Sting Jets in severe Northern European Windstorms

Sting Jets in severe Northern European Windstorms
NERC funded 3 year project. Sue (principal investigator) Oscar (Post Doctoral Research Assistant) started Feb ‘08 Laura (PhD student) started Oct ‘07 Suzanne Gray, Oscar Martinez-Alvarado, Laura Baker (Univ. of Reading), Peter Clark (collaborator, Met Office) June 2009

Outline Review Project aims and tools New sting jet cases Ongoing work
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

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)

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 Figure 1. Conceptual model of the life cycle of an extratropical cyclone: (I) incipient frontal cyclone, (II) frontal fracture, (III) bent-back front and frontal T-bone, and (IV) warm-core frontal seclusion. Upper diagram: sea-level pressure, full lines; fronts, bold lines; cloud signature, shaded. Lower diagram: temperature, full lines; and cold and warm air currents, full and dashed arrows, respectively. (From Shapiro and Keyser 1990.) Shapiro and Keyser (1990)

Review – existing cases : October 1987 storm, observations
Figure 9. (a) Raw data and (b) mesoanalysis of the distribution of peak surface wind gusts (m s-1) at 0130 UTC 16 October 1987, derived from anemograph traces at 20 stations. Station locations are indicated by the circles in (a) where half-hourly peak-gust measurements for each station have been converted to a series of measurements at equivalent spatial locations based on the system velocity at this time of 23 m s-1 from 217B . (The three stations enclosed by two circles indicate the stations whose anemographs are shown in Fig. 10.) Isotachs in (b) are at 5 m s-1 intervals. Some of the fine structure in (b) is based on observations extracted at finer temporal resolution than plotted in (a). Isotachs shown dashed deviate from the data in (a) to take into account French data in Fig. 8. The anomalously high underlined values at one of the stations (Portland RNAS) are discussed in the text. Four regions of strong winds, referred to in the text, are labelled A, B, C and D. The thick line in (b) represents the warm and bent-back front and the secondary cold front CF2 as derived from thermograph records. The dashed line in (b) shows the extent of dry air (RH<80%) as derived from hygrograph records. The peak-gust analysis in Fig. 9(b) draws attention to four regions of strong winds—A, B, C and D—which, with the exception of D, were all associated with the dry air: Region A: a region containing localized areas (individually labelled A in Fig. 9(b)) with gusts up to about 35 m s-1. It is shown in section 3(c)(i) that this region was associated with cumulonimbus clouds just ahead of CF2. Region B: a region containing localized areas (individually labelled B) with gusts up to about 40 m s-1. This region was associated with only shallow non-precipitating cloud in the dry slot behind CF2. Region C: a larger region of even stronger winds, with gusts from 40 to 50 m s-1. It is shown in section 3(c)(ii) that this region was located in the dry slot in the region of very shallow or no cloud close to the tip of the hooked cloud head. Region D: an extension of Region C, behind the bent-back front, but associated with moister air under the tip of the cloud head. Regions C and D are associated with the sting jet Mesoanalysis IR imagery Browning (2004)

Review – existing cases : October 1987 storm, modelling
Clark et al. (2005) Pseudo-IR from predicted broadband radiance temperature Isolines of thetaw also at 825 hPa Model system-relative 825hPa windspeed at 0300 UTC Pseudo-IR at 0300 UTC and system-relative track of the maximum descending trajectory.

