Large-scale wind and precipitation extremes in the Mediterranean: dynamical aspects of five selected cyclone events

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1 Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: , October 216 B DOI:1.12/qj.2891 Large-scale wind and precipitation extremes in the Mediterranean: dynamical aspects of five selected cyclone events Shira Raveh-Rubin* and Heini Wernli Department of Environmental Systems Science, Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland *Correspondence to: S. Raveh-Rubin, Universitätstrasse 16, 892 Zürich, Switzerland. shira.raveh@env.ethz.ch Cyclones impacting the densely populated Mediterranean region have been a continuous research focus, mainly for investigating either the associated heavy precipitation or the damaging wind gusts. In this study we examine five Mediterranean cyclones with combined large-scale impact of strong 1 m gusts and heavy precipitation. The selected events occurred in (i) December 23 in the northeastern Mediterranean; (ii) October 27 in the central Mediterranean; (iii) January 29, known as storm Klaus, in the western Mediterranean; (iv) December 21 in the eastern Mediterranean; and (v) October 211 in the central-northern Mediterranean. European Centre for Medium-range Weather Forecasts (ECMWF) reanalyses and 7 km resolution regional model simulations (COSMO) are analysed for each event. A Lagrangian viewpoint is employed to focus on interacting mechanisms that contribute to the joint impact on different spatial and temporal scales. In all cases, widespread strong wind gusts occur in the southwestern parts of the cyclone, while the precipitation field has localized peaks, with variable distribution in the central, southern, eastern and northern parts of the cyclone. Convective precipitation, significant in the cases in 27, 21 and 211, is limited to the southern areas. In all cases, nonconvective precipitation is associated with ascent in a warm conveyor belt. Intense gusts are found within unstable air, below a low tropopause in a region with strong vertical wind shear, favouring downward momentum flux by turbulent mixing. Strongly descending dry intrusions are located coherently to the south and west of strong gusts. Much variability exists with regard to the emergence of convection, where strong winds and convective precipitation co-occur: In the 27 case, the dry intrusion is central in producing shallow convection in the cold frontal region. In the 21 and 211 cases, convective activity at high topography and in coastal regions leads to co-location of both types of impact. Key Words: Mediterranean cyclones; extreme precipitation; wind gusts; trajectories; warm conveyor belt; cold conveyor belt; dry intrusion; COSMO Received 26 November 2; Revised 13 July 216; Accepted 2 July 216; Published online in Wiley Online Library 2 October Introduction Heavy precipitation and strong surface winds are central types of extreme weather, adversely impacting the natural and human environment. Extratropical cyclones and their associated wind footprint and storm surge induce a major part of economic and insured losses from natural hazards in Europe, and cause disruption to the transport, trade, and energy supply, as well as human fatalities (e.g. Schwierz et al., 21; Pinto et al., 212). In the Mediterranean region, flood risk is the most prevalent meteorological hazard, both due to the frequent occurrence of floods and the vulnerability of the densely populated regions (Llasat et al., 21). The Mediterranean region is highly heterogeneous topographically, as well as with regard to the diversity of systems that generate wind and precipitation on the different spatio-temporal scales. They include cyclones, often associated with favourable upper-level features, and mesoscale convective systems, which are in some cases orographically enhanced (Doswell et al., 1998; Trigo et al., 22; Delrieu et al., 2; Fita et al., 27; Romem et al., 27; Romero et al., 27; Nuissier et al., 28; Funatsu et al., 29; Nissen et al., 21; Ziv et al., 21;Buzziet al., 214; Dayan et al., 2; Raveh-Rubin and Wernli, 2). High-impact events in the Mediterranean have a clear seasonality, with extreme precipitation in autumn and winter, in the western and eastern Mediterranean, respectively, and extreme surface winds in the winter throughout the region (Funatsu et al., 29; Nissen et al., 214; Raveh-Rubin and Wernli, 2). The regional amplitude of climate change in the Mediterranean region is projected to be stronger than the global average (Giorgi, 26), affecting the frequency and extremeness of high-impact weather. The occurrence of winter cyclones is expected to decrease in many parts of the Mediterranean (Ulbrich et al., 29; Zappa c 216 Royal Meteorological Society

2 398 S. Raveh-Rubin and H. Wernli et al., 213, 2). Accordingly, the frequency of windstorms is expected to decrease in most regions in the Mediterranean, except Morocco and the Levant (Nissen et al., 214), and a tendency towards drier conditions is expected (Giorgi and Lionello, 28; Bengtsson et al., 29; Raible et al., 21). However, natural variability in the region is high and large uncertainties are associated with the trends of the most extreme categories of events (Ulbrich et al., 29; Raible et al., 21; Zappa et al., 213; Nissen et al., 214), some suggesting an increase in the most extreme windstorms (Nissen et al., 214) and precipitation intensities (Scoccimarro et al., 213). Improved predictability of such highimpact Mediterranean extremes in a changing climate requires better understanding of the underlying interacting mechanisms, on the different spatio-temporal scales. To this end, it is insightful to examine coherent Lagrangian flow features in the vicinity of an extratropical cyclone (Carlson, 198). These coherent airstreams consist of the warm conveyor belt (WCB), cold conveyor belt (CCB) and dry-air intrusion (DI), and are central for understanding the processes that are involved in generating precipitation and strong surface winds, as summarized in the next paragraphs. WCBs are coherent large-scale ascending and moist airstreams ahead of the cold front in extratropical cyclones (e.g. Browning, 199). In the Mediterranean region, more than 9% of 6- hourly accumulated extreme precipitation events are associated with either a cyclone or a WCB (Pfahl et al., 214). The remaining fraction of extreme precipitation can be attributed to slantwise ascending and precipitating air parcels, which do not fulfil the stringent WCB criterion (ascent of more than 6 hpa in 48 h, cf. Madonna et al., 214), as well as convective precipitation. Furthermore, convection can be embedded within the synopticscale systems, and be accounted for as part of WCB-related precipitation (Flaounas et al., 2b). Furthermore, the cyclonerelative location of impact due to wind or precipitation is not symmetric. In a composite study based on satellite observations of oceanic cyclones, Field and Wood (27) identified distinct locations of rain and intense surface winds. Precipitation is found in a comma-shaped pattern in the southern, eastern and northern parts of the cyclone (Field and Wood, 27; Bengtsson et al., 29), a common pattern for midlatitude cyclones (e.g. Carlson, 198), as well as for Mediterranean cyclones (Flaounas et al., 2a). Strong surface winds associated with extratropical cyclones are produced by different mechanisms during the cyclone lifetime, and thus they occur at variable cyclone-relative locations and times, and on different spatio-temporal scales. In a composite study based on satellite observations, the strongest surface winds were located km southwest of the cyclone centre (Field and Wood, 27). Strong winds in early stages of cyclone development are found ahead of the cold front, within the cloudy region of the WCB (Martínez-Alvarado et al., 214; Hewson and Neu, 2). However, the large-scale signature of strong surface wind gusts is found in the region of the bent-back warm front below the cloud head region in later stages of the cyclone (Grønås, 199), where the so-called sting jet, which is mesoscale and short-lived, and/or the CCB are nested (Browning, 24; Schultz and Sienkiewicz, 213; Martínez-Alvarado et al., 214; Smart and Browning, 214; Hewson and Neu, 2). The CCB is a synoptic-scale air stream that originates in the lower troposphere on the cold side of the warm front, curves cyclonically rearward towards the cyclone northwest quadrant behind the cold front, with no significant vertical motion (Schultz, 21). Surface wind maxima due to the CCB were found on the cold side of the bent-back front, west and south of the cyclone centre, where it is colder and moister, compared with sting-jet associated winds (Martínez-Alvarado et al., 214; Smart and Browning, 214). Using an eddy kinetic energy budget, Rivière et al. (2a, 2b) offered a framework for explaining the strong winds ahead of the cold front in early stages of the cyclone lifetime, and their development to strong westerlies south of the bent-back front in later stages of storm Klaus, while it crossed the mean jet axis. Dry intrusions (DIs) are distinct airstreams, which descend from the vicinity of the tropopause to middle and low tropospheric levels, while travelling equatorward. In the midlatitudes, DIs interact with low-level baroclinic zones and serve as a central component of airflow around cyclones (Browning, 1997). DIs with stratospheric origin were shown to contribute to severe wind gusts in the narrow cold-frontal rain bands ahead of them (Browning and Reynolds, 1994) and in other cases to downstream convection (Doswell et al., 1998). However, little is known about the mechanistic relationship between DI and severe wind gusts. Co-occurrence of precipitation and winds in cyclones has been highlighted recently for the Mediterranean region (Raveh- Rubin and Wernli, 2), where a significant proportion of the large-scale wind and precipitation extremes occurred together. Events with such combined impact are important, as they may create particularly large damage; however, the mechanisms for this type of extreme event have not been studied in detail. In the event set of Raveh-Rubin and Wernli (2), the gust maximum occurred on average 12 h after and to the southwest of the peak in precipitation. A composite mean suggested coherent differences between the western and eastern Mediterranean when considering the top 2 events with combined impact in each region; events in the western Mediterranean are associated with a larger, deeper cyclone, and largely separated regions of wind and precipitation impact, with some overlap in the cyclone centre and to its south, while a smaller cyclone, and confined extent of combined impact near the cyclone centre were found in the eastern Mediterranean events. However, much case-to-case variability exists with regard to the spatio-temporal overlap of intense precipitation and gusts (Raveh-Rubin and Wernli, 2), as well as embedded transient, smaller-scale features and local topographic influences. Therefore, this study aims to explore the detailed underlying mechanisms by investigating a set of selected cases, and identify the possible mechanisms that are important contributors for the combined impact. However, as demonstrated in Raveh-Rubin and Wernli (2), a single detailed case-study cannot be sufficient for exploring the various mechanisms that may be relevant in the Mediterranean as a whole. Hence, to allow for a broader sampling of potential mechanisms, we investigate five Mediterranean events, all with co-occurring large-scale wind and precipitation extremes but with different additional characteristics (see section 2.1 for the rationale behind the case selection). For each of the five cases, we specifically address the following questions: 1. What is the spatial and temporal relation between the co-occurrence of precipitation (both large-scale and convective) and strong wind gusts? 2. Are the Lagrangian conveyor belts (WCB, CCB and DI) linked to the resulting heavy precipitation and strong winds? 3. Where does moist convection occur? How does it influence the distribution of precipitation and strong gusts? 4. What are the involved mechanisms that cause the overall co-occurrence of strong wind gusts and heavy precipitation? Addressing these questions for five different cases will provide insight into the actual mechanisms involved in producing the overall combined impact, on different spatio-temporal scales. This approach, however, cannot provide a climatological analysis that is representative for all Mediterranean combined impact events. 2. Selection of cases, data and methods 2.1. Rationale for the case selection Five events of extreme precipitation and gusts are taken from the top 4 combined large-scale events identified objectively in c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

