Large-scale surface wind extremes in the Mediterranean Shira Raveh-Rubin and Heini Wernli Institute...
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Transcript of Large-scale surface wind extremes in the Mediterranean Shira Raveh-Rubin and Heini Wernli Institute...
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