Strong surface winds in Storm <i>Eunice</i>. Part 2: airstream analysis

Storm Eunice caused severe wind impacts on the United Kingdom 18 February 2022. This article contains second of a two-part study and consists detailed analysis Eunice's airstream structure, confirming presence sting-jet (SJ) activity during its lifecycle. By time crossed Wales southern England, several other airstreams, including cold conveyor belt dry intrusion, interacted in generating observed damaging winds. These results illustrate wind-damage potential multiple SJs, intense cyclones like (as displayed figure), highlighting importance accurate forecasts. Strong, winds over widespread areas, or windstorms, are generally associated with cyclones. Research has shown that highly asymmetric nature extratropical leads to strong different regions storm at stages storm's presents an additional forecasting challenge. Not only do we need predict path overall strength storm, but also when hence where, geographically, these within might occur. Three were identified many decades ago as structure was elucidated (Dacre, 2020). The ‘warm belt’ (WCB; Browning, 1971) starts warm-sector boundary layer jet trailing front before rising above warm front. ‘cold (CCB), side occluded ‘bent-back’ front, wraps cyclonically around cyclone centre low levels (Schultz, 2001) often produces once it turns broadly into direction is moving. Strong upper-level descend towards ‘dry intrusion’ (DI) can, sometimes, contribute surface gusts, via deep convection (Browning Roberts, 1994; Raveh-Rubin, 2017). DI can overrun leading ‘shallow moist zone’ (mainly due humidity contrast) ahead (see fig. 11 Browning 1994). A fourth region, called ‘sting jet’ (SJ), first proposed by (2004) from ‘Great Storm’ 1987. SJ occurs ‘frontal-fracture’ region storms particular structure; follow Shapiro–Keyser model (Shapiro Keyser, 1990). SJs ‘cloud head’ and, if they reach all, so CCB for few hours. especially difficult detect unequivocally observations, much our understanding them stems simulations sufficiently high resolution represent them. example identification simulations, again 1987 Storm’, reported Clark et al. (2005) shows narrow (~50km) descending mid-levels surface. In case responsible most damage. three named affect (UK) week 2022 although not devastating Great (not least because actions taken result warning), produced record-breaking gusts England led weather UK Ireland fatalities, power transport disruption, thousands felled trees, closure schools businesses. An account observations all storms, together historical context Eunice, given Met Office report (Kendon, 2022). part this conducted Eulerian approach, is, assessing motion airstreams specific locations, rather than following air parcels time. As previously explained 1 study, slantwise circulations descent, which cloud head banding interpreted evidence, be release mesoscale instabilities such conditional symmetric instability (CSI). Symmetric (SI), counterpart CSI, more immediately related motions, saturation needed release. However, SI defined stricter conditions while implies opposite true CSI will have SI. Recent work idealised (Volonté al., 2020) uncommon even generated then released descent. indicated negative values vorticity (PV). Forecast PV 700hPa between 2100 utc 17 0300 global Figure 1. evolution selected clear tip times figs. 3b c discussion article). illustrates mid-tropospheric contours wet-bulb temperature, ? w $$ {\theta}_w , indicating location sectors fronts, black dashed enclosing areas where clouds present (both fields 700hPa). Elongated structures PV, SI, develop late hours highlighted sharp gradient north (where instead values, well zero). features move rearwards respect develops bending back (Figures 1a b). early February, those tip, same wrapping consequent seclusion (Figure 1c). edge collocated gap hole 1d, see arrow). break consistent satellite imagery relevant article), feature likely realistic, artefact. flow later analysis) thus evaporation occurring descent (i.e. SI). summary, form elongated bands symmetrically unstable travelling descending, drying airstream, confirmed development. Considering horizontal maps vertical cross-sections 2, now identify various low-level maxima describing their 3-dimensional structures. At 0100 still Atlantic strongest would recorded ?10h later. mentioned earlier, stage formed tip. 2(a) two speed exceeding 44ms?1 pressure level 850hPa, either centre, higher values. cross-section 2b) western maximum, 276 278K, centred ~900hPa, peak 46ms?1 30ms?1 envelope extending down maximum located bent-back tight . typical gradually centre. eastern mostly cloud-free Its 278 279K, characteristic frontal though forms maximum; just beyond cloud-head entrance developing frontal-fracture increasing contours. being SJ, contribution cannot excluded without further evidence. time, should noted tropopause fold extend way lower troposphere 50ms?1 contour ~700hPa). similar seen 4 Volonté (2018), (around 600hPa). downward, slanted, protrusion aligned notch cloud, suggesting linked overrunning relatively weak (the contour) about 900–950hPa east ? 