3 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes 399 Raveh-Rubin and Wernli (2). They are selected such that (i) they occur near a cyclone in the winter or autumn season, (ii) the peak in the spatially integrated 1 m gust field follows shortly after the peak in precipitation (consistent with the average development inferred from the composite in Raveh-Rubin and Wernli (2), and (iii) the cyclone occurs in the Mediterranean basin. The main variability between the cases arises from the location of impact, the cyclone origin and the relative contribution of convective precipitation. The selected events are: (i) 19 December 23 in the northeastern Mediterranean, with impact in Greece and Turkey, extending towards the Black Sea and Iran; (ii) 2 24 October 27 in the central Mediterranean, with impact in Italy, Greece and Malta; (iii) January 29, with impact in the western Mediterranean (France and Spain); this event, known as storm Klaus, was studied by Liberato et al. (211) and Bertotti et al. (212); (iv) 9 13 December 21 in the eastern Mediterranean, impacting Lebanon, Israel and Egypt; and (v) 7 12 October 211, in the central-northeastern Mediterranean, with wind impact that extended over most of the Mediterranean basin, and precipitation impact mainly in Turkey Data and methods The analysis of the synoptic situation during each of the events is based on the European Centre for Medium-range Weather Forecasts (ECMWF) ERA-Interim reanalysis dataset (Dee et al., 211), with 6-hourly temporal resolution and 6 vertical hybrid levels. All fields have been interpolated to a 1 1 grid. Wind gust computation in ERA-Interim employs the resolved wind at 1 m, with an empirical multiplier from turbulence scheme, and is consistent with the World Meteorological Organization (WMO) definition for wind gust observation, namely the maximum in wind, averaged over 3 s intervals (ECMWF, 27). To gain insight on the mesoscale processes during each event, the limited-area numerical weather prediction model COnsortium for Small scale MOdelling (COSMO: Steppeler et al., 23) version 4.18 was run for each case with a.62 ( 7 km) horizontal grid with a rotated pole at 7. N, 17 E, 39 hybrid vertical levels, and a time step of 4 s. Output from the simulations is available every hour. Initial and boundary conditions were taken from operational ECMWF analysis data at.2 horizontal resolution and 91 vertical levels for cases 2 and ERA-Interim at.2 horizontal resolution and 6 vertical levels for case 1 (at the time of this event the operational IFS model was older than the one used for ERA-Interim). Additional set-up parameters are summarized in Table S1. The 1 m wind gusts in COSMO are calculated by taking the maximum of two parametrized values obtained from dynamical and convective parametrizations. The dynamical parametrization follows Equation 3 in Born et al. (212), with a newer model development which employs the maximum wind speed at 1 m rather than 3 m. Moist convection in COSMO is parametrized using the Tiedtke (1989) mass-flux scheme. Air-parcel trajectories are calculated using the Lagrangian analysis tool LAGRANTO (Wernli and Davies, 1997; Sprenger and Wernli, 2), which calculates trajectories using the threedimensional wind fields with an iterative predictor corrector procedure. Trajectories are calculated with the ERA-Interim data and COSMO model output. Evaporative moisture sources for precipitation are diagnosed based on a Lagrangian technique (Sodemann et al., 28) using the ERA-Interim dataset. In this method, 2-day backward trajectories are calculated from a regular initial grid in the precipitation target area, along which an increase in specific humidity during a 6 h time step is diagnosed as a moisture uptake event. Since the specific humidity increase is diagnosed along air parcel trajectories, the moisture uptake can take place within or above the boundary layer, i.e. it depends on the trajectory s threedimensional position and is not pre-restricted to the boundary layer or to water surfaces. The uptakes have been gridded and weighted to obtain a quantitative map of the overall moisture uptake for a specific precipitation event. This method has been applied successfully, for instance, to diagnose moisture sources of extreme precipitation events in the western Mediterranean (Winschall et al., 214) and of a flood event in central Europe (Grams et al., 214). Meteosat satellite imagery of the water vapour (.3 7. μm) and infrared (11 13 μm) channels are accessed via the Natural Environment Research Council (NERC) Satellite Receiving Station, Dundee University, Scotland ( The European Severe Weather Database (ESWD) of the European Severe Storms Laboratory ( which provides detailed and quality-controlled information on severe convective storm events over Europe, is accessed to confirm reported impact during the selected events. 3. Results The results for Cases 1 are presented hereafter, outlining first their impact and synoptic configuration, followed by the associated Lagrangian flow features and insight on the mesoscale dynamics from analysis of the COSMO model output Synoptic overview Case 1: 19 December 23 During 17 December, high wind gusts dominated the central and eastern Mediterranean, and heavy precipitation spread across the northeastern Mediterranean, extending to the Turkish coast of the Black Sea, towards eastern Turkey and northwestern Iran (Figure 1 (c)). In Cyprus, 36 mm of rainfall during 2. h and wind gusts exceeding 27 m s 1 were reported at Larnaca airport on December and 17 December, respectively (ESWD). During this period, a quasi-stationary surface trough extended from the Black Sea towards Turkey and the central Mediterranean, with a cyclone forming within its edge, while a wide upper-level trough was present over central and eastern Europe (Figures S1 and S2). The upper-level trough narrowed due to wave breaking during December (Figures S1 and 2) while the surface cyclone deepened (Figure 3) Case 2: 2 24 October 27 During October, widespread precipitation covered southern Italy, Greece and the Balkan area, spreading later to the west coast of Turkey (Figure 1(e) and (f)). At the same time, gust maxima occurred mostly over the central Mediterranean basin, affecting Corsica, as well as the northern Adriatic coastline and Greece (Figure 2(d)). Heavy rainfall, severe floods, galeforce winds and minor landslides were reported in Greece (ESWD). Surface cyclogenesis occurred at UTC 21 October 27 within a trough of low pressure extending from North Africa, impinging upon a large blocking anticyclone over northern Europe (Figures S1 and S2). Two centres of low pressure were formed. The eastern centre weakened and the western one further deepened and remained in the central Mediterranean basin (Figures 2 and 4), propagating cyclonically around southern Italy (pink line in Figure 1(d)). Two days prior to the time of maximum precipitation and gusts, a potentialvorticity (PV) streamer, with a northeast southwest orientation, reached the area of Corsica (Figure S1). A strong jet was present along its upstream flank, while a weaker, subtropical jet merged with the southern tip of the streamer over the coast of Libya. The streamer then wrapped cyclonically around the cyclone centre, while the jet winds weakened (Figure 2). c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

4 31 S. Raveh-Rubin and H. Wernli 1 m gust (c) Total precipitation Convective precipitation Case 1: December 23 (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) Case 2: October 27 Case 3: January 29 Case 4: December 21 Case : October Figure 1. Overview of the five events, based on ERA-Interim data and calculated over the event duration: 1 m gust maxima (m s,, (d), (g), (j), (m)) and cyclone track (pink line, position marked every 6 h. The starting location is marked with a large dot), accumulated total precipitation (mm,, (e), (h), (k), (n), colour bar is at the bottom) and accumulated convective precipitation (mm, (c), (f), (i), (l), (o), colour bar is at the bottom). The event periods are UTC UTC 19 December 23 (case 1, first row), UTC 2 UTC 24 October 27 (case 2, second row), UTC 23 UTC 26 January 29 (case 3, middle row), 12 UTC 9 12 UTC 13 December 21 (case 4, forth row) and UTC 7 UTC 12 October 211 (case, last row). The orange rectangles mark the area for the integrations used in Figures Case 3: January 29 During January, storm Klaus brought record-breaking and damaging wind gusts to the Bay of Biscay, the Pyrenees and the whole western Mediterranean basin (Figure 1(g) and ESWD). At the same time, precipitation was spread mainly over the Pyrenees and the north-western Mediterranean coasts (Figure 1(h) and (i)). Major damage and financial loss were caused by the strong winds, summing to over 6 billion US dollars (Liberato et al., 211). The cyclone formed in a short-wave perturbation over the North Atlantic between a deep Icelandic low and a stationary anticyclone to its south, at 6 UTC 23 January 29, before it intensified rapidly and moved eastwards (Figures S1 and S2). The cyclone attained its minimum pressure and made landfall 24 h later, and further propagated across the Mediterranean (Figures 2(c) and ) all the way to the Black Sea (pink line in Figure 1(g)). At upper levels a stationary ridge prevailed over the c 216 Royal Meteorological Society North Atlantic, during the 24 h preceding cyclogenesis. A strong, anticyclonically curved jet stream was oriented along the ridge. The cyclone crossed the jet axis towards the northeast, and the tropopause structure was perturbed and attained a weak trough structure (Figure 2(c)) Case 4: 9 13 December 21 During 1 12 December, extended precipitation spread from the Black Sea to the Turkish Mediterranean coast, extending later along the eastern Mediterranean coast (Figure 1(k) and (l)). Strong wind gusts prevailed generally to the southwest of the location of heavy precipitation (Figure 1(j)). Damage due to winds and flooding, including injuries, were reported during December in Lebanon and Israel (ESWD). A deep North Atlantic cyclone reached Portugal on 7 December and propagated northeastwards to France, Germany and Russia. A trough towards the northern Adriatic Sea developed on Q. J. R. Meteorol. Soc. 142: (216)

5 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes 3 W 3 W 3 E (c) 3 W 3 E 6 N 6 N 6 N 4 N 4 N 4 N 3 N 3 N 3 N N N 12 UTC 17/12/23 (d) 3 W 3 E (e) 3 W 6 N 6 N 4 N 4 N 3 N 3 N N N 12 UTC 11/12/21 N 12 UTC 22/1/ E 12 UTC 24/1/29 3 E 6 UTC 1/1/211 Figure 2. Water vapour satellite images from Meteosat (.3 7. μm channel), overlaid with the 2-PVU contour on the 32 K isentropic surface (red) and SLP (blue contours, at 4 hpa intervals), at 12 UTC 17 December 23, 12 UTC 22 October 27, (c) 12 UTC 24 January 29, (d) 12 UTC 11 December 21, and (e) 6 UTC 1 October 211. Satellite images are not available for case central pressure (hpa) gust (m s 1) tot. prec. (mm h 1) conv. prec. (mm h 1) central pressure (hpa) gust (m s 1) tot. prec. (mm h 1) conv. prec. (mm h 1) Figure 3. Temporal evolution of central SLP of the tracked cyclone (hpa, black), average 1 m gust maximum (m s 1, red), average accumulated total precipitation (mm h 1, blue), average accumulated convective precipitation (mm h 1, light blue), from 6-hourly ERA-Interim data and 1-hourly COSMO simulation data. The average is calculated for every time step during UTC UTC 19 December 23 (case 1) within the orange rectangle marked in Figure 1(a c). The dates on the x-axes follow the format dd-hh (UTC). 9 December and a transient closed cyclone formed there at 12 UTC (Figure S1). This trough then propagated along the Adriatic and had a persistent closed sea-level pressure (SLP) minimum, which propagated along the Aegean Sea during 1 c 216 Royal Meteorological Society 2- Figure 4. As Figure 3, but for Case 2 (time period UTC 2 UTC 24 October 27). Values are averaged within the orange rectangle marked in Figure 1(d f). December and intensified strongly with central SLP decreasing from 12 to 986 hpa within 24 h (Figures S1, S2, 1(j) and 6), reaching a particularly low central SLP minimum compared to other Mediterranean cyclones (Trigo et al., 1999; Flaounas et al., 2a). During the intensification phase, an upper-level trough formed over the central Mediterranean, and on 11 December extended to the southeast and overlapped with the Q. J. R. Meteorol. Soc. 142: (216)

