What is this IMOS Sub-Facility all about?
High precision satellite altimeter missions including TOPEX/Poseidon (T/P), Jason-1 and now OSTM/Jason-2, have contributed fundamental advances in our understanding of regional and global ocean circulation and its role in the Earth's climate and regional applications. These altimeter satellites essentially observe the height of the global oceans – as such, they have become the tool of choice for scientists to measure sea level rise – both at regional and global scales as well as giving information about ocean currents and large- and small-scale variability.
The determination of changes in global mean sea level is of fundamental importance in understanding the response of the ocean to a continuing warming climate – both through thermal expansion of the ocean, melting of the major ice sheets of Greenland and Antarctica, and mountain glaciers, and redistribution of water over the continents and atmosphere. As with all scientific observations, it is vital that the measurement tool is operating within its specifications – calibration and validation is therefore an important component in this regard.
This IMOS sub-facility provides the sole southern hemisphere in situ calibration site to provide an ongoing calibration and validation data stream directly to the international (NASA and CNES sponsored) Ocean Surface Topography Science Team (OSTST). This contribution, importantly, spans multiple altimeter missions, thus enabling the assimilation of multi-mission satellite data to determine ongoing changes in sea level with flow-on effects to other uses of this data. The OSTST collectively oversees the operation and calibration of the precision altimetry missions, and ensures each of these missions is performing as accurately as possible in order to meet mission objectives.
The IMOS supported calibration site in Australia is one of four primary in situ calibration/validation sites that contribute to the Ocean Surface Topography Science Team (OSTST). The remaining sites include Harvest (USA), Corsica (France), and Gavdos (Greece).
The Australian calibration site includes two comparison points where in situ data is compared against the altimeter – Bass Strait and Storm Bay. These two locations both lie on descending (N -> S) pass 088 of the satellite altimeter, and thus share similar satellite orbit characteristics. The use of these two sites allows detailed investigation into the accuracy of the altimeter over two different wave climates. The average significant wave height at Storm Bay is approximately double that observed at the comparatively sheltered Bass Strait location.
One of the ongoing issues with satellite altimeter missions is the “Sea-state bias” correction which is related to wave shape and height. We plan to use the different wave climates at the two locations, coupled with the fact that some other things (such as orbit errors) are the same at both locations to improve the quality of this correction.
How does this IMOS Sub-Facility work?
This IMOS sub-facility directly estimates the performance of the altimeter (and its sub-systems) at the Bass Strait and Storm Bay comparison points, thus providing a continuous calibration and validation record to the OSTST to assess altimeter accuracy. Specifically, the sub-facility determines the “absolute bias” of the altimeter for each over flight. The absolute bias is the difference between sea surface height (SSH) measured by the altimeter, and SSH measured in situ (using an independent technique). Understanding the contributions to absolute bias assists in ensuring an accurate and reliable altimeter data record. By monitoring changes in bias, or “bias drift”, the sub-facility is able to track any spurious drifts that may otherwise express as erroneous changes in regional or global mean sea level.
To determine the altimeter absolute bias, a range of tools and in situ instruments are used. Our technique involves a novel combination of moored oceanographic instrumentation (pressure gauges, temperature and salinity sensors), surface based GPS equipped buoys, and coastal tide gauges and continuously operating Global Positioning System (GPS) sites. The moored oceanographic instrumentation (deployed at specific offshore comparison points under the orbiting altimeter) provides a highly precise SSH record by converting the water pressure measured by the pressure gauge to height (i.e. taking into account the density of the water column), yet lack an “absolute” datum that makes them suitable for direct comparison with the altimeter. To derive this absolute datum, GPS equipped buoys are deployed approximately every three months over the mooring locations. The combination of the GPS buoy data and mooring derived SSH, enables the generation of a sea level time series that can be compared to the altimeter, for each over flight (once every 9.9 days). Outside of the period of operation of the mooring, we use a coastal tide gauge that has been tidally corrected and transformed to the datum that exists at the offshore comparison point. This has enabled the generation of absolute bias estimates in Bass Strait back to the time of the launch of TOPEX/Poseidon in 1992.
The computation of bias drift requires an alternate approach that takes advantage of many more comparison points, thereby reducing the noise and revealing subtle drifts in the altimeter bias. To achieve this, the altimeter record is compared with specific long running tide gauges across the global network. Importantly, the land motion of these tide gauges is computed using analysis of Global Positioning System (GPS) data that is either recorded co-location with the tide gauge, or in a nearby location.