Relocatable Ocean Atmosphere Model ROAM

The Relocatable Ocean Atmosphere Model (ROAM) is used by the RAN to generate high resolution oceanic and atmospheric forecasts over limited domains of interest to the Australian Defence Force (ADF). ROAM is designed to be set up by non-expert users, with minimal input, anywhere in the Australian region (Herzfeld 2009), and is used routinely by RAN forecasters. The ROAM ocean model is initialised and forced by data from OceanMAPS, and is typically implemented at resolutions of 1-2 km. Figure 24.8 shows Sea Surface Temperature (SST) and currents calculated by ROAM for a domain in the vicinity of Hobart, Tasmania (see Fig. 24.1), which was used for the RAN mine warfare exercise 'DUGONG'. Exercise 'DUGONG' involved the Mine Hunter Coastal (MHC) vessels HUON and DIAMANTINA, which provided mine sweeping and hunting capabilities, the auxiliary minesweeper BANDICOOT, clearance diving teams and US Navy salvage divers. It took place over two weeks in October 2009 in the Derwent River, and the approaches to Hobart.

In this example, the current characteristics were of primary importance to the exercise, which involved an underwater survey of the historic wreck of MV Lake Illawarra in the Derwent River. The water temperature was also of interest to the diving teams, to ensure that they were suitably prepared for the prevailing conditions. ROAM was used to generate current forecasts at intervals down to one hour. Additionally, the ROAM atmospheric model provided high resolution forecasts of the wind strength and direction, also at one hour timesteps, which allowed changes in the sea state to be anticipated. These also proved to have a significant impact on the exercise. Note that Fig. 24.8 does not show the full resolution of the ROAM model, as it has been expanded to show conditions in Storm Bay.

As well as providing oceanographic data for graphical products, the output from ocean forecasting systems can be used in sonar range prediction systems, in order to produce assessments and forecasts of acoustic conditions which take account of the spatial and temporal variability of the ocean environment. Figure 24.9 shows a series of sonar range predictions, which have been generated by the RAN's Tactical Environmental Support System (TESS 2) using ROAM data at 1 km resolution. The domain is in the vicinity of Jervis Bay, which is around 130 km south of Sydney (Fig. 24.1). It is an area where the RAN frequently conducts ASW and MW exercises. The sonar range predictions are displayed as 'Probability of Detection' plots (PODgrams), where a 90% or greater probability of detection is shown in red.

Fig. 24.8 Sea Surface Temperature (SST) and current forecast produced by the ROAM system for the mine warfare Exercise DUGONG in October 2009

Figure 24.9 shows ROAM sea surface temperature and currents as the background. The three PODgrams are for an ASW frigate leaving Jervis Bay and tracking to the northeast, searching for a submarine at Periscope Depth (PD). Similar calculations may be run at any depth required by the user. The capabilities of the sonar used for the calculation are fictional. The PODgrams seem to make sense intuitively, since they show the greatest ranges inshore, where the water is shallow and with a relatively homogeneous thermohaline structure, and bottom losses are low from the sandy sea bed. Offshore, where the temperature gradient is greater, detection ranges are less. The scale of Fig. 24.9 can be gauged by considering that the current vectors are shown at the ROAM resolution of 1 km. The PODgrams have hollow centres because echoes cannot be received whilst the sonar is transmitting. This gives rise to

Fig. 24.9 Sonar performance predictions produced by ROAM and TESS 2 for 1000 UTC on 06 October 2009, in the vicinity of Jervis Bay, NSW. The background shows sea surface temperature (colour .stretch and contours) and current vectors. The three 'Probability of Detection' plots (POD-grams) are over-plotted, with a probability of detection of 90% or more shown in red

Fig. 24.9 Sonar performance predictions produced by ROAM and TESS 2 for 1000 UTC on 06 October 2009, in the vicinity of Jervis Bay, NSW. The background shows sea surface temperature (colour .stretch and contours) and current vectors. The three 'Probability of Detection' plots (POD-grams) are over-plotted, with a probability of detection of 90% or more shown in red a 'dead zone' of varying radius, depending on the duration of the transmitted pulse, and the speed of sound in water.

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