1. Large-scale surface wind extremes in the Mediterranean Shira Raveh-Rubin and Heini Wernli Institute for Atmospheric and Climate Science (IACETH), ETH Zurich WWOSC Montreal, 20.8.2014

2. Impact of large-scale extremes Large scale = 500-1000 km, 3 days Relevant for wind and precipitation extremes Current studies focus on station/grid-scale extremes of short duration Lacking systematic understanding of mechanisms leading to large-scale extremes Are there typical dynamical precursors of wind extremes at this scale? What mechanisms are involved? Mediterranean is a climate change hot spot (Giorgi, 2006, GRL)

3. Objective identification method of large-scale wind gusts ECMWF ERA-Interim reanalysis data (1º X 1º, 6-h resolution) of 10-m gust and atmospheric fields for 1979-2012 anomaly from climatological monthly mean and std 500-km circle, 3-day moving averages Select the top events in different subdomains 1. General dynamical features 2. selected case study ‘Klaus’

4. Top large-scale gust events 30 N 45 N 0 E 30 E

5. Top large-scale gust events westcentreeast N. Africa Vivian 28.2.1990 (Schuepp et al., 1994, Th. App. Clim.) Klaus 24.1.2009 (Liberato et al., 2011, Weather) Martin, Lothar (Wernli et al., 2002, QJRMS) Reported damage in the Mediterranean Main occurrence in winter, rest in transition seasons

6. Composite for western Mediterranean extremes -36 h m s -1 kg kg -1 Anomalies -850-hPa q -SLP [hPa] -330-K PV -1,2, +1,2 PVU -330-K wind vectors -330-K Wind mag. -SLP [hPa] -330-K PV [2,3,4 PVU]

7. Anomalies -850-hPa q -SLP [hPa] -330-K PV -1,2, +1,2 PVU -330-K wind vectors -330-K Wind mag. -SLP [hPa] -330-K PV [2,3,4 PVU] -36 h0 h m s -1 kg kg -1 Composite for western Mediterranean extremes

8. Composite [0 h] – [36 h] -vertically integrated specific humidity (kg m -2, color) -850-hPa θ (only -3 and -2 K contours) -850-hPa wind vectors kg m -2

9. westcentre east N. Africa Composite [0 h] – [36 h] kg m -2

10. Summary 1 Successful method for identifying high-impact wind events Spread over Mediterranean domain Large fraction in winter, rest in spring and autumn Coherent flow structures upstream prior to extremes : vertically aligned cyclonic and anticyclonic anomalies over Atlantic, ridge with strong jet and a cyclone Typical evolution: jet breaks down, cold and dry northerly flow in low levels regional variability Case study Details of the dynamical precursors and mechanisms, temporal evolution, trajectory analysis

11. Liberato et al., 2011 Weather Storm ‘Klaus’, January 2009 Identified as combined large-scale precipitation and gust extreme 26 killed >10 8 € damages >34 hPa decrease in 24 h 199 km h -1 measured 28-33 33-39 39-44 44-50 >50 m s -1

12. 23.1.2009 00 UTC -WV channel ( copyright EUMETSAT, NERC Satellite receiving station, University of Dundee) -SLP [hPa] -330-K PV [2,4,6,8 PVU] -Average 850-500 hPa PV [1,2 PVU] -300-hPa wind mag. [m s -1 ] -6-h maximum gusts [>20 m s -1 ] [m s -1 ] Warm conveyor belt

13. 23.1.2009 12 UTC Rapid intensification phase Jet break up, divergence Low-level PV Dry intrusion [m s -1 ]

14. 24.1.2009 00 UTC Deepest low pressure Maximal precipitation now [m s -1 ]

15. 24.1.2009 12 UTC Landfall 06 UTC Maximal gusts 06-12 UTC [m s -1 ]

16. 25.1.2009 00 UTC [m s -1 ] Cyclone and strong gusts inside Mediterranean

17. 25.1.2009 12 UTC [m s -1 ] Cyclone and strong gusts inside Mediterranean

18. Intensification phase 23.1.2009 12 UTC PVU -PV (color) -Wind magnitude [m s -1 ] -θ [K] -330-K PV [2,4,6,8 PVU] -Average 850-500 hPa PV [1 PVU] 2 high-PV centres in low levels split jet strong winds reach the boundary layer stronger winds at surface near high PV

19. Intensification phase – source of low-level PV Stratospheric PV (dry intrusion) Diabatically-produced PV (warm conveyor belt) Trajectories reaching PV>1 in low levels RH [%]

20. Maximum Mediterranean gusts 25.1.2009 00 UTC hPa Trajectories reaching gusts>25 m s -1 Descending dry intrusion advects momentum to low levels in Mediterranean Unstable boundary layer allows mixing down to the surface 2 PVU RH [%] θeθe Time [h] P [hPa]

21. Maximum Mediterranean gusts 25.1.2009 00 UTC hPa Warm and moist low-level flow accelerates near high-PV anomaly Circulates around cyclone centre

22. Summary 2 - Case study ‘Klaus’: 23-25 January 2009 Co-occurrence of both extreme large-scale gusts and precipitation Large ridge at outflow of a WCB Rapidly deepening cyclone Ascending/descending airstreams involved in low-level PV production/advection during cyclone intensification Cyclone with high-PV anomaly intensifies winds in its surrounding Gusts are produced by converging airstreams: 1.Dry intrusion, descending air destabilizes air beneath it, imports momentum into Mediterranean along sloping isentropes 2.Moist and warm low-level jet accelerates near strong PV gradients

23. Thank you!

24. Cross section composite for western Mediterranean PVU

25. Maximum gusts 2 hPa Dry and cold air in mid-levels accelerates near high-PV anomaly

26. COSMO model verification copyright EUMeTrainLiberato et al., 2011 Weather

27. COSMO model simulation Accumulated precipitation 23.1 00 UTC – 24.1 12 UTC [mm] Convective precipitationTotal precipitation