Categorisation of synoptic environments associated with mesoscale convective systems over the UK

Atmospheric Research 97 (2010) 194–213

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Categorisation of synoptic environments associated with mesoscale convective systems over the UK Matthew W. Lewis, Suzanne L. Gray ⁎ Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK

a r t i c l e

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Article history: Received 17 September 2009 Received in revised form 1 March 2010 Accepted 7 April 2010 Keywords: Baroclinic life cycle Climatology Spanish plume

a b s t r a c t Mesoscale convective systems (MCSs) are relatively rare events in the UK but, when they do occur, can be associated with weather that is considered extreme with respect to climatology (as indicated by the number of such events that have been analysed as case studies). These case studies usually associate UK MCSs with a synoptic environment known as the Spanish plume. Here a previously published 17 year climatology of UK MCS events is extended to the present day (from 1998 to 2008) and these events classified according to the synoptic environment in which they form. Three distinct synoptic environments have been identified, here termed the classical Spanish plume, modified Spanish plume, and European easterly plume. Detailed case studies of the two latter, newly defined, environments are presented. Composites produced for each environment further reveal the differences between them. The classical Spanish plume is associated with an eastward propagating baroclinic cyclone that evolves according to idealised life cycle 1. Conditional instability is released from a warm moist plume of air advected northeastwards from Iberia that is capped by warmer, but very dry air, from the Spanish plateau. The modified Spanish plume is associated with a slowly moving mature frontal system associated with a forward tilting trough (and possibly cut-off low) at 500 hPa that evolves according to idealised life cycle 2. As in the classical Spanish plume, conditional instability is released from a warm plume of air advected northwards from Iberia. The less frequent European easterly plume is associated with an omega block centred over Scandinavia at upper levels. Conditional instability is released from a warm plume of air advected westwards across northern continental Europe. Unlike the Spanish plume environments, the European easterly plume is not a warm sector phenomena associated with a baroclinic cyclone. However, in all environments the organisation of convection is associated with the interaction of an upper-level disturbance with a low-level region of warm advection. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Mesoscale convective systems (MCSs) are convective systems that form from the amalgamation and/or upscale organisation of individual storms into a single cloud system with a very large cirrus anvil and contiguous precipitation areas. Houze (2004), in a review article on MCSs, gives a broad description as ‘a cumulonimbus cloud system that produces a contiguous precipitation area ∼100 km or more in ⁎ Corresponding author. Tel.: +44 118 3786791; fax: +44 118 3788905. E-mail address: [email protected] (S.L. Gray). 0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2010.04.001

at least one direction’. Maddox (1980) sub-classified MCSs according to their type (linear or circular) and location (tropical or extratropical) and used the anvil extent and storm duration to define the most severe type of MCS, the circular type termed a mesoscale convective complex (MCC). Mesoscale convective systems occur frequently in many parts of the globe. Laing and Fritsch (1997) established that the typical MCC is nocturnal and occurs over land during the warm season. About 400 such MCCs occur annually over the globe. Many regional climatologies of MCSs and MCCs exist. These usually consider regions where these phenomena are frequent (such as North America and Africa) and focus on the

M.W. Lewis, S.L. Gray / Atmospheric Research 97 (2010) 194–213

more severe MCCs. MCSs are comparatively rare in Europe (and MCCs very rare). Despite this, Morel and Senesi (2002) were able to fully track nearly 4700 MCSs that occurred over Europe during five warm seasons using an automated method based on satellite imagery and defining MCSs as systems that exceeded, at least once, an area of 10 000 km2 with a brightness-temperature below−45 °C. The UK was identified as one of the continental areas with the least number of MCS occurrences, with the greatest density of UK MCSs occurring over eastern England. An alternative climatology of UK MCSs was produced by Gray and Marshall (1998). They diagnosed an average frequency of just under two systems per year, from analysis of MCSs that occurred between 1981 and 1997, and also found MCSs to be most frequent in Eastern England (more specifically in south-east England, central Southern England and East Anglia). MCSs were defined as features satisfying several criteria: widespread lightning reports covering an area of greater than 100 km in at least one direction (an indicator of thunderstorm activity), some reports of heavy, and/or persistent, rainfall (N10 mm) in the storm area and continuous reports of a trace covering an area greater than 100 km in at least one direction, high surface temperatures (N25 °C, a proxy for high CAPE — Convective Available Potential Energy) and identification of a cloud anvil in a satellite image (these were not always available at the times the suspected MCS was over the UK). MCS events are a small subset of the total thunderstorm events. van Delden (2001) analysed reports from weather stations (available four-times daily) in western Europe from the summer seasons (April–October) to construct a four-year climatology. He diagnosed a peak thunderstorm frequency for the UK (found in south-east England) of 7 per 1000 weather reports, implying about six events per year at that particular weather station (compared to the average MCS frequency of just under two systems per year over the whole UK diagnosed by Gray and Marshall (1998). Several authors have analysed the large-scale environment associated with mid-latitude MCCs finding similarities in those that favour large-scale convective development. Based on analysis of ten MCCs that developed over the central US, Maddox (1983) concluded that the systems developed within a region of mesoscale convergence and lifting that is forced primarily by low-level warm advection. Although this lifting occurs ahead of a weak eastward moving mid-level trough, Maddox (1983) found the forcing from low-levels to dominate over that due to differential vorticity advection from upper-levels. However, the trough helps to enhance convective development by its contributions to the conditionally unstable thermodynamic structure and veering of the low-level winds (necessary for organisation of the developing MCC). Laing and Fritsch, (2000) built upon this and several other studies of US MCCs, extending their analysis to the five global centres of MCC development: Africa, Australia, China, South America and the US. Based on 12 events for each centre, they found the genesis environments to be very similar comprising prominent baroclinic zones with locally large values of lower-tropospheric wind-shear and CAPE. Triggering of convection occurs where there is a maxima in absolute humidity and a minima in static stability within a low-level jet of air with high wet bulb potential temperature (θw) that is oriented nearly perpendicular to the baroclinic zone and

