Frequency and distribution of sting jets in intense winter North-Atlantic cyclones
Sting Jets in severe Northern European Windstorms
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Transcript of Sting Jets in severe Northern European Windstorms
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www.met.reading.ac.uk/~sws98slg
Sting Jets in severe Northern European WindstormsSuzanne Gray, Oscar Martinez-Alvarado, Laura Baker (Univ. of Reading), Peter Clark (collaborator, Met Office)
June 2009
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Outline• Review
Severe Northern European windstorms.
Currently identified sting jet cases
Climatological importance Mechanisms A brief guide to conditional
symmetric instability Synthesis
• Project aims and tools
• New sting jet cases Potential cases Observations Synoptic and mesoscale
evolution Mechanisms for sting jet
development
• Ongoing work Towards a climatology of
sting jet cyclones Idealised modelling
• Conclusions
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Outline• Review
Severe Northern European windstorms.
Currently identified sting jet cases
Climatological importance Mechanisms A brief guide to conditional
symmetric instability Synthesis
• Project aims and tools
• New sting jet cases Potential cases Observations Synoptic and mesoscale
evolution Mechanisms for sting jet
development
• Ongoing work Towards a climatology of
sting jet cyclones Idealised modelling
• Conclusions
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Review – severe Northern European windstorms
Conceptual model of cyclone (undergoing transition from stage III to IV of Shapiro-Keyser evolution) showing principal air streams:
• Warm conveyor-belt (W1, W2)
• Cold conveyor-belt (CCB)
• Dry intrusion
Browning (2004)
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• Insurance losses for extreme windstorms are significant: e.g. 3.4 billion Euro for the Christmas 1999 storms Lothar and Martin
• Some of the most damaging winds in extratropical cyclones are found in the dry slot of cyclones evolving according to the Shapiro-Keyser conceptual model.
• A recent series of papers has attributed these winds to a coherent mesoscale feature – a sting jet
Review – severe Northern European windstorms
Shapiro and Keyser (1990)
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Review – existing cases : October 1987 storm, observations
Browning (2004)
Mesoanalysis IR imagery
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Review – existing cases : October 1987 storm, modelling
Model system-relative 825hPa windspeed at 0300 UTC
Pseudo-IR at 0300 UTC and system-relative track of the maximum descending trajectory.
Clark et al. (2005)
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Review - existing cases :Windstorm Jeanette, observations
Windspeed from MST radar
IR satellite imagery
Parton et al. (2009)
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Review - existing cases:Windstorm Jeanette, modelling
MST radar wind fields overlaid by operational UM fields.Sting jet present in model fields due to assimilation of MST data.
Parton et al. (2009)
Enhanced UM synthesis showing sting jet, CCB, and dry intrusion.
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Review - climatological importance
Extracted from PhD thesis by Parton
Algorithm developed to extract mesoscale strong wind events from MST radar data – classified by structure and synoptic setting
9 potential sting jets passed over radar (in 7 years)
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Review - mechanisms: evaporative cooling
• Browning (1994) suggested that evaporation associated with slantwise convection could enhance the surface winds byi. Intensifying the slantwise
circulations and so amplifying the latent heat sources and sinks on the mesoscale
ii. Reducing the static stability in the dry slot (where there is potential instability so leading to upright convection) and/or closer to the cloud head so leading to turbulent momentum transfer.
Clark et al. (2005)
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Review – mechanisms: Conditional symmetric instability (CSI)
• Browning (2004) noted that the multiple slantwise circulations inferred from banded cloud tops near the tip of the cloud head in the Oct. 87 storm are suggestive of CSI release.
• Parton et al. (2009) found that the sting jet in windstorm Jeanette started at the tip of the region of CSI in the cloud head.
Parton et al. (2009)
Browning (2004)
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Review – a brief guide to CSI:theory
• CSI is the due to the combination of inertial and conditional instability (gravitational instability) for air parcels displaced along a slantwise path.
• It will only be released if the atmosphere is inertially stable to horizontal displacements and conditionally stable to vertical displacements.