6 312 S. Raveh-Rubin and H. Wernli central pressure (hpa) gust (m s 1 ) tot. prec. (mm h 1 ) conv. prec. (mm h 1 ) central pressure (hpa) gust (m s 1 ) tot. prec. (mm h 1 ) conv. prec. (mm h 1 ) Figure. As Figure 3, but for Case 3 (time period UTC 23 UTC 27 January 29). Values are averaged within the orange rectangle marked in Figure 1(g i). Figure 6. As Figure 3, but for Case 4 (time period 12 UTC 9 12 UTC 13 December 21). Values are averaged within the orange rectangle marked in Figure 1(j l) Large-scale trajectory analysis surface cyclone (Figure 2(d)), which reached its deepest SLP value then (Figure 6). The cyclone remained stationary near the Mediterranean Turkish coast for 2 days, until UTC 13 December. The jet in the southern tip of the trough merged on 12 December with a pre-existing subtropical jet over the Arabian Peninsula (not shown) Case : 7 12 October 211 During this period, strong wind gusts dominated the Mediterranean basin, from Palma to east of Crete, and precipitation concentrated over Greece and western Turkey (Figure 1(m) (o)). Exceptionally heavy precipitation was reported in Turkey during 9 11 October, with 24 h accumulated precipitation exceeding 3 mm in Antalya and 6 mm in other stations in the southwestern parts of Turkey, causing flooding and four fatalities (ESWD). The convective peak of 238 mm of precipitation during 6 h over Antalya has been recently studied by Demirtaş (216). Hail occurred over central Turkey and along its northern Black Sea shore, reaching a size of 3. cm during October. Moreover, tornadoes were observed in northwest Turkey, directly causing casualties (one fatal) and severe damage to houses and forests. Tornadoes were also reported over Italy, but without impact (ESWD). Upstream, extreme Alpine precipitation occurred, analysed recently in detail by Piaget et al. (2). However, this Alpine precipitation event is excluded from the current study (see extent of the orange rectangle in Figure 1(n)). The cyclone formed over northern Italy within a trough that extended from the northeast (Figure S1). The cyclone propagated along the Adriatic Sea and deepened, crossed Greece and deepened again while remaining in the Aegean Sea (Figures S1,S2,1(m)and7).On7October,anarrowupper-leveltrough stretched over western Europe. A day later, a large ridge formed upstream and wave breaking occurred, creating an elongated PV streamer, which developed into a cut-off on 1 October (Figure 2(e)). The first part of the Lagrangian analysis involves trajectory calculations with ERA-Interim data. This allows identification of moisture sources for precipitation, as well as a systematic analysis of synoptic-scale air streams during each event. The identification of the moisture source regions is based on 2- day backward trajectories from the target area, which covers the main region of cyclone-related precipitation (red rectangles in Figure 8). In the second part of the Lagrangian analysis, three types of trajectories are calculated, starting every 6 h during each event: (i) strongly ascending 48 h forward trajectories that rise at least hpa (note that this can be regarded as a weak criterion for identifying WCBs, which is appropriate in the Mediterranean); (ii) strongly descending 96 h backward trajectories that descend at least 4 hpa (which will select DI-like air streams); and (iii) 96 h backward trajectories from strong gust areas near the surface (gust >22 m s 1 and pressure >9 hpa). These results are presented now for each event Case 1: December 23 Evaporative moisture sources for this event span from the Mediterranean basin itself (with a local peak in its southeast corner) to the northeast Atlantic, the Black Sea, and the Middle East (Figure 8). Unique to this event is that significant contributions from the Red Sea are identified, as well as from the Persian Gulf and sources over land in the Middle East, all located to the south of the main precipitation areas over the Zagros Mountains (Figures 1 and 8). WCB trajectories are found persistently throughout the event. During 17 December, WCBs occur in the eastern Mediterranean basin, and extend towards the Black Sea (not shown). At the timing of peak impact, 12 UTC 17 December, the locations of all WCB trajectories are gridded and their density is presented along with total precipitation during the previous 6 h (Figure 9). The WCB overlaps the entire precipitation spatial distribution through the duration of the event, demonstrated c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

7 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes central pressure (hpa) gust (m s 1 ) tot. prec. (mm h 1 ) conv. prec. (mm h 1 ) indeed convective precipitation prevails (Figure 1(f)). This area will be examined in greater detail in section 3.4. DI trajectories are detected throughout the event. They typically start their descent north of Scandinavia, and continue to descend southwestward over Europe, while curving cyclonically over the western Mediterranean and the North African coast. Gridded DI trajectories are presented at the time of maximum surface gust (12 UTC 22 October), when they are found south and west of the area of strong gusts (Figure 9(e)). Their average pressure is 8 hpa; however, these trajectories descend further after the time of maximum surface gusts. Importantly, the DI trajectories come close to but do not directly reach the region with strong surface gusts. When explicitly calculating backward trajectories from the strong gust locations, a distinct low-level air stream, akin to the CCB concept, rotates around the cyclone and approaches the southwest side of the cyclone from the north (Figure 9(f)), and a second, similar northerly low-level air stream reaches the second gust maximum in the northern Adriatic. These trajectories remain at low levels and on average descend slightly from 89 to 9 hpa Case 3: January Figure 7. As Figure 3, but for Case (time period UTC 7 UTC 12 October 211). Values are averaged within the orange rectangle marked in Figure 1(m o). here for the time of peak precipitation, 12 UTC 17 December 23 (Figure 9). DI trajectories are located on the northern and western sides of the cyclone, spanning from Scandinavia to northern Africa, throughout the event. The DIs are located around and to the west of the area of the strongest gusts in the eastern Mediterranean, with only limited overlap south of Italy and Greece (Figure 9). In comparison, calculated back trajectories directly from the area of the strong gusts (Figure 9(c)) indicate that the vertical motion of these air parcels is more limited. They originate at mid lower levels in the Norwegian Sea, and descend further or remain at low levels for the 96 h period. A part of this air mass accelerates strongly (from 2 to over 2 m s 1, not shown) when channelled between the high orography of the Balkans and Turkey, while moving from the Black Sea towards the Aegean Sea Case 2: October 27 The identified moisture source regions for this event are localized mainly in the central Mediterranean (Figure 8). Some remote sources are also found over the eastern North Atlantic and northern Africa, but they are much weaker than those in the Mediterranean. The maximal uptake is located around Crete, southeast of the cyclone centre, with another maximum in southern Italy and the Adriatic Sea, which overlaps with the centre of the rather stationary cyclone. WCB trajectories are identified throughout the duration of the event, in two main coherent air streams: the first starts at UTC 2 October from the Aegean Sea and rises northwards towards Russia, while a day later, a second WCB rises cyclonically from southern Greece along the Balkan region and towards northern Italy (not shown). At the time of maximum precipitation (12 UTC 22 October), WCB air parcels are found mainly in the centre, north and northeast of the cyclone (Figure 9(d)) and their average pressure is 47 hpa. It can be seen that some areas that receive strong precipitation do not overlap with the WCB, especially southeast of the cyclone over the Aegean Sea where The moisture sources for precipitation in the western Mediterranean are strongest in the central subtropical Atlantic between 2 and 3 N (Figure 8(c)) and extend even into the Caribbean. This pattern is fundamentally different from the other cases and it follows partly the track of the cyclone (Figure 1(g)). Clearly, the very mobile and fast-moving cyclone is inducing ocean evaporation and collecting moisture all along its track, leading to an extreme case of tropical moisture export to the Mediterranean, consistent with the brief case-study of Klaus in Knippertz and Wernli (21). WCB trajectories are identified over the central North Atlantic already during the two days before the cyclone entered the Mediterranean basin. A WCB rises while turning anticyclonically, reaching the Bay of Biscay at an average pressure level of 3 hpa (not shown). At the time of peak precipitation in the Mediterranean (12 UTC 24 January), the location of a laterforming Atlantic WCB overlaps with precipitation (Figure 9(g)), which is in this case almost entirely non-convective (Figure 1(h) and (i)). DI trajectories are identified throughout the event. They follow the anticyclonically curved ridge across the Atlantic and then descend and turn cyclonically near the Iberian and Moroccan coasts (not shown). At the time of maximum gusts in the Mediterranean, these DI trajectories are concentrated to the southwest of (but outside) the location of maximum gusts (Figure 9(h)), similar to the October 27 case. Moreover, back trajectories from the high gust areas indicate two low-level air streams, one that originates near Newfoundland and another one that moves anticyclonically from around 2 N, 4 Wto the western Mediterranean, where the two airstreams converge. Their average pressure remains above 9 hpa along the entire path (Figure 9(i)) Case 4: December 21 Evaporative moisture sources for this event are localized along the Mediterranean Turkish coast. A second maximum is farther away in the Gulf of Sidra near the Libyan coast (Figure 8(d)). The lower values of uptake in this case, compared to the central Mediterranean case of October 27, are in agreement with the lower precipitation amount. WCB trajectories are identified throughout the duration of the event, originating in the central and eastern Mediterranean basins (not shown). At the time of maximum precipitation, the location of the gridded WCB trajectories (shaded area in Figure 9(j)), which have an average pressure of 4 hpa, overlaps partially with precipitation in the northern part of the cyclone (orange c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

8 314 S. Raveh-Rubin and H. Wernli 7 N 6 N N 4 N 3 N 2 N 8 N 7 N 6 N N 4 N 3 N (c) 6 N N 4 N 3 N 2 N 1 N 1 W 7 W W 2 W 2 E 1 N W 2 W 2 E E (d) 7 N 2 N 1 N 2 W (e) 6 N 2 E 4 E N N N 4 N 6 4 N 3 N 4 3 N 2 N 2 2 N 1 N 4 W 2 W 2 E 4 E W E 3 E 4 E Figure 8. Moisture uptake regions (mm (6 h) 1 ) for precipitation within the red rectangle, during UTC UTC 19 December 23, 6 UTC 2 UTC 24 October 27, (c) UTC 23 UTC 2 January 29, (d) UTC 1 UTC 13 December 21 and (e) UTC 7 UTC 12 October 211. See text for technical details. contour in Figure 9(j)). Precipitation south and east of the cyclone cannot be explained by this slantwise ascent (it does not overlap with WCB trajectories) and is indeed substantially convective (Figure 1(l)). DI trajectories reach the eastern Mediterranean from the North Sea, moving southeastward towards central Europe and sweeping cyclonically through the central and later eastern Mediterranean (not shown). At the time of maximum activity, the DI trajectories clearly surround the area of strong gusts, from its northwest to the south (Figure 9(k)), remarkably similar to the other cases. Here too, trajectories that directly reach the area of strong gusts are associated with a CCB-like airstream, which remains at low levels, arrives from the north and moves cyclonically around the cyclone (Figure 9(l)) Case : October 211 The evaporative moisture sources of the precipitation of this event are spread across the Mediterranean basin, as well as over continental regions. A substantial amount of moisture originates locally within the target area of precipitation (red rectangle in Figure 8(e)), mainly in its southern parts. In addition, longdistance transport of moisture is evident from two branches of moisture sources that stretch out to the northwest, i.e. between the Gulf of Lion and the Gulf of Genoa and France, and to the southwest, with continental (and even tropical) sources over central North Africa (Figure 8(e)). WCB trajectories are identified throughout the event, mainly over the eastern Mediterranean, Turkey, the Black Sea and further west. They overlap mostly with the precipitation pattern at the timing of the first peak in precipitation, 6 UTC 9 October (not shown). However, at the time of the second peak, 6 UTC 1 October, it is evident that fewer WCB trajectories are identified, and those mostly do not overlap with the precipitation signature, which is indeed largely convective (Figure 1(o)). DI trajectories are identified to surround the areas of strong 1 m gusts, similar to all other cases (Figure 9(n)), and are located to the north and west of the cyclone, with an additional occurrence to the south, over North Africa. Trajectories that directly reach the location of the strong gusts originate from the North Sea, remain at low levels while curving anticyclonically and finally arrive from the northeast to the southern Adriatic (Figure 9(o)). These low-level trajectories descend slightly from 8 to 99 hpa after crossing the Balkans, and accelerate from 1 m s 1 to over 2 m s 1 (not shown). In summary, this analysis of large-scale air streams, based on ERA-Interim data, has shown that: (i) WCB-like air streams are identified in all five cases, and overlap spatially with precipitation in the eastern and northern parts of the cyclones. When convective precipitation occurs (in cases 2, 4 and ), mostly south and east of the cyclone centre, there is no such overlap between WCBlike trajectories and convective precipitation. (ii) DI trajectories are found in all five cases, southwest of the cyclone centre, surrounding the regions with strong gusts. (iii) Back trajectories from the regions with strong surface gusts originate at low levels and share some characteristics of the CCB COSMO model verification The COSMO simulations are first validated with respect to the cyclones central SLP (black lines in Figures 3 7) and track (Figure 1), followed by the spatio-temporal distribution of 1 m wind gusts and precipitation (File S1). Both the central SLP values and cyclone tracks agree very well with ERA- Interim. The cyclone tracks are almost identical, and differ mainly when the cyclone is located over complex topography, which is represented coarsely in ERA-Interim (e.g. over Turkey in the December 23 and 21 cases). The simulated SLP evolution is generally also very similar to the reanalysis, but given at a higher temporal resolution (1-hourly). However, some differences are found. In the December 23 case, the tracks in the two datasets diverge towards the end of the event on 18 December (Figure 3). The minimum SLP for the October 27 case is lower in the simulated data (99 hpa, compared c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

9 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes (c) (e) (f) (h) (i) (k) (l) (n) (o) 3 Case 1: 12 UTC 17 December 23 (d) Case 2: 12 UTC 22 October 27 (g) Case 3: 12 UTC 24 January 29 (j) Case 4: 12 UTC 11 December 21 (m) Case : 6 UTC 1 October Figure 9. SLP (black contours, 4 hpa intervals) at 12 UTC 17 December 23, 6 h accumulated total precipitation (orange contours, mm intervals starting at mm) and gridded WCB trajectories at the same time (shaded, in number of trajectories per 1 1 grid box); SLP as in, and 6 h 1 m gust maximum (red contours, m s 1 intervals starting from 2 m s 1 ) and gridded DI trajectories at the same time (shaded, in number of trajectories per 1 1 grid box); (c) backward trajectories reaching high gust locations (black dots, see text for details) at 12 UTC 17 December 23, coloured according to pressure (hpa). The same fields are shown in (d f) at 12 UTC 22 October 27, (g i) at 12 UTC 24 January 29, (j l) at 12 UTC 11 December 21 and (m o) at 6 UTC 1 October 211. to 996 hpa in the reanalysis) and is reached about 8 h earlier (compare Figure 4 and ). On the other hand, the lowest simulated minimum SLP for Klaus (Figure ) is underestimated in the model (973 hpa) compared to measurements (963 hpa, see Liberato et al., 211) and the reanalysis (968 hpa). Nonetheless, the rapid cyclone intensification, and the slower weakening after 12 UTC 24 January are captured well by the simulation. For the last case in October 211 the passage of the cyclone over Greece on 1 October causes a greater weakening of c 216 Royal Meteorological Society the cyclone s central SLP in the COSMO simulation than ERA-Interim. The simulated precipitation and gust fields are consistent with those in the reanalysis, while the simulation offers more detailed structures and often higher and more localized values of both gusts and precipitation (compare Figures 1 and 1). Please note that the precipitation and gust values in Figures 3 7 are obtained by averaging the data within the orange rectangles shown in Figure 1, and therefore they are lower than local maxima in Q. J. R. Meteorol. Soc. 142: (216)

