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Challenging Aspects of Challenging Aspects of Severe Tropical Cyclone Severe Tropical Cyclone George George’ ’s Track Forecasts s Track Forecasts
Jim DAVIDSON Jim DAVIDSON
Former Affilliation Former Affilliation
Setting the Scene Setting the Scene STC George was both very intense and physically large 3 fatalities – numerous injuries – localised damage 10 minute mean wind = 194 km/h recorded just offshore Equivalent to wind gusts reaching 275 km/h “Very destructive winds” to about 115 km inland 5 metre storm tide but landfall near the bottom of the tide Substantial but not significant rainfalls occurred No major flooding due lack of previous rainfall and the steady movement of the cyclone Track forecasts were very challenging and the focus of this presentation
WA TC Forecast Accuracy : 200607 season
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Forecast Time (h)
Accuracy (km)
George Jacob Kara 01/0205/06 Average
Track Forecast Performance
Best Track in Black with Multimodel Forecast Tracks in various colours
Selection of Forecast Tracks & Best Track at base time 060000 UTC
Best Track in Black with Multimodel Forecast Tracks in various colours
Selection of Forecast Tracks & Best Track at base time 061200 UTC
Best Track in Black with Multimodel Forecast Tracks in various colours
Selection of Forecast Tracks & Best Track at base time 070000 UTC
Best Track in Black with Multimodel Forecast Tracks in various colours
Selection of Forecast Tracks & Best Track at base time 0712000 UTC
One exception to the generalisation … The UKMO ensemble did indicate the possibility of a tight recurvature but this
prediction was made earlier on 3 March. Amidst other guidance, including later runs of the UKMO, it did not influence the forecast to any degree except maybe to raise
the probability of such a scenario from zero to still quite a very low number.
Warning Performance Warning Performance The possibility that George could recurve to the south was identified late morning on 7 March and a Cyclone Watch was issued for coastal and island communities The section of coast where George ultimately made landfall was first put under Cyclone Warning late on 7 March, approximately 24 hours prior to landfall Warnings consistently indicated that George was expected to impact the coast as a Severe TC Moreover – the warnings specifically stated that the system was expected to produce “very destructive winds” for more than 100 km inland
TCWC Methodology TCWC Methodology Meteorologists in the TCWC construct track forecasts based on a consensus approach, combining available dynamical and statistical forecast aids The forecasts issued by the TCWC drew directly upon guidance from 9 independent models Meteorologists also informed their decisionmaking by accessing output from models run in ensemble mode Despite being the most skilful track forecasting methodology available – and practised in all major TCWCs – it is still dependent on the skill of the component guidance
TCWC Methodology TCWC Methodology Remarkedly, the available guidance had a “small spread” (very little variation in direction and speed of the tracks) None of the available guidance foreshadowed the abrupt southerly shift in motion This was considered a surprising outcome even in 2007 It was extremely rare for all available guidance to fail “Small spread” lends to higher confidence in the track forecast Hence this is the worst situation a TCWC meteorologist can face – namely a “small spread” leading to high confidence, but ultimately large error
1. Small Spread/Large Error: Nightmare
2. Large Spread/Large Error: Largest opportunity for improvement
3. Small Spread/Small Error: Ideal Case
4. Large Spread/Small Error: Opposing errors cancel each other out
Error Vs Spread
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A Puzzling Question A Puzzling Question
Assuming a model has a fair and realistic representation of the TC and the environment, why/how does it steer the system in a direction inconsistent with the steering flows it depicts?
TC Steering 101 TC Steering 101 (largely courtesy of NRL Monterey) (largely courtesy of NRL Monterey)
TC motion is the result of a complex interaction between a number of internal and external influences Largescale environmental steering is typically the most prominent external influence on a TC, accounting for as much as 70 to 90% of the motion – and is computed by separating the TC wind fields from the largescale environmental wind fields (which is easier said than done) Separation of flow is dependent on the situation but can be anywhere from 17 degrees from the centre of the TC Deeplayer mean steering is obviously best but a single layer computation can sometimes provide a crude estimate Internal influences (such as the Beta Effect) steer the TC poleward and westward in both hemispheres
Possible Significant Factors Possible Significant Factors leading to the Sharp Recurvature leading to the Sharp Recurvature TC Humba (undergoing ETT well upstream) TC Jacob (Fujiwhara Effect) Beta Effect Poorly represented monsoonal flow in the models (Assymetrical Convection not a consideration here)
Possible Significant Factors Possible Significant Factors leading to the Sharp Recurvature leading to the Sharp Recurvature TC Humba (undergoing ETT well upstream) TC Jacob (Fujiwhara Effect) Beta Effect Poorly represented monsoonal flow in the models
TC Humba was undergoing ETT well upstream (near
90E) and is unlikely to have influenced George’s track to
any measurable degree
Possible Significant Factors Possible Significant Factors leading to the Sharp Recurvature leading to the Sharp Recurvature TC Humba (undergoing ETT well upstream) TC Jacob (Fujiwhara Effect) Beta Effect Poorly represented monsoonal flow in the models
TC Tech Bulletin issued 0112 UTC 07/03/2007 by the Perth TCWC:
....“No model has effectively captured TC Jacob in the analysis fields, and therefore all guidance is considered with some caution. Interaction between the two systems may imply a greater level of SW motion in the short term, but may also hinder any recurvature in the longer term.”...
“I am almost 100% convinced that this was all down to the model's handling of Jacob.
I am giving a talk at a symposium later this week on why tropical cyclone forecasts sometimes go wrong. I plan to make this one of my prime examples! It illustrates that modelling the environment around the tropical cyclone is just as important as modelling the cyclone itself to get a good forecast.”
Julian Heming Tropical Prediction Scientist UK Met Office
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Track of TC Jacob
Jacob was relatively weak and moving further away from George during the critical forecast period
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TC George TC Jacob interaction
TC George TC Jacob Interaction TC George was larger and more intense
Expect greater effect on TC Jacob’s track
Break in interaction as cause of S’ly motion?
Ø Systems don’t breach ‘direct interaction’ threshold
Ø From late on the 5 th interaction would have decreased as separation increased
Ø Systems continued to separate around time of TC George’s abrupt change in direction towards the south
Ø Independent or causal?
Ø Weak argument given distance and relative strengths
Possible Significant Factors Possible Significant Factors leading to the Sharp Recurvature leading to the Sharp Recurvature TC Humba (undergoing ETT well upstream) TC Jacob (Fujiwhara Effect) Beta Effect Poorly represented monsoonal flow in the models
Beta Effect 101 Beta Effect 101 (largely courtesy of NRL Monterey) (largely courtesy of NRL Monterey)
Local change of the vertical component of relative vorticity – maximum value at the equator and zero at the poles Causes TCs to move polewards and westward with a speed of several degrees per day (in the absence of the large scale steering flow) Function of the TC size but not necessarily the TC intensity When the TC size is large (as with George), the Beta Effect may have some impact on the motion and should therefore be considered when developing track forecast strategy
Physically large – therefore expect greater beta effect
Likely to have influenced the motion to some degree but difficult to argue that the beta effect contributed in a significant way to the abrupt change in direction
Far more likely that the beta effect would have lead to a more gradual change in direction towards the southwest
Beta Effect on George
Possible Significant Factors Possible Significant Factors leading to the Sharp Recurvature leading to the Sharp Recurvature TC Humba (undergoing ETT well upstream) TC Jacob (Fujiwhara Effect) Beta Effect Poorly represented monsoonal flow in the models
4 Sector Vector Analysis at 071200 UTC – EC 500 hPa
North – 300/50 East – 330/30 South – 100/60 West 130/20 SUM – 025/35
TC Justin in 1997 was centred in the Coral Sea off Queensland. The system was located north of a strong 500 hPa ridge and all computer models forecast it to move under the influence of the middlelevel easterlies on to the Queensland coast. However the monsoon westerly flow to the north was very strong and deep such that the easterly flow was balanced by the monsoon flow and the system remained stationary.
Animation of Synthetic and Actual IR Imagery (Sun and Rikus, 2004)
ACCESSTC Operational Forecast of TC Lua, Base Time 12UTC, 15 March 2012
Courtesy Lawrie Rikus, ESM Program, CAWCR
Sun, Z. and L.J. Rikus, 2004: Validating model clouds and their optical properties using Geostationary satellite imagery. Mon. Wea. Rev., 132, 20062020
Animation of Model & Actual IR Imagery (TC LUA)
Track of TC YASI with extent of hurricane, storm & gale force winds
Bright RED = Hurricane force winds
TC YASI – a really good example of Small Spread & Small Error for almost a week
leading up to landfall
Model forecasts for SANDY were exceptional out to 7 days
Deterministic EC
MultiModel Ensemble Strike Probability Map
In conclusion In conclusion … … The influence of strong and deep monsoonal westerlies on the steering flow of TCs is not always fully captured by numerical models (as highlighted with TC George) Consensus track forecasting has been largely a positive experience but still value from detailed synoptic analysis Increased availability of near realtime wind field analyses from a suite of numerical models together with remote sensed wind data over the oceans should hopefully lead to careful synoptic analysis being a routine function in TCWCs (if of course forecaster numbers allow)