Guidance Products for Vessels Sailing in Antarctic Waters
Feb 3, 2006
Author: Ian Hunter
SA Weather Services
At the beginning of every austral summer polar research and relief vessels head south on their annual cruises into Antarctic waters. Although there were 6 flights from Cape Town to Queen Maud Land during the 2005/6 summer - and this number is likely to increase - ships will always be needed to transport the high-volume, heavy cargo to the various Antarctic bases. Then of course there is also the increasing number of passenger cruise vessels.
SA Agulhas on the Antarctic ice shelf – this image courtesy Smit/SA Weather Services
At the National Center for Atmospheric Research (NCAR) in Boulder a numerical weather prediction model is run in real-time mode - specifically for Antarctica. This is the Antarctic Mesoscale Prediction System (AMPS). Although primarily focused on support for aviation operations, the model has also proved very useful for marine activities as well.
NCAR has very kindly made several new products available to the South African Weather Service (SAWS) for its prediction service to the SA Agulhas. These include a 20 km nested model covering the Queen Maud Land coast and another of 15 km resolution extending over the area covered by the vessel during the cruise.
Figure 1 is a prognostic chart of pressure and rainfall rate, made available on the AMPS website. During her buoy deployment cruise the Agulhas also had to replace an AWS on South Thule Island. The AMPS model accurately predicted the required weather window.
In addition to aiding helicopter flight planning through its predictions of reduced visibility due to heavy precipitation or fog, the model's surface wind predictions also help to plan marine operations. Apart from the direct effect of the wind, the resulting seas can cause problems for a vessel alongside the ice shelf, unloading cargo. Another important consideration relating to predicted wind conditions is pack ice movement. In a relatively short space of time a vessel may find its manoeuvring space significantly reduced.
Navigation through the pack ice has been made a lot easier thanks to satellite imagery of ever-increasing resolution. A feature as large as the polynya which forms annual at roughly 65S 00E can be easily spotted by microwave sensors such as the AMSR-E flying on NASA's Aqua satellite. Both the Agulhas and the Polarstern made good use of this 'hole' in the pack ice on their way south.
But to see smaller ice features one needs to make use of the full 250m resolution of the MODIS sensor, also located on the Aqua platform - see Figure 2. These images are made available on the Internet by the National Snow and Ice Data Centre (NSIDC), also based in Colorado.
Unfortunately, in the case of MODIS one needs cloud-free conditions - a relatively uncommon condition at these latitudes - to get a detailed picture of the pack ice patterns.
The fact that these products are made freely available is a tribute to international co-operation, global concern for maritime safety - and the institutions that provide them.
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