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forced to ascend over a relatively shallow surface-based layer of relatively cool air. Similar studies relating to the broader category of mid-latitude MCSs are rare despite evidence that MCCs are not a discrete category of MCSs, i.e. a gap does not exist in the distribution of maximum extent and duration of MCSs (see Morel and Senesi, 2002 and references therein; Augustine and Caracena, 1993). Despite frequent European MCS events, a composite study of the environments in which these MCSs form has not been performed (to the authors' knowledge) although a strong correlation between preferential locations of MCS triggering and orography has been found (Morel and Senesi, 2002). Case studies of UK MCSs usually associate them with a synoptic situation known as the Spanish plume (hereafter the classical Spanish plume). As documented by Carlson and Ludlam (1968), Morris (1986), Gray and Marshall (1998) and Bennett et al. (2006), this situation has two main precursor components: an upper-level trough propagating towards Biscay and Iberia associated with a surface cold front and a low-level zone of strong temperature gradient over Iberia. The key synoptic features of the Spanish plume, at the time of convective triggering, are shown here in a schematic (Fig 12(a)). The approaching trough leads to limited dynamical ascent (through positive differential vorticity advection) and surface pressure falls over Iberia. The resulting low-level circulation leads to the development of warm advection over Iberia, leading to further pressure falls and low-level forced dynamical ascent. Cold advection is created in the western coastal zone. A sharp discontinuity is thus created over Iberia between the regions of cold and warm advection and associated descending and ascending air. This advection sharpens the pre-existing region of strong temperature gradient leading to the distinct Spanish plume, a plume of convectively unstable air that moves northwards towards northern France and southern England. The plume effectively becomes the main frontal zone and the cold front originally associated with the upper-level trough becomes less distinct. A positive feedback is then created whereby the enhanced low-level thermal gradient strengthens the winds ahead of the upper-level trough which in turn increases the positive differential vorticity advection and its associated ascent. Large CAPE builds in the Spanish plume because the warm humid surface air (high θw) is capped by a stable layer of warmer but very dry air (lower θw but higher θ) at about 850 hPa (1500 m) that has its origins over the Spanish plateau; this elevated mixed layer acts as a convective lid inhibiting CAPE release and allowing the boundary-layer θw to increase on sunny days, especially when soil moisture is high. Cooling aloft ahead of the advancing upper-level trough enhances the CAPE. The ascent forced by the low-level warm advection and upper-level positive vorticity advection (due to the upper-level trough) can eventually allow the CAPE to be released, triggering convection. Alternatively, as described by Carlson and Ludlam (1968), triggering can occur where the low-level flow reaches the western edge of the convective lid (marked by the frontal zone). Strong vertical wind shear near the front enables the organisation of the thunderstorms resulting in a MCS. The mechanism of creation of a low-level jet of lowstability, high θw air (the Spanish plume) and the resultant triggering of severe convection within a region of significant low-level vertical wind shear that leads to the development

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of a UK MCS is composed of the same key ingredients as identified by Laing and Fritsch (2000) for the development of MCCs globally. One difference is the relative importance of the upper-level trough (its strength and dynamical role appears to be possibly more important in the Spanish plume). Analogous features to the Spanish plume occur in other parts of the world e.g. the Mexican plume (a plume of potentially warm air from the Mexican plateau) which can lead to longlived squall lines in the southern US plains. The development of a UK MCS is a sufficiently rare event that several detailed case studies have been published (e.g. Browning and Hill, 1984; McCallum and Waters, 1993; Young, 1995; Elsom and Webb, 1993; Webb and Pike, 2007). All these cases can be associated with the Spanish plume synoptic situation. However, in their climatology of UK MCSs, Gray and Marshall (1998) found only 25 (plus possibly an additional two) of their 32 cases are associated with a Spanish plume. They noted that the upper-level trough extended south towards Iberia and formed a cut-off low in a third of these cases and in one further case the cold advection into Iberia was provided by a cut-off low. These are modifications of the classical Spanish plume conceptual model. The remaining four cases studied (comprising two pairs of MCSs that formed on consecutive days and hence only two synoptic environments) formed in strongly blocked flow over the UK with a cut-off low situated over north-west Iberia in one case and to the west of the UK in the other case. These results suggest that a significant number of UK MCSs do not form during a classical Spanish plume event. However, the proportion of such events is still uncertain and detailed descriptions of the alternative synoptic environments are lacking; we address this knowledge gap in this paper. Knowledge of the synoptic environments associated with MCS development could potentially be used to infer the changes to the MCS climatology in future climates given the changes predicted by climate models in summertime North Atlantic weather patterns. Although the synoptic environments associated with UK MCSs have not all been defined, Roberts (2000) suggested a framework for the typical large-scale pressure patterns associated with thunderstorms over western Europe and the North Atlantic from a subjective classification of more than 300 thunderstorm events over a summer and autumn season according to their triggering location relative to water vapour imagery. He defined these pressure patterns by reference to the two paradigms of baroclinic life cycle behaviour (termed LC1 and LC2) described by Thorncroft et al. (1993) which differ in their basic state by the addition of a cyclonic barotropic shear in LC2 and demonstrate anticyclonic and cyclonic behaviour respectively; this behaviour is particularly evident in the evolution of their upper-level troughs. Wernli (1995) also considered cyclones evolving from a basic state with anticyclonic barotropic shear (termed LC3). As described by Shapiro et al. (1993) (and references therein), LC1 cyclones evolve according to the Shapiro–Keyser conceptual model (Shapiro and Keyser, 1990), developing a T-bone frontal structure with a bent-back warm front. LC2 cyclones evolve according to the Norwegian conceptual model (Bjerknes and Solberg, 1922; Bergeron, 1928) developing a bent-back polar warm frontal occlusion. LC3 cyclones are open frontal-wave cyclones with well-defined cold fronts and weak warm fronts that do not form occluded cyclones. The presence of confluence or