10 316 S. Raveh-Rubin and H. Wernli 1 m gust Total precipitation Convective precipitation (c) Case 1: December 23 (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) Case 2: October 27 Case 3: January 29 Case 4: December 21 Case : October Figure 1. As Figure 1, but for the 1-hourly COSMO simulation data. Figures 1 and 1. The peak precipitation and gust features are described for each event separately in the File S Mesoscale dynamics In this section, using the high-resolution COSMO simulation output, we focus on the time of peak gust and precipitation for each of the five events, and analyse vertical sections across locations of intense precipitation and/or strong gusts. We compute backward and forward trajectories from the vertical cross-sections to understand the origin and dynamical characteristics of different air streams that contribute to strong surface wind gusts and precipitation Case 1: 12 UTC 17 December 23 The times of the deepest central SLP, the maxima in total precipitation and 1 m gust coincide in this event (Figure 3, around 12 UTC 17 December). A vertical cross-section at this time, which cuts through the gust maximum in the central Mediterranean, and the non-convective precipitation and an c 216 Royal Meteorological Society additional local gust peak in the southern Black Sea, is analysed here (see location in Figure 11 (c)). A prominent tropopause fold is visible at 37 N (Figure 12, cf. 2-PVU contour in green), i.e. where the peak in 1 m gusts and high wind speeds at 4 hpa occur. Wind speeds of 2 m s 1 reach down to 7 8 hpa in the vicinity of the fold, below which the layer is well mixed, as seen by the homogeneous equivalent potential temperature (θ e ) profile (Figure 12). This configuration enables high-momentum air to be mixed down to the surface and create a widespread maximum of wind gusts. Interestingly, strongly descending trajectories (Figure 13 and black asterisks in Figure 12) are found around 7 hpa, at 38 N, where the wind gusts have lower values, and larger spatial variability. Further north, strong pressure gradient forces are evident to create strong winds near 42 N. The wind gust maximum there is associated with a low-level northerly jet (Figures 12 and 14) which accelerates due to the strong pressure gradient forces and the low friction over the Black Sea. This low-level jet is fundamentally different from the extended wind gust maximum behind the cyclone, as it is detached from the upper levels; wind speeds of up to 2 m s 1 are restricted to the layer below 7 hpa (orange contour near 42 N in Figure 12). Q. J. R. Meteorol. Soc. 142: (216)

11 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes 1 m gust Total precipitation (c) 317 Convective precipitation Case 1: 12 UTC 17 December 23 (d) (e) (f) (g) (h) (i) (j) (k) (l) (n) (o) Case 2: 12 UTC 22 October 27 CS2 CS1 Case 3: 12 UTC 24 January 29 Case 4: 12 UTC 11 December 21 (m) Case : 6 UTC 1 October Figure 11. COSMO model fields for the 6 h time period prior to peak impact, noted to the left of each row., (d), (g), (j), (m) maximum 1 m gusts over the 6 h period (shaded for values above m s 1 )., (e), (h), (k), (n) 6 h accumulated total precipitation (starting at mm). (c), (f), (i), (l), (o) 6 h accumulated convective precipitation (starting at mm) and SLP (grey contours, hpa intervals). The location of the vertical cross-sections is shown with red lines. WCB-like trajectories are found throughout this area (green asterisks in Figure 12). The associated strong diabatic heating creates low-level positive PV anomalies below (green contours) and co-locates with the strong peaks in precipitation. Here, the bulk of precipitation corresponds to the WCB slantwise ascending motion. It is plausible that the prominent diabatically produced PV anomaly at low levels reinforces the cyclonic circulation and the associated wind maximum at 42 N Case 2: 12 UTC 22 October 27 The first vertical cross-section cuts through the large area of strong gusts southwest of the cyclone, the convective precipitation in the cyclone centre, and the major area of non-convective precipitation over the Balkans north of the cyclone (Figure S3, location labelled CS1 in Figure 11(e)). The tropopause wraps around the cyclone, reaches as low as 6 hpa and is accompanied by a strong jet at 36 N, south of the cyclone centre (Figure S3). The strongest gusts are found at the surface of the potentially unstable atmospheric column, where the vertical θ e gradient is negative. This region is vertically aligned with the low tropopause. In the northern part of the unstable region, at 38 N, convective precipitation occurs. No strongly ascending WCB or descending c 216 Royal Meteorological Society DI trajectories cross the vertical cross-section at this time in the area of strong gusts between 3 and 39 N. Back trajectories from high gusts at the surface arrive coherently from the north and are restricted to low levels (Figure 14). The CCB-like air that reaches the location of strong gusts is a large-scale feature, consistent with trajectories calculated based on the reanalysis data (Figure 9(f)). Moreover, the model indicates that CCB trajectories are deflected by the Alps, such that some trajectories pass over and some around (east) of the Alps (Figure 14), both of which reach velocities larger than 2 m s 1 when they enter the Mediterranean (not shown). Interestingly, the CCB branch that flows east of the Alps descends from 7 to 9 hpa over Slovenia towards the northern Adriatic, as a bora wind (Grisogono and Belusic, 29). Further north, in the cyclone centre (41 42 N), precipitation, which is mostly convective, is found together with fast-ascending trajectories through the whole column (Figure S3 and ). To its north, at 44 N, the most significant large-scale (non-convective) precipitation prevails. Indeed, WCB trajectories crossing the vertical cross-section in this area are found in the layer between 6 and 3 hpa (green asterisks in Figure S3). The resulting condensation creates large positive PV anomalies below it, reaching all the way down to the surface and stabilizing the low levels, as well as eroding the high Q. J. R. Meteorol. Soc. 142: (216)