Review - existing cases : Windstorm Jeanette, observations
Satellite – black dots indicate positions of Aberystwyth and Cardington MST – downward protrusion of strong winds just before the passage of the main cold front at ~0400 UTC at 4km followed by a secondary wind maximum from UTC. Strong upper level jet at 10 km (> 50 ms-1). Windspeeds + echo power (maximum in echo power due to gradient in RH and enhanced static stability) indicate cold front passed between 3-8 km from UTC (only evident above 3 km). ~2 hours later the winds strengthened again in the midtroposphere. Between UTC windspeeds in excess of 37 ms-1 measured below 5 km sometimes increasing to 50ms-1. Prominent slantwise banding evident in observed winds. 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. Figure 16. MST radar wind speeds (colour) with corresponding values from the operational UM fields overlaid as contours. Note that below the main jet stream the operational UM depicted two separate features between 0700 UTC and 0900 UTC: a wind maximum at 4–5 km and the CCB below 2 km. Unreliable MST radar winds have been blanked as described in the text. Figure 25. A synthesis of data from the enhanced UM above the pseudo- Cardington site (see text for details). Relative humidity is shown by the coloured contours, while black lines depict isotachs at 2 m s−1 intervals. The white lines are θw (white) contours every 0.5 K and the hashed regions depict areas of dry potential vorticity ≥ 1.5 PV units. Regions identified as the cold conveyor belt, sting jet and dry intrusion are marked CCB, SJ and DI respectively. Enhanced run 90 levels instead of 38. In Figure 25 a synthesis of various parameters from the high-resolution model is presented, in an analogous way to Figure 17. The high θw, low relative humidity and elevated PV values at the top left of the diagram are typical of the dry intrusion. Below this is the low-level wind maximum that would at first appear to be the CCB, by examination of θw, but its core is both dry (<50% relative humidity) and above a region of high potential vorticity. There is a moist layer below this PV anomaly, in which wind speeds remain high, with the tongue of high winds at 0800 UTC where θw is slightly lower. This region is more consistent with the CCB. We propose that the low-level wind maximum is in fact a merger of two airflows – the descending (and therefore dry) sting jet and the moister CCB beneath it. Enhanced UM synthesis showing sting jet, CCB, and dry intrusion. Parton et al. (2009)

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) Black cold frontal Blue warm sector Orange sting jet Green tropopause fold Red Plus – unclassified Figure 2.8: Schematic diagrams of event locations and relative wind directions for events where a synoptic location could be established. Principle clouds are represented by the grey outlines. Extracted from PhD thesis by Parton