diffluence in the upper-level jet (present at the entrance and exit regions respectively) is also used to diagnose the associated baroclinic life cycle as this has been shown to affect the evolution of the surface frontal structure (Schultz et al., 1998). Shapiro–Keyser model evolutions (associated with LC1-type developments) are expected to occur in confluence regions of the upper-level jet, Norwegian model evolutions (associated with LC2-type developments) are expected to occur in diffluence regions, and LC3-type developments are expected to occur in regions without diffluence or confluence. An alternative method of categorising the large-scale environment is through the upper-tropospheric jet stream pattern. Degirmendžić and Wibig (2007) identified the 15 most frequent jet types from NCEP/NCAR reanalysis data (warm season, 1950–2001) with respect to their associated sea-level pressure, vertical velocity and temperature fields. They also examined the seasonal variability in frequency and duration (and trends in this); hence, determining the jet stream types associated with MCS-favourable synoptic environments could provide further information on their prevalence. The principle aim of this study is to categorise MCSs that affect the UK according to the synoptic environment in which they form. The three-stage procedure for achieving this is reflected in the structure of the paper that follows. The data sources and methodology are described in Section 2. In Section 3 the 17 year climatology of UK MCSs performed by Gray and Marshall (1998) is extended to the present (up to and including the summer of 2008). In Section 4, two MCS events that are not associated with the classical Spanish plume synoptic environment are analysed. In Section 5 the events presented in Section 3 are categorised using the insights developed from the case study analyses in Section 4; at the start of Section 5 we term the synoptic environments associated with the two MCS events analysed in Section 4 ‘modified Spanish plume’ and ‘European Easterly plume’. Composites of the three synoptic environments associated with the MCSs are presented. The associated cyclones are related to the idealised baroclinic life cycles described above and the upper-level jet stream pattern matched to those in the climatology of Degirmendžić and Wibig (2007). 2. Methodology The criteria used here to identify MCSs are reflective of the widely accepted definition given in Houze (2004). From the available data, lightning reports must be observed over a region greater than 100 km, lasting for a period greater than that of single cell convection (N3 h). Additionally, the precipitation associated with the system must be contiguous in one dimension of greater than 100 km. This approach largely follows that used by Gray and Marshall (1998) for the period after 1994, albeit in retrospect rather than in real-time. Initially, daily lightning plots drawn from the Met Office ATD (Arrival Time Difference) network (Lee, 1989) are used to identify days with lightning activity satisfying the above criteria over the UK. We note the caveat that, due to the pronounced contrast in lightning activity between convection over land and sea (e.g. Williams and Stanfill (2002)), the lightning activity within a UK MCS may be dependent on the origin of the airmass in which it develops (since the UK is a

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maritime country). Radar data, surface synoptic reports and Meteosat 7 and 9 imagery is used to remove disorganised convection cases. An attendant cirrus anvil is an essential identifiable attribute of MCSs that can be verified via geostationary and polar orbiting satellite imagery. Given the current temporal resolution of satellite imagery (up to 15 min), the identification of each event is often easily discernible. Access to archive radar imagery and sferics data over the UK for dates prior to 2004 was not readily available and greater reliance was placed on other data sources; this limitation potentially leads to the exclusion of some events due to a lack of conclusive evidence (as specified in the caption of Table 1). We note that these criteria, although chosen for consistency with those used by Gray and Marshall (1998), are somewhat arbitrary; the lightning criterion in particular leads to the exclusion from the climatology of some significant MCS-like events (for example the storm of 20 July 2007, that led to exceptional rainfall and flash flooding over southern England and Wales, did not exhibit a continuous region of lightning and in the storm of 28 July 2005 the lightning appeared to remain offshore). Two cases from the climatology, 7–8 June 2007 and 19 June 2007, that do not appear to be associated with a classical Spanish plume synoptic environment have been analysed in detail to determine their initiation, development and maintenance. This is based on numerical weather prediction model data from six-hourly operational analyses from the Met Office Unified Model (MetUM) run over their North Atlantic European domain. This configuration uses a rotated grid with 0.11° × 0.11° horizontal gridboxes (0.11° is equivalent

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to ∼ 12 km) with 38 vertical levels. This model data was used in conjunction with archive surface synoptic analyses, radar and lightning observations, and infra-red and water vapour satellite imagery. Six-hourly data from the NCEP (National Centers for Environmental Prediction) GFS (Global Forecast System) was also used to assess the synoptic environment over the days prior to the event. This is available on a grid with approximately 0.5° × 0.5° horizontal gridboxes and the model uses 64 vertical levels. The NCEP GFS 1° × 1° data was used to determine the synoptic scale features attributable to the initiation and subsequent development of each MCS identified in the climatology. NCEP data was used here, and as described above, in preference to Met Office data for convenience due to the ease of access of the NCEP data and its reliable availability over the time period covered by the climatology. Common features such as the mid-to upper-level flow pattern, surface frontal features and low-level plumes of warm air were key determinants in categorising each case. Pattern matching of features at the upper-, mid-and low-levels was used to the group events according to their synoptic configuration. For example, the presence of a cut-off low feature at 500 hPa was used to distinguish a ‘modified’ Spanish plume event from a ‘classic’ plume. Archive vicinity radiosonde sounding data provided indications of airmass origin, type and modification as well as vertical profiles of horizontal winds. This subjective categorisation process is an appropriate method to cluster the synoptic environments associated with this relatively small number of cases and is a necessary pre-requisite to future objective analyses.

Table 1 List of probable MCS events over the UK from 1998 to 2008. Date

13 May 1998 20 July 1998 27/28 May 1999 29 May 1999 2/3 July 1999 25 August 1999 11 September 2000 9/10 May 2001 25/26 June 2001 26/27 June 2001 3/4 July 2001 15/16 August 2001 25/26 August 2001 22/23 June 2003 22 July 2004 5 August 2004 19 June 2005 28 June 2005 31 August/1 September 2005 22 July 2006 7/8 June 2007 19 June 2007 14/15 July 2007 27/28 May 2008 2/3 June 2008

Advected (A) or initiated over the

Time of onset (over the UK)

UK (I)

(UTC)

A A A A I A I I A A A A I A A I I A I A A A A A A

1500 1600 1600 0300 2200 0700 1800 2200 2200 2000 2000 0800 1500 0900 1500 1400 1500 0600 0900 0800 1600 1300 2200 1100 1300

(1900) (1700) (2000) (0500) (1500)

(2300) (2300) (0400*) (2300) (2200) (1700)

(0900) (1100) (0000*) (1700) (0400*) (1900) (1600)

Category

EEP SP MSP MSP SP MSP MSP EEP MSP SP MSP SP SP SP SP EEP MSP MSP SP UC EEP MSP SP MSP EEP

Categories are classical Spanish plume (SP), modified Spanish plume (MSP), European easterly plume (EEP) and unclassified (UC). Starred times indicate that they refer to the second of the dates associated with that MCS. Note that events occurring on 20 June 1998, 1/2 June 1999, 2 August 2001 and 16/17 May 2002 were discounted from the analysis due to a lack of supporting evidence concerning the spatial and temporal dimensions of the systems (including verification of their track over the UK) due to the data availability limitations described in Section 2.

