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1.1.2 Scientific Background

The main objective of the EnviSat Radar Altimetry Mission is to ensure the continuity of the altimetric observations started with the ERS-1 satellite in 1991. The science mission objectives are similar to that of ERS but the length of the altimeter record will exceed 15 years and will permit the examination of changes on interannual to decadal time scales of:

  • global and regional sea level
  • dynamic ocean circulation patterns
  • significant waveheight and wind speed climatology
  • ice sheet elevation, sea-ice thickness

 

Another objective is to provide for the enhancement of the ERS mission, notably in ocean and ice missions, by improving the quality of the measurements and monitoring capabilities for:

  • Ocean mesoscale, significant wave height and wind speed in Near Real Time
  • Marine geophysics - Polar oceans
  • Ice sheet margins - sea ice
  • Lakes, wetlands and river levels
  • Land
  • Ionosphere, water vapor

 

The EnviSat Mission is part of a coherent European Earth Observation Programme ensuring the long-term provision of continuous data sets, essential for addressing environmental and climatological issues. As such, the ENVISAT Altimetry Mission is a contribution to international Earth Observation programmes such as International Geosphere Biosphere Programme and World Climate Research Programme. ENVISAT also aims at the promotion of applications and commercial use of Earth Observation data, namely for Altimetry: the operational sea-state and ocean circulation forecasting.

 

Oceanographic Applications

 

The Ocean covers 70% of the Planet and plays a key role in regulating the global climate. The ocean is the main reservoir for heat as well as a powerful vehicle to transport warm water masses poleward. It has the capacity to intake (but also reject) significant amounts of carbon dioxide, one of the greenhouse gases. It also supports a cost-effective type of transportation of goods, is a milieu where to discover new oil fields, is a feeding ground for fish and sea-food to nourish the ever-growing human population. The Oceanographic mission objectives of EnviSat are derived from the ERS results. To resolve low frequency signals in the ocean spectrum and to further understand oceanic processes a longer time series is however needed. The Oceanographic mission objectives of EnviSat Altimetry are dynamic topography monitoring, mesoscale variability, seasonal and interannual variability, mean global and regional sea level trends, marine geophysics -especially in polar oceans even covered with sea-ice-, sea-state monitoring. The objectives are to be met with data products available either in near real time (3 hours), in quasi near real time (2-3 days) or with the highest precision off-line products (50 days).

 

Seasonal and interannual variability has an important impact on climate. Planetary waves propagate from months to seasons across basins to adjust the ocean in response to wind forcing. Interannual variations of the seasonal or annual cycles have a direct and sometimes dramatic impact of the global climate, well illustrated by the El Niño-Southern Oscillation (ENSO) (Figure 1.1 ). The data serve mainly to tune and evolve global ocean and atmosphere models, to better understand the ocean-atmosphere interaction and the underlying processes.

image
Figure 1.1 Series of Sea Level Anomaly (cm) in the Tropical Pacific. Each row is one year -'97, '98, '99, '00- with one sample of the sea level anomaly field at each season, by column: March, June, September and December. The strong El Niño Event of late '97 followed by a La Niña event is clearly visible. A film of such 3D vignettes helps the researcher "visualize" the wave propagation involved in such events. Each weekly field can be assimilated in an ocean model. (SLA data processed by R. Scharroo, DEOS, NL, graphics processed at ESA/ESRIN.)

The ocean is vast and hosts a full spectrum of signals. One orbiting satellite alone cannot pretend to cover that spectrum. There are significant advantages in merging the data from two or more Altimetry missions sampling the Earth with different orbital patterns. A good illustration is the enrichment in space resolution of the mesoscale variability field computed with merged data from ERS and Topex-Poseidon (Figure 1.2 ) which are in the same orbital configuration as EnviSat and Jason will be [1].

imagefull size
Figure 1.2 Root mean square of sea level anomaly (in cm) obtained from merged ERS and T/P data from 0ctober `92 to October `97. Note the high resolution of the map brought by the denser ground track mesh of ERS. (Courtesy of PY LeTraon, CLS.)

Ice and Sea-Ice Applications

Polar ice sheets and sea ice play a vital role in the global climate system due to both their effectiveness in reflecting incoming solar radiation, and as a huge store of freshwater. Sea Ice acts as a barrier between the ocean and the atmosphere, cutting off exchanges of heat, moisture and momentum. Brine expulsion during seasonal sea ice formation and intense cooling of the sea surface through polynyas drive the thermohaline circulation of the oceans. This process creates the dense bottom water in the Pacific, Indian and Atlantic Oceans. It is responsible for the poleward transport of heat in the North Atlantic, which ensures mild winters for Western Europe.

 

This critical component of the climate system is not well represented in current climate models but is clearly important if accurate predictions of the consequences of global warming are to be made. Global warming is predicted to be greatest in the Arctic region. If Arctic sea-ice is lost, it could change the circulation pattern of the North Atlantic resulting in severe winters for Western Europe. Melting of the Greenland and Antarctic ice sheets would contribute to global sea level rise.