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. (i) Slantwise circulations in the cloud head are associated with evaporative cooling in the descending parts, particularly where precipitation falls into them from the overlying ascending flows. The cooling is strong just to the south and south-east of the cyclone near the tip of the cloud head. This leads to the intensification of potential vorticity sheets and perhaps also to the generation or intensification of moist symmetric instability (see section 4(b)). As a result, the evaporative cooling acts to intensify the transverse slantwise circulation which in turn amplifies the latent-heat sources and sinks on the mesoscale. Evaporative cooling is most effective where there is sublimation of ice, as shown in the high-resolution modelling study of FASTEX cyclones by Forbes and Clark (2003). However, evaporation of cloud water may be important in enhancing the slantwise circulations closer to the surface. (ii) The evaporative cooling of the air leaving the tip of the cloud head reduces the static stability in the dry slot, which is a region characterized by potential instability created as described in section 2(c). It thus contributes to the triggering of the upright convection in region A where the associated downdraughts bring high-momentum air down to the surface. Closer to the tip of the cloud head, in regions B and C, the static stability, although not reduced sufficiently to permit the development of convective showers, may be reduced enough to produce very low Richardson numbers and strong turbulence capable of mixing the strong winds in the descending branches of the slantwise mesoscale circulations down to the ground. Figure 12. Pressure increase between the modelled minimum trajectory pressure and first reaching a pressure greater than 800 hPa plotted against potential-temperature change over the same period, for those trajectories which exhibit a change in wet-bulb potential temperature of less than 1 K over this time. Fig. 12 is consistent with the idea that the region of strong winds is associated with at least some air that has descended substantially (from as high as 500 hPa in the extreme case). The strong adiabatic warming associated with this descent may be offset to some extent by potential cooling (due to evaporation) and potential warming (due to mixing).There is some evidence that cooling may be by as much as 8 K in the extreme, but in parts of the SJ may be small or absent. 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) The sketches in (d) show: the northwestern boundary of the polar-front cloud head (hatched); the southern portion of the cloud head (stippled for TB < 240 K), with axes of mesoscale cloud bands near the tip of the cloud head (for TB < 270 K) shown by thin full lines labelled 1, 2, 3 and 4; and shallow shower clouds over southern England, with anvils trailing downwind from the cold front. The overall frontal analysis from Fig. 9(b) is superimposed on the 0130 UTC sketch, along with the 40 m s¡1 gust isopleth for the strong-wind region C. J denotes Jersey. Banding, believed to be associated with multiple slantwise convective circulations, is seen at the top of the cloud head as described earlier in connection with Figs. 3 and 4. Axes of the main bands near the tip of the cloud head are shown by curved lines labelled 1, 2, 3 and 4 in Fig. 11(d). These extend as shallow features some way to the east of the tips of the upper cloud in the cloud head (stippled region). The cloud-head bands terminate over and close to Brittany where they begin to hook around the cyclone centre and warm seclusion. The main region of strong gusts (enclosed by the small dashed curve in Fig. 11(d), top panel) is located close to the leading edge of these cloud-head bands. The overall structure in this region broadly resembles that in Figs. 15(c) and (d) of Neiman and Shapiro (1993). Figure 29. Trajectories initiated at 800 mb around the Cardington pseudo-location and taken back to 2300 h, overlaid on to simulated broad-band infrared and conditional symmetric instability diagnostic at 2300 h model time on 26 October. The white star denotes the Cardington pseudo-location. Evidence of the narrow, moist updrafts is seen by the narrow width of the bands in the CSI diagnostic in Figure 29. This is consistent with the argument that the banding within the cloud head seen in satellite imagery was also CSI-generated. CSI is also consistent with the vertical banding along θw surfaces inferred from the MST radar. These bands were not coincident with the cores of the strongest winds, but were located between the wind cores, and so could be an indication of the upward-moving branches of the CSI circulations. The evidence presented here is necessarily circumstantial and does not prove that CSI caused the sting jet; further investigation is required to identify a causal link. Nevertheless, given the proximity of the sting jet trajectories to the tip of the areas of active CSI, that these trajectories follow θ surfaces and that banding was seen in both the satellite and MST radar data, we cannot rule out a role for CSI in the generation of the sting jet. Parton et al. (2009)

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. For a region to be susceptible to the release of CSI, the relative slope of the saturated equivalent potential temperature ( ) surfaces must be steeper than the geostrophic absolute momentum (Mg) surfaces 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. FIG. 2. An ingredients-based methodology for forecasting gravitational or slantwise convection. FIG. 4. Stability regimes often observed near frontal zones. Contours represent typical values often present: Mg (thick black lines) and (thin gray lines). Based on a figure originally constructed by Prof James Moore and Sean Nolan. 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. 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
Sting jet is a transient mesoscale feature that occurs during the process of frontal fracture Based primarily on one case study (October ’87 storm) Figure 17. Conceptual model of the near-surface flows in an extratropical cyclone. (a) Early stage of frontal wave cyclone development. L denotes low-pressure centre with direction of movement shown by thin arrow. Grey arrows show the system-relative low-level jets; WJ is the warm-conveyor-belt jet (WCB in text) and CJ the cold conveyor- belt jet (CCB in text). (b) Frontal fracture phase, when the sting jet (SJ) first appears at the surface. (c) As the cloud head wraps round further the SJ region extends. (d) Eventually the distinct SJ disappears and the dominant low-level wind in this region is due to the CJ. Positions of cross-sections shown in Fig. 18 are marked in (b). Frontal analysis based on the Shapiro and Keyser (1990) lifecycle model in which the surface cold front fractures from the surface warm front so as to leave behind a bent-back warm front (part of which is analysed here as a cold front. Secondary WCB omitted for clarity. Clark et al. (2005)

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.

