# Processes leading to heavy precipitation associated with two Mediterranean cyclones observed during the HyMeX SOP1

Emmanouil Flaounas, Konstantinos Lagouvardos, Vassiliki Kotroni, Chantal

Claud, Julien Delanoë, Cyrille Flamant, Erica Madonna, Heini Wernli

► To cite this version:

Emmanouil Flaounas, Konstantinos Lagouvardos, Vassiliki Kotroni, Chantal Claud, Julien Delanoë, et al.. Processes leading to heavy precipitation associated with two Mediterranean cyclones observed during the HyMeX SOP1. Quarterly Journal of the Royal Meteorological Society, 2016, 142 (S1), pp.275-286. 10.1002/qj.2618 . insu-01175685

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# Processes leading to heavy precipitation associated with two Mediterranean cyclones observed during the HyMeX SOP1

Emmanouil Flaounas,\* Konstantinos Lagouvardos,\^a Vassiliki Kotroni,\^a Chantal Claud,\^b Julien Delanoë,\^c Cyrille Flamant,\^d Erica Madonna\^e and Heini Wernli\^e

Two deep cyclones occurred in the Mediterranean between 25 and 31 October 2012, during the first Special Observation Period (SOP1) of the Hydrological cycle in the Mediterranean Experiment (HyMeX). Both cyclones were associated with extreme rainfall covering a large part of the western Mediterranean Sea, where 24 h accumulated precipitation measurements exceeded 150 mm. We combine complementary observations from airborne radar and lidar systems, ZEUS lightning detection network and meteorological surface stations along with satellite diagnostics on deep convection, for a detailed microphysics and (thermo-)dynamical analysis of the two extreme rainfall cases. In addition, we use operational analysis data from the European Centre for Medium-range Weather Forecasts (ECMWF) for analysing the synoptic conditions and diagnosing strongly ascending air masses in the vicinity of the cyclones, so-called warm conveyor belts (WCBs). The analysis revealed the different physical characteristics of the two cyclones responsible for the extreme rainfalls. Both cyclones were associated with a WCB and a comma cloud, but deep convection, intense lightning and very cold cloud tops occurred only for the first case while the second cyclone was mostly associated with stratiform rainfall, a strong WCB, and only a few embedded cells of deep convection.

Key Words: heavy rainfall; Mediterranean cyclones; warm conveyor belts; deep convection: lightning; microphysics

Received 23 March 2015; Revised 18 June 2015; Accepted 29 June 2015; Published online in Wiley Online Library 9 September 2015

## 1. Introduction

Mediterranean cyclones are a particular category of extratropical cyclone that develop over the Mediterranean basin, a relatively small region characterized by the Mediterranean Sea and the surrounding high mountains. Due to the complex geography of the region, Mediterranean cyclogenesis is influenced by a variety of factors including synoptic-scale Rossby waves and their breaking, air-flow steering due to mountains, and intense air-sea interactions. Compared to extratropical cyclones developing over the main storm track regions, Mediterranean cyclones are of smaller size, lifetime and intensity (e.g. Trigo, 2006; Čampa and Wernli, 2012). Indeed, Mediterranean cyclones have a radius of typically 200–500 km, a life span of the order of a day or two, and intensities rarely reaching the standards of tropical hurricanes (Trigo et al., 1999; Emanuel, 2005; Tous and Romero, 2013). Despite their smaller magnitude and size, Mediterranean cyclones are often linked with high-impact weather, resulting in floods,

fatalities and construction damage (Lagouvardos et al., 1996, 2007; Kotroni et al., 1999; Llasat-Botija et al., 2007, Llasat et al., 2013; Papagiannaki et al., 2013; Jansà et al., 2014; Raveh-Rubin and Wernli, 2015).

Recently, Flaounas et al. (2015) performed a composite analysis of the 200 most intense cyclones in a mesoscale model simulation during a 20-year period (1989–2008). The majority of the selected cyclones developed over the Mediterranean Sea and under the influence of an upper-level potential vorticity (PV) streamer. PV streamers are filamentary features of stratospheric air originating from higher latitudes and intruding to lower tropospheric levels (Appenzeller and Davies, 1992). They typically form during the nonlinear breaking of Rossby waves propagating from the North Atlantic towards Europe. The life cycle of the intense Mediterranean cyclones was shown to be similar for most cases and characterized by the cyclonic wrapping of the PV streamer around the cyclone’s centre. This common life cycle was suggested to result from the strong horizontal cyclonic

<footnote>\^aNational Observatory of Athens, Greece</footnote>

<footnote>\^bInstitut Pierre Simon Laplace/Laboratoire de Météorologie Dynamique, CNRS and Ecole Polytechnique, Palaiseau, France</footnote>

<footnote>\^cInstitut Pierre Simon Laplace/Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS, Université Versailles Saint Quentin, Guyancourt, France</footnote>

<footnote>\^dUMR 8190 LATMOS, CNRS and UVSQ, Sorbonne Universités, UPMC Univ Paris 6, France</footnote>

<footnote>\^eDepartment of Environmental Sciences, Institute for Atmospheric and Climate Science (IACETH), ETH Zurich, Switzerland</footnote>

<footnote>\*Correspondence to: E. Flaounas, National Observatory of Athens, Institute of Environmental Research and Sustainable Development, Lofos Koufou, P. Penteli, 15236, Athens, Greece. E-mail: flaounas@noa.gr</footnote>
wind shear north of an intense subtropical jet, similar to that proposed by Thorncroft et al. (1993) in their LC2 scenario. Fita et al. (2006, 2007) confirmed the dominant role of large-scale forcing in Mediterranean cyclogenesis, but also pointed out the significant impact of diabatically generated PV in low and mid tropospheric levels for the deepening of cyclones, especially over the eastern Mediterranean. Such diabatic processes can be associated with latent heating in deep convective systems or in more slantwise ascending warm conveyor belts (WCBs: Browning, 1990; Lagouvardos et al., 1993; Madonna et al., 2014) – or in a combination of the two (WCBs with embedded convection).