 

The vast, remote and inhospitable polar regions can only be effectively monitored through a global remote sensing system. Polar regions experience between 50 and 90% cloud cover and spend long periods in darkness, which limits the observations of optical and thermal infrared instrument. However, this task is particularly well served by satellite-borne active Radar instruments.

 

Techniques developed using the ERS Radar Altimeters have allowed the monitoring of ice sheet mass balance and the derivation of sea ice thickness through the measurement of freeboard. Continuous Altimetric measurement of the Antarctic ice sheet since 1992 has revealed for the first time a significant thinning of a West Antarctic glacier Figure1.3 . The Pine Island Glacier has retreated and thinned inland by as much as 10 metres. It is important to continue this monitoring with the ENVISAT RA-2 to establish if this retreat will accelerate the mass discharge from the West Antarctic Ice Sheet.

image
Figure 1.3 Rate of elevation change of the lower 200km of the Pine Island Glacier. The coloured dots are located at crossover points and have an area equal to the RA footprint. The grey shading is the velocity field derived from ERS SAR data. (Produced at MSSL/UCL.)

 

Balance velocities, i.e., depth-averaged velocity required to maintain the ice sheet in a state of balance at a given point for a given surface mass flux, have been estimated over the Antarctic grounded ice sheet using ERS altimeter data (Figure1.4 ). Balance velocities depend mainly on the surface slope and are modulated by surface mass balance and ice thickness. Their study contributes to the understanding of ice sheet dynamics and their response to climatic forcing.

 

image
Figure 1.4 Balance velocities estimated from ERS Altimeter. (Produced by F. Remy, LEGOS)

Sea-ice thickness can be sampled using moored or submarine mounted Upward Looking Sonar (ULS). Moored ULS are only sampling a fixed location and Submarines tend to sample limited areas for only a few weeks each year. This is not sufficient to deduce full regional and seasonal variations. Freeboard measurement by satellite is the only technique which can measure sea ice thickness at the time and length scales that climate investigation demands. The technique has been developed using ERS Altimeter data, verified using the ULS measurements and implemented in the ENVISAT RA-2 ground processing. Results from ERS suggest that the recently reported thinning of Arctic sea ice may be localised (Figure1.5 ). Continued monitoring by the EnviSat RA-2 is critical to establish long term trends.

imagefull size
Figure 1.5 A comparison between RA derived sea ice thickness (m) and sparse measurements from Upward Looking Sonar on submarines (dots). Both sets of measurements are from October 1996. (Produced at MSSL/UCL.)

The EnviSat Altimetry mission will extend and improve the monitoring of the cryosphere in the climatically important Polar Regions.

 

Land Applications

Over land the Radar Altimeter echoes have a non-predictable shape. This is why this field of applications has slowly and painstakingly matured. A remarkable result from the ERS 1 Geodetic phase is the Altimeter Corrected Elevation Model ACE [Berry et al., 2000] which replaces more than 28 % of the most precise Global Digital Elevation Model with an altimeter derived height dataset and corrects another 17 % (Figure1.6 ). The Envisat Altimeter , even if it will not fly an orbit as dense as the ERS-1 Geodetic mission, will enhance this result in terms of accuracy and new areas never measured before, thanks to its enhanced tracking capacity.

image
Figure 1.6 This figure compares the ERS altimeter derived map (ACE) of part of the Amazon basin together with the GLOBE map of the same area (73-68W, 11-6S). Note the 100m contours in the GLOBE histogram and the fine detail -rich histogram- in ACE. (Produced by P. Berry, De Montfort University, UK)

Another Land application that has been painstakingly attempted since SeaSat is river and lake levels monitoring. Radar Altimetry is a powerful tool for this application as it unifies all the worldwide river and lake level measurements, even the ones most remote or inaccessible, with a unique gauge. Being able to measure the global river levels, be it only once or twice a month would be a significant contribution to Hydrology. It has been demonstrated with ERS Altimeters that echoes from an continental water surface are clearly discernible and convertible to river or lake levels. The inclusion of an Ice mode on ERS-1 and ERS-2 has led to a huge increase in the percentage of the earth's land surfaces from which valid altimeter echoes have been obtained. This has also resulted in coverage of the majority of the world's river systems, raising the exciting possibility of a ten-year time series of river height data. The inclusion of a third tracking mode on EnviSat should further increase the land hydrology potential of Altimetry with even greater river coverage, as well as continuing the hydrology time series. To illustrate the ERS/EnviSat contribution, Figure1.7 shows part of the Amazon River system, with 35-day river crossings from ERS-2 superimposed on an altimeter derived rivers map.

image
Figure 1.7 River echoes superimposed on high-resolution ERS-1 Altimeter derived topography and river network. The plot shows a 11x12 degree square of South America containing part of the Amazon basin (15-4S, 75-63W) with the ACE GDEM heights (high: yellow through red low: green to dark blue) overlaid with ERS-1 Geodetic Mission "water type" returns in sulphur yellow. Note that this part of the ACE GDEM is totally derived from Altimetry, which has provided a huge increase in spatial and vertical resolution over previous GDEM models for this region rich in river networks. (Produced by P. Berry, De Montfort University, UK)


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry