1. Ensemble tropical cyclone and windstorm forecast applications
Ken Mylne, Helen Titley, Piers Buchanan
Ensemble Forecasting Applications, Met Office, Exeter, Devon, UK
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2. Introduction and General overview of ensemble products
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3. MOGREPS – The Met Office short-range ensemble
24-member ensemble designed for short-range forecasting
Operational Regional ensemble over N. Atlantic and Europe (NAE) (24km resolution, 38 levels) to T+54
Operational Global ensemble (~90km resolution, 38 levels) to T+72
Also runs to 15 days at ECMWF for THORPEX multi-model ensemble research MOGREPS-15
ETKF for initial condition perturbations
Stochastic physics
MOGREPS-G run at 0Z and 12Z: MOGREPS-R run at 6Z & 18Z
MOGREPS-15 run at 0Z and 12Z
NAE
MOGREPS became fully operational in Sep 2008 after 3 years of trials
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4. MOGREPS 2010 upgrades Planned for implementation mid Feb 2010
Resolution upgrade
Global ~90km L38 -> ~60km L70
NAE km L38 -> 18km L70
Version 2 of SKEB stochastic physics scheme
Backscatter for unresolved convection and excessive diffusion
Vertical localisation
Allow inflation factor to vary in the vertical
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5. Tropical cyclones Piers Buchanan, Helen Titley
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6. Tropical cyclone ensemble charts
Tropical cyclones are identified and tracked using 850hPa relative vorticity maxima
Tracks cyclones out to T+360 (from MOREPS-15)
Cyclone George: Landfall 12/03/07 near Port Headland, winds 195km/hr, 3 deaths
Mean reduction in forecast errors for ensemble mean compared to deterministic:
Similar up to T • 12% at T • 23% at T+120
Although this is an older example from the southern hemisphere, it is shown because it is a good example of where an ensemble forecast’s spread is useful. The deterministic forecast here (in green on the right hand slide) has the storm heading east whereas the reality was that the storm turned towards the south (shown in red on the right hand plot)
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7. 15-day Tropical cyclones genesis
The right blob is Gustav. The middle one is Ike
Animation of TC strike probabilities. 30th August Z
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8. Gustav August 30th 2008 at 12Z Multi-model ensemble products
MOGREPS
ECMWF
Benefit of THORPEX collaboration in this case.
There was considerable debate at the time as to whether Gustav would impact greatly on New Orleans.It can be seen in the above strike probability plots that both MOGREPS (left) ,ECMWF (right) and multimodel combination of the two (bottom) have the storm coming uncomfortably close to the city. In the bottom right the actual storm path is shown in red.
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9. Multimodel Verification Results.
The key result here is that ECMWF is currently the best model but MOGREPS-15 still adds value to it. Also MOGREPS provides extra spread (as shown in the previous slides).
Track error (km) against forecast lead time in hours for all available storms for 2008, for MOGREPS-15, ECMWF EPS and 2 multimodel combinations.
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