Satellite Remote Sensing

Summary

An altimetry-derived map of sea surface height anomaly (LHS) and geostrophic current vectors overlaid on an SST image off Sydney in 2011 (IMOS).

Earth observing satellites provide a uniquely efficient means of making repetitive observations automatically over vast areas of the ocean.  Although confined to the surface layer, over the past 30 years these observations have been a vital tool for both biological and physical oceanographers and have played a key role in advancing both sciences. The objective of the IMOS Satellite Remote Sensing Facility is to make more of these data, of better quality, more available and easier to use for research. It does this through supporting collection, management, processing, calibration and validation of satellite data, and providing properly documented data in standard formats that are widely supported and familiar to the research community.

Satellite-borne sensors measure spatial and temporal properties of the sea surface using ranges of the electromagnetic spectrum in a variety of different ways. Instruments using the infra-red and microwave spectrum observe changes global, regional and local in sea surface temperatures, revealing both the short term dynamics of ocean currents and the long term variation of the ocean climate. Optical sensors use sunlight reflected from the upper ocean layers to infer and quantify biological activity and, in coastal regions, to detect the presence of other optically active constituents such as suspended solids and dissolved organic matter.  Radar altimeters provide measurements of sea surface height with unmatched precision over the global ocean, revealing regional variations in water density and giving one of the most reliable measurements of the long term change in ocean volume. Other specialised, and increasingly innovative, sensors measure surface roughness, wind direction, and even surface salinity.

Current research priorities for satellite remote sensing include:

  • The global carbon cycle
  • Role of the ocean in climate change
  • Operational ocean modelling using sea surface temperature (SST) and altimetry data
  • Water quality applications, such as land run-off effects on aquaculture, fisheries, habitat and coral reefs
  • Data assimilation for ocean biogeochemical modelling
  • Phytoplankton functional types
  • Development of regional algorithms for complex coastal waters

IMOS’ satellite remote sensing facility supports all of these by supporting data provision, management and quality.

Instrumentation and Data

In the absence of an independent Australian national satellite capability, IMOS has adopted a dual strategy to support remote sensing infrastructure for marine research:

  1. Supporting the production of and access to standardised quality controlled products that are in high demand by researchers
  2. Collecting and organising in situ calibration and validation data that is available not only for local algorithm development, but is also provided to international space agencies for the improvement of global algorithms and products, especially making them more accurate in Australasian regional waters.

There are five satellite remote sensing sub-facilities operating infrastructure and delivering data for IMOS. Three of these focus on sea surface temperature, ocean colour and sea surface altimetry data, while the other two undertake data acquisition, management and provision activities.

The GHRSST SST products sub-facility operated by the Bureau of Meteorology produces sea surface temperature products for the Australasian region, Southern Ocean and Indonesian Seas from polar orbiting and geostationary satellites.  These data are processed, quality controlled and formatted to conform to the global standards set by the Group for High Resolution Sea Surface Temperature (https://www.ghrsst.org/) to whom they are provided, making a significant contribution to the global data set. A variety of gridded and temporally aggregated products are available.

The ocean colour sub-facility, hosted primarily within CSIRO, produces a 1km daily ocean colour product for the Australian region using the global algorithms. The production infrastructure, located at the national eResearch peak facility in Canberra, supports a variety of research activities by maintaining base data archives and data processing capabilities that can be used to produce customised data products. Through this sub-facility IMOS is also supporting the assembly of a bio-optical database of Australian waters, a compilation of in situ observations from multiple institutes and agencies over more than a decade, that can be used to validate algorithms. IMOS also supports the operation of autonomous spectral radiometers on research vessels to collect underway in situ data used to develop and validate atmospheric correction algorithms. In addition the Lucinda Jetty Coastal Observatory is an established tropical site with both above and below water radiometry and atmospheric monitoring that contributes a time series of measurements to the global algorithm development community. A further important activity is supporting Australian participation in the SeaHARRE round robin inter-comparison experiments that ensures Australia’s lab capability for the analysis of in situ samples is at the forefront of international best practice.

The Satellite Altimetry Calibration and Validation sub-facility operates the only Southern Hemisphere calibration site for satellite altimetry missions, and contributes two data streams to the international Ocean Surface Topography Science Team (OSTST). The calibration site incorporates a series of moored oceanographic sensors that enable the determination of a precise sea surface height time series. These data are used to assist in the assessment of the quality and accuracy of the primary geophysical data record as provided by the mission agencies. 

