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Predictability of a large-scale flow conducive to extreme precipitation over western Alps*

Predictability of a large-scale flow conducive to extreme precipitation over western Alps*. Federico Grazzini ARPA – Servizio IdroMeteorologico Emilia-Romagna, Bologna, Italy *work done at ECMWF, Shinfield Park, Reading (UK).

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Predictability of a large-scale flow conducive to extreme precipitation over western Alps*

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  1. Predictability of a large-scale flow conducive to extreme precipitation over western Alps* Federico Grazzini ARPA – Servizio IdroMeteorologico Emilia-Romagna, Bologna, Italy *work done at ECMWF, Shinfield Park, Reading (UK) Thanks to ONR Global for financial support in the framework of VSP program Grazzini F., 2007: Predictability of a large-scale flow conducive to extreme precipitation over the western Alps, Meteorol. Atmos. Phys., 95, 123-138

  2. The quality of numerical medium-range forecast has improved considerably since its beginning. However this remarkable achievement has to be considered true for average conditions since it is calculated over many days/seasons with very different flow patterns and atmospheric states. Has the skill increased also in specific high impact weather conditions ? A study has been carried out to examine the skill of ECMWF global forecasting system in predicting a specific flow configuration that is believed to be associated with extreme precipitation events over the Alpine region. We were not investigating the precipitation itself. Motivations

  3. Location of areas exposed to heavy and prolonged precipitation events

  4. Widespread demages during the main flood of October 2000 Maximum discharge up to 12.000 m3/s 40 casualities 32.000 have been evacuated Demages in the order of billion of Euro

  5. Po river catchment's area ( 105 Km2 )

  6. REFERENCE Anomaly from ERA40 60°W 60°E 80°E 60°W 60°E 80°E 528 -15.1 80°E 80°E -10 80°N 80°N -10 540 540 528 552 528 60°W 60°W 70°N 70°N -1.8 60°E 60°E 552 540 10 552 60°N 60°N 13.0 6.1 552 564 10 564 L 40°W 564 40°W -10 50°N 50°N -10 40°E 40°E 576 576 552 40°N 40°N -24.2 30°N 30°N 564 576 30°N 30°N -10 576 20°W 0° 20°E 20°W 0° 20°E Definition of the reference pattern in 500 hPa Z (SSF) The reference pattern has been defined as a composite of 6 major EAP events, it is consistent with others patterns in literature based on objective precipitation clustering or averaging over EAP cases (see for example Martius et al., 2006, Int. Journal of Climatology)

  7. In the period 1958-2003 312 days have been classified as SSF Selection based on ACC and RMS criteria of 500 hPa Z

  8. RMSE over Europe in Spring and Autumn days only 68 cases 70 cases

  9. ERA40 reforecast suggests that SSF events are indeed more predictable Grey curves represents the average skill in individual years (180 days Spring+Autumn) of ERA40 reforecast, 43 years from 1958-2001. Black curve is the average skill during SSF days only during the whole periods (223). During SSF days RMSE is lower.

  10. Trend of (D+4/D+6) RMSE over Europe in Spring+Autumn 3 years running mean

  11. An example : 1 December 2003

  12. Seasonal dependence of the predictive skill of SSF Spring Autumn

  13. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 7.1 -3.0 12.85 11.53 11.4 -11.2 -13.1 6 11.5 7 1.9 6 60°N 60°N 60°N 60°N 7 9.2 10 11 8 -9.0 -7.3 10.3 9 12.8 10 -9.9 -11.9 10.2 -12.6 -7.1 15.2 40°N 40°N -11.8 40°N 40°N 8.5 9 12 -6.5 8 4.1 9 8.9 6 7.1 7.6 7 -10.4 7 -6.8 6 7 8.4 20°N 20°N 20°N 20°N 3.6 -0.1 6 8 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions Lag composite of 250 hPa v-component and envelope* SSF cases between 1980-2001 (ERA40) Spring: April/May, 65 cases Autumn: October/November, 45 cases D-6 Spring Autumn *As defined in Zimin, Szunyogh et. al., Mon. Wea. Rev, May 2003

  14. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 7.1 14.98 12.61 -3.9 12.3 -13.3 9 13 3.0 60°N 60°N 60°N 60°N 8 9 13 11 -8.9 -5.7 -6.1 17.2 12.1 -15.2 11 -16.3 -13.3 11.6 10.8 9 10.0 9 40°N 40°N 40°N 40°N -12.8 -7.0 9 8 5.3 8.0 9 8.4 5.1 -8.6 -7.2 7 6.2 10.4 20°N 20°N 20°N 20°N -0.2 2.3 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D-5 Spring Autumn

  15. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 6.6 6.5 13.98 15.42 6 -11.1 6 -15.1 6 11 12.0 8 9 12.0 13.5 13 60°N 60°N 60°N 60°N -3.5 8 9.0 14.6 13 9 -17.2 -16.1 13 -7.3 6 16.6 16.1 -14.0 4.6 11 11 11.6 -8.7 40°N 40°N 40°N 40°N -9.6 9 8 -7.7 5.6 3.3 6 8.0 9 8.5 5.9 -8.7 -6.1 6 3.6 6.2 20°N 20°N 20°N 20°N 3.1 3.4 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D-4 Spring Autumn

  16. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 6.2 -3.5 16.57 19.06 6 -13.2 6 11 -13.9 19 16 7.6 -7.3 14.5 60°N 60°N 13 60°N 60°N 11.6 6 -18.9 14.1 -11.8 -19.8 -19.6 13.5 16 14.7 19.9 13.8 -10.9 13 40°N 40°N 40°N 40°N 11 -8.4 13 11 6 6.8 5.0 -3.8 -10.8 8.5 10 -6.3 5.6 12.2 5.6 20°N 20°N 20°N 20°N 6 2.0 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D-3 Spring Autumn

