Category: ANMN, ANFOG, Q-IMOS, Home Slider
Experimental drifting wave buoy observations of Severe Tropical Cyclone Debbie
Tropical Cyclone Debbie left a well documented trail of destruction in north Queensland. IMOS observations have enhanced our understanding of its impact on the marine environment.
On the morning of 25 March 2017 a tropical low pressure system developed in the Coral Sea that formed into a low level category 1 Tropical Cyclone named Debbie. Two days earlier the forecast track for Debbie was to make landfall between Innisfail and Lucinda as a Category 1 cyclone.
Despite the obvious pending destruction of Debbie, this was not seen as all bad news at the time. The Great Barrier Reef (GBR) had been experiencing a prolonged bleaching event in the north and central GBR for the second successive year. With almost 90 percent of Queensland being officially declared in drought, the potential benefits were that the cyclone would take up the ocean heat energy, winds would mix and cool the reef waters with a rain depression over land filling the dams.
From an ocean observing point of view, this provided the possibility to observe cyclone generated waves in the GBR lagoon and also a glimpse of how the shelf stratification would break down through mixing.
An IMOS Slocum glider was deployed a few weeks earlier and was profiling along the Palm Passage where IMOS have two delayed mode Moorings. Glider Unit 209 was positioned in 150–200 m of water at the top of Palm and Magnetic Passage. It performed well through cyclone Debbie, and with a measure of glider wrangling collected some storm data. Notable changes in density and currents were recorded even though Cyclone Debbie passed considerably to the south of the glider.
The Coastal Impacts Unit (CIU) of the Queensland Department of Science, Information Technology and Innovation (DSITI) operates a storm tide and wave monitoring network along the Queensland coast. In recent years they have acquired a number of drifting buoys fondly referred to as a “BOB” (Baby Ocean Buoy), a smaller 40 cm diameter version of the Datawell Waverider buoy. These are stationed in regional coastal centres and are available for deployment in extreme events. For example in 2015, Tropical Cyclone Nathan crossed over Lizard Island in the north, well away from any coastal measurement infrastructure. The CIU deployed a BOB off Cooktown by helicopter and it was drawn toward the cyclone and yielded a maximum wave measurement in the GBR lagoon of over 9m within the 40 km radius of maximum winds.
The Australian Institute of Marine Science (AIMS) also acquired a new drifting buoy, capable of measuring waves, after discussions with Luca Centurioni from Scripps Institution of Oceanography in the USA. This buoy is smaller than the BOB, being only 35 cm in diameter. We chose to name our buoy “Tim”. On Thursday 23 March with the predicted track of TC Debbie being to the north of Townsville, AIMS experimental scientist Matthew Beck successively deployed BOB#3 and Tim with the assistance of the Palm Island barge company MV Lady Fraser on Friday 23 March.
A third buoy, BOB#5, was deployed by Grant Millar of the CIU the next morning from the end of the 6 km long Lucinda Jetty where the IMOS ocean colour infrastructure is located. Deployment was successful as they drifted with the wind and currents northwards up the lagoon toward Cairns. However, TC Debbie continued to move slowly to the south and delayed her turn to the southwest so that the drifters were well north of the cyclone path in quite a benign weather and wind environment rather than the planned for area to the southwest of Debbie where the strongest winds and waves occur.
The track of Tim shows it negotiating the coastal islands and reefs up the GBR lagoon moving northwards and finally reversing direction once TC Debbie crossed the coast. Initial data reveals that the maximum significant wave height (Hsig) was only 1.5 m for the deployment. In contrast the maximum wave height recorded by the Hay Point Waverider buoy was over 6 m, with a Hsig of ~3.9 m. Note that these are preliminary data subject to verification. The relatively calm conditions to the north has meant that the cooling of the GBR waters was confined to the south of Townsville and offshore in the Coral Sea as seen in the NOAA Coral Reef Watch SST anomaly product (Figure 4).
Nevertheless these experimental deployments are important as in situ wave measurements in extreme events are seldom made and can be used to inform operational agencies of the conditions at sea and also to help improve and validate models of these extreme events. This activity is one of a number that DSITI, Scripps and AIMS are planning over the next year as a budding collaboration to evaluate and inter-compare existing and emerging wave measuring technologies and platforms.
AIMS has partnered with Boeing and DSITI to trial a Wave glider. The Wave Glider is a new autonomous device that utilises wave and solar energy for self propulsion; so its movement can be remotely controlled and not completely at the whim of the currents and weather. The drifters can also be anchored to provide fixed point observations. We are also planning to inter-compare the Scripps Wave buoy with existing DSITI coastal Wave buoys and a bottom mounted ADCP off the Gold Coast and Cape Cleveland near Townsville.
With each drifting buoy deployment it is incumbent on us to make reasonable efforts to recover the units. A request was made and the MV Hero recovered BOB#3 and Tim, with BOB#5 recovered in the early hours of April 1 by the MV Kalinda whilst on a charter with AIMS assessing coral bleaching near Rib Reef in the early hours of 1 April. This exercise also shows the enthusiasm of industry and tourism operators to engage and assist with the logistics of ocean observations and so we thank them for their assistance.
More detailed analyses of TC Debbie are being made by the Remote Sensors and Oceanographers that will also access the IMOS glider and moorings data, the latter being recovered in early June.
This article was written by Craig Steinberg, AIMS Oceanography and Shelf Processes Research Team Leader and IMOS Australian National Mooring Network Facility Leader.
Further information is also available on Tropical cyclone Nathan.