Sting Jets in severe Northern European Windstorms

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

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

Outline• Review








development



• Conclusions

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)

• 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)

Review – existing cases : October 1987 storm, observations

Browning (2004)

Mesoanalysis IR imagery

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)

Review - existing cases :Windstorm Jeanette, observations

Windspeed from MST radar

IR satellite imagery

Parton et al. (2009)

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.


Enhanced UM synthesis showing sting jet, CCB, and dry intrusion.

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)

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)

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)

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)

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)

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)

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

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)

Outline• Review








development



• Conclusions

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

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.

Outline• Review








development



• Conclusions

• 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

New cases – observations:satellite

Gudrun, 7th to 9th January 2005 A storm on 26th February 2002

IR satellite imagery (Shapiro-Keyser stage III)

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.

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

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

New cases – synopticand mesoscale evolution


Top of boundary layer system-relative winds and w



WCBWCB

Sting Jet

Sting JetCCB

UL Jet

UL Jet

CCB?


Gudrun

26th February 2002

Pressure evolution RH evolution

Back trajectories

Modelled ascending and descending sting jet branches.


Conceptual pictureBrowning (2004)

26th February 2002

New cases – mechanisms: role of evaporational cooling

Gudrun

26th February 2002

evolution w evolution

New cases – mechanisms: role of CSI (SCAPE)


SCAPE (lifting from low-levels) prior to descent of sting jet with midlevel RH (cloud

head) and low-level w

New cases – mechanisms: role of CSI (DSCAPE)


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

New cases – mechanisms: role of CSI (MPV)


MPV (at level of sting jet trajectories) at onset of descent of sting jet with midlevel RH (cloud head)

and low-level w


Gudrun 7th/8th January 26th February 2002

MPV evolution


PVU

Pre

ssure

(h

Pa)

Sting jet

Ascending branch

Moist PV along trajectories

26th February 2002

New cases – mechanisms : role of CSI (MPV)

PVU

Pre

ssure

(h

Pa)

Moist PV along trajectories

26th February 2002

Outline• Review








development



• Conclusions

Ongoing work – towards a sting jet climatology

DCAPE

Global model (0.4o) Limited area model (0.11o)

Sting jet

26th February 2002

DSCAPE

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)

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

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.

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Sting Jets in severe Northern European Windstorms

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