Deep convection is a common feature among the most intense Mediterranean cyclones, which often are also associated with extreme rainfall (Lagouvardos et al., 1999; Jansà et al., 2001; Claud et al., 2010, 2012). For instance, Lagouvardos and Kotroni (2007) and Miglietta et al. (2013) used lightning and satellite observations to analyse deep convection associated with intense cyclones over the Mediterranean Sea. They showed the existence of numerous lightning impacts prior to and during the mature stage of cyclones, indicating the development of deep convection. This result agrees with the study by McTaggart-Cowan et al. (2010), who used lightning and radar observations in order to analyse convection during the development of an intense Mediterranean cyclone. The interaction between cyclones and convective systems is still an open question that deserves further investigation. This would help to better understand the linkage between cyclone dynamics and thermodynamics, with the eventual aim of increasing predictability of high-impact weather in the region (e.g. Hohenegger and Schär, 2007).

The composite analysis in Flaounas et al. (2015) revealed that intense Mediterranean cyclones present clear surface frontal structures and associated comma-shaped rainfall patterns. Such a frontal structure is consistent with the existence of coherently ascending air masses, from the boundary layer to the upper troposphere, so-called WCBs. This result is in accordance with Ziv et al. (2010), who analysed several case-studies of WCBs, based on satellite images, reanalyses fields and air-mass trajectories. In a climatological study, Pfahl et al. (2014) showed that WCBs in the Mediterranean may be involved in more than 60% of the local, short-duration rainfall extremes (taken as the top percentile of 6-hourly accumulated precipitation values). As a side remark we note that the moisture in the inflow of WCBs is poorly constrained by operational observations (Schäfler et al., 2011; Schäfler and Harnisch, 2015), with potentially significant consequences for WCB-related precipitation and the evolution of the downstream flow.

In order to better understand cyclone-induced extreme rainfall, we investigate the dynamical and thermodynamical characteristics of two deep Mediterranean cyclones. Both cyclones reached their mature stage over the Mediterranean Sea south of the Alps, during the 9-day period from 24 October to 1 November 2012, and were associated with comma-shaped cloud coverage and high rainfall amounts over the western Mediterranean coast (Figure 1). For both cyclones, several stations in southern France and Italy reported daily rainfall of more than 150 mm. Our purpose is to better understand the qualitative characteristics of cyclone-related convection and its connection with extreme rainfall in the region. Furthermore, we aim to analyse the role of the WCBs for the rainfall. For this reason, we use a plethora of observations from the first Special Observations Period (SOP1: Ducrocq et al., 2014) of the HyMeX project (Hydrological cycle in the Mediterranean Experiment: Drobinski et al., 2014), and also operational analyses from the European Centre for Medium-range Weather Forecasts (ECMWF). More specifically, this study will address the following questions:

1. Are both processes of WCBs and deep convection present during the formation of heavy rainfall in the two investigated Mediterranean cyclones?

2. What (thermo-)dynamical conditions are mainly associated with WCBs and deep convection during the development of the cyclones?

3. How do special observations from the HyMeX SOP1 document these conditions?

## 2. Observations and datasets

### 2.1. Rainfall

Rainfall observations are taken from numerous surface stations along the northern coasts of the Mediterranean. Moreover we use rainfall estimates provided by the 3B42 algorithm, which takes into account the Tropical Rainfall Measuring Mission (TRMM) measurements as well as observations by other satellite platforms. This algorithm provides rainfall rate estimates (in mm h⁻¹) every 3 h on a regular grid with 0.25° horizontal resolution for the regions south of 50°N (Huffman et al., 2007; Romilly and Gebremichael, 2011). A statistical verification of 3B42 precipitation estimates over the central and eastern Mediterranean against rain-gauge data performed by Katsanos et al. (2004) has shown almost unbiased results for the low and medium precipitation thresholds, especially during the wet period of the year. However, when considering accumulated rainfall of more than 40 mm (12 h)⁻¹, the relative error of 3B42 against rain-gauge data can be as large as 80%.

### 2.2. Deep convection diagnostics

In order to diagnose areas of deep convection, we use brightness temperature measurements from the Microwave Humidity Sounder (MHS) radiometer, which collects data at five microwave frequencies on board the MetOp-A and B, and National Oceanic and Atmospheric Administration NOAA-18 and 19 polar-orbiting satellites. Here we follow the methodology of Funatsu et al. (2008) and interpolate measurements from the five channels without any smoothing operation to a regular latitude–longitude grid with 0.2° resolution over the broader Mediterranean region. Deep convection diagnostics are provided through brightness temperature thresholds applied to the difference of measurements between channels 3, 4 and 5, which measure in the water vapour absorption band around 183 GHz. Deep convection areas are detected whenever B3m5 (i.e. MHS channels 3 minus 5), B4m5 and B3m4 are simultaneously equal to or larger than 0. By comparing with TRMM 3B42, it was shown that in 70–80% of the cases when deep convection was detected, TRMM observed precipitation, and that in more than 50% of the cases accumulated rainfall amounted at least to 20 mm (3 h)⁻¹ (Funatsu et al., 2007). In addition, we use the Meteosat Second Generation (MSG) rectified (level 1.5) Meteosat Spinning Enhanced Visible and InfraRed Imager (SEVIRI) rapid scan image data. The baseline scan region covers the whole Mediterranean region with measurements every 5 min. The SEVIRI satellite observations used here cover the infrared and the visible spectrum. The infrared spectrum provides cloud-top temperatures, where colder values denote high clouds and are thus indicative of deep convection.

Finally, we use observations of lightning occurrences as detected by the ZEUS system, which is operated by the National Observatory of Athens. ZEUS is a long-range lightning detection system based on a network of six receivers deployed in Europe, providing a very good coverage of Europe and the Mediterranean. Lagouvardos et al. (2009) compared the performance of ZEUS against a dense network of LINET lightning sensors over Germany and found that the mean location error of ZEUS is ~6.8 km. They also showed that ZEUS data can be used as a good indicator of lightning activity due to their high storm-detection efficiency. Further details on the ZEUS system are given in Kotroni and Lagouvardos (2008) and Lagouvardos et al. (2009).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Visible satellite images of cyclones (a) C1 at 1200 UTC 26 October 2012 and (b) C2 at 1200 UTC 31 October 2012. Coloured dots represent 24 h accumulated rainfall measured by the HyMeX surface stations.</figcaption></figure>

## 2.3. WCB diagnostics

In order to associate observations of rainfall and convection with WCBs, we consider WCBs identified with the method outlined in Madonna *et al.* (2014) based upon trajectories calculated with ECMWF operational analyses. WCBs are defined here as trajectories ascending at least 500 hPa in 48 h in the vicinity of a cyclone. This threshold is lower than the 600 hPa used by Madonna *et al.* (2014), but useful for the purpose of this study and consistent with the study by Flaounas *et al.* (2015), who detected WCBs for the 200 most intense Mediterranean cyclones in the period 1989–2008. Physical quantities (specific and relative humidity, liquid and ice water content, temperature, and potential vorticity (PV) are tracked along the identified WCB trajectories).

