Archive for the ‘tropical cyclones’ Category

TC Debbie

March 29, 2017

TC Debbie hit the Whitsunday coast and areas to the south and inland yesterday.  As I spent nearly half my life in places not far from Mackay and have many friends in the region, I was very interested to see what was happening.   I began checking online from 5 a.m. Tuesday morning.

Here is some initial analysis of TC Debbie.  Firstly, here is the table of cyclone intensities as found at http://www.bom.gov.au/cyclone/faq/index.shtml#definitions .

Fig. 1:  Cyclone Intensity

TC Intensity

I began checking online from 5 a.m. Tuesday morning.

Fig. 2:  0500 forecast cyclone track map.

Debbie 5am

How accurate was the Bureau’s forecast?  Here is the forecast 22 hours later, at 0300 Wednesday morning.

Fig. 3:  Wednesday 0300 forecast cyclone track map.

Ex TC Debbie

The track forecast was pretty good.

The next images show Debbie’s progress across the Whitsunday Islands until the eyewall crossed the coast near Airlie Beach.

Fig. 4:  0720 Eyewall about to hit Hamilton Island

radar 720am debbie hayman is eye

Fig. 5:  0910  Hamilton Island near the eyewall, Hayman Island in the eye

radar 910am debbie hamilton eyewall

Fig. 6:  10.30  Hamilton Island near the eyewall, Hayman Island in the eye, and the eyewall about to pass over Airlie Beach

radar 1030am debbie hamilton eyewall

And four and a half hours later, the worst is over at Hamilton and Hayman Island and the eye is collapsing over Proserpine.

Fig. 7:  1510  Debbie weakening near Proserpine

radar 310pm eye breakup

Note the “gap” in the image in the northwest sector.  The Bowen radar failed and the Mackay radar was blocked by high mountains to the west.

What about forecasts of the cyclone’s intensity?

The next figures show plots of wind gusts, pressure, temperature, and rain at Hamilton Island, Proserpine, and Bowen, the closest stations to the cyclone’s track.

Fig. 8:  Wind gusts at Hamilton Island

wind hamilton

The black line shows the period from just before 8.00 a.m. until about 2.30 p.m. during which Hamilton Island was close to the eyewall, the area of maximum wind strength.   For nine hours from before 6.00 a.m. until nearly 3.00 p.m. wind gusts were of Category 3 strength.  From 8.00 a.m. until 12.30 p.m. gusts approached or exceeded 225 km/hr, bordering on category 4, and between 10.35 and 10.30 reached 263 km/hr three times at least- and the Bureau had forecast winds up to 270 km/hr.  While the station at Hamilton Island is too high to be completely reliable, these data are indicative that winds at 10 metres were at cat 4 level for some time.

Fig. 9:  Air Pressure at Hamilton Island

pressure hamilton

The red line shows the period from just before 8.00 a.m. until about 2.30 p.m. during which Hamilton Island was near the eyewall, the area of maximum wind strength.    From 2.00 a.m. until 5.00 p.m.  pressure was below 985 hPa (Cat, 2) and from 10.00 a.m. until 1.30 p.m. was below 970 hPa (Cat.3) but did not reach 955 hPa (Cat. 4).  Remember however that Hamilton Island was some 50 km from the centre of the eye, so 955 hPa is quite possible for central pressure.

On the basis of wind gusts and pressure at Hamilton Island, I believe Debbie was a strong Category 3, weak Category 4 system.

Fig. 10:  Air temperature at Hamilton Island

T hamilton

Note the sudden jump in temperature from 8.12 a.m.- 3 degrees in 3 minutes- coinciding with a wind gust of 212 km/hr, and kept climbing to unbelievable values.  (Compare with Proserpine below.)  It is likely that the AWS probe malfunctioned, and failed altogether at 12.00 noon.

Fig. 11:  Rain at Hamilton Island

rain hamilton

Rain measurement is unlikely to be accurate in such ferocious winds.  Note how rainfall levelled off from 11.00 a.m until 2.00 p.m., then increased after 3.00 p.m.

Fig. 12:  Wind gusts at Proserpine

wind proserpine

Proserpine Airport is some 20 km inland, 41 km west of Hamilton Island and 56 km from Bowen.  As the cyclone arrived over land it began losing strength and the eye began to shrink.  From 10.00 a.m. until 2.00 p.m. gusts were at Category 2 strength and at 1.00 p.m. reached the magic 165 km/hr of Cat 3 strength.  They were very probably much stronger in the town itself 9.1 km north.

Fig. 13:  Pressure at Proserpine Airport

pressure proserpine

From 12.30 p.m. until 5.00 p.m. the pressure at the airport, some 20-30 km from the centre, was below the Category 3 value of 970 hPa.

Wind gust and pressure data indicate Debbie was very likely still Category 3 as it passed over Proserpine town.

Fig. 14:  Air temperature at Proserpine

T proserpine

Fairly stable temperature with only about 1.5C range all day.

Fig. 15:  Rain at Proserpine

rain proserpine

Steady rain all day, fairly typical of cyclonic conditions.  At Strathdickie not far from Proserpine, 193mm fell in one hour that morning, and at Dalrymple Heights about 50km south 814mm fell in 24 hours.

Fig. 16:  Wind gusts at Bowen

wind bowen

For four and a half hours wind gusts reached Category 2 strength, and were above 100 km/hr from 9.00 a.m. to 8.00 p.m.

Fig. 17:  Pressure at Bowen

pressure bowen

Pressure was at Category 2 levels from 9.00 a.m.

Fig. 18:  Air temperature at Bowen

T bowen

Winds were west south west most of the day, but as Debbie passed and winds turned northwest (over the ocean), the temperature climbed.

Fig. 19:  Rain at Bowen

rain bowen

Steady rain all day: 12 inches in 12 hours.

While no stations were directly in the cyclone’s path, nearby station data indicate that Debbie was a large Category 3 to Category 4 tropical cyclone when it hit the coast and brought very strong winds, very heavy rainfall, and widespread destruction.  It is still lingering as a tropical low 300 km inland, bringing more strong winds and very heavy rain, and will head south over the next couple of days.  The clean up begins.  We await the report from James Cook University engineers who will provide their assessment of damage and wind loadings in a few weeks’ time.

Give credit where credit is due: the Bureau of Meteorology got this one pretty right.