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Dynamically-Based Seasonal Forecasts of Atlantic Tropical-storm Activity

This article discusses the recent hurricane activity in the Tropical Atlantic and the technology used for forecasting tropical storms. It also explores the impact of long-range forecasts and the use of climate models for seasonal forecasting. The article concludes by discussing future potential for improving hurricane intensity forecasts and the use of statistical techniques.

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Dynamically-Based Seasonal Forecasts of Atlantic Tropical-storm Activity

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  1. Dynamically-Based Seasonal forecasts of Atlantic Tropical-storm Activity Matt Huddleston With thanks to: Frederic Vitart (ECMWF), Ruth McDonald & Met Office Seasonal forecasting team 14th March 2007

  2. Seasonal forecasts of tropical storms • Recent hurricane years in the Tropical Atlantic • Forecasting technology • Forecast skill • Future work

  3. Atlantic Hurricanes • 2004 Four hurricanes struck Florida • Unprecedented 10 tropical cyclones struck Japan • 2005 Record hurricane activity (28) in the Atlantic • Four category 5 hurricanes (Emily, Katrina, Rita, Wilma) Activity in other regions was either quiet or normal • 2006 Nothing out of the ordinary – 9 storms c/o & thanks to NASA (also previous animation) • Atlantic has been active for last decade, but no change in other regions • Natural variations in activity are likely to mask any clear climate change link for the foreseeable future • Observational studies suggest no change in frequency, but an increase in intensity in recent decades • Historical database is not considered robust enough to use for detailed climate studies

  4. 2005 tropical cyclone activity

  5. Seasonal Forecasting technology

  6. Current seasonal tropical storm forecasting: statistical • E.g. Gray/Klotzbach – June to November Atlantic season forecasts • Use a range of statistical predictors for each forecast • Wide range of predictions made: numbers of storms, strength, landfall etc Taken from Klotzbach P.J. and Gray W.M., Extended Range Forecast of Atlantic Seasonal Hurricane Activity and U.S. Landfall Strike Probability

  7. The Daily Telegraph – 7th October 2006 Impact of long-range forecasts Amaranth Advisors, the US-based hedge fund that lost about $6bn (£3.2bn) betting on gas prices, has sought help as it liquidates its remaining assets.BBC News 2nd Oct 2006

  8. Global Coupled Climate model Ensembles Seasonal forecasting using climate models:Multi-model seasonal forecasting Recent global atmospheric wind, rain, solar heating etc Ocean observations Including ARGO floats EUROSIP Sea Surface Temperature

  9. Model Tropical Storms Tangential Winds Radial Winds Average observed typhoon from Gray (1979) Upper level anticyclonic Out flow In flow Low level cyclonic Summary: Despite their low horizontal resolution climate models are able to simulate some of the features of tropical cyclones Single tropical storm from HadAM3 N144 Model Anticyclonic flow too far from centre Upper level Too strong, too low and confined near centre Low level Model winds speeds are too low, max is too far out Top row: Gray (1979), Bottom row: McDonald et al. (2005) Climate Dynamics

  10. Examples of model cyclone tracks Tropical Storms for 15 years – AGCM + observed SST Tracks look sensible, despite low resolution (~100Km / N144 ) and poor simulation of individual cyclones

  11. Tropical storm genesis in N144 HadAM3 Observations:- NHC best track data Too many Too few Model:- 1980s Units = TS per grid box per 10 years Only one TS observed

  12. Forecast skill

  13. Impact of El Niño/wind shear in the Atlantic 1995: 19 tropical storms 11 hurricanes 5 major hurricanes 1997: 7 tropical storms 3 hurricanes 1 major hurricane Strong El Niño prevented hurricane development due to high wind shear

  14. Key to forecasting climate – sea surface temperature Pacific (NINO3.4) SSTs • Forecast of August to October SSTs from 1st June • Correlation of forecast ensemble mean and observed SSTs • 0.87 in Pacific • 0.81 in Atlantic • Persistence is • 0.47 in Pacific • 0.73 in Atlantic

  15. EUROSIP – June forecast for July-November (95% population)

  16. June forecast of number tropical Atlantic storms Jun-Nov 1993-2006 1987-2006

  17. Recent predictions • 2005 was an extremely active year • 2006 was an (just-below) average

  18. Climate forecasts & risk mitigation • Chances of an extreme Atlantic tropical storm season in 2005: • Above 20 storms was forecast as twice as likely • 1st June forecast: 37% chance of being above 20 storms where 1993-2004 average chance was 18% • Above 27 storms was forecast as 3 times as likely • 1st June forecast: 13% chance of being above 27 storms where 1993-2004 average chance was 4.2%

  19. DEMETER Probabilistic verification 1959-2001 Forecasts that differ from climatology frequency for above or below normal activity are reliable

  20. Future potential • Increasing model resolution • Land fall predictions • Better representation of hurricane intensity • Statistical techniques could be used now: • Calibrate inter-annual variability of ensemble • Calibrate land-falling hurricanes • Assessment in more locations and lead times

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