## 2.4. Airborne observations

Three aircraft participated in the HyMeX SOP1: the French ATR-42, the French Falcon-20 and the German DO128. The ATR-42 and Falcon-20, operated by Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE), were based in Montpellier (at 3.9°E/43.6°N).

The primary mission of the ATR-42 was to characterize the water vapour and aerosol inflow feeding heavy precipitating systems. Its main payload consisted of the airborne differential absorption lidar LEANDRE 2, capable of profiling the water vapour mixing ratio above or beneath the aircraft. Details concerning the design of LEANDRE 2 and the standard lidar signal processing are given in Bruneau *et al.* (2001a, 2001b). The system includes a tunable laser whose emission is tuned upon a water vapour absorption line selected from two rotation-vibration bands in the near infrared (727–770 nm). The system allows the performing of water vapour mixing-ratio measurements with a precision ranging from less than 0.1 g kg⁻¹ at 4.5 km above sea level to less than 0.4 g kg⁻¹ near the surface for a vertical resolution of 150 m and accumulation of 100 individual profiles. For an ATR-42 flight speed of 100 m s⁻¹, this corresponds to an along-track resolution of nearly 1 km. Systematic errors associated with the Centre Nationale de la Recherche Scientifique (CNRS) Differential Absorption Lidar (DIAL) system are described in Bruneau *et al.* (2001b) and typically do not exceed 0.1 g kg⁻¹. On 25–26 October, the ATR-42 performed a survey of moist inflow towards southern France over the western Mediterranean between the Balearic Islands and Montpellier, along the Spanish coastline (from 2325 UTC 25 October to 0125 UTC 26 October).

LEANDRE 2 was no longer operational after this flight, and did not perform measurements during the 30–31 October period.

The French Falcon-20 was deployed in order to characterize ice microphysical properties in precipitation systems. The state-of-the-art of *in situ* microphysical probes was available on board the Falcon-20, including the stereo probe (2DS) and the particle imaging probe (PIP), measuring particles with a diameter from 25 µm to 2 mm. This *in situ* setting was complemented by the Radar Airborne System Tool for Atmosphere (RASTA) radar, a research multi-beam 95 GHz Doppler radar developed at the Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS). It measures the radar reflectivity (the fraction of power backscattered by the clouds and precipitation) and the Doppler velocity (the velocity at which the cloud particles move along the radar beam). The RASTA radar is part of the radar-lidar project RALI (Protat *et al.*, 2004; Tinel *et al.*, 2005; Delanoë *et al.*, 2013).

Once the cross track, along track and vertical wind fields are retrieved above and below the aircraft, the vertical air velocity is used to quantify the level of turbulence at different height levels beyond the flight altitude, and to characterize the updraught/downdraught structures (intensity and size) associated with the convective cells. The calibrated radar reflectivity and retrieved terminal fall speed are used to derive estimates of the ice water content (IWC), mean volume diameter, and number concentration using the RadOnvar technique (based on Delanoë *et al.*, 2007; Plana-Fattori *et al.*, 2010). The terminal fall velocity of the ice crystals is directly linked to the particle size while the reflectivity is related to the water content.

## 3. Cyclone dynamics and rainfall during SOP1 of HyMeX

In order to investigate the evolution of the main synoptic features during the development of the two cyclones, we use ECMWF analyses and remote sensing observations. Figure 2 shows the wind field and the equivalent potential temperature ($\theta_e$) at 850 hPa, as well as the mean-sea-level pressure daily at 1200 UTC from 25 October to 1 November 2012. To complement the atmospheric dynamics analysis of the two events, Figure 3 shows the 500 hPa geopotential height and the 300 hPa PV and winds at the same times as in Figure 2. Figure 4 presents the 24 h 3B42 rainfall estimates, all deep convection cells developed within a day as observed by the MHS measurements, and the daily lightning activity as observed by ZEUS, for the same period. Finally, again for the whole period, Figure 5 presents WCBs and hydrometeor profiles along the WCB trajectories and Figure 6 maps of averaged
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Mean-sea-level pressure (contour interval 3 hPa, blue for 1000 hPa and less, and red for more than 1000 hPa), equivalent potential temperature (in colour) and horizontal winds (arrows) at 850 hPa at 1200 UTC on (a) 25, (b) 26, (c) 27 and (d) 31 October 2012. All data are from operational ECMWF analyses.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Same as Figure 2, but for PV (in colour) and horizontal wind (arrows) at 300 hPa, as well as geopotential height at 500 hPa (black contours every 5 dam, thick contour for 550 dam).</figcaption></figure>

vertical velocity in the WCBs at the time of ice formation (further discussed in the following section).

