Hobart

Seminar Abstract

Friday 19 June, 11.30am (Tas time)
CSIRO Auditorium, Hobart

Benoit Legresy
Senior Scientist/Researcher
CNRS LEGOS
Toulouse, France
and
Visiting Scientist (2009)
ACE-CRC
University of Tasmania

Measuring the present mass balance of Antarctica and Greenland: the satellite altimetry and gravimetry approaches

The mass balance of ice caps of Antarctica and Greenland is of major concern as to their dynamical state as well as to sea level change. Satellite radar altimetry has shown to be of great interest to survey ice sheets and in particular their topography and temporal surface height variation. Height is tricky to recover over land since the surface slope induced error affect widely the absolute height measurement much more than the classical atmospheric or orbital errors on ocean surfaces, penetration of the radar waves in the snow complicate a bit more the height measurement. The difficulty to compare 2 successive measurements made people concentrate on crossover points where at least the 2 measurements must be exactly at the same location and hence receive exactly the same slope/topographic error. However the error budget is in general large over continental surfaces. Here, we applied a method which accounts for both the local errors like the slope induced and the time variation of the radar echoes shape, at each point along track using the long time series. This method has the advantage to open the view on much more data (eg about 100 times more over Antarctica). We applied this processing to both ERS and ENVISAT, to Antarctica and to Greenland. We show the results in terms of seasonal variations both in amplitude and phase and compare the ENVISAT (2003-) period to the ERS2 period (1995-2003). We also computed trends over the 2 periods and show the similarity and differences. Besides, satellite gravimetry (GRACE) is a way to measure time variations of the gravity field thus of the mass changes of the ice caps. We used 10-day GRACE products computed by GRGS (07/2002 – ) grace data to compute maps of ice caps surface height change comparable to the trends from radar altimetry. The comparison shows some significant agreements and large differences. Mainly, some big structures visible in the altimetric trends show up shifted in space in the gravimetric trends. We decompose the interpretation in terms of snow, ice and crust changes. We developed a formalism to investigate the consistency of the 2 datasets, show the potential and limitations of each techniques and put in perspective the known and unknown parts of the mass balance. Levels of measurement errors, reliability and significance of inferred trends as well as implications for the state of balance of the ice sheet are discussed. We will then discuss the availability of appropriate data to monitor the state of mass balance of the large ice caps in the future with upcoming or potential satellite missions.

Seminar Recording

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Location:
CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
To schedule a seminar, contact:
Clothilde Langlais, (Oceanographic seminars) CSIRO Marine and Atmospheric Research (03) 6232 5399
Natalie Kelly, (Biology/Modelling seminars) CSIRO Marine and Atmospheric Research 0438 452 483
Jillian Enraght-Moony, (seminar administrator) CSIRO Marine and Atmospheric Research (03) 6232 5320
Communications Manager, Antarctic Climate and Ecosystems CRC (03) 6226 2265
Margaret Hazelwood, Institute of Antarctic and Southern Ocean Studies (IASOS) University of Tasmania (03) 6226 2971

Last updated 21/07/09