1 / 40

Professor Lennart Bengtsson ESSC, University of Reading, UK MPI for Meteorology, Hamburg

Predictability and predictive skill of weather systems and atmospheric flow patterns -25 years of progress- -The legacy of FGGE-. Professor Lennart Bengtsson ESSC, University of Reading, UK MPI for Meteorology, Hamburg Many thanks to colleagues at ESSC and ECMWF

meena
Download Presentation

Professor Lennart Bengtsson ESSC, University of Reading, UK MPI for Meteorology, Hamburg

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Predictability and predictive skill of weather systems and atmospheric flow patterns-25 years of progress--The legacy of FGGE- Professor Lennart Bengtsson ESSC, University of Reading, UK MPI for Meteorology, Hamburg Many thanks to colleagues at ESSC and ECMWF and especially to Kevin Hodges, ESSC THORPEX Conference December 2004

  2. Predictability and predictive skill of weather systems and atmospheric flow patterns The improvements in NWP over the last 25 years • The impact of observations on forecast skill • The importance of data-assimilation • How much better can the forecasts be? • The need for a new initiative in weather forecasting THORPEX Conference December 2004

  3. Improvements in NWP from Miyakoda (1972) to 2002. Courtesy ECMWF How long to get to D+10 in winter? THORPEX Conference December 2004

  4. What are the causes of the improvements? • More accurate models • Advanced data-assimilation • Better observational coverage THORPEX Conference December 2004

  5. THORPEX Conference December 2004

  6. The principle of error reduction in data assimilation THORPEX Conference December 2004

  7. Same model and data- assimilation but different observations. Results from ERA40 Courtesy ECMWF THORPEX Conference December 2004

  8. Note that the error growth is virtually unchanged between the time of a good forecast (r=0.90) and the time of a useful forecast (r=0.60). This fall in skill takes ca. 3.5 days THORPEX Conference December 2004

  9. Possible causes? • When errors have reached a certain size the growth rate is determined by internal dynamics. If so the skill can only be increased by a more accurate initial state. • Or, there are errors due to down-scale cascade, say from tropical forcing, which not yet is properly handled by models • Or, perhaps errors, due to aliasing or to incorrect handling of the up-scale cascade of physical processes, such as convection, projecting the errors on synoptic scale modes leading to an overly rapid growth? THORPEX Conference December 2004

  10. There are large variations in forecast skillECMWF 1000 hPa height anomaly correlation Cumulative frequency distribution for Europe. Winter 1988 and 1998 THORPEX Conference December 2004

  11. There are large variations in forecast skillECMWF 500 hPa height anomaly correlation, Europe. Winter 1998 and 2004. Courtesy H Böttger, ECMWF 5% 25% Jan-Mar 1998 Jan-Mar 2004 50% 95% 75% THORPEX Conference December 2004

  12. Improvements in predictive skill in atmospheric flow patterns since FGGE • NH extra-tropics has improved by more than three days • SH extra-tropics is as good as the NH • Five day forecasts in the tropics are as good as a one day forecast 25 years ago But There are large differences in skill from day to day The time it takes for a forecast at 90% correlation to reach 60% correlation in the extra-tropics is about 3.5 days. This has not changed. Forecast improvements is due to more accurate initial states. THORPEX Conference December 2004

  13. The impact of observations on forecast skillExperiments with different observing systemsDJF 1990/91 using ERA40 observations • The control system (using all observations) • A terrestrial based system ( radio-sondes and aircraft obs.) • A satellite based system ( satellites and surface pressure) • A surface based system ( surface observations) THORPEX Conference December 2004

  14. Global forecasts DJF 90/91 • 7- day forecasts, every 6hr. • Later ECMWF model T159/L60 • Extra-tropics 20-90N and 20-90S • 500 hPa Z, normalized SD for the period • Tropics 20N-20S • Wind vector field 850 and 250hPa THORPEX Conference December 2004

  15. Observing systems and predictive skillNorthern Hemisphere extra-tropicsBengtsson and Hodges, 2004 THORPEX Conference December 2004

  16. Observing systems and predictive skillSouthern Hemisphere extra-tropicsBengtsson and Hodges, 2004 THORPEX Conference December 2004