### 3.1. *The first cyclone*

On 25 October, the first cyclone (C1, hereafter) was located in the North Atlantic offshore Portugal (Figure 2(a)), alongside a strong PV streamer at 300 hPa with peak values of approximately 6 Potential Vorticity Units (PVU) (Figure 3(a)). In the following two days, C1 intrudes into the western Mediterranean basin

beneath the diffluent area of a continuously deepening 500 hPa trough (Figure 3(b)). On 26 October, C1 intensifies as indicated by the deepening of its central pressure, forming a comma-shaped rain band over the western Mediterranean and causing more than 80 mm of rainfall in 24 h over southern France (Figure 1). Eventually C1 reaches its mature stage on 27 October, when ECMWF analyses show a sea-level pressure minimum of about 985 hPa over the leeward side of the Alps (Figure 2(c)). The C1 mature stage is characterized by the formation of a strong PV streamer to the northeast of its centre (Figure 3(c)), while the
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Same as Figure 2, but for 24 h accumulated precipitation (grey shading) from TRMM, areas of lightning activity (red contours, denoting areas with at least five impacts) from ZEUS, and areas of deep convection (green contours) as diagnosed from the MHS.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 5.** (a–c) WCB trajectories in which cloud ice occurs for the first time at 1200 UTC 26 October (a, cyclone C1), 1200 UTC 27 October (b, cyclone C1), and 1200 UTC 31 October (c, cyclone C2). The black dots denote the cyclone centres at the indicated times. (d–f) The average mixing ratio of different cloud and precipitation particles (in mg kg⁻¹) as a function of pressure along the WCBs shown in (a–c).</figcaption></figure>

decay of C1 is associated with the arrival of the streamer over the cyclone centre on 28 October (Figures 2(d) and 3(d)). This type of co-evolving life cycles of the surface cyclone and the upper-level PV streamer is typical for intense Mediterranean cyclones as shown by Flaounas et al. (2015).

On 26 and 27 October (Figure 2(c)), C1 is associated with an intense low-level circulation that led to a broad northward flow of warm and humid air (i.e. high θc values) over southern France and the central Mediterranean. In several case-studies of extreme rainfall in this region it has been shown that such atmospheric conditions are favourable for the development of heavy precipitation events (e.g. Nuissier et al., 2008; Lagouvardos

et al., 2013). Indeed, 3B42 rain estimates for these days (Figure 4(b,c)) show that heavy precipitation has affected almost the entire western Mediterranean (on 26 October surface stations in southern France measured a maximum of 180 mm in 24 h) associated with intense lightning activity and deep convection (as observed by ZEUS and MHS, respectively). In fact, about 1000 lightning impacts were detected close to the C1 centre on 26 October over southern France.

In order to associate rainfall with WCBs, first we identified all WCB trajectories during the period from 25 October to 1 November. Then, for each WCB trajectory we determined the time and location when ice formed for the first time in the WCB
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Same dates as shown in Figure 2, but here for the average vertical velocity of the WCB trajectories, calculated at the time and location where cloud ice occurs for the first time along the trajectory (in colour), daily averaged lightning activity of more than five impacts (black contour, same as the red contours in Figure 4) and 2 PVU contour on 300 hPa at 1200 UTC of the day (green contours).</figcaption></figure>

air parcel. This criterion is useful to capture the time when the WCBs (i) start ascending to higher tropospheric levels, (ii) are associated with mixed-phase clouds, and hence (iii) contribute to intense precipitation. Eventually, all WCB trajectories form ice (due to the strong ascent) but typically the onset of ice formation is expected to occur when air masses are strongly cooled due to relatively fast ascent at about 700 hPa (see Fig. 7c in Madonna et al. (2014)). Figure 5(a,b) show all WCB trajectories, irrespective of their starting time, along which ice first formed at 1200 UTC on 26 and 27 October, respectively. Evidently, the WCBs ascend over the Mediterranean Sea, close to the cyclone centre. Figure 5(d,e) show that this ascent causes an increase of the WCBs' cloud water and rain content. To investigate further the connection between WCBs and deep convection, Figure 6 shows the daily spatial coverage of ascending air parcels that correspond to WCBs. During the intensification of C1 (Figure 6(b,c)), it appears that there is a partial overlapping between the areas affected by deep convection and the WCBs. For instance, on 26 October (Figure 6(b)), deep convection and intense lightning activity match the regions of fast ascending WCB trajectories. On the other hand, on 27 October (Figure 6(c)), the western Mediterranean Sea presents no WCBs, although deep convection is present. It is likely that extreme rainfall in C1 occurs due to the combined effect of local deep convection and of large-scale slantwise ascent piloted by the WCBs.

## 3.2. *The second cyclone*

The pressure centre of the second cyclone (C2, hereafter) deepens faster than C1, from about 1005 hPa on 30 October (when the cyclone is located over Portugal as a weak pressure perturbation) to approximately 985 hPa on 31 October (when the cyclone has moved over the western Mediterranean Sea, Figure 2(d)). While C1 and C2 exhibit similar minimum sea-level pressure values in their mature stage, the intensification of C2 takes place under different large-scale conditions. Indeed, on 31 October the C2 centre is associated with a weak 500 hPa geopotential cut-off low (Figure 3(d), at 3°E/40°N) and a weak 300 hPa PV streamer not

exceeding 4 PVU (Figure 3(d)). The cyclone C2 reaches its deepest central pressure on 31 October, in parallel with the intensification of a much stronger North Atlantic cyclone, located north of the United Kingdom (Figure 2(d) at 0°W/60°N). As a result, the mature stage of C2 appears as a secondary low that is embedded within the greater North Atlantic cyclone (Figure 2(d)). In the presence of such a strong cyclone, it comes as no surprise that the upper tropospheric geopotential and PV fields are dominated by a strong 300 hPa PV tongue over the eastern North Atlantic, evident at 8°W/55°N on 31 October (Figure 3(d)). This PV tongue is however located far enough from the Mediterranean so that it does not absorb the C2-related weak PV streamer on 31 October (C2 mature stage).

In the lower troposphere, the mature stage of C2 is associated with tighter closed sea-level pressure contours than C1, causing a narrower region with intense northward advection of high $\theta_e$ values (Figure 2(d)). As a result, C2 is associated with a strong zonal temperature gradient and consequently with a clearer warm front. Under these conditions, C2 presents a clear comma-shaped cloud coverage, which is characteristic for extratropical cyclones with an intense warm front and which caused more than 80 mm of daily rainfall over the Italian west coast (Figure 1). Figure 4 shows that on 31 October, rainfall mainly affects the southern coast of France and Italy, when the C2 cyclone reaches its lowest core pressure (Figures 2(d) and 4(d)). Similar to the C1 case, the heavy rainfall affects a large area of the western and central Mediterranean. However, lightning and MSG observations show limited lightning activity and very few deep convection cells. Clearly the absence of deep convection suggests the existence of large-scale ascent causing heavy stratiform rainfall (instead of deep convection). Indeed, WCBs are shown to be present over the centre of C2 (Figure 5(c)), matching the areas affected by rainfall (Figure 4(d)).

