Coastalwatch :: Reference :: Features :: The East Coast Low. May 19-25, 2009

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The East Coast Low. May 19-25, 2009

Meteorological analysis. Noosa vid. Over 30 pics and charts.

- By Jeff Callaghan.

Noosa 23-25 May 2009
a Tim Bonython Film

In this report we have used data from the Bureau of Meteorology and data from the National Centres for Environmental Prediction/ National Centre for Atmospheric Research (NCEP/NCAR) Reanalysis Project, which is available at :http://nomad3.ncep.noaa.gov/ncep_data/index.html

Wave data was obtained from the Queensland Government Environment Protection Agency's web site at: http://www.epa.qld.gov.au/environmental_management/coast_and_oceans/waves_and_storm_tides/wave_monitoring/
and the New South Wales Manly Hydraulics laboratory web site at:
http://mhl.nsw.gov.au/www/wave_data_plot.htmlx .
Quikscat satellite wind observations were also obtained from the US Government site:-
http://manati.orbit.nesdis.noaa.gov/quikscat/.

The May 2009 event was significant in that it caused large seas over a wide area of the east coast of Australia. Wave rider buoys recorded significant wave heights (average of the top third wave heights) of over 5 metres between the Sunshine Coasts and Sydney. Disastrous flooding and damaging winds also occurred in Southeast Queensland and Northeast New South Wales. The Bureau of Meteorology did a great job issuing timely warnings which undoubtedly led to saving many lives. However two lives were unfortunately lost, one due to flying debris on the Gold Coast and one due to drowning in Northern NSW.

Josh Downthwaite Shacked up the beach at Noosa on Sat May 23.

Photo: Matt Brockie.

The development of east coast lows (ECLs)

The evolution of ECLs is reasonably easy to understand if we consider the role of a tropopause undulation in their development. Although this sounds like a complicated meteorological phenomenon it is one of the easiest features to identify on weather charts at upper levels. The tropopause undulation is very easily detected on weather charts at the 200hPa (elevation near 12km) level as a large warm temperature anomaly centre. Figure 1 shows such an undulation over Southwest Queensland just before the development of the May 2009 ECL. The temperatures at the centre of this undulation were warmer than -440C, which is an indication of its strength. Typical temperatures in these undulations during ECL development are generally not much warmer than -500C.

Strong upper westerly winds flowing through this undulation cause surface pressures to fall to the region east of the undulation if there are rain areas there. This is usually the case as there is cool air in the middle levels of the atmosphere beneath the undulation which extends eastwards and helps lift tropical moist air flowing down from the north. This is displayed in Figure 2 where the actual wind observations flow is plotted on the NCEP/NCAR temperature charts.

At 200hPa from 0000UTC 19 to 20 May (9am EST 19th to 20th and left hand frames in Figure 2) the winds are bringing warmer air at that level over the Southeast Queensland/ area which should produce mean sea level pressure falls there if heavy rain is falling. In the right hand panels the 700hPa levels (elevation 3km) are shown and the arrows and speeds in red represent vector wind differences (called wind shear) between the 500hPa (elevation 6km) and 850hPa levels (elevation 1.5km). A property of this vector wind is that it is parallel to the isotherms at 700hPa with cold air to the right looking towards the way the arrows are pointing. So over the period covered by Figure 2, the air over Southeast Queensland at 3km elevation, originated from warm tropical areas and then moved to cooler temperate areas being lifted in the process. This type of configuration produces extreme rainfall especially when the wind at 700hPa is normal to the shear as at Brisbane at 0000UTC 20 May (10am EST and in the lower right hand frame of Figure 2) when flood rains affected the Brisbane region over the next few hours.

In summary mean sea level pressure falls leading to ECL development can occur when warm air moves over a location at elevations around 12km. This is especially the case when air flow beneath this at elevations near 3km was being lifted after originating in warmer areas.

FIG 1
The temperature at 200hPa over Australia at 0000UTC 19 May 2009 which is 10am 19 May Eastern Standard Time (EST).

Fig 2
Temperature charts at 200hPa and 700hPa during the development of the ECL. Actual wind observations are plotted in black with arrows indicating direction and speeds underlined with temperatures when available. In the 700hPa charts the arrows and speeds in red are for the vector wind difference between the 500hPa and 850hPa levels.