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 ..... 7th November 2005 case: 3 bands identified by masking technique but none satisfy the sting jet model. None of them descend much, MPV>0 and not a clear frontal fracture region. 11th January 2005 case: weak descent, +ve MPV, theta_w not very well conserved. Kyrill – Norwegian model evolution?

New cases – observations: satellite
Gudrun, 7th to 9th January 2005 A storm on 26th February 2002 Gudrun at 0536UTC 8th January 2005 and (b) 0518UTC 26th February 2002, both from NERC satellite receiving station, University of Dundee. Gudrun: Satellite imagery shows a hooked cloud head feature with a banded structure and ‘tails’ at the cloud head tip. 26th Feb: Satellite imagery shows the hooked cloud head and clear dry intrusion and ‘tails’ at the cloud head tip. IR satellite imagery (Shapiro-Keyser stage III)

New cases – observations: Gudrun wind gusts
Battered northern Europe from Ireland to Russia on 7-9 January 2005, killing at least 17 people and severely disrupting sea, air and land transport Cut power to around 500,000 homes. Widespread property damage. Strong winds and floods caused widespread damage in UK. Statistics from Guy Carpenter – speciality practice briefing IR satellite image from Dundee Satellite receiving station Forest damage in Sweden was the worst recorded in recent history Oil production hampered at three offshore fields. 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.

New cases – observations: Gudrun frontal passage

New cases – observations: 26th February storm, wind gusts
0518 UTC 0300 0500 0700 0200 0400 0800 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 Highest gust map , numbers near the circles indicate time of occurrence ( Observed surface wind gusts

New cases – synoptic and mesoscale evolution
Gudrun 04 UTC 8th January 07 UTC 26th February 2002 Figure 2: Earth-relative wind strength, with over-laid contours of relative humidity. (a) 04UTC 8th January 2005: wind strength at 850mb, relative humidity at 600mb; (b) 07UTC 26th February 2002: wind strength at 750mb, relative humidity at 600mb. Top of boundary layer Earth-relative winds and midlevel relative humidity

Gudrun 04 UTC 8th January 07 UTC 26th February 2002 Figure 3: System-relative wind strength, with overlayed contours of θw. (a) 04UTC 8th January 2005: wind strength and θw both at 850mb; (b) 07UTC 26th February 2002: wind strength and θw both at 750mb. Top of boundary layer system-relative winds and qw

Gudrun 04 UTC 8th January 07 UTC 26th February 2002 UL Jet UL Jet CCB? CCB Sting Jet WCB WCB Sting Jet

Back trajectories Gudrun Figure 4: Time series of variables along trajectories: (a) and (b) from 16UTC 7th January to 04UTC 8th January 2005; (c) and (d) from 01UTC to 12UTC 26th February (a) and (b) show pressure and relative humidity respectively terminating in the region of strong winds marked in Figure 3 (a) at 04UTC; (c) and (d) as (a) and (b) but for trajectories passing through the region marked in Figure 3 (b) at 07UTC. Lines show ensemble mean (blue) and ensemble mean plus and minus one standard deviation (red). 26th February 2002 Pressure evolution RH evolution

26th February 2002 Ascending branch (black) and descending sting jet (cyan) as part of slantwise circulation. Obtained from model trajectories (each point is can be considered as an air parcel that is advected with the resolved winds). Conceptual figure - Conceptual model depicting stacked slantwise convective circulations wrapping around the bent-back from of an intense extra-tropical cyclone. The cyclone is shown at the stage of minimum central pressure when the bent-back front (thick line) has almost encircled the warm air near the cyclone centre (L). Three slantwise convective circulations are sketched, each consisting of a slantwise ascending, SU (hatched), and slantwise descending, SD (stippled), branch. Each (SU+SD)-pair can be interpreted as a transverse circulation (radially outward from the cyclone centre) superimposed on a strong flow along the axis of arrows approximately parallel to the bent-back front. See paper text for details. Conceptual picture Browning (2004) Modelled ascending and descending sting jet branches.