12 318 S. Raveh-Rubin and H. Wernli Pressure (hpa) h accum. conv. prec 2 1 6h accum. prec 2 1 6h max. gust SLP Latitude ( N) Figure 12. Model vertical cross-section (marked in Figure 11(a c)) at 12 UTC 17 December 23. Relative humidity (%, shaded), θ e (black contours, 28 to 34 K in 4 K intervals, above 34 K in 1 K intervals), 2-PVU contour (green), wind speed (orange contours starting at 2 m s 1,in1ms 1 intervals,). 2-D fields along the cross-section, SLP (hpa, in black), 6 h maximum 1 m gust (m s 1, in red), 6 h accumulated total precipitation (mm, in dark blue) and 6 h accumulated convective precipitation (mm, in light blue). The 6 h period refers to the 6 h prior to the specified time. PV at the upper levels and thus lifting the tropopause. Topography reaches 8 hpa in this region, above which the densely-packed θ e isentropes indeed indicate a stable column. A possible outcome of the stable layer enhanced by the WCB above is a minimum in the 1 m gust field, which coincides with the area of heavy non-convective precipitation (Figure S3 and ). The second vertical section is across two areas of high gusts (Figure, see label CS2 in Figure 11(e)), which differ in terms of the involved dynamics. The eastern centre, off the southwestern Turkish coast at 26 E, coincides with convective precipitation, while the western gust centre in the northern Adriatic is precipitation free. The vertical cross-section passes through the cold front at the location of the eastern gust and precipitation maximum at 26 E (Figures and S2). A shallow potentially unstable layer is visible on both sides of the front, below 7 hpa. Many descending DI trajectories are found coherently below 8 hpa, confined to the cold side of the front (Figure 13 and black asterisks in Figure ), which undercuts the warm air mass, possibly triggering/enhancing the rise of moist surface air ahead. Indeed, above the DI air mass, the warm-sector air reaches saturation, and shallow convection occurs, along with strong gusts. Shallow convection in this area is verified by satellite imagery, as a dark area in the water-vapour channel and indications of relatively warm (shallow) clouds in the infrared (not shown). It is important to note that here the convective precipitation is not linked with a WCB, but rather, the shallow convection is located beneath a very dry air column, extending from the stratosphere down to 7 hpa. The western gust centre in the northern Adriatic is in an area with high relative humidity and diabatically produced PV at low and mid-levels. In this region, the strong pressure gradient itself contributes to the strong surface winds. There is no precipitation but many WCB trajectories, already in the outflow region (green asterisks in Figure ), with stable air below the WCB. In the cyclone centre there is weak convection and some strongly rising trajectories Case 3: 12 UTC 24 January 29 The vertical section chosen crosses the cyclone centre and the central location of precipitation and gusts (Figure S4). At all altitudes the wind is roughly orthogonal to the cross-section. A prominent tropopause fold is visible at 41 N, reaching down to below 6 hpa. A tilted PV tower connects it with the cyclone centre at 43 N, where the gust velocities reach m s 1.The boundary-layer height rises from 8 hpa at 37 Nto6hPaat 41 N where it is in direct contact with the folded tropopause. Such behaviour has been reported previously for folds over the Tibetan Plateau (Chen et al., 213) and for cold air outbreaks from Antarctica (Papritz and Pfahl, 216). Due to the large temperature gradients throughout the atmospheric column, the westerly winds are coherent in the vertical, and extend to low levels (Figures 13(c) and 14(c)). Specifically, below the dry air mass south of the cyclone centre, between 3 and 41 N, strong downward momentum fluxes are probable where the jet axis reaches the deep mixed boundary layer. Large-scale descending trajectories are spread across and confined vertically to the boundary layer (Figure 13(c) and black asterisks in Figure S4). Non-convective precipitation is found near the cyclone centre, associated with WCB trajectories (green asterisks c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

13 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes (d) (e) 319 (c) Figure 13. COSMO SLP (grey contours at hpa intervals, unlabelled) and DI trajectories crossing the vertical cross-sections (intersection points marked in blue, see text for details). Trajectories are coloured according to their pressure (hpa), and the times of intersection are 12 UTC 17 December 23, 12 UTC 22 October 27, (c) 12 UTC 24 January 29, (d) 12 UTC 11 December 21, and (e) 6 UTC 1 October 211. (d) (e) (c) Figure 14. Same as Figure 13, but for gust trajectories with pressure larger than 9 hpa and 1 m wind gust of at least 2 m s 1 at the time of intersection. in Figure S4). However, compared to the other cases, precipitation amounts at this time are relatively weak and not widespread Case 4: 12 UTC 11 December 21 Analysis of the vertical cross-section (Figure S, red line in Figure 11(j) (l)) exhibits characteristics similar to the other cases. c 216 Royal Meteorological Society Namely, the tropopause reaches down to 6 hpa, creating a layer with strong vertical wind shear and potential instability, leading to peak gusts at the surface. DI trajectories are found southwest of the maximum gusts location, within the mixed boundary layer, below 8 hpa (Figure 13 (d) and black asterisks in Figure S). Moreover, convective precipitation is co-located with peak gusts between mainland Greece and Crete. Back trajectories indicate a northerly flow at low levels, recognized as a CCB Q. J. R. Meteorol. Soc. 142: (216)

14 311 S. Raveh-Rubin and H. Wernli Pressure (hpa) h accum. conv. prec h accum. prec h max. gust SLP Longitude ( E) Figure. Same as Figure 12, but at 12 UTC 22 October 27; the position of the cross-section CS2 is marked in Figure 11(d f). (Figure 14(d)). These finding resemble the convection found near CCBs in Ziv et al. (21). The DI trajectories also come from the north (Figure 13(d)), coherently to the west of the CCB pathway; however, they diverge over the North African coast, while the CCB airstream accelerates around the cyclone, and later rises significantly above eastern Turkey (Figure 14(d)). The cyclone centre is located at the Turkish Mediterranean coast, near steep topography. Non-convective precipitation is co-located with WCB trajectories at to 4 hpa and moderate wind gusts (not shown) Case : 6 UTC 1 October 211 At the time of peak activity, there is a notable convective precipitation maximum over Antalya. We analyse this area, together with the maximum gusts south of Italy, with a vertical cross-section connecting the two maxima and the cyclone centre (red line in Figure 11(m) (o)). The upper tropospheric cut-off is accompanied by a tropopause fold and a strong jet at E, and a smaller fold on its eastern side, at 24 E (Figure 16). The tropospheric column in a wide area below the cut-off and its vicinity is characterized by low static stability. High gust values dominate the western segment of the cyclone, and follow the same scenario as in the other events, whereby momentum from the tropopause-level jet (which is lowered and folded) reaches the top of the elevated mixed boundary layer, where dry, highmomentum air is mixed down by turbulence. The strongest gust peak is reinforced by moist convection near the cyclone centre, at 18 E. The flow in the western segment of the cyclone is northerly, exemplified here by the low-level CCB that reaches the most intense gusts maximum (Figure 14(e)), and descending trajectories (Figure 13(e) and black asterisks in Figure 16), to the west of the gust peak. The eastern side of the cross-section is located above the high orography of the Taurus Mountains. The flow there is southerly (not shown), thus originates over the Mediterranean Sea, rising sharply over the steep orography, and releasing the potential instability. The intense and sustained convection produced unprecedented precipitation (note the different scale of precipitation in Figure 16, compared with Figures 12,, S3 and S4), as well as a local peak in convective gusts. The importance of the orography and the moist airflow from the warm sea is consistent with the analysis by Demirtaş (216) of the V-shaped convective cell that caused the Antalya flood. 4. Summary and discussion The central role of surface cyclones for Mediterranean highimpact weather is demonstrated through a detailed investigation of five events with co-occurring large-scale wind and precipitation extremes. The cyclonic environment organizes coherent largescale air streams and co-locates precipitation and strong winds, which is important for the overall impact. Four Mediterranean cyclones affected the central and eastern Mediterranean in the autumn and early winter, while the fifth, storm Klaus, was an explosive North Atlantic cyclone that entered and impacted the western Mediterranean. The detailed analyses of the five cases provide insight on the variety of the possible mechanisms that control the precipitation and wind gust impact. As expected, the examination of five events in different regions of the Mediterranean samples a diversity of involved mechanisms, especially when convective precipitation and/or complex orography are present. Nevertheless, some aspects are common to several events. The range of potential mechanisms, c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)