Mean Sea Level Development

In figure 3 the development of the ECL at mean sea level is shown over the period up to 6am 21 May and it can be seen that the centre of the low was complex with several transient centres. From a Quikscat satellite pass around 5pm 21 May (not shown) an extensive area of 40 to 60knot easterly winds had developed east of Brisbane and the Gold Coast and this is reflected by the increased pressure gradient at 10pm 20 May (lower left frame in figure 3). Gales at the time were reported from both Automatic Weather Stations (AWS) on the Gold Coast and this was around the time a man was killed at Surfers Paradise in his office as flying debris crashed through his office window. By 6am 20 May (lower right frame figure 3) from Quikscat satellite observations, an extensive area of easterly gales can be seen east of Brisbane and the Gold Coast and a small low developed north of Byron Bay where mean winds to 62knots were reported.

Figure 4 illustrates this small low in more detail using AWS wind observations and Brisbane radar images. The low had a curved rainband signature as viewed from radar and the mean winds reached 62 knots which is almost hurricane force.

Fig 3
Mean sea level pressure distribution (hPa) and selected wind observations from 10pm (EST) 19 May 2009 to 6am 21 May 2009 where a half barb/barb /flag represents 5knots/10knots /50knots. The lower right panel at 6am 21 May also contains Quikscat wind observations.

Fig 4
Radar images from Brisbane Radar at 1936 UTC 20 May 2009 (left) and 2030UTC 20 May 2009 highlighting 10 minute average winds at Byron Bay (bottom), Coolangatta (centre) and Gold Coast Seaway (top).

Significant wave heights readings from the Brisbane Buoy (off the Northern tip of North Stradbroke Island) increased during the 20 May and reached 5 metres around 3pm EST. The Gold Coast Buoy reached 5 metres later at 10pm while there were some problems with the Tweed buoy. NSW buoys at, Byron Bay, Coffs Harbour and Crowdy Heads all reached 5 metres earlier in the day. All this indicates that the off shore pressure gradient at 0000UTC 20 May should have been stronger than indicated in the top right frame of figure 3. The spacing of the isobars over the ocean in this frame was taken from the US high resolution analyses obtained from the NCEP/NCAR site.

During the day on 21 May (see figure 5) the low moved south and the pressure gradient increased east of the NSW coast and this was when some of the largest waves were recorded.
The Tweed buoy reached 6.2metres significant wave height at 8am 21 May while the buoy at Byron reached 7.4 metres later at 6pm. The Gold Coast buoy failed to report over this period.

Over the 24 hour period up to 10am 23 May 2009 (figure 6) the low had weakened by 10am 22 May but quickly reformed out to sea and then a small centre moved towards the Gold Coast where it stalled. Quikscat observations at 6am 23 May 2009 indicated a very extensive area of gales off the coast. The Buoy at Coffs recorded its peak significant wave height of 6.4metres around 10am 23rd while Crowdy reached 6.1metres at 7am. A redevelopment of waves occurred off the Gold Coast with the Buoy there recording as significant wave height of 6.2metres at 4am 23 May. The waves at Sydney had been increasing over this period and reached a significant height of 5.9 metres by 3am 23 May.

The buoys indicated that the waves came from the east. However the peak period of the waves over the period varied from 12 to 14 seconds which equates to wavelengths of 224metres to 306metres. As most of the buoys are located in depths of water of less than 80 metres the waves which feel the bottom at depths of between a quarter and half a wavelength would be becoming orientated parallel to the coast by the time they reached the buoys. From the Quikscat observations it would appear that initially the waves came from the east northeast and eventually came from the east southeast.

- By Jeff Callaghan.
2008 Christopher Taylor Award winner.
Jeff won the CTA award for contributions by meteorologists for all kinds of operational forecasting and support activities. Jeff recently retired from the position of Manager of the Severe Weather Section at The BOM. Jeff will now be a regular Coastalwatch contributing editor.

Fig 5
As in figure 3 except for 10am EST 21 May 2009 (left) and 10pm 21 May 2009 (right).

Fig 6
As in figure 3 except for period 10am EST 22 May2009 to 10am EST 23 may 2009 with Quikscat satellite observations shown at 6am 23 May 2009 (lower left frame).