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Composites (average fields) of the three synoptic environments determined as associated with UK MCSs were produced from the 40 year NCEP/NCAR reanalysis dataset (Kalnay et al.,

1996) using the daily mean composite online plotting tool (available via http://www.cdc.noaa.gov/data/composites/ day/). Reanalysis fields were taken from the first day for MCS events that occurred over two days (as specified in Table 1). Compositing smoothes the detailed, case dependent features of the fields and is less appropriate for small numbers of cases. Hence, consistency checks were also made by examining the fields for the individual days. Following Shapiro et al. (1993) (in particular see their Fig. 4), the relative positions of the polar and subtropical jets streams (indicated by the wind fields at 300 hPa and 200 hPa respectively) were used to infer the sense of the environmental barotropic shear (i.e., cyclonic shear, no shear or anticyclonic shear) associated with each synoptic situation and hence the associated idealised baroclinic life cycle. Finally, the upper-tropospheric jet stream pattern was matched to one of those identified by Degirmendžić and Wibig (2007).

3. Climatology of UK MCSs The 25 MCSs identified to have affected the UK between 1998 and 2008 are listed in Table 1. The annual frequency, seasonal distribution and diurnal pattern of these MCSs are described below and compared with those from Gray and Marshall (1998) for the period 1981 to 1997. Together these two datasets form a continuous 28 year climatology of UK MCS events.

3.1. Annual frequency The 25 MCS events identified to have affected the UK between 1998 and 2008 equate to an annual average of 2.3 cases per annum. This compares well with the 1.8 cases/year found by Gray and Marshall (1998). The inter-annual range of events is considerable: six cases occurred in 2001 compared to none in 2002. The results of the current study are amalgamated with those of Gray and Marshall (1998) in Fig. 1(a); there is no obvious trend in annual frequency during the 28 year period. The MCSs tend to occur in clusters such that all events within a given year can potentially occur within a few days. There have been five years (1985, 1992, 1993, 1997 and 2001) when three or more MCSs occurred within a three week period, suggesting that the conditions conducive to MCS formation can often be persistent for several weeks and so associated with synoptic-scale weather patterns. The correspondence between peaks in MCS frequency and strong El Niño events noted by Gray and Marshall (1998) (they found peaks of frequency in 1983, 1992 and 1997 when strong El Niño events occurred) does not extend into our analysis period.

Fig. 1. (a) Annual frequency of UK MCS events. Data for the period 1981 to 1997 is from Gray and Marshall (1998) (filled bars) and for 1998 to 2008 is from this study (open bars). Error bars indicate borderline events which require further observational evidence to determine if the MCS criteria are met. (b) Monthly variation of UK MCS events. Filled and open bars as in (a). (c) Time variation of the initiation of MCS events initiated (I, filled bars) or advected (A, open bars) over the UK for the period 1998–2008.

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3.2. Seasonal distribution The seasonal distribution of the MCSs in the current study is very similar to that found by Gray and Marshall (1998). The combined dataset shows no significant pattern in MCS frequency during the summer months of June to August (Fig. 1(b)). The lack of organised convection in the form of MCSs in the autumn, winter and spring is due to the lack of the high boundary-layer θw required for deep convection. A significant number of MCSs also occurred in late-May and a further two cases occurred in mid-September. This seasonal distribution compares well with that for European MCSs (Morel and Senesi, 2002) but with a reduced relative frequency of events at the beginning and end of the summer. As highlighted by Gray and Marshall (1998), and also evident in this study, the majority of MCSs initiate over the continent and are advected northwards over the UK. As water has a much greater thermal inertia than that of the neighbouring land-mass, the English Channel is relatively cold in the late spring/early summer — typically averaging a sea surface temperature of 11 °C in late May (e.g. see Reynolds SST analysis available at the National Hurricane Centre: http:// www.nhc.noaa.gov/aboutsst.shtml). This leads to stable stratification in the boundary layer that acts to inhibit surface-based convection from being advected across the English Channel. However, a limited number of events have occurred when instability is released from elevated unstable layers and are therefore less likely to be impeded by a lack of surface-based positive buoyancy (e.g. case II described in Section 4.2). Elevated convection requires strong mid-level wind shear for cell organisation which limits the frequency of occurrence. 3.3. Diurnal pattern The initiation time of an MCS is defined here as the first time when lightning strikes associated with the developing MCS are recorded (over the UK or elsewhere). Several authors have related the time of onset of lightning activity to radar reflectivity and Doppler measurements. For example, Zipser and Lutz (1994) consider continental mid-latitude, continental tropical, and oceanic tropical thunderstorms and propose that a necessary condition for rapid electrification is that the updraught speed in a convective cell exceeds a threshold value. This suggests that the onset time of lightning activity can be used as a proxy for the onset time of rapid convective development. This lightning-based definition of MCS initiation time differs from that used by Gray and Marshall (1998) (specifically the first time the spatial criterion for precipitation was satisfied within the UK coastline) and hence the two datasets are not directly compared. Here we also distinguish between those MCSs that are initiated over the UK and those that are advected over it. Some caution should be exercised when analysing such a small sample of cases. With this caveat, a pronounced diurnal cycle is observed with the peak period of MCS initiation in the late afternoon/early evening (1500 to 2100 UTC, Fig. 1(c)) but with a significant number of cases also initiated at other times. The timing of this peak is consistent with that found by Dai (2001) using 3 hourly present weather reports for thunderstorm frequency globally over land areas (a peak between 1500–1900 LST that was