Morcrette (2004)
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Review – a brief guide to CSI:prevalence
• Single and multi-banded clouds in frontal zones.
• Trailing precipitation regions of mesoscale convective systems.
• Hurricane eyewalls
• Cloud heads in extratropical cyclones.
Schultz and Schumacher (1999)
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Review – a brief guide to CSI:Diagnosis
• SCAPE (slantwise convective available potential energy): large values of SCAPE indicate that CSI is present.
• DSCAPE (downdraught SCAPE): large values indicate that CSI could be released by a descending air parcel.
• MPV (moist potential vorticity): negative MPV in the absence of gravitational and inertial instability indicates regions of CSI.
1eMPV
Schultz and Schumacher (1999)
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Review – synthesis: key features
• Mesoscale (~100 km) region of strong surface winds occurring in the most intense class of extratropical cyclones
• Occurs at the tip of the hooked cloud head• Distinct from warm and cold conveyor belt low level jets• Transient (~ few hours), possibly composed of multiple
circulations• Evaporative cooling of cloudy air and the release of condition
symmetric instability (a mixed gravitational/ inertial instability) hypothesized to be important
• Vertical transport of mass and momentum through boundary layer needed to yield surface wind gusts
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Review – synthesis: conceptual model
Clark et al. (2005)
Sting jet is a transient mesoscale feature that occurs during the process of frontal fracture
Based primarily on one case study (October ’87 storm)
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Outline• Review
Severe Northern European windstorms.
Currently identified sting jet cases
Climatological importance Mechanisms A brief guide to conditional
symmetric instability Synthesis
• Project aims and tools
• New sting jet cases Potential cases Observations Synoptic and mesoscale
evolution Mechanisms for sting jet
development
• Ongoing work Towards a climatology of
sting jet cyclones Idealised modelling
• Conclusions
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Project aims
• To determine the dominant mechanisms leading to sting jets
• To determine the environmental sensitivities of sting jets• To develop diagnostics that can be used to predict the
development of sting jets and the likelihood of the existence of a sting jet from synoptic-scale data
• To develop and analyse a climatology of sting jet events• To explore the effect of climate change on sting jets
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Project tools
• (UK) Met Office operational numerical weather forecast model (Unified Model), used in case study and idealised modes– Case study configuration: limited area (North Atlantic
European domain), 0.11o horizontal gridboxes, enhanced vertical resoution (76 levels), full physics, initial conditions from Met Office or ECMWF analyses.
• Observational validation from satellite, radar (MST radar, Chilbolton radar, wind profilers) and surface station observations (radiosonde ascents).
• Trajectory analysis and diagnostic tools for CSI
• Climatological data from re-analyses datasets such as ERA-40.
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Outline• Review
Severe Northern European windstorms.
Currently identified sting jet cases
Climatological importance Mechanisms A brief guide to conditional
symmetric instability Synthesis
• Project aims and tools
• New sting jet cases Potential cases Observations Synoptic and mesoscale
evolution Mechanisms for sting jet
development
• Ongoing work Towards a climatology of
sting jet cyclones Idealised modelling
• Conclusions
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• Gudrun/Erwin 7th-9th January 2005
• 26th February 2002
• Tilo: 7th/8th January 2007
• 11th January 2005
• Kyrill 18th/19th January 2007
• Klaus 23rd January 2009
• .....
New cases - potential cases
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New cases – observations:satellite
Gudrun, 7th to 9th January 2005 A storm on 26th February 2002
IR satellite imagery (Shapiro-Keyser stage III)
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New cases – observations:Gudrun wind gusts
Gudrun/Erwin was a powerful windstorm that exhibited strong surface winds and gusts of over 40ms-1, and caused significant damage as it passed over land in the UK and Northern Europe.
