Sting Jets in severe Northern European Windstorms

1
Sting Jets in severe Northern European Windstorms

• Suzanne Gray, Oscar Martinez-Alvarado, Laura
  Baker (Univ. of Reading), Peter Clark
  (collaborator, Met Office)

June 2009
2
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

3
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

4
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)
5
Review severe Northern European windstorms

• 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

Shapiro and Keyser (1990)
6
Review existing cases October 1987 storm,
observations
Mesoanalysis
IR imagery
Browning (2004)
7
Review existing cases October 1987 storm,
modelling
Clark et al. (2005)
Model system-relative 825hPa windspeed at 0300 UTC
Pseudo-IR at 0300 UTC and system-relative track
of the maximum descending trajectory.
8
Review - existing cases Windstorm Jeanette,
observations
Windspeed from MST radar
IR satellite imagery
Parton et al. (2009)
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Review - existing casesWindstorm Jeanette,
modelling
MST radar wind fields overlaid by operational UM
fields. Sting jet present in model fields due to
assimilation of MST data.
Enhanced UM synthesis showing sting jet, CCB, and
dry intrusion.
Parton et al. (2009)
10
Review - climatological importance
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)
Extracted from PhD thesis by Parton
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Review - mechanisms evaporative cooling

• Browning (1994) suggested that evaporation
  associated with slantwise convection could
  enhance the surface winds by
• Intensifying the slantwise circulations and so
  amplifying the latent heat sources and sinks on
  the mesoscale
• 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.

Browning (2004)
Parton et al. (2009)
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Review a brief guide to CSItheory

• 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 CSIprevalence

• 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 CSIDiagnosis

• 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.

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

17
Review synthesis conceptual model
Sting jet is a transient mesoscale feature that
occurs during the process of frontal
fracture Based primarily on one case study
(October 87 storm)
Clark et al. (2005)
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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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New cases - potential cases

• 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
• .....

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New cases observationssatellite
A storm on 26th February 2002
Gudrun, 7th to 9th January 2005
IR satellite imagery (Shapiro-Keyser stage III)
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New cases observationsGudrun 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.
25
New cases observationsGudrun frontal passage
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New cases observations26th February storm,
wind gusts
0518 UTC
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
Observed surface wind gusts
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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 qw
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New cases synopticand mesoscale evolution
07 UTC 26th February 2002
Gudrun 04 UTC 8th January
UL Jet
UL Jet
CCB?
Sting Jet
CCB
WCB
WCB
Sting Jet
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New cases synopticand mesoscale evolution
Back trajectories
Gudrun
26th February 2002
Pressure evolution
RH evolution
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New cases synopticand mesoscale evolution
26th February 2002
Conceptual picture Browning (2004)
Modelled ascending and descending sting jet
branches.
32
New cases mechanisms role of evaporational
cooling
Gudrun
26th February 2002
q evolution
qw 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 qw
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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 qw
35
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 qw
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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)
Moist PV along trajectories
26th February 2002
Sting jet
Pressure (hPa)
Ascending branch
PVU
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New cases mechanisms role of CSI (MPV)
Moist PV along trajectories
26th February 2002
Pressure (hPa)
PVU
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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

40
Ongoing work towards a sting jet climatology
DSCAPE
Sting jet
DCAPE
26th February 2002
Global model (0.4o)
Limited area model (0.11o)
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Ongoing work idealised modelling theory
Polar jet stream
Subtropical jet stream
LC2 cyclonic shear cyclone Norwegian frontal
cyclone
Shapiro et al. (1999)
LC1 nonshear cyclone Shapiro-Keyser frontal
cyclone
LC3 anticyclonic shear cyclone
42
Ongoing work idealised modelling application
qw at 850 mb
Day 7 of baroclinic lifecycle 1
Surface pressure deviation from 1000 mb
Limited area UM simulations east-west periodic
domain, wave-number 6 perturbations
43
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.