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relatively insensitive to season). He attributed this peak to corresponding timing peaks in lower-tropospheric temperature and CAPE due to solar heating. Five MCSs were found to have initiated around sunrise—this is perhaps surprising given the weak solar heating at this time. However, these MCSs were all advected over the UK (rather than being initiated here) and their timing is thus consistent with the ∼ 0600 LST peak in thunderstorm occurrence found by Dai (2001) over the North Atlantic adjacent to the European continent. He attributes this to triggering resulting from a land-breeze circulation that is set up when the air over the oceans becomes warmer than that over the continent. The reverse process, a sea-breeze, occurs over land in the late afternoon and evening (when the air over the continent is warmer than that over the nearby ocean) and can also lead to convective triggering. 4. Case study analyses The synoptic background and initiation and development of convection in two MCS events that were not associated with the classical Spanish plume synoptic environment are described below. The first case, 7–8 June 2007, was notable for the lack of a coupled upper-level and surface pattern. This MCS was advected over the UK from Northern France. The second case, 19 June 2007, was composed of several MCSs that formed along the leading edge of an upper-level cold front that acted to destabilise a deep warm moist plume across western France and southern UK. 4.1. Case I: 7–8 June 2007 Quiescent conditions dominated much of the UK and northwest Europe in the days preceding 7 June 2007 with high pressure established over the Faroe Islands and Scandinavia forming a weak omega block (Fig. 2(a)). Largely cloudless skies over central Europe led to strong surface heating with temperatures across much of Germany, France and Benelux reaching the mid- to high-20 s °C (not shown). Despite abundant moisture and steep lapse rates in the lower troposphere (below 700 hPa), only disorganised, single cell convection developed (probably due to the weak vertical wind shear observable in the radiosonde ascent from Trappes at 1200 UTC 6 June, see Fig. 3(a)). The following developments altered the synoptic environment during the morning of the 7 June leading to a situation conducive for organised convection. Surface pressure falls over southwest Germany and eastern France (a surface low is marked here on the Met Office 0000 UTC analysis for 7 June, Fig. 2(a)) strengthened the weak easterly surface flow that prevailed across central Europe due to the high pressure centred east of Iceland (Fig. 4(a)). Widespread subsidence over southern central Europe allowed boundarylayer moisture to increase north of the Alps; this was advected westward on the amplified easterly flow towards Benelux/northern France leading to the development of a low-level plume of air with high θw (θw N 289 K in Fig. 4(a)). Two small-scale cut-off lows and attendant upper-level cold pools developed off the western coasts of England and France, and over the England/Scotland border (centred at

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the coastal regions of northern France and Belgium was cool due to the long sea fetch (Fig. 4(a)). A shallow cold pool developed in the lowest kilometre over southern England as a result (Fig. 6(a)). This persisted while temperatures increased over mainland Europe away from the northwest coast (sharpening the temperature gradient between the two regions). During the afternoon of the 7 June, further disorganised, short-lived convection became established over central and eastern France. The low-level plume of high θw, with a depth of approximately 200 hPa, continued to be advected westward (not shown). Shortly after sunset, the nose of the plume engaged with the surface cold pool over southern England

Fig. 2. Met Office surface analysis at (a) 0000 UTC 7 June 2007 and (b) 0000 UTC 8 June 2007. Crown copyright.

about 6°W, 49°N and 2°W, 56°N respectively in Fig. 4(b)). The southern cut-off low caused cold advection aloft over northwest France, which acted to steepen mid-level lapse rates, and forced very weak ascent through positive differential vorticity advection (the 700 hPa vertical velocity due to quasi-geostrophic forcing from upper levels (600–100 hPa) — assumed to be the result of positive differential vorticity advection — peaked at ∼0.006 m s− 1 along the west coast of England at 0000 UTC 8 June, Fig. 5(a)). The changing lowlevel flow was primarily the result of the development of a high pressure region over southern France and eastern Spain during 7 June, visible on the Met Office analysis at 0000 UTC 8 June, Fig. 2(b)). The air advected over the southern UK and

Fig. 3. Atmospheric profiles (skew-T plots) from the radiosonde ascents for (a) Trappes 1200 UTC 6 June 2007 and (b) Bordeaux 0000 UTC 19 June 2007. Courtesy of the University of Wyoming.

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Fig. 4. Geopotential height (black contours, interval 20 m), θw (shaded) and velocity vectors at (a) 850 hPa and (b) 500 hPa for 1200 UTC 7 June 2007. Data from Met Office analyses.

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Fig. 5. CAPE (grey shaded for values exceeding 500 J kg− 1) and vertical velocity at 700 hPa due to quasi-geostrophic forcing from 600 to 100 hPa (solid contours for positive values, dashed contours for negative values, contour interval 0.004 m s− 1) for (a) 0000 UTC 8 June 2007 and (b) 1800 UTC 19 June 2007.

(see cross-section at 0000 UTC 8 June, Fig. 6(a)). The plume became elevated through isentropic ascent over the cold pool and weak synoptic forcing, lifting unsaturated parcels near to

the level of free convection and resulting in convection that was observable on radar by 2200 UTC (not shown). CAPE values reached ∼ 1700 J kg− 1 in the region of triggering at

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Fig. 6. (a) Vertical cross-section at 0000 UTC 8 June 2007 of θw (shaded), horizontal windspeed (solid contours at 10 and 20 m s− 1 and positive vertical velocity (dashed contours at 0.1–0.5 m s− 1 with contour interval of 0.1 m s− 1), data from Met Office analyses, and (b) IR AVHRR (channel 5 — 11.5–12.5 μm) satellite image at 0126 UTC 8 June 2007 (courtesy of Dundee Satellite Receiving Station). The approximate location of the cross-section shown in panel (a) is marked in (b) by X–XX.

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0000 UTC 8 June (peak values were found from parcel ascents from 1000 hPa, Fig. 5(a)) with negligible CIN (Convective INhibition), not shown.1 Note that the convective instability is sufficiently strong that its release is resolved explicitly by the MetUM (see the vertical velocity field in Fig. 6(a)) despite a convective parameterisation scheme operating at this resolution. Further east, under greater synoptic-scale subsidence, the lack of interaction with the upper-levels precluded widespread convection. This environment of a weak upperlevel trough and low-level warm advection is that typically associated with MCCs development in the US (Maddox, 1983). Initially, convection was disorganised with individual convective elements being steered by the mid-level flow to the northwest. Rainfall estimates derived from radar (not shown) showed the strongest cells were producing rain rates in excess of 50 mm h− 1 and the Météo-France Rapid Developing Thunderstorm product reported over 1200 strikes h− 1 at maximum extent. Between 0000 and 0300 UTC 8 June 2007, individual convective cells became more organised with an expanding stratiform rain region extending northwest (Fig. 6(b)). The location of the MCS was marked as a weak low with associated trough line on the 0000 UTC Met Office analysis (Fig. 2(b)). 4.2. Case II: 19 June 2007 In the preceding days, much of the UK and Ireland had been influenced by a number of slow moving cut-off upperlevel lows. A high amplitude trough, identified at 500 hPa at 0000 UTC 17 June with a strong upper-tropospheric jet on its southern flank (30 m s− 1 at 500 hPa, not shown), stretched equator-ward. By the morning of the 18 June, this feature had become cut off, equivalent barotropic and almost stationary to the west of the Bay of Biscay (Figs. 7(a) and 8 show the Met Office surface analysis and corresponding upper-and lowerlevel features a day later at 0000 UTC 19 June). The surface features consisted of a moderately active cold front (with attendant widespread precipitation, not shown) with warm sector ahead (Fig. 8(a)). Ahead of the surface low, a slack pressure gradient across Iberia and western France allowed a shallow heat low to develop during the course of the 18 June (not shown). Warm advection at low levels, ahead of an increasingly cyclonic upper-level flow caused the northwards ejection of a plume of high-θw air from northern Spain (θw N 290 K in Fig. 8(a)). The warm and dry air at 900 hPa led to a well-defined elevated mixed layer above a convective lid (that inhibited convective triggering) observable on the Bordeaux radiosonde ascent at 0000 UTC 19 June 2007 (Fig. 3(b)). The following developments took place during the course of the 19 June which provided a focus for the initiation of widespread organised convection during the afternoon and evening. A well-defined upper-level shortwave trough at 300 hPa developed on the southern extent of the parent cutoff feature over the eastern Atlantic and tracked eastwards inducing ascent through positive differential vorticity ad-