Compared to C1, C2 presents about 10 times more trajectories that correspond to WCBs (ECMWF analysis air parcel trajectories that ascend more than 500 hPa in 48 h). In addition, the WCBs related to C2 present stronger vertical motions that may exceed -40 hPa h⁻¹, compared to C1 where the WCB ascent was shown to be less than -24 hPa h⁻¹ (Figure 6(b,c) compared
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Q-vector convergence (in colour) at 700 hPa, mean-sea-level pressure (black contours with interval increments of 3 hPa; thick contour for 1000 hPa) and WCB intersection points with 700 hPa (red crosses) at 1200 UTC on (a) 26, (b) 27 and (c) 31 October 2012. Black dots denote the cyclone centres defined as locations with minimum sea-level pressure.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Vertical cross-sections along the segments A–B (dashed lines in Figure 7(a)–(c)) of θₑ (in colour), θₑ* (red contours) and PV (black contours, starting from 2 PVU with 2 PVU increments) at 1200 UTC on (a) 26, (b) 27 and (c) 31 October. Vertical black line denotes the cyclone centre.</figcaption></figure>

to Figure 6(d)). Moreover, the WCBs associated with C2 have higher values of liquid and ice water content and rain within the layer from 900 to 600 hPa. This suggests the formation of mixed-phase clouds in the lower troposphere that are very effective in producing surface precipitation. In the upper troposphere, from 500 to 300 hPa, both the C1 and C2 WCBs have equally high values of snow and ice content, indicative of cirrus-like clouds with cold cloud tops (Spichtinger *et al.*, 2005).

# 4. The physical relation between atmospheric dynamics and extreme rainfall

## 4.1. Cyclone dynamics related to extreme rainfall

In Figures 2 and 6 we showed that C2 is associated with a stronger warm front and a more intense WCB. Consequently, C1 and C2 are also associated with different quasi-geostrophic (QG) forcing of ascent. For a more quantitative understanding of the QG-forced ascent, we introduce in Figure 7(a)–(c) the convergence of the Q-vectors at 700 hPa, the PV streamers at 300 hPa and the sea-level pressure for cyclones C1 (at 1200 UTC on 26 and 27 October) and C2 (at 12 UTC 31 October). Therefore, the Q-vectors’ convergence (divergence) reveals the regions where ageostrophic circulation will compensate temperature modifications by provoking ascent (descent) of air masses in order to cool (warm) the atmosphere (Hoskins *et al.*, 1978). Using thus the Q-vector convergence as a proxy for QG-forced vertical motions of air masses, it is evident that ascent is favoured close to the centre of C2 (Figure 7(c)) and within a larger area, when compared to C1 (Figure 7(a,b)). Areas of favoured ascent, as

the ones shown around C2, are characteristic for comma clouds associated with extratropical cyclones (Martin, 1999; Rauber *et al.*, 2014).

At upper levels, we showed that C1 is associated with a stronger and clearer PV streamer than C2 (compare Figure 3(c) and (d)). To gain further insight on the streamers’ differences, Figure 8 shows vertical sections of PV across the PV streamers, the cyclone centres and the fronts. The C1-related PV streamer (Figure 8(a,b)) penetrates the 300 hPa level to the northwest of the C1 centre with PV values that exceed 8 PVU (Figure 8(b)). On the other hand, the C2-related PV streamer presents weaker values at the 300 hPa level, but the 2 PVU surface reaches down to lower levels, to approximately 700 hPa (Figure 8(c)) over the C2 centre. The high PV values in Figure 8(c) in the middle and upper troposphere are characterized by relative humidity of less than 30% (not shown), suggesting that the high PV values are likely due to the deep penetration of stratospheric air into the middle troposphere.

To further analyse the thermodynamic profiles of the two cyclones and their connection with heavy rainfall, Figure 8(a)–(c) present θₑ and θₑ for a saturated atmosphere (θₑ*). Negative vertical gradients of θₑ denote potentially unstable atmospheric layers, while negative vertical gradients of θₑ* denote atmospheric levels where air masses are conditionally unstable. Conditional instability is strong close to the centre of C1, where the vertical gradient of θₑ* is strongly negative within a thick layer between 1000 and 700 hPa. Further analysis shows that these conditionally unstable regions, close to the C1 centre, are indeed collocated with deep convection and intense lightning activity shown in Figure 4(c), suggesting that the C1 extreme rainfall is related to strong in-column atmospheric instabilities. Conditional
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Cloud-top temperatures from SEVIRI satellite at (a,b) 0750 and 1340 UTC 26 October and (c,d) 1000 and 1300 UTC 31 October 2012. Black lines represent flight tracks of Falcon-20 around the time of the satellite image. Circles and squares denote the position of the beginning and end of the flight track, respectively. Red crosses represent the lightning impacts detected within ±5 min of the time of the satellite images.</figcaption></figure>

instability is also observed close to the centre of C2, but in contrast to C1, it is limited within a narrow layer between 1000 and 900 hPa. On the other hand, C2 presents a well-marked layer that is potentially unstable. Indeed, Figure 8(c) presents a potentially unstable θe layer with values of approximately 310 K located between 1000 and 800 hPa (i.e. between the cyclone centre and point B in Figure 8(c)) over the cyclone warm front. Below 900 hPa, θe values are as high as 340 K. Such a layer is potentially unstable and, induced by strong ascent, rainfall will likely result. It is thus plausible that the strong QG forcing for ascent in C2 (Figure 7(c)) induces large-scale ascent over the C2 warm front, in the form of WCBs, and consequently the extreme rainfall that was observed over the western Mediterranean.

## 4.2. Cloud properties and water content of the two cyclones

In this section we analyse the airborne observations of the cloud properties of the two cyclones. Figure 9 shows the four Falcon-20 flight tracks on 26 October (Figure 9(a,b)) and on 31 October (Figure 9(c,d)), together with the SEVIRI infrared imagery and lightning impacts. Infrared images provide cloud-top temperatures and therefore they may be used as a proxy for the presence, or absence, of deep convection. For all flights, the cyclone-related comma-shaped cloud coverage is evident: over the western Mediterranean Sea for C1 and over Italy for C2. Cloud-top temperatures and lightning detection show large areas

of organized convection close to the centre of C1 (Figure 9(a,b)). Especially on 26 October, the Falcon-20 passed through the convective cells, close to the cyclone centre and close to the regions of intense lightning activity. On the other hand, C2 cloud-top temperatures are more homogeneous in the comma-shaped cloud, with about 20 °C warmer values than for C1. The warmer cloud-top temperatures are consistent with the absence of deep convection in C2. The airborne radar observations were taken in the comma-shaped cloud and within the regions of the diagnosed WCBs.

