How Strong Was Yasi?-Update 11 Feb

Ken Stewart, February 2011

Update Friday Feb 11 8.12pm:  On ABC news tonight, a report from James Cook University, whose scientists have established Yasi’s speed, at

http://www-public.jcu.edu.au/news/JCUPRD1_071493

“The CTS team leader in the field, Dr Geoff Boughton, said that the analysis of damage to simple structures throughout the region indicated that the wind speeds on the ground in Tropical Cyclone Yasi were less than those expected in a Category 5 event.

Using techniques developed and refined in previous cyclones, the CTS team has formed a preliminary view that the maximum wind speed in Cyclone Yasi was about the same as that in Tropical Cyclone Larry.

“We estimate that the gust wind speeds in some of the most affected areas were about 220 km/h” Dr Boughton said.”

In fact that’s at the low end of my estimate!

On Wednesday 02 February 2011, TC Yasi struck Far North Queensland.  Despite the enormous devastation in the Cardwell-Mission Beach-Tully area, thanks to Emergency Services Queensland, Anna Bligh, and the Bureau of Meteorology (BOM), people were well prepared, evacuated when it was thought necessary, and only one life was lost.

However, many people have noticed that Yasi was nowhere near as strong as expected.

The Bureau of Meteorology estimated that windspeeds would reach 290kph near the centre, and at 8.00pm warned these could be expected between Cairns and Ingham.  This estimate was made from analysing radar and satellite data.  But at 4.33p.m. BOM’s technical advice estimated winds at the centre as being 205kph (my emphasis):

Technical brief issued by BOM at 6:33 UTC.

REMARKS:
The latest imagery shows Yasi as very symmetric showing a well defined eye with surrounding deep convection. Dvorak intensity: Eye pattern white surround [6.0] with OW/W [0.5] eye adjustment giving DT=6.5, adj. MET=6.0. FT/CI=6.5. Max winds estimated at 110 knots [205 km/h] [AMSU/SATCON estimates at 125kn [1min]].
Surface observations indicate hurricane force winds extend about 50 nm [95 km] to the southwest of the track.

As well, they had access to data from Willis Island which was right in Yasi’s path and which transmitted data until just before the eyewall passed over it.  Yet they still maintained Yasi at Cat 5, probably as a precaution.    They received unjustified criticism over the Toowoomba and Lockyer floods so government and BOM were taking no chances.   What most people don’t realize is that cyclone category is not an exact statement but a warning.  From the BOM website:

The severity of a tropical cyclone is described in terms of categories ranging from 1 to 5 related to the zone of maximum winds. An estimate of cyclone severity is included in all tropical advices. Remember that the Warning Service is not designed to give an exact statement of conditions at individual locations but will give a general idea of the expected worst conditions.

http://www.bom.gov.au/cyclone/about/

And while no more precise estimate of wind speed can be made until completion of engineering analysis of the damaged areas, news reports are still universally reporting winds of 290kph.  In the meantime, here is an alternative estimate.

Although the maximum recorded windgust at Lucinda was 185kph, this was 50km south of the centre and about 35km from the eyewall, and a look at the data shows the wind changing direction every few minutes, possibly an effect of the nearby topography.  These speeds are probably not reliable.

I have used data from Willis Island to estimate windspeeds in the lead  up to the crossing at Mission Beach.

Willis Island was directly in the path of Yasi’s eye.  It transmitted data right from  the edge of the eyewall.  This is its final radar image (courtesy of Watts Up With That):

Here’s the graph of windspeed:

and the table of observations:Notice the wind suddenly changed from south at 6.00 a.m  to north-west at 6.15 a.m.  with no drop in speed (102kph sustained).  I can think of 3 explanations.

1. Possibly the north face of the eyewall passed over it.  However, the radar image at 8.00 shows the island right in the firing line, and still some distance away.  The cyclone would have had to change course.
2. Alternatively, the eye passed over for about 15 minutes and the calm was not recorded.  This is odd.  We were told the calm eye would take an hour to pass over.  Besides, the radar image shows the eye still to arrive.  The wind increased to 141kph, gusting to 185kph, at 8.10a.m. The anemometer then ceased operations, although the barometer recorded decreasing pressure (to 938hPa at 9.00) then increasing to 940.1 at 10.00a.m.  The continued drop in pressure means the wind might have increased even further by perhaps 20 to 30kph, making the top gust at around 215kph.
3. Perhaps the wind vane was damaged at 6.15?  Then for two hours the cyclone travelled 60-70km closer and the wind increased by 40kph sustained and 46kph gusts.  This is most likely.

The 8.00 a.m. radar image shows Willis island was approximately 15-20km from the eyewall and 30-35km from the centre.  Maximum speed is reached at the eyewall.  I used the data to make a plot of the data including an interpolation for 5.45 a.m., and  extrapolating to 8.30 a.m., the approximate time of the eyewall’s arrival.

As you can see I used 3 estimates for the 8.30 windspeed: the BOM 4.33 p.m. estimate of 205kph; the 8.12 p.m. cyclone advice estimate of 290kph, and 231kph, the AMSU/SATCON estimate and just above the borderline Cat 3/Cat 4 speed.

I believe that the estimate of 290kph cannot be supported- it appears too high.  Further, this is not supported by BOM’s 4.33 p.m. technical brief.  However it appears that 205kph could be too low for 8.30pm at Willis Island.  230kph appears to be closer.

The cyclone was travelling at about 30kph at this time, so every hour is approximately 30km.  Using 9.00 a.m. as the time of the cyclone centre passing just a few km to the south of Willis Island with a QNH (barometric pressure) of 938hPa, I constructed a plot of observed windspeeds at approximate distances from the centre.

The horizontal scale is distance from the centre in kilometres.

Therefore, a defensible estimate for windspeed in the Cardwell-Mission Beach area would be in the range 220-240kph.  This indicates it was a low Cat 4, not Cat 5.

Yasi was indeed an enormous system in area covered by cloud, the largest we’ve seen in the satellite era.  The zone of maximum destruction (and winds) extended from roughly Silkwood to Cardwell, a distance of about 60km.  The storm was roughly the same strength as Cyclone Larry but took longer to pass.  Remarkably, it was still classed as a cyclone at Julia Creek, the furthest inland a cyclone has been recorded.

Contrary to many alarming reports, it was not the deepest cyclone (<926hPa, Mourilyan Mill, 1918) nor the highest storm surge (Bathurst Bay >10m, 1899 or Mission Beach 3.6m 1918) nor the the most rain (907mm in 24 hours at Crohamhurst, 1893) nor the deadliest (307 known fatalities, Bathurst Bay, 1899).

Which is no consolation for the residents of Cardwell, Tully, and Mission Beach.