  17. Observing systems and predictive skillTropics V 250 hPaBengtsson and Hodges, 2004 THORPEX Conference December 2004

  18. Observing systems and predictive skillNorthern Hemisphere Z 500 hPaDJF 1990/91 (full line)and DJF 2000/01 (dashed)) THORPEX Conference December 2004

  19. Observing systems and predictive skillSouthern Hemisphere Z 500 hPaDJF 1990/91 (full line)and DJF 2000/01 (dashed)) THORPEX Conference December 2004

  20. Forecast skill and observations • The terrestrial system is the best at the NH extra-tropics • The satellite system is crucial for the SH extra-tropics • In the tropics the terrestrial system and the satellite system are equally useful and highly complementary • Between 1990 and 2000 the satellite system has increased its information content and the terrestrial system has decreased it THORPEX Conference December 2004

  21. The impact of observations on forecast skillExperiment with ERA40 The impact of observations of humidity on NWP 360 global forecasts DJF 1990/91 THORPEX Conference December 2004

  22. Impact of humidity observationsNH Z 500 hPaFull observing systemNo humidity observations in data-assimilation THORPEX Conference December 2004

  23. Impact of humidity observationsSH Z 500 hPaFull observing systemNo humidity observations in data-assimilation THORPEX Conference December 2004

  24. Impact of humidity observationsTropics wind at 850 hPaFull observing systemNo humidity observations in data-assimilation THORPEX Conference December 2004

  25. Impact of humidity observationsTropics wind at 850 hPa (mean error at day 5)Full observing systemNo humidity observations in data-assimilation Control No humidity THORPEX Conference December 2004

  26. Humidity observationsin present data-assimilation • Moisture observations have no detectable influence on the overall large scale predictive skill • During the cause of the data-assimilation the large scale moisture field is mainly controlled by the model dynamics • There is an urgent need to develop techniques for a better assimilation of humidity observations THORPEX Conference December 2004

  27. Assessment of predictability • Growth of small perturbations inserted in a model • Comparing how consecutive forecasts separate from each other ( Lorenz, 1982) THORPEX Conference December 2004

  28. THORPEX Conference December 2004

  29. Estimation of predictability ( Lorenz, 1982) The error between consecutive forecasts separated by a day ( or shorter) is a suitable expression for predictability. The initial error is then the difference between the analysis an day 1 and the forecast from the day before THORPEX Conference December 2004

  30. Predictive skill ( Z 500 hPa) for the NHand predictability estimates ( for 6 ( red) and 24 hr (blue) increments) THORPEX Conference December 2004

  31. Predictive skill ( Z 500 hPa) for the SHand predictability estimates ( for 6 ( red) and 24 hr (blue) increments) THORPEX Conference December 2004

  32. Predictive skill ( V 850 hPa) for the Tropicsand predictability estimates ( for 6 ( red) and 24 hr (blue) increments) THORPEX Conference December 2004

  33. THORPEX Conference December 2004

  34. What are the predictive skill and predictability of storm-tracks? • Storm- tracks as a proxy for predicting the transient weather • Predictive skill of storm-tracks as a function of the observing system • Estimation of predictability following Lorenz (1982) THORPEX Conference December 2004

  35. NH, MSLP, Cyclones Tracks Intensities THORPEX Conference December 2004

  36. NCEP Ensemble Storm TracksCourtesy Z Todt, NCEP and L Froude, ESSC THORPEX Conference December 2004

  37. Predictive skill and predictability of storm tracksfor different observing systems NH SH THORPEX Conference December 2004

  38. Conclusions • Major progress have been achieved following the FGGE in 1979 but significant improvements are still feasible • Predictability estimates indicate a possible gain of several days compared to the present best forecasts • The largest potential improvements are in the tropics • The Thorpex program is a timely initiative and should be strongly supported THORPEX Conference December 2004

  39. What is needed? • Weather independent observations of wind and temperature profiles • Research to better identify priorities of the observing system • Higher resolution in model integrations to reduce numerical errors and representation problems and to simplify the parameterization of physical processes • Continue dedicated process studies and associated field experiments THORPEX Conference December 2004

  40. END THORPEX Conference December 2004

More Related