To gain further insight into the physical and microphysical properties of the clouds, Figure 10 presents the RASTA radar reflectivity observations for the four Falcon-20 tracks. Differences are striking when comparing the cloud structure of the two cyclones. In the case of C1 (Figure 10(a,b)), cloud tops reach approximately 12 km and reflectivity exceeds 20 dBz, while for C2, the cloud tops rarely overshoot 8 km in altitude and reflectivity is lower (by approximately 5–10 dBz). Figure 11 presents the RASTA measurements of IWC and vertical wind speed along the flight tracks. To ease the comparison of the cloud observations between the four flights, Figure 11 presents composite scatterplots of the measurements versus the altitude. The observations show distinct cloud formations for C1 and C2. IWC in C1 has values between 0.5 and 1 g m⁻³ fairly mixed along the atmospheric column. In contrast, the shallower clouds of C2 have lower values of IWC that do not exceed 0.5 g m⁻³. Different patterns are also
<figure><img src="image_12.png" /><figcaption>**Figure 10.** (a–d) Time series (UTC) of RASTA radar reflectivity (in colour) and flight altitude (thick black line) along the flight tracks shown in Figure 9(a,b) on 26 October, and Figure 9(c,d) on 31 October, respectively.</figcaption></figure>

observed in the scatterplots of vertical wind speed. C1 presents a large variability of observed vertical velocities, regardless of altitude, which clearly indicates deep convection characterized by strong updraughts and downdraughts which might exceed 1 m s⁻¹. On the other hand, the comma-shaped cloud in C2 is characterized by lower and mainly upward-directed vertical velocities of less than 0.5 m s⁻¹. Such speeds are consistent with the slantwise ascent of WCBs.

To better understand the mechanism that determined the evolution of water vapour in WCBs, Figure 12(a) shows the WCBs' water vapour mixing ratio at 850 hPa at 0000 UTC 26 October (corresponding to an altitude of approximately 1500 m above sea level (asl)). Around this time the LEANDRE 2 measurements of water vapour mixing ratio from the ATR-42 capture a strong moisture ascent towards higher atmospheric levels (see the structure of the moisture field between the two vertical lines in Figure 12(b)). Indeed, within approximately 20 min of the ATR-42 flight, air masses with a water vapour mixing ratio in excess of 10 g kg⁻¹ are observed to rise from approximately 500 to 2000 m asl (Figure 12(b)). This sloping structure of the water vapour field co-occurs with the increasing height of the cloud tops as detected by the lidar between the southern and northern parts of the flight track, as observed previously by Schäfler *et al.* (2011)

for a lucky WCB encounter during the European THORPEX (THe Observing system Research and Predictability EXperiment) Regional Campaign (ETReC 2007). The typical average air parcel's ascent in WCBs is about −10 to −15 hPa h⁻¹ (Madonna *et al.*, 2014). Given the ATR-42's speed (~10 m s⁻¹), the airborne lidar measurements shown in Figure 12(b) capture different stages of ascending motions associated with air parcels within the WCB, rather than following the ascent of the WCB in a Lagrangian sense. Nevertheless, the balloon sounding launched from Palma de Majorca at 0000 UTC 26 October reveals that the flow below 700 hPa is south-southeasterly (roughly along the aircraft flight track) and the horizontal wind speed is about 5 m s⁻¹. Hence, in a Lagrangian framework, the vertical difference of the top of the moist layer observed by the lidar (1500 m over a distance of 150 km) leads to an estimate of the vertical motion of −18 ± 4 hPa h⁻¹ for a wind speed of 5 ± 1 m s⁻¹, which is comparable to the vertical motions derived from the ECMWF analyses. This analysis shows that the lidar-derived water vapour observations capture the rise of the moist air masses as well as the formation of clouds along the ascending parcels of the WCB, which eventually extend to higher levels than shown in Figure 12(b) (because the lidar measurements are limited to the lower 3000 m of the atmosphere).
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Composite vertical profiles of RASTA measurements of ice water content (top panels) and vertical wind speed (bottom panels) for the clouds shown in Figure 10. Letters a, b, c and d in the panel titles correspond to the flight tracks and reflectivity measurements shown in the corresponding panels of Figures 9 and 10, respectively. For each panel, the composite vertical profile are represented in the form of a probability density function, where the colours indicate the relative frequency, normalized with respect to the maximum value in each panel.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 12.** (a) Water vapour mixing ratio (g kg⁻¹) of WCB trajectories at 850 hPa at 0000 UTC 26 October. The black line represents part of the flight track of ATR-42, starting at 2330 UTC 25 October (circle) and ending at 0100 UTC 26 October (in square). (b) Water vapour measurements from LEANDRE 2 along the ATR-42 flight track. Vertical black lines delimit the part of the flight track shown as the thick black line in (a).</figcaption></figure>

5. Concluding remarks

In this article we analysed the (thermo-)dynamics and aspects
of the microphysics of two Mediterranean cyclones that led to
heavy rainfall over the western Mediterranean in autumn 2012.
In fact, both cyclones developed over the same region, within
the same period, shared the same intensity (in terms of central
mean-sea-level pressure) and caused more than 150 mm of daily-
accumulated rainfall. The cyclones occurred during the HyMeX
SOP1, thus providing a great opportunity to analyse the two cases
by exploiting numerous observations not commonly available
for such a study. Our results highlight two different mechanisms
for the production of heavy rainfall. In C1, the heavy rainfall
was mainly associated with deep convection and secondly to
WCBs. In contrast, the C2-induced rainfall was mainly related to
WCBs' ascent while deep convection was quasi-absent. Deeper
analysis revealed different (thermo-)dynamical conditions that
were associated with the development of the two cyclones:

- During its reinforcement phase, on 26 and 27 October, C1 was centred over the Alps and was fed by a broad area of warm and moist air advected over the western Mediterranean. Such advection was favoured by the presence of a strong PV streamer that preceded the cyclone displacement. In parallel, C1 exhibited a weak warm front, but high conditional instability over the Mediterranean Sea and within the warm sector. The release of conditional instability caused deep convection, embedded in the relatively weak WCB (in terms of the identified WCB trajectory number and vertical motion) and characterized by intense lightning activity, high IWC, strong updraughts and cold cloud tops that reached the altitude of 12 km. Airborne lidar observations also revealed the intense vertical motion in the region of the C1 WCBs.
- The mature stage of C2 occurred over the Mediterranean Sea and under the influence of a strong North Atlantic cyclone. The presence of a WCB in C2 was associated with a well-defined warm front and strong potential instability in the lowest troposphere. In contrast to C1, the clouds in C2 were not associated with deep convection or with strong convective updraughts, and cloud tops did not reach altitudes higher than 8 km. All observations suggested that rainfall related to C2 was associated with stratiform clouds, 'fuelled' by the humid air masses ascending along the WCB.

While cyclones in the Mediterranean region are known to be among the main weather systems that lead to extreme rainfall, little is found in the literature on observations of the physical processes causing the intense precipitation. In this study we made extensive use of airborne and space-borne observations, providing deeper insight into the physical and microphysical properties of the clouds that were associated with both deep convection and WCBs. Our results will thus be valuable for in-depth validations of atmospheric model simulations of high-impact weather in the region as well as the climatological analysis of Mediterranean cyclone-related processes leading to extreme rainfall.

## Acknowledgements

The HyMeX SOP1 was supported by many French agencies through the large interdisciplinary international programme MISTRALS (Mediterranean Integrated STudies at Regional And Local Scales) dedicated to the understanding of the Mediterranean Basin environmental process (www.mistrals-home.org). Airborne data was obtained using the ATR-42 Environment Research Aircraft operated and managed by Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE), which is a joint entity of CNRS, Météo-France and CNES. The SAFIRE staff are thanked for their support during the SOP1. The authors are grateful to D. Bruneau, P. Genau, C. Merlet, R. Meynadier, T. Deleporte, S. Bastin, C. Kocha, C. Lavaysse (LATMOS) as well as F. Blouzon and A. Abchiche (DT/INSU) for operating the LEANDRE 2 system aboard the ATR-42 during the HyMeX SOP1. The airborne field campaign was sponsored by grants MISTRALS/HyMeX and ANR-11-BS56-0005 IODA-MED. The Microwave Humidity Souder (MHS) was obtained with support from the INSU-CNES French Mixed Service ICARE via Climserv. The ANR MUSIC grant ANR-14-CE01-014 is also acknowledged.

Finally we would like to thank Marty Baxter and an anonymous referee for their constructive reviews that helped improve the overall quality of the article.

## References

Appenzeller C, Davies HC. 1992. Structure of stratospheric intrusions into the troposphere. *Nature* **358**: 570–572.

Browning KA. 1990. Organisation of clouds and precipitation in extratropical cyclones. In *Extratropical Cyclones: The Erik Palmén Memorial Volume*, Newton CW, Holopainen EO (eds.): 129–154. American Meteorological Society: Boston, MA.

Bruneau D, Quaglia P, Flamant C, Meissonnier M, Pelon J. 2001a. The airborne lidar LEANDRE 2 for water vapor profiling in the troposphere. Part I: Description. *Appl. Opt.* **40**: 3450–3461.

Bruneau D, Quaglia P, Flamant C, Pelon J. 2001b. The airborne lidar LEANDRE 2 for water vapor profiling in the troposphere. Part II: First results. *Appl. Opt.* **40**: 3462–3475.

Čampa J, Wernli H. 2012. A PV perspective on the vertical structure of mature midlatitude cyclones in the Northern Hemisphere. *J. Atmos. Sci.* **69**: 725–740.

Claud C, Alhammoud B, Funatsu BM, Chaboureau JP. 2010. Mediterranean hurricanes: Large-scale environment and convective and precipitating areas from satellite microwave observations. *Nat. Hazards Earth Syst. Sci.* **10**: 2199–2213, doi: 10.5194/nhess-10-2199-2010.

Claud C, Alhammoud B, Funatsu BM, Lebeaupin-Brossier C, Chaboureau JP, Béranger K, Drobinski P. 2012. A high resolution climatology of precipitation and deep convection over the Mediterranean region from operational satellite microwave data: Development and application to the

evaluation of model uncertainties. *Nat. Hazards Earth Syst. Sci.* **12**: 785–798, doi: 10.5194/nhess-12-785-2012.

Delanoë J, Protat A, Bouniol D, Heymsfield AJ, Bansemer A, Brown PR. 2007. The characterization of ice cloud properties from Doppler radar measurements. *J. Appl. Meteorol.* **46**: 1682–1698.

Delanoë J, Protat A, Jourdan O, Pelon J, Papazzoni M, Dupuy R, Gayet J-F, Jouan C. 2013. Comparison of airborne *in situ*, airborne radar–lidar, and spaceborne radar–lidar retrievals of polar ice cloud properties sampled during the POLARCAT campaign. *J. Atmos. Oceanic Technol.* **30**: 57–73, doi: 10.1175/JTECH-D-11-00200.1.

Drobinski P, Ducrocq V, Alpert P, Anagnostou E, Béranger K, Borga M, Braud I, Chanbis A, Davolio S, Delrieu G, Estournel C, Filali Boubrahmi N, Font J, Grubisic V, Gualdi S, Homar V, Ivancan-Picek B, Kottmeier C, Kotroni V, Lagouvardos K, Lionello P, Llasat MC, Ludwig W, Lutoff C, Mariotti A, Richard E, Romero R, Rotunno R, Roussot O, Ruin I, Somot S, Taupier-Letage I, Tintore J, Uijlenhoet R, Wernli H. 2014. HyMeX, a 10-year multidisciplinary program on the Mediterranean water cycle. *Bull. Am. Meteorol. Soc.* **95**: 1063–1082, doi: 10.1175/BAMS-D-12-00242.1.