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0300
0500
0700
0200
0700
0400
08000400
New cases – observations:26th February storm, wind gusts
Observed surface wind gusts
This storm passed over the UK during 25th to 26th February 2002 and was associated with strong winds over northern England and Wales, with wind gusts of over 40ms-1 recorded
0518 UTC
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New cases – synoptic and mesoscale evolution
Gudrun 04 UTC 8th January 07 UTC 26th February 2002
Top of boundary layer Earth-relative winds and midlevel relative humidity
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New cases – synopticand mesoscale evolution
Gudrun 04 UTC 8th January 07 UTC 26th February 2002
Top of boundary layer system-relative winds and w
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New cases – synopticand mesoscale evolution
Gudrun 04 UTC 8th January 07 UTC 26th February 2002
WCBWCB
Sting Jet
Sting JetCCB
UL Jet
UL Jet
CCB?
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New cases – synopticand mesoscale evolution
Gudrun
26th February 2002
Pressure evolution RH evolution
Back trajectories
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Modelled ascending and descending sting jet branches.
New cases – synopticand mesoscale evolution
Conceptual pictureBrowning (2004)
26th February 2002
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New cases – mechanisms: role of evaporational cooling
Gudrun
26th February 2002
evolution w evolution
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New cases – mechanisms: role of CSI (SCAPE)
Gudrun 18 UTC 7th January 22 UTC 25th February 2002
SCAPE (lifting from low-levels) prior to descent of sting jet with midlevel RH (cloud
head) and low-level w
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New cases – mechanisms: role of CSI (DSCAPE)
Gudrun 23 UTC 7th January 04 UTC 26th February 2002
DSCAPE (DSCAPE maxima in sting jet region falling from level of sting jet trajectories) at onset of descent
of sting jet with midlevel RH (cloud head) and low-level w
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New cases – mechanisms: role of CSI (MPV)
Gudrun 23 UTC 7th January 04 UTC 26th February 2002
MPV (at level of sting jet trajectories) at onset of descent of sting jet with midlevel RH (cloud head)
and low-level w
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New cases – mechanisms: role of CSI (MPV)
Gudrun 7th/8th January 26th February 2002
MPV evolution
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New cases – mechanisms: role of CSI (MPV)
PVU
Pre
ssure
(h
Pa)
Sting jet
Ascending branch
Moist PV along trajectories
26th February 2002
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New cases – mechanisms : role of CSI (MPV)
PVU
Pre
ssure
(h
Pa)
Moist PV along trajectories
26th February 2002
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Outline• Review
Severe Northern European windstorms.
Currently identified sting jet cases
Climatological importance Mechanisms A brief guide to conditional
symmetric instability Synthesis
• Project aims and tools
• New sting jet cases Potential cases Observations Synoptic and mesoscale
evolution Mechanisms for sting jet
development
• Ongoing work Towards a climatology of
sting jet cyclones Idealised modelling
• Conclusions
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Ongoing work – towards a sting jet climatology
DCAPE
Global model (0.4o) Limited area model (0.11o)
Sting jet
26th February 2002
DSCAPE
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Ongoing work – idealised modelling: theory
Subtropical jet stream
Polar jet stream
LC3 anticyclonic shear cyclone
LC1 nonshear cyclone: Shapiro-Keyser frontal cyclone
LC2 cyclonic shear cyclone: Norwegian frontal cyclone
Shapiro et al. (1999)
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Ongoing work – idealised modelling: application
Surface pressure deviation from 1000 mb
w at 850 mb
Day 7 of baroclinic lifecycle 1
Limited area UM simulations: east-west periodic domain, wave-number 6 perturbations
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Conclusions
• New sting jet cases have been presented that are consistent with the conceptual model developed from the two cases already published.
• The new cases show some evidence of evaporational cooling occurring along the sting jet.
• A detailed analysis of the role of CSI release has demonstrated its importance in generating slantwise descending motions from cloud level. This is a modification to the conceptual model of the sting jet as the slantwise descending branch of a circulation arising from the release of CSI by the ascending branch.
• Ongoing work is examining potential diagnostics to develop a climatology of sting jet cases and sting jets in idealised baroclinic lifecycles.