New cases – mechanisms: role of evaporational cooling
Gudrun 26th February 2002 q evolution qw evolution

New cases – mechanisms: role of CSI (SCAPE)
Gudrun 18 UTC 7th January 22 UTC 25th February 2002 Figure 5: (a) SCAPE at 18UTC 7th January 2005, lifting from mb. (b) SCAPE at 22UTC 25th February 2002, lifting from mb. (c) Downdraft SCAPE from level of sting jet trajectories at 23UTC 7th January (d) As for (c) but at 04UTC 26th February (e) MPV at 700mb at time corresponding to (c). (f) MPV at 600mb at time corresponding to (d). All figures are plotted system-relative, and show position of sting jet trajectories with a black circle, and red contours of wet-bulb potential temperature at 825mb. Purple contours in figures (a), (c) and (e) are 80% relative humidity at 500mb, and in (b), (d) and (f) are 200Wm-2 outgoing longwave radiation. Both cases show slantwise convective available potential energy (SCAPE) near the horizontal location of the sting jet trajectories before the sting jet air starts to descend. SCAPE (lifting from low-levels) prior to descent of sting jet with midlevel RH (cloud head) and low-level qw

New cases – mechanisms: role of CSI (DSCAPE)
Gudrun 23 UTC 7th January 04 UTC 26th February 2002 Both cases show a peak in downdraft SCAPE (DSCAPE) occurring at the level and horizontal location of the trajectories when the sting jet air is descending. Scan down from 500 mb but max value at level of sting jet for peak in this region. 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

New cases – mechanisms: role of CSI (MPV)
Gudrun 23 UTC 7th January 04 UTC 26th February 2002 There is also a corresponding region of negative moist potential vorticity at this position. MPV (at level of sting jet trajectories) at onset of descent of sting jet with midlevel RH (cloud head) and low-level qw

Gudrun 7th/8th January 26th February 2002 MPV evolution

Moist PV along trajectories 26th February 2002 Sting jet Pressure (hPa) Ascending branch PVU

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

Ongoing work – towards a sting jet climatology
DSCAPE Sting jet Global model 38 levels LAM 76 levels SJ marked on global model plots at same Lat/Lon as on LAM plots Theta_w on LAM/global plots at 825mb DCAPE 26th February 2002 Global model (0.4o) Limited area model (0.11o)

Ongoing work – idealised modelling: theory
Polar jet stream Subtropical jet stream LC2 cyclonic shear cyclone: Norwegian frontal cyclone A conceptual hypothesis for the influence of upper-level jet stream and PV alignments on frontal structure within extratropical cyclones: Upper plane (light shading): 200 mb planetary wave and subtropical jet stream (white ribbon) with associated PV anomalies suspended below. Middle plane (heavy shading): 300 mb synoptic wave and polar jet stream (white ribbon) with associated PV anomalies suspended below. Lower plane: Earth’s surface with three characteristic cyclone frontal configurations; frontal symbols are conventional with fronts aloft entered as open symbols. Left cyclone: The anticyclonic shear cyclone (LC3) located south of the polar jet stream with its westward trailing cold front and a weakly defined warm front. Middle cyclone: The nonshear cyclone (LC1) located beneath the vertically aligned polar and subtropical jet streams and associated coupled tropopause folds; the T-bone polar occlusion and bent-back warm-frontal seclusion cyclone. Right cyclone: The cyclonic-shear cyclone (LC2) situated north of the subtropical jet stream; the Norwegian frontal cyclone with its back-bent polar warm-frontal occlusion. Shapiro et al. (1999) LC1 nonshear cyclone: Shapiro-Keyser frontal cyclone LC3 anticyclonic shear cyclone

Ongoing work – idealised modelling: application
qw at 850 mb Day 7 of baroclinic lifecycle 1 Surface pressure deviation from 1000 mb 0.4 degree horizontal gridboxes and 38 levels. Shapiro-Keyser stage III Frontal fracture evident 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.

Sting Jets in severe Northern European Windstorms

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