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Www.met.reading.ac.uk/~sws98slg Sting Jets in severe Northern European Windstorms Suzanne Gray, Oscar Martinez-Alvarado, Laura Baker (Univ. of Reading),

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Presentation on theme: "Www.met.reading.ac.uk/~sws98slg Sting Jets in severe Northern European Windstorms Suzanne Gray, Oscar Martinez-Alvarado, Laura Baker (Univ. of Reading),"— Presentation transcript:

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 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)

6 Review – existing cases : October 1987 storm, observations Browning (2004) MesoanalysisIR imagery

7 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)

8 Review - existing cases : Windstorm Jeanette, observations Windspeed from MST radar IR satellite imagery Parton et al. (2009)

9 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.

10 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)

11 Review - mechanisms: evaporative cooling Browning (1994) suggested that evaporation associated with slantwise convection could enhance the surface winds by i.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)

12 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)

13 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)

14 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)

15 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. Schultz and Schumacher (1999)

16 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 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)

18 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

19 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

20 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), 0.11 o 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.

21 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

22 Gudrun/Erwin 7 th -9 th January th February 2002 Tilo: 7 th /8 th January th January 2005 Kyrill 18 th /19 th January 2007 Klaus 23 rd January New cases - potential cases

23 New cases – observations: satellite Gudrun, 7 th to 9 th January 2005 A storm on 26 th February 2002 IR satellite imagery (Shapiro-Keyser stage III)

24 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.

25 New cases – observations: Gudrun frontal passage

26 New cases – observations: 26 th February storm, wind gusts Observed surface wind gusts This storm passed over the UK during 25 th to 26 th February 2002 and was associated with strong winds over northern England and Wales, with wind gusts of over 40ms -1 recorded 0518 UTC

27 New cases – synoptic and mesoscale evolution Gudrun 04 UTC 8 th January 07 UTC 26 th February 2002 Top of boundary layer Earth-relative winds and midlevel relative humidity

28 New cases – synoptic and mesoscale evolution Gudrun 04 UTC 8 th January 07 UTC 26 th February 2002 Top of boundary layer system-relative winds and w

29 New cases – synoptic and mesoscale evolution Gudrun 04 UTC 8 th January 07 UTC 26 th February 2002 WCB Sting Jet CCB UL Jet CCB?

30 New cases – synoptic and mesoscale evolution Gudrun 26 th February 2002 Pressure evolutionRH evolution Back trajectories

31 Modelled ascending and descending sting jet branches. New cases – synoptic and mesoscale evolution Conceptual picture Browning (2004) 26 th February 2002

32 New cases – mechanisms: role of evaporational cooling Gudrun 26 th February 2002 evolution w evolution

33 New cases – mechanisms: role of CSI (SCAPE) Gudrun 18 UTC 7 th January 22 UTC 25 th February 2002 SCAPE (lifting from low-levels) prior to descent of sting jet with midlevel RH (cloud head) and low-level w

34 New cases – mechanisms: role of CSI (DSCAPE) Gudrun 23 UTC 7 th January 04 UTC 26 th 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

35 New cases – mechanisms: role of CSI (MPV) Gudrun 23 UTC 7 th January 04 UTC 26 th 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

36 New cases – mechanisms: role of CSI (MPV) Gudrun 7 th /8 th January26 th February 2002 MPV evolution

37 New cases – mechanisms: role of CSI (MPV) PVU Pressure (hPa) Sting jet Ascending branch Moist PV along trajectories 26 th February 2002

38 New cases – mechanisms : role of CSI (MPV) PVU Pressure (hPa) Moist PV along trajectories 26 th February 2002

39 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 DCAPE Global model (0.4 o ) Limited area model (0.11 o ) Sting jet 26 th February 2002 DSCAPE

41 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)

42 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

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.


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