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,

Impact of large-scale extremes Large scale = 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)

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 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’

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

Top large-scale gust events westcentreeast N. Africa Vivian (Schuepp et al., 1994, Th. App. Clim.) Klaus (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

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]

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

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

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

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

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 >50 m s -1

UTC -WV channel ( copyright EUMETSAT, NERC Satellite receiving station, University of Dundee) -SLP [hPa] -330-K PV [2,4,6,8 PVU] -Average 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

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

UTC Deepest low pressure Maximal precipitation now [m s -1 ]

UTC Landfall 06 UTC Maximal gusts UTC [m s -1 ]

UTC [m s -1 ] Cyclone and strong gusts inside Mediterranean

UTC [m s -1 ] Cyclone and strong gusts inside Mediterranean

Intensification phase UTC PVU -PV (color) -Wind magnitude [m s -1 ] -θ [K] -330-K PV [2,4,6,8 PVU] -Average 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

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 [%]

Maximum Mediterranean gusts 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]

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

Summary 2 - Case study ‘Klaus’: 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

Thank you!

Cross section composite for western Mediterranean PVU

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

COSMO model verification copyright EUMeTrainLiberato et al., 2011 Weather

COSMO model simulation Accumulated precipitation UTC – UTC [mm] Convective precipitationTotal precipitation