1 CAPE is defined here as the energy obtained from lifting from the parcel origin level to the level of neutral buoyancy assuming pseudo-adiabatic ascent (i.e. the sum of the positive and negative (CIN) areas on a tephigram).

Fig. 7. Met Office surface analysis at (a) 0000 UTC 19 June 2007 and (b) 0000 UTC 20 June 2007. Crown copyright.

vection (the 700 hPa vertical velocity due to quasi-geostrophic forcing from upper levels (600–100 hPa) peaked at ∼ 0.015 m s− 1 in this region at 1800 UTC 19 June, Fig. 5(b)) and cooling of the mid-levels above the warm moist plume. This yielded strong vertical wind shear over northern France and southeast England (this windshear is already apparent at 0000 UTC on 19 June from comparison of the wind vectors in Fig. 8(a) and (b)). A developing dry intrusion (identified by a marked dry slot on water vapour imagery; see Fig. 9(a) for model representation) overran the surface cold front (forming a kata-front), introducing mid-level positive potential vorticity (PV) anomalies (not shown) and associated low values of θw (Fig. 9(a)). The PV anomaly induced further cyclogenesis and potential instability was generated between

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Fig. 8. Geopotential height (black contours, interval 20 m), θw (shaded) and velocity vectors at (a) 850 hPa and (b) 500 hPa for 0000 UTC 19 June 2007. Data from Met Office analyses.

the upper-and surface-cold fronts (see Browning, 1997 for a description of the role of the dry intrusion in cyclone development).

Convection initiated over western France at approximately 1300 UTC approximately 250 km east of the surface frontal precipitation which had significantly weakened since the

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Fig. 9. (a) Vertical cross-section of θw (shaded), horizontal windspeed (solid contour 30 m s− 1 and positive vertical velocity (dashed contours at 0.1–0.5 m s− 1 with contour interval of 0.1 m s− 1), data from Met Office analyses, and (b) SEVIRI IR 10.8 μm image from the Eumetsat Meteosat Second Generation (MSG-2) satellite both at 1800 UTC 19 June 2007. The approximate location of the cross-section shown in panel (a) is marked in (b) by X–XX.

over-running of the dry intrusion. To the east of the developing convective precipitation the elevated plume remained capped by a temperature inversion which had yet to be eroded by the onset of dynamical forcing and subsequent cooling by the eastward propagating shortwave trough. By 1500 UTC, there were widespread reports of

thunderstorms in a line from Devon, Cherbourg to Paris forming four distinct clusters (labelled A to D in Fig. 9(b)). CAPE values increased throughout the day. At 1800 UTC CAPE values reached ∼ 2400 J kg− 1 in the region of triggering (peak values were found from parcel ascents from 1000 hPa, Fig. 5(b)) with CIN values of up to ∼ 100 J kg− 1,

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Fig. 10. Composites of mean sea level pressure (hPa, left column) and 500 hPa geopotential height (gpm, right column) for MCSs (between 1998 and 2008) classified as developing in the (a),(b) classical Spanish plume; (c),(d) modified Spanish plume; (e),(f) European easterly plume synoptic environments. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/.

not shown. The southerly steering flow caused individual cells to move northwards. As the cap was being eroded from west to east by the upper front, secondary cells formed on the southeast side of each system where instability and

moisture were greatest. Radar derived rain rates suggested these cells were producing small hail and very heavy rain (N 80 mm h− 1) and there were widespread reports of frequent lightning.

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Fig. 11. Composites of vector winds at 300 hPa (m s− 1, left column) and 200 hPa (m s− 1, right column) for MCSs (between 1998 and 2008) classified as developing in the (a),(b) classical Spanish plume; (c),(d) modified Spanish plume; (e),(f) European easterly plume synoptic environments. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/.

5. Categorisation of UK MCSs The two case studies are examples of two particular synoptic situations, distinct from that termed the classical

Spanish plume here, that have been identified to be associated with a subset of the MCSs identified in the 1998–2008 climatology. Cases I and II are examples of situations termed the ‘European easterly plume’ (due to the origin region and