The long-term site in Bass Strait spans multiple satellite altimetry missions, providing a vital means of ensuring the continuity in this key satellite record for understanding ocean circulation, sea level rise, and the role of the ocean in the global climate system.  The gridded sea level anomaly (GSLA) product produced by IMOS OceanCurrent ingests altimeter data that has been assessed and improved based on the international cal/val effort undertaken by the OSTST.

The primary data sets supported by each of these three sub-facilities, SST, ocean colour and GSLA, can all be explored via the IMOS OceanCurrent web site.

IMOS makes a contribution to the ongoing operation, by the Australian Institute of Marine Science, of a satellite data receiving station outside Townsville in Queensland. This station is ideally located to provide coverage of the Coral Sea and the Great Barrier Reef. Data streams collected include the NOAA polar orbiting satellites that provide the backbone of the SST products, the NASA MODIS sensors used for ocean colour, and other satellites that relay in situ environmental data. All Earth Observation data collected are made available through IMOS.

Lastly, the Remote Sensing Collections sub-facility supports generic data acquisition, processing and management for both the SST and Ocean Colour sub-facilities, as well as collecting and preparing additional satellite data sets from around the world to enable their access via IMOS. Remote sensing data are amongst the largest (by volume and number of files) data sets served by IMOS.  The collections sub-facility performs a data presentation quality and consistency check on these large data sets, and maintains a primary super-archive from which the IMOS Portal draws the data sets that it serves directly.  While primary  access to Satellite Remote Sensing data discovery and exploration is through the Australian Ocean Data Network (AODN) Portal, Remote Sensing Collections can provide advice and assist with specialised data needs, such as complex requests, direct access to large volumes or subsets of non-gridded data (eg for matchup analyses). The sub-facility plays a role in managing and merging the Australian NOAA/AVHRR data set that underpins the SST sub-facility, and in capturing and curating remote sensing data sets from overseas that are of interest to Australian marine researchers, such as those from the new sea surface salinity missions or the NASA global chlorophyll data sets.

Application of Data

  • In May 2015, our bias drift contribution to the OSTST was published in a new paper in the journal Nature Climate Change , together with a commentary in The Conversation. This work investigates the altimeter derived trend in global mean sea level by comparing altimeter data with sea level observations from the global tide gauge network. There are three main findings:

    • First, we confirm that sea levels have risen faster over the altimeter era compared with proceeding decades over the twentieth century as observed by tide gauges.
    • Second, after taking into account land motion, our results suggest that the first six years of the altimeter record slightly overestimates the sea-level trend. Our revised estimate over the full altimeter period (1993-mid 2014) is +2.6-2.9 mm/yr (with the exact value depending on the method used to estimate vertical land motion), slightly less than the previous estimate of +3.2 mm/yr.
    • Third, without corrections for bias-drift, the previous record showed a slower rate of sea-level rise over the most recent decade, relative to the proceeding decade. Our revised record is significantly different and suggests that the rate has actually increased in line with accelerating contributions from the Greenland and West Antarctic ice sheets. Our acceleration is also consistent with the acceleration determined from the IPCC sea level projections.

    The IMOS team is one of a number of international groups that actively participate in the process of validating and further understanding this important climate data record.

  • NSW floodwaters seen from space and by an IMOS glider. The storms that hit the Hunter Valley region of NSW on 21-22 April 2015 caused much flooding as well as damage from winds and huge waves. River discharge mixes with seawater to form a buoyant mixture that can take some time to disperse, depending on the influence of winds and ocean currents. To read more click here.

Useful Links

The Satellite Remote Sensing Facility Publication Report  - If you have any questions regarding the data, or corrections, or would like to add a publication or presentation that uses IMOS data please contact the IMOS office via email: publication(at)emii.org.au.

Contacts:

Dr Edward King (Facility Leader)

Dr Helen Beggs

Mr Craig Steinberg

Dr Thomas Schroeder

Dr  Nick Hardman-Mountford 

Dr Christopher Watson

Partners:

Bureau of Meteorology

CSIRO Oceans and Atmosphere

Geoscience Australia

Australian Institute of Marine Science

University of Tasmania 

Your access to the IMOS Satellite Remote Sensing data discovery and exploration is through the Australian Ocean Data Network (AODN) Portal.