285 K {\theta}_w\sim 285\mathrm{K} Being sector could WCB, nearby transporting slower upwards ridge green close complicate picture (and therefore ‘WCB?’ cross-sections). Both maxima, eastward westward (associated respectively CCB), 0600 2c d). While retain intensity previous stage, ~4ms?1 weaker extended horizontally. Contextually, slanted less earlier on. lies bottom area buckling isentropes ( contours). indication coherent airflow, reinforcing hypothesis conserved along distinct parcel does experience substantial radiative heating cooling turbulent mixing assume airflow isentropes). 1200 mature near-surface affecting England. Figures 2(e) (f), (CCB) Bristol Channel intense, southeast corner weakening continues begins close, separation masses sector, becomes clear. result, heights across covered 2(f). closer primary dry-air near excluded. Wind 750hPa exceed top oscillations base height indicate convective and/or motions. features, real artefact struggling simulate strongly vertically sheared static stability, played role aiding downward transfer momentum formation local gusts. dynamics case-study 2015) (Rivière research. Along comes possible influence it. WCB albeit sloping rightmost dip 40ms?1 filled contour). Therefore, end indicates contributions reason ‘SJ+’ (f)). slices through output suggests separate jets: west east. utc, mid-troposphere 70ms?1 500hPa) descended troposphere. behind fold, making interpretation cause difficult. Lagrangian trajectory analysis, described next, used unpick regions. alternative characteristics flows jets Using field components each grid point ‘parcels’ traced forwards backwards produce trajectories. Clusters, sometimes ‘coherent ensembles’, trajectories define jets. Although starting any point, typically ‘start points’ restricted geographically thresholds (e.g. system). must period calculated frequent output. Here calculation code LAGRANTO (Sprenger Wernli, been calculate using hourly forecast 3 start points (black dots) (including London). threshold 42ms?1 select cores (a balance having interpret representative). coloured change cooler warmer colours, moves eastwards component. 3(a) clustered sets, south Wales/the Cornish Peninsula map 2(e). sets broad overlapping clusters, crossing southwards originating northward component approaching points. descends More information obtained adding selection criteria 3(b–d). For panels, locations slightly compared lead winds, reaches levels), captured. Comparison 3(b–d) captured criteria; schematically (which synthesis Eunice). 3(b) originate cloudy (defined relative ice >80%) 100hPa six utc. atmospheric layer. Note shown), forming re-moisten intersect 3(c) weakly ascend (descent <100 hPa) clusters identified. Analysis shown) cluster set Eunice. trajectories, Peninsula, coldest jet. temperatures majority lower, 2(f), gradients fracture. mix characteristics. Some portions DI, become interact air, whereas others may spreading out (warmer) branch moved undercut SJ; labelled ‘possible CCB2’ 4. quite spread horizontally CCB, hard distinguish third France/Belgium border, pressures altitude They warmest entire shown); comparison embedded upper surface, line (visible radar imagery, brought some contributing Finally, 3(d) <60%). lie (have pressures) (to east) 3(c), offshore continent. right-hand Trajectories (using 40ms?1, shown). times, there little evidence (just meet utc) far (compare 2b, d f). meeting exist both times. reveals that, attributable attributed single Instead, possibly (CCB2). windstorm affected 2022, triggering red warning since National Severe Weather Warning Service up wake 2023), impacts, fatalities. station provided indications likelihood taking place, did precursor diagnostic tool, Gray (2021) available operational forecasters, days advance. advance, uncertainty exact remained forecasts passage. Starting indications, main underlying fully addresses key questions (listed Introduction section) longevity airstream(s) short-lead-time highlight development opening filaments, type (SI) head, travel one cloud. sign head. Focusing troposphere, denoting stable intermediate region. concluded respectively. entering beginning becoming clearly separated storm. levels. situation, individual causing large speeds Back-trajectories (>42ms?1) reveal mixed origin. detected nearby. complex correspondence passage synthesised, paths, schematic final remark showed SJ-capable cyclones, number airstreams. inherent complexity adds beauty systems, interaction their, potentially very damaging, effects terms remain significant challenges. performed Ambrogio funded pump-priming fund Department Meteorology, University Reading. Oscar Martínez-Alvarado's supported Natural Environment Council member Centre Atmospheric Science. Duncan Ackerley Joint BEIS/Defra Hadley Climate Programme (GA01101) facilitated access datasets. authors thank Dan Suri providing helpful insight version work. No new datasets created study. Operational outputs archived Office; please contact details. Trajectory files upon request. AV designed retrieved data wrote draft article. SLG co-designed PAC introduction OM-A analysed DA, All took discussing scope results, revising document peer-review process.