  17. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 6.4 22.45 21.80 6 8 6 9 9 6 -13.0 -14.5 21 21 7.9 8 60°N 60°N 60°N 60°N 8 17 13.4 17 -16.4 6 6 17 18.8 9 15.5 18.7 6 -25.0 19.5 17 17 9 -25.4 11.8 -11.2 8 40°N 40°N -2.3 40°N 40°N 9 13 13 8 6 4.9 5.4 6.4 -4.0 9.5 -6.1 -12.4 9 9 -5.9 6.0 20°N 20°N 20°N 20°N 9 3.4 6 0.7 2.1 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D-2 Spring Autumn

  18. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 26.35 25.33 6.2 8 11 11 8 -16.9 26 7.9 13.5 -15.1 21 60°N 60°N 60°N 60°N 11 -13.7 11 21 21 14.3 25.8 21 26.1 -30.2 20.5 -29.8 -8.5 16 21 13.5 40°N 40°N 40°N 40°N 8 11 16 -5.8 6.2 8 -6.0 11 12.4 -14.7 11 -6.0 6.2 5.4 20°N 20°N 20°N 20°N 11 2.1 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D-1 Spring Autumn

  19. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E -1.2 24.38 23.61 6 7.1 8 6 9 6 9 7.6 21 21 8 -15.1 8 -18.7 9 13.7 6 60°N 60°N 60°N 60°N 17 8 6 -10.6 17 17 15.8 10.9 26.8 17 17 28.4 8 -26.7 9 -25.9 7.6 -9.7 -6.1 15.5 40°N 40°N 40°N 40°N 13 13 6 9 9 9 9 8 9 8 -15.1 13.7 9 9 7.9 -4.9 20°N 20°N 20°N 20°N 1.8 2.8 6 6 8 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D-0 Spring Autumn

  20. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E -0.2 18.72 17.71 2.5 14.0 -6.6 6 6 -11.4 7 7.5 6.9 10.2 7 16 -14.4 6 13 16 60°N 60°N 60°N 60°N 13 12.1 -19.5 6 13 22.3 13 19.9 -5.2 -20.4 15.3 -19.7 7.0 -8.4 4.5 13 40°N 40°N 40°N 40°N 3.7 10 6 7 -11.0 -7.5 -13.4 10 7.8 7 -5.6 13 7 20°N 20°N 20°N 20°N 1.9 4.7 2.7 6 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D+1 Spring Autumn

  21. 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E 3.9 15.08 10.99 11.2 -10.9 6 6.4 13.5 60°N 60°N 60°N 60°N 13.2 -9.6 11 10.5 15.7 -15.3 8 8.5 9.75 6.8 8.5 8 -6.5 8 8.5 -18.7 -8.0 14.7 3.4 11 -16.3 8 40°N 40°N 40°N 40°N -9.9 5.5 6 8.5 11 5.1 -6.7 -13.6 3.9 8.5 2.8 10.9 14.8 7.3 20°N 20°N 20°N 20°N 2.5 3.9 4.7 6 8 6 120°E 120°E 160°E 160°E 160°W 160°W 120°W 120°W 80°W 80°W 40°W 40°W 0° 0° 40°E 40°E 80°E 80°E Propagation of wave packets leading to SSF conditions D+2 Similar to North Atlantic Jet waveguide* Spring Similar to North African – Asian Jet waveguide* or circumglobal waveguide** Autumn *As defined in Hoskins and Ambrizzi., J. Atmos. Sci., 50 1993 **As defined in Branstator, J. of Climate, 2001

  22. 120°E 160°E 160°W 120°W 80°W 40°W 0° 40°E 80°E -12 41.68 -12 -12 40 0 60°N 60°N 0 0 35 20 20 30 0 20 12 40 40°N 40°N 30 20 30 20 20 12 30 25 20 24 20 12 20°N 20°N 20 20 20 120°E 160°E 160°W 120°W 80°W 40°W 0° 40°E 80°E 120°E 160°E 160°W 120°W 80°W 40°W 0° 40°E 80°E 49.81 -12 45 -12 -12 60°N 60°N 0 20 40 20 0 30 30 20 0 40 35 40°N 40°N 30 20 40 12 30 20 30 30 12 12 20 25 20°N 20°N 20 20 120°E 160°E 160°W 120°W 80°W 40°W 0° 40°E 80°E Seasonal change in the Jet stream Spring Autumn

  23. The predictive skill of SSF conditions has increased during the years, especially in the medium-range where the improvement has been greater than normal conditions. Summary Error reduction over Europe from 80’ to 90’ D+6 ALL SSF ACC RMS Gain (hours)

  24. The predictive skill of SSF events is higher than average conditions. We argue that this could be explained by the prolonged linear growth of the wave packet induced by a strong wave guiding effects of the jet. Wave breaking and others highly non linear processes may act to reduce predictability in the decaying stage. Which is the predictability limit of these events ? Will it be possible to predict them beyond 10 days ? Biggest improvement in the autumn cases. The different propagation of wave packets in the two seasons may induces different responses to model changes and availability of observation. Spring events, for example, propagate from N-America where known difficulties in correctly simulating the interaction between deep convection and large-scale flow might have delayed improvements in the forecast quality. Increase and better usage of satellite data may have had greater impact in correctly defining the initial conditions in data sparse regions, like the Pacific Ocean, where autumn wave packet seems to originate. Summary

  25. Thanks for your attention

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