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flow direction of the warm, moist low-level plume) and the ‘modified Spanish plume’ (due to its similarity to the classical Spanish plume) respectively here. Of the 25 MCSs, 24 were classified as occurring in one of these three synoptic situations (the remaining case was unclassified). Fig. 10 shows the composite mean sea level pressure and 500 hPa geopotential height pattern associated with each synoptic configuration and Fig. 11 shows the vector wind fields at 300 hPa and 200 hPa (from which the positions of the polar and subtropical jet streams respectively are inferred). 5.1. Classical Spanish plume Nine cases followed the conceptual model of the classical Spanish plume described in Section 1. Consistent with the conceptual model, a slow moving, high amplitude, approximately north–south oriented upper-level trough is observed in the east Atlantic with a surface low pressure centre south of Iceland with a trough extension towards Ireland and the UK (Fig. 10(a) and (b)). For the Spanish plume situation the polar and subtropical jets are approximately collocated (Fig. 11(a) and (b)). This implies a lack of barotropic shear (no shear) and hence is consistent with cyclones evolving according to the LC1 baroclinic life cycle. The position of the surface low at the left entrance of the upper-level jet over the UK (with the trough extension towards the right jet entrance), and hence embedded in confluent flow, is also consistent with LC1 development. The (geostrophic) flow is southwesterly at 500 hPa and southerly at the surface over the UK (inferred from the mean sea level pressure and 500 hPa geopotential height fields). Hence, the Spanish plume environment is favourable for MCSs over southeast England; the steering flow at mid-levels is typically south-westerly so that convection initiated over France and the Bay of Biscay propagates northeastwards towards southeast England. The analysed composites match to jet stream type F of Degirmendžić and Wibig (2007). As described by these authors, the most outstanding feature of the temperature field associated with this jet stream pattern is the advection of warm air crossing central Europe towards its northern edges. 5.2. Modified Spanish plume Ten cases followed the synoptic configuration described here as the modified Spanish plume and exemplified by the second case study. This synoptic configuration is similar to the classical Spanish plume in that an upper-level trough propagating towards Biscay and Iberia is associated with a surface cold front and a low-level zone of strong temperature gradient over Iberia. Unlike the classical Spanish plume the upper-level trough has stretched and thinned equator-ward and is forward (northwest–southeast) tilted (Fig. 10(d)). This can lead to a cut-off upper-level feature which is typically centred to the northwest of the Bay of Biscay (as seen for case II in Fig. 8(b)). The subtropical jet peaks in intensity to the south of polar jet implying cyclonic barotropic shear, consistent with cyclones evolving according to the LC2 baroclinic life cycle (Fig. 11(c) and (d)). The surface low is at the left exit region of the subtropical jet streak (and hence embedded in diffluent flow) which is also consistent with LC2 development. Animations of the potential temperature on the

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2 PVU (1 PVU = 10− 6 m2 s− 1 K kg− 1) surface for case II (from ECMWF analyses, not shown) show the evolution of this cyclone with cyclonic wrap-up of air on the poleward side of the jet, again consistent with LC2. The cyclone is mature and vertically stacked — the low-level centre can be seen in Fig. 10(c). Consequently, the surface frontal features are often occluded; in case II a kata-front is diagnosed as the dry intrusion has overrun the surface cold front. The differences in the synoptic configuration, compared to the classical Spanish plume, lead to differences in the evolution of plume and triggering of MCSs. Compared to the classical Spanish plume there is often greater northward advection of warm air ahead of the approaching surface cold front because the cut-off has become detached from the more progressive zonal flow. This leads to more MCSs affecting western and northern regions of the UK. The mid-level flow is also more meridionally oriented which allows the high θw plumes to be advected over more of the UK than just southeast England (as found for the classical Spanish plume). This effect is seen in Table 1; eight out of ten modified Spanish plume cases (compared to six of the nine classical Spanish plume cases) were advected over the UK. As shown in Case Study II, widespread MCS formation can occur along the trailing cold front in the modified Spanish plume configuration. However, those MCSs closest to the low pressure centre over the southwest English Channel can be subsumed into the occluding system with further development south and east along a pre-frontal convergence line or upper-level front where the warm sector airmass is deeper. The analysed composites match to jet stream type M of Degirmendžić and Wibig (2007); warm thermal advection is steered by the cyclone centred to the south of Ireland. 5.3. European easterly plume Five cases followed the synoptic configuration described here as the European easterly plume and exemplified by the first case study. Northwest Europe is influenced by weak upper-level disturbances and central Europe is dominated by a high amplitude omega block (Fig. 10(f)). This displaces the upper-level jets southwards resulting in a zonal flow over the Mediterranean region. At the surface, there is a strong Scandinavian anticyclone which allows boundary-layer moisture to increase as a subsidence inversion caps convective growth from the surface over much of central Europe (Fig. 10(e)). This also leads to strong easterly warm advection across northern continental Europe (the low-level flow had a strong easterly component over this region due to a high pressure region centred to the northeast of the UK in all five cases). Thunderstorms initially develop in a broad region of weakly upper-level forced ascent over northeast France. However, the vertical wind shear is weaker than found in the Spanish plume configurations due to comparatively weak upper-level winds (compare the horizontal geopotential height gradient in Fig. 10(f) with that in Fig. 10(b) and (d)). This suggests that storm organisation is limited by the lack of a deep layer (0–6 km) shear. Specifically, this was found in case I where disorganised convection (single cell thunderstorms that formed in response to diurnal heating) was widespread across northern France but only developed into an organised

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MCS due to an upper-level disturbance in the form of a cut-off low. The upper-level cut-off low forced weak ascent through positive differential vorticity advection and led to cold advection aloft. In general, the wind field at upper-levels becomes modified such that there is sufficient vertical shear for constituent cells to become organised; a jet with a strong southerly component exists over northern France and eastern England at both 200 and 300 hPa in both the composites (Fig. 11(e) and (f)) and in all five cases individually). The analysed composites most closely match to jet stream type N of Degirmendžić and Wibig (2007): an omega block with the upper-level anticyclone situated north of the UK. However, there is some discrepancy between the exact positions of the features in the jet stream and European easterly plume composites; in particular the upper-level anticyclone is slightly further west in the jet stream composite resulting in upper-level southerly flow over Ireland rather than eastern England. 5.4. Synthesis of environments conducive to MCS development Notable variations exist between the European easterly plume and the two Spanish plume synoptic environments although all three environments have been identified to be conducive for MCS development. Schematics of these environments, derived from the composites and case studies (and, for the classical Spanish plume, with reference to previously published literature) are shown in Fig. 12. Note that there is case-by-case variability in the position of the marked features relative to the map on which they are shown. The classical Spanish plume environment (Fig. 12(a)) is defined by an open wave frontal system with a broad warm sector extending over Western Europe. Warm advection from Iberia in response to dynamical interactions between the lower- and upper-levels results in the advection of a conditionally unstable plume towards the UK. The modified case (Fig. 12(b)) is defined by a mature frontal system and forward tilted trough (with possibly a cut-off low) at 500 hPa. In contrast, in the European easterly plume environment (Fig. 12(c)) convection develops on the southwestern periphery of a strongly blocked pattern over central Europe. Unlike the Spanish plume environments, the European easterly plume environment is not a warm sector phenomenon in association with a mid-latitude synoptic system. Instead, widespread convection develops within the region of strongest warm advection irrespective of upper-level dynamics. However, in all environments the organisation of constituent cells is associated with the onset of an upper-level disturbance coupled with the region of warm air advection (the region of synoptic-scale forcing for ascent due to this upper-level feature is marked by the black circles in Fig. 12). The European easterly plume environment most closely resembles that conducive to the formation of Mesoscale Convective Complexes in the United States (as analysed by