15 Dynamical Aspects of Mediterranean Wind and Precipitation Extremes Pressure (hpa) SLP 9 3 6h max. gust 1 6h accum. prec h accum. conv. prec Longitude ( E) Figure 16. Same as Figure 12 but at 6 UTC 1 October 211; the position of the cross-section is marked in Figure 11 (m o). based on the five cases, is summarized in the schematic Figure 17 and in Table 1. We now address the four questions posed in the introduction, in light of the five events investigated: 1. On the synoptic scale, the spatial distributions of strong wind gusts and precipitation are largely separated, with some limited overlap. In the cases analysed here, strong winds are located south of the cyclone track, in agreement with previous observations (Born et al., 212; Pinto et al., 212) and/or to their southwest (Field and Wood, 27; Pfahl, 214). In comparison to the wind, precipitation is more limited in scale (Raveh-Rubin and Wernli, 2; Martius et al., 216), and its spatial distribution varies from case to case, with peaks occurring anywhere in the cyclone centre, and to the south, east and north, and some of them enhanced by topography. Convective precipitation, when present (see Table 1), occurs in diverse locations within the precipitation areas south and east of the cyclone centre, and/or in the cyclone centre itself. The emergence of convective precipitation in the cyclone centre agrees with a recent composite study (Flaounas et al., 2a). The temporal evolution of the spatially averaged winds and precipitation shows that from the large-scale perspective (based on ERA-Interim reanalysis data), the peak in precipitation coincides with the deepest minimum in SLP for the five events investigated here. The wind gust maximum follows within the next 6 h, as expected by the adopted case selection. However, analysis of the fields at higher spatio-temporal resolution (i.e. from the COSMO simulations) shows that the better resolved topography may modify the evolution of the fields. c 216 Royal Meteorological Society 2. A systematic trajectory analysis has been carried out for the five events, and coherent conveyor belts were identified: (i) Strongly ascending trajectories are found for all five events (see Table 1). They are typified by slantwise ascent and correspond to WCBs, although with a weaker ascent (less than 6 hpa) than diagnosed typically over the main storm-track regions (Madonna et al., 214). A substantial fraction of the total precipitation can be explained by WCBs, mainly in the northern parts of the precipitation signature, for all five cases. This indicates that intense precipitation is produced directly by slantwise ascending moist air in the warm sector and cloud head region. The overlap between WCB trajectories and precipitation is temporally persistent, and demonstrated in this study at the time of peak precipitation. It is plausible that the temporal persistence of this characteristic overlap demonstrates how WCBs enable prolonged precipitation production and thus a long and largescale precipitation event, as opposed to purely convective precipitation. By occurring on a larger temporal and spatial scale, WCBs supply moisture over a long duration of time, and exploit larger and more distant moisture reservoirs compared to convective precipitation. The strong association between heavy precipitation and WCBs is consistent with the climatological correspondence between WCBs and extreme (6-hourly) precipitation in the Mediterranean region (Pfahl et al., 214). Q. J. R. Meteorol. Soc. 142: (216)

16 3112 S. Raveh-Rubin and H. Wernli 3 DI Gust trajectory 2 9 topography 4 1 L 1 m wind gusts 7 precipitation 6 8 WCB Figure 17. Schematic illustration of possible features related to combined precipitation and wind impact of a cyclone in the Mediterranean, based on the five events studied here. Note that this schematic does not represent any individual event, but rather summarizes the variable possible features of importance. The cyclone centre is denoted by the letter L, accompanied by a cold and a warm front. Shading shows areas with precipitation impact (light blue), and with 1 m gust impact (light red). Areas with convection (and thus co-located precipitation and wind gust impact) are shaded blue and encircled by a red line. High topography is represented by grey regions. The 32 K 2-PVU contour is shown as a dashed line and typical WCB, DI and gust trajectories are denoted as green arrows. The numbers mark the location of: (1) a prominent upper-level feature (PV streamer/trough/ridge), (2) tropopause fold and downward momentum transfer, (3) DI trajectories around high gusts, (4) low-level gust trajectory, () WCB slantwise ascent associated with precipitation, (6) convective precipitation at cold front, (7) convective precipitation in cyclone centre, (8) orography enhancing precipitation (and/or convection), (9) orography accelerating gust trajectories. Please refer to Table 1 for the representation of features 1 9 in the five events, and the text for details regarding the individual representation of the features in the five case-studies. (ii) Strongly descending trajectories constitute the DI airflow to the rear of the cyclones in all five events. They are found to descend to the outer periphery of strong gusts, mainly southwest to northwest of the cyclone centre, following the upstream branch of the large-scale PV streamer (dark areas in Figure 2). DIs reach below 7 to 8 hpa at the time of maximum gusts and to even lower levels thereafter. The general cyclone-relative spatial distributions of DIs and gusts are consistent with the results of Field and Wood (27), who found surface wind divergence >1 km west of the centre of oceanic cyclones, while strong winds were observed at km distance from the centre. When examining ERA-Interim data, a systematic displacement between DI air and the location of strongest gusts is apparent. However, colocation at the edge of both features is present, as some DI trajectories overlap with areas with gusts above 2 m s 1 in all cases (Figure 9). Analysis of 7 km resolution COSMO data reveals a larger degree of overlap in the cold front region, where DI trajectories undercut the cold front (in case 2). However, DIs are also found near local minima of wind gusts (in case 1 and ), and it is yet unclear how they interact with surface winds. (iii) Back trajectories from maximum gust areas exhibit coherent characteristics of a low-level airstream. The air parcels originate at low levels, to the north (in cases 1, 2, 4 and ) or to the west (in the fast eastwardmoving case 3 cyclone), i.e. outside of the warm sector, and thus can be considered a CCB (Schultz, 21). They are shown to be influenced by topography and at times descend slightly, causing further acceleration 8 topography Table 1. Summary of the different features that contribute to the extreme precipitation and wind impact (numbers correspond to the same notation as in Figure 17). The occurrence of a certain aspect in an individual case in marked with a + sign. Please refer to the text for details regarding the case-to-case variability in the representation of the different features. Feature Case 1 Case 2 Case 3 Case 4 Case 1 Upper level PV streamer Upper level trough Upperlevel ridge + 2 Tropopause fold and downward momentum transfer 3 DI trajectories around high gusts 4 Low-level gust trajectory WCB slantwise ascent associated with precipitation 6 Convective precipitation at + + cold front 7 Convective precipitation in cyclone centre 8 Orography enhancing precipitation (and/or convection) 9 Orography accelerating gust trajectories 1 Mediterranean moisture sources 11 Non-local moisture sources (see Table 1). These findings agree with Ziv et al. (21) who identified CCBs in winter Mediterranean cyclones. (iv) Lagrangian analyses to identify evaporative moisture sources show in cases 1, 2, 4 and a combination of mainly local Mediterranean sources and additional highly diverse remote sources. In contrast, for case 3 (which has the smallest fraction of convective precipitation) the main maximum of the moisture source region is located south of 3 N in the central North Atlantic. These results demonstrate that although all five events share the characteristic of non-local moisture sources (Table 1), the case-tocase variability in the moisture uptake distribution is dramatic, a feature that has been emphasized even for a larger set of cases with similar large-scale conditions (Winschall et al., 214). 3. Convective precipitation is found in all five cases, with a large degree of variability of its extent, in the southern parts of the total precipitation distribution. Inspection of the fine structures of the model output indicates that substantial convective precipitation may be preferably found along the cold front, in the cyclone centre and in coastal regions (see Table 1 for representation in each case). This study suggests that convective activity is important not only for the localization and peak values of extreme precipitation, but also for their co-occurrence with strong wind gusts. This is mainly true for the cold frontal region (for cases 2 and ), where convection along extended segments of the front impacts surface conditions with both strong wind gusts and heavy precipitation. Similarly, convective precipitation occurred along the cold front of storm Kyrill which affected western and central Europe in January 27 (Ludwig et al., 2). It is yet unclear whether shallow or deep convection is often embedded in large-scale slantwise vertical motion (cf. WCBs, see Flaounas et al., 2b) and this remains an important open question for further investigation. Generally, with respect to the fraction of precipitation produced by the convection schemes, the c 216 Royal Meteorological Society Q. J. R. Meteorol. Soc. 142: (216)