIS data are being used to monitor the dynamics of snow and ice cover over large areas (greater than 10 km2) and, on a weekly basis, to report the maximum area covered by both. The resulting snow maps should be available within 48 hours of MODIS data collection.

Passive microwave instruments such as SSM/I and AMSR/E (on Aqua until October 2011, and on GCOM-W, launched 2012) have all-weather and day/night monitoring capability, and are able to estimate the thickness of dry snow up to about
80 cm deep.

Data from RADARSAT and ERS-2 have shown the usefulness of SAR remote sensing techniques to determine snow area extent and to monitor the physical conditions of snow. Envisat and RADARSAT-2 have provided continuity of such snow information.

Click to view the Snow Cover, Edge and Depth mission timeline.

Sea-Ice Cover, Edge and Thickness

Essential Climate Variables: Sea Ice

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 ICESat, launched in 2003 and completed in 2010, has provided high-resolution ice thickness maps. CryoSat-2, launched in April 2010, has a radar altimeter that provides precise ice-thickness maps. A precise first map of Arctic sea-ice thickness was produced by CryoSat-2 in June 2011, complementing observations of the dramatic decrease of sea-ice extent at the end of summer from around 8 million sq km in the early 1980s to 2007’s historic minimum (nearly equalled in 2011) of just over 4 million sq km.

ICESat-II (Ice, Cloud and land Elevation Satellite-2) is the second-generation of the laser satellite ICESat, scheduled for launch in 2016.

All-weather, day and night active radar, including the low-resolution QuikSCAT (complete 2010) 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, with thin seasonal ice replacing thick older ice as the dominant type between the winters of 2004 and 2008 for the first time on record.

  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-2, 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 provided polarimetric data, which improved 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, such as ESA’s Sentinel-1 (first launch 2014, so there will be a service gap for European missions given the recent conclusion of Envisat), is important in providing complementary high-resolution data to further elucidate sea-ice processes.

JAXA’s AMSR-E radiometer on Aqua (failed October 2011) and GCOM-W (launched 2012) 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.

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.

Click to view the Sea Ice Cover, Edge and Thickness mission timeline.