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Maddox (1983)) as in this environment weak ascent is forced through positive differential vorticity advection by a weak, upper-level trough feature but the primary cause of the system is strong low-level warm advection. A synthesis of the features associated with the three synoptic environments is given in Table 2. The detailed descriptions of the modified Spanish plume and European easterly plume synoptic environments presented here are consistent with those briefly sketched out by Gray and Marshall (1998) as alternatives to the ‘classical’ Spanish plume, as is the relative rarity of the European easterly plume environment compared to the Spanish plume environments. These authors also note that weak short-wave troughs can be important in assisting the development of MCSs in these alternative environments. 6. Discussion and conclusions Mesoscale convective systems (MCSs) are a common feature throughout much of continental Europe in the warm-season months. An objective climatology of MCS events affecting the UK between 1998 and 2008 has been developed. This extends a climatology developed by Gray and Marshall (1998) for the period 1981–1997. Consistent with this earlier work we find an average of about two MCS events per year with considerable inter-annual range and with events predominantly (∼70%) advected over the UK from the Bay of Biscay or France rather than initiated locally. Two contrasting cases from the climatology were identified that did not develop in the classical Spanish plume environment (as defined by, for example, Morris (1986) and Gray and Marshall (1998)) and detailed case studies of those events were presented. Based on these analyses the synoptic environments associated with all of the MCSs in the climatology were subjectively categorised. Three distinct synoptic environments were identified to be associated with 24 out of the 25 cases, here termed the classical Spanish plume, the modified Spanish plume and the European easterly plume. Composites of upper-level winds, 500 hPa geopotential height and mean sea level pressure have been used to analyse and compare the large-scale features of these environments and relate them to idealised baroclinic life cycles (described by for example Thorncroft et al., 1993) and diagnosed jet stream types (Degirmendžić and Wibig, 2007). The relative frequency of the jet stream types identified here as associated with the classical Spanish plume, modified Spanish plume and European easterly plume synoptic configurations are 4.0, 2.0 and 1.8% respectively according to Degirmendžić and Wibig (2007) (note that the 15 most frequent jet streams selected by this study together account for 60.8% of the days in the data analysed). Given the small number of MCS cases analysed caution must be applied in comparing the frequency of the associated synoptic environments. However, given that caveat, there is some consistency here with the relative frequency of MCSs associated with these configurations in that the jet stream types associated with the

Fig. 12. Schematics of the large-scale features associated with the (a) Spanish plume, (b) modified Spanish plume and (c) European easterly plume synoptic environments at the time of convective triggering. Marked are the mean sea level low and high pressure centres (marked L and H respectively), low-level cold fronts (blue with filled triangles), low-level warm fronts (red with filled semi-circles), an upper-level cold front (blue with open triangles), representative contours of 500 hPa geopotential height (thick black contours marked Z500), locations of the main jet axes (solid green arrows, a weaker jet is shown by a dashed line in panel (c)), axes of cold (CA) and warm (WA) advection (thick blue and red arrows respectively) and the region of low-level ascent forced by upper-level positive differential vorticity advection in the vicinity of the warm advection (marked by a black circle labelled PVA).

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Table 2 Synthesis of the three synoptic environments associated with UK mesoscale convective systems. Feature

Classical Spanish plume

Modified Spanish plume

European Easterly Plume

Baroclinic wave life cycle Upper-level flow pattern

LC1 N–S orientated trough approaching from E Atlantic Cyclone centred south of Iceland (left jet entrance) with trough extension over Ireland/UK Iberia NE across SE England

LC2 Stretched NW–SE tilted trough (possible cut-off) Cyclone centred over Bay of Biscay (occluded fronts, left jet exit)

No baroclinic cyclone High amplitude omega block over Scandinavia Strong Scandinavian anticyclone

Iberia N across W and N England

Northern continental Europe NW across S England

Low-level flow pattern

Low-level plume origin Mean MCS propagation

classical and modified Spanish plume configurations occur more frequently than that associated with the European easterly plume configuration (nine, ten and five MCSs in the climatology were associated with the classical Spanish plume, modified Spanish plume and European easterly plume respectively). The position of the low-level centre in the modified Spanish plume configuration is more favourable for MCS advection over the UK than that in the classical Spanish plume configuration which may account for the similar numbers of MCSs attributed to these two configurations despite the difference in the frequency of the associated jet stream pattern. The classical and modified Spanish plume synoptic configurations have been associated with the evolution of cyclones according to the LC1 and LC2 baroclinic lifecycles respectively. The association found here between the classical Spanish plume environment and LC1-type cyclones is consistent with the finding of Roberts (2000) that ‘dry edge’ thunderstorms associated with the breakdown of a hot spell over Europe (especially southwest Europe) in summer seem mostly to be of the LC1 type with the thunderstorms occurring on the leading edge of an upper-level cut-off region of high potential vorticity at the end of the life cycle. Dry edge thunderstorms were defined as occurring on the forward edge of dry intrusions in water vapour imagery and accounted for nearly two-thirds of severe thunderstorms (many of which are likely to be MCSs according to the definition used by Roberts (2000)) when all events with a possible or clear relationship to water vapour imagery were included. Here we assume that the classical Spanish plume environment is the dominant environment in which MCSs occur when considering the whole of southwest Europe rather than just the UK (consistent with the prevalence of the associated jet stream type). The association found here between the modified Spanish plume environment and LC2-type cyclones is also consistent with Roberts (2000) in the location of the MCS development. He found that when dry edge thunderstorms were associated with developing cyclones, the cyclones usually appeared to be of the LC2 type with the thunderstorms occurring in the region of the kata-front; the MCS was initiated along the kata-front in the detailed case study of a MCS developing in a modified Spanish plume environment presented here (case II). This kata-front is included in the schematical representation of this environment (Fig. 12b). Acknowledgements We are grateful to the Met Office for making available the MetUM analyses, and to NCAS (National Centre for Atmo-

spheric Science) CMS (Computational Modelling Support) for providing computing and technical support.

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