23 year sea surface temperature products from IMOS Helen Beggs1, Leon Majewski1, Christopher Griffin1, Edward King2 and Janice Sisson1 1Bureau
of Meteorology, Melbourne, Victoria, Australia 2CSIRO Oceans and Atmosphere Flagship, Hobart, Tasmania, Australia
Introduction
Applications
Validation
Satellite imagery for sea surface temperatures (SST) has been acquired around Australia on a daily basis since the early 1990’s. Building on the archives and processing expertise of CSIRO and the Bureau of Meteorology, IMOS has made these data available as a consistently processed, calibrated and formatted archive of SST maps for the Australasian region. The collection consists of all overpasses recorded in the Australian and Antarctic region (Fig. 1) from Advanced Very High Resolution Radiometers (AVHRR) aboard all satellites in the National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental series from April 1992 to the present, and stored with full resolution (1 km at nadir). The SST values are derived by regressing the AVHRR brightness temperatures to SST observations from drifting buoys in the region. The system smoothly and automatically corrects for changes in each AVHRR sensor's bias using a running 1-year calibration window adjusted monthly. The data are formatted and flagged according to the conventions and standards of the international Group for High Resolution SST (GHRSST) collaboration (www.ghrsst.org). The data records contain time varying error estimates and quality level flags for each SST value, calculated using matchups with drifting buoy SST data and proximity to cloud, respectively. These records form a unique 23-year data set that supplies qualityassured SST values to within 2 km of coasts.
The real-time IMOS L2P products are currently ingested into operational level 4 (L4) SST analyses – NASA's 1 km G1SST (Fig. 3(a)) and the Bureau of Meteorology's 9 km regional and 25 km global daily SST analyses (www.bom.gov.au/marine/sst.shtml). The 2 km resolution of the IMOS AVHRR L3 products makes them particularly useful in coastal applications where the coarser (4 km) globally-available L3, and 1 km statistically interpolated level 4 (e.g. Fig. 3(a)), SST products are unable to provide sufficient detail. Dayonly or night-only IMOS SST products such as those shown in Fig. 3 can be used to measure diurnal warming in coastal regions and SST changes over 2 km spatial scales to within 2 km of coasts.
Time series validation plots for the IMOS L2P SST are available on-line for each NOAA satellite, with verification against drifting and moored buoys, Argo floats and ships. Fig. 6 indicates that there appears to be no temporal drift in the mean or median bias compared with drifting buoy observations. The IMOS SST calibration therefore appears stable across satellites. More recent satellites (NOAA17, 18, 19) produced SSTs with significantly smaller biases and standard deviations than earlier NOAA satellites. See http://imos.org.au/sstproducts.html for links to validation web pages.
(a)
The foundational data product, known as Level 2 Pre-processed (L2P), is an unmapped swath of geo-located and calibrated data from a single satellite overpass. For many applications, users prefer re-projected (mapped) products that can be more easily related and compared with other observations (and model outputs), and also products without gaps due to cloud. To meet these needs, a hierarchy of mapped (0.02º x 0.02º) and time-averaged data sets are derived from the L2P data. These data are known as Level 3 (L3) and comprise a variety of combinations of day-time only, night-time only, or both, formed from single sensors and multiple sensors (Fig. 2). The multiple sensor data sets (“L3S”) are available daily, averaged over 1, 3, 6 and 14-day intervals, and are also available averaged monthly. The IMOS L2P and L3 products are generated routinely in near real-time by the Bureau of Meteorology. The L3 products are available from the IMOS data portal (https://imos.aodn.org.au/imos123), and also directly from OPeNDAP and WMS servers or as individual L2P files. See http://imos.org.au/sstproducts.html for data access and further information.
L1B
Swath SST L2P
Single swath L3U
(c)
(d)
(c)
Fig. 3. Sea surface temperatures from (a) NASA’s OurOcean Global daily 1 km resolution SST “L4” analysis, G1SST, for 1st January 2014 over the Great Barrier Reef near Cairns, Australia. G1SST ingested IMOS 1 km resolution L2P SST data, which were also used to produce (b) day-time and (c) night-time composite “L3S” products for the same date.
IMOS night-only and day-only level 3 products are being used in PhD projects by Xiaofang Zhu (University of Miami) and Haifeng Zhang (UNSW@ADFA) to study diurnal warming over the Great Barrier Reef and the Tropical Warm Pool, respectively. Some applications that currently use IMOS-GHRSST level 3 products are regional seasonal climatologies, real-time maps for recreational fishers (www.fishtrack.com), real-time maps of SST and ocean currents (IMOS OceanCurrent, http://oceancurrent.imos.org.au/sst.php Fig. 4) and nowcasting of coral bleaching (eReefs MWQ Dashboard, www.bom.gov.au/marinewaterquality and ReefTemp NextGen, http://www.bom.gov.au/environment/activities/reeftemp/reeftemp.shtml - Fig. 5).
(e)
Multi-swath, multi-sensor, (1-day) L3S
Fig. 2. Different types of IMOS AVHRR SST products (following GHRSST conventions).
Fig. 6. Validation plots showing time series of the (a) number of matchups and (b) mean, (c) median, (d) standard deviation and (e) robust standard deviation of the difference between colocated measurements of IMOS L2P SST from NOAA satellites and drifting buoys over a 90 day period. Measurements are considered co-located if the satellite observation is within 2 km and 60 minutes of the in situ observation. Only SST data flagged as being of best quality (quality level = 5), signifying observations are at least 5 km horizontally from the nearest detected cloud, are included here. Known biases with respect to the drifting buoy SST data have also been removed.
N19: 02:01:49 N19: 03:40:20 N18: 04:01:33 N19: 05:22:46 N18: 05:42:09 N18: 04:01:33
L3C
(b)
Fig. 1. Location and approximate range of Australian ground-stations that receive direct broadcast "HRPT" AVHRR data from NOAA polar-orbiting satellites.
Range of Products
Swath BT
(b)
(a)
L3S
Future Plans N18: 0094:01:33 N18: 05:42:
Multi-swath, single sensor (1-day)
Multi-swath, multi-sensor, multi-day (3-day)
Fig. 4. IMOS OceanCurrent map of the real-time 2 km SST and surface ocean current vectors for 6th May 2015 around Sydney, Australia, generated using IMOS single swath L3U SSTs.
IMOS is a national collaborative research infrastructure, supported by Australian Government. It is led by University of Tasmania in partnership with the Australian marine and climate science community.
Fig. 5. ReefTemp NextGen map of the 2 km SST Anomaly generated from IMOS 1-day night-only L3S SSTs for 1st January 2014.
www.imos.org.au
NOAA-19, launched in 2009, is the last in the series of NOAA Polar Orbiting Environmental Satellites (NPOES). In order to continue the IMOS 2 km L3 SST data record, during 2016 to 2017 we aim to include direct broadcast, high resolution SST observations from the European Space Agency's METOP-B polar-orbiting satellite and NASA's Suomi National Polar-orbiting Partnership (S-NPP) satellite.
FOR FURTHER INFORMATION
ACKNOWLEDGEMENTS
e e w
We acknowledge the provision of raw AVHRR data from ground-stations operated by the Bureau of Meteorology, Australian Institute of Marine Science, Western Australian Satellite Technology Applications Consortium, Geosciences Australia and Commonwealth Scientific and Industrial Research Organisation (CSIRO).
[email protected] [email protected] http://imos.org.au/sstproducts.html
