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

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)

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’

Top large-scale gust events

30 N

45 N

0 E 30 E

Top large-scale gust events

west centre east

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

Composite for western Mediterranean extremes

-36 hm 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 h 0 hm s-1 m s-1

kg kg-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

west centre

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>108 € damages

>34 hPa decrease in 24 h199 km h-1 measured

28-33 33-39 39-44 44-50 >50 m s-1

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

23.1.2009 12 UTC

Rapid intensification phase

• Jet break up, divergence• Low-level PV• Dry intrusion[m s-1]

24.1.2009 00 UTC

• Deepest low pressure• Maximal precipitation now

[m s-1]

24.1.2009 12 UTC

• Landfall 06 UTC• Maximal gusts 06-12 UTC

[m s-1]

25.1.2009 00 UTC

[m s-1]

• Cyclone and strong gusts inside Mediterranean

25.1.2009 12 UTC

[m s-1]

• Cyclone and strong gusts inside Mediterranean

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

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 25.1.2009 00 UTChPa

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 [%]

θe

Time [h]

P [h

Pa]

Maximum Mediterranean gusts 25.1.2009 00 UTC

hPa

• Warm and moist low-level flow accelerates near high-PV anomaly

• Circulates around cyclone centre

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

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 EUMeTrain Liberato et al., 2011 Weather

COSMO model simulation

Accumulated precipitation 23.1 00 UTC – 24.1 12 UTC [mm]

Convective precipitationTotal precipitation