Ducrocq V, Braud I, Davolio S, Ferretti R, Flamant C, Jansa A, Kalthoff N, Richard E, Taupier-Letage I, Ayal PA, Belamari S, Berne A, Borga M, Boudevillain B, Bock O, Boichard JL, Bouin MN, Bousquet O, Bouvier C, Chiggiano J, Cimini D, Corsmeier U, Coppola L, Coquerez P, Defer E, Delanoë J, Di Girolamo P, Doerenbecher A, Drobinski P, Dufournet Y, Fourrié N, Gourley J, Labat L, Lambert D, Le Coz J, Marzano F, Molinié G, Montani A, Nord G, Nuret M, Ramage K, Rison B, Roussot O, Said F, Schwarzenboeck A, Testor P, Van Baelen J, Vincendon B, Aran M, Tamayo J. 2014. HyMeX-SOP1, the field campaign dedicated to heavy precipitation and flash flooding in the northwestern Mediterranean. *Bull. Am. Meteorol. Soc.* **95**: 1083–1100, doi: 10.1175/BAMS-D-12-00244.1.

Emanuel K. 2005. Genesis and maintenance of 'Mediterranean hurricanes'. *Adv. Geosci.* **2**: 217–220, doi: 10.5194/adgeo-2-217-2005.

Fita L, Romero R, Ramis C. 2006. Intercomparison of intense cyclogenesis events over the Mediterranean basin based on baroclinic and diabatic influences. *Adv. Geosci.* **7**: 333–342.

Fita L, Romero R, Ramis C. 2007. Objective quantification of perturbations produced with a piecewise PV inversion technique. *Ann. Geophys.* **25**: 2335–2349.

Flaounas E, Raveh-Rubin S, Wernli H, Drobinski P, Bastin S. 2015. The dynamical structure of intense Mediterranean cyclones. *Clim. Dyn.* **44**: 2411–2427, doi: 10.1007/s00382-014-2330-2.

Funatsu BM, Claud C, Chaboureau JP. 2007. Potential of Advanced Microwave Sounding Unit to identify precipitating systems and associated upper-level features in the Mediterranean region: Case studies. *J. Geophys. Res.* **112**: D17113, doi: 10.1029/2006JD008297.

Funatsu BM, Claud C, Chaboureau JP. 2008. A 6-year AMSU-based climatology of upper-level troughs and associated precipitation distribution in the Mediterranean region. *J. Geophys. Res.* **113**: D15120, doi: 10.1029/2008JD009918.

Hohenegger C, Schär C. 2007. Predictability and error growth dynamics in cloud-resolving models. *J. Atmos. Sci.* **64**: 4467–4478, doi: 10.1175/2007JAS2143.1.

Hoskins BJ, Draghici I, Davies HC. 1978. A new look at the ω-equation. *Q. J. R. Meteorol. Soc.* **104**: 31–38.

Huffman GJ, Bolvin DT, Nelkin EJ, Wolff DB, Adler RF, Gu G, Hong Y, Bowman KP, Stocker EF. 2007. The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-global, multiyear, combined-sensor precipitation estimates at fine scales. *J. Hydrometeorol.* **8**: 38–55.

Jansà A, Genovés A, Picornell MA, Campins J, Riosalido R, Carretero O. 2001. Western Mediterranean cyclones and heavy rain. Part 2: Statistical approach. *Meteorol. Appl.* **8**: 43–56.

Jansà A, Alpert P, Arbogast P, Buzzi A, Ivancan-Picek B, Kotroni V, Llasat Botija MdC, Ramis C, Richard E, Romero R, Speranza A. 2014. MEDEX: A general overview. *Nat. Hazards Earth Syst. Sci.* **14**: 1965–1984.

Katsanos D, Lagouvardos K, Kotroni V, Huffman GJ. 2004. Statistical evaluation of MPA-RT high-resolution precipitation estimates from satellite platforms over the central and eastern Mediterranean. *Geophys. Res. Lett.* **31**: L06116, doi: 10.1029/2003GL019142.

Kotroni V, Lagouvardos K. 2008. Lightning occurrence in relation with elevation, terrain slope and vegetation cover over the Mediterranean. *J. Geophys. Res.* **113**: D21118, doi: 10.1029/2008JD010605.

Kotroni V, Lagouvardos K, Kallos G, Ziakopoulos D. 1999. Severe flooding over central and southern Greece associated with pre-cold frontal orographic lifting. *Q. J. R. Meteorol. Soc.* **125**: 967–991, doi: 10.1002/qj.49712555511.

Lagouvardos K, Kotroni V. 2007. TRMM and lightning observations of a low-pressure system over the eastern Mediterranean. *Bull. Am. Meteorol. Soc.* **88**: 1363–1367.

Lagouvardos K, Lemaitre Y, Scialom G. 1993. Dynamical structure of a wide cold frontal cloudband observed during the FRONTS 87 experiment. *Q. J. R. Meteorol. Soc.* **119**: 1291–1319.

Lagouvardos K, Kotroni V, Dobricic S, Nickovic S, Kallos G. 1996. On the storm of 21–22 October 1994 over Greece: Observations and model results. *J. Geophys. Res.* **101**: 26217–26226, doi: 10.1029/96JD01385.

Lagouvardos K, Kotroni V, Nickovic S, Jovic D, Kallos G. 1999. Observations and model simulations of a winter sub-synoptic vortex over the central Mediterranean. *Meteorol. Appl.* **6**: 371–383.
Lagouvardos K, Kotroni V, Defer E. 2007. The 21–22 January 2004 explosive cyclogenesis over the Aegean Sea: Observations and model analysis. *Q. J. R. Meteorol. Soc.* **133**: 1519–1531.

Lagouvardos K, Kotroni V, Betz HD, Schmidt K. 2009. A comparison of lightning data provided by ZEUS and LINET networks over western Europe. *Nat. Hazards Earth Syst. Sci.* **9**: 1713–1717.

Lagouvardos K, Kotroni V, Defer E, Bousquet O. 2013. Study of a heavy precipitation event over southern France, in the frame of HYMEX project: Observational analysis and model results using assimilation of lightning. *Atmos. Res.* **134**: 45–55.

Llasat MC, Llasat-Botija M, Petrucci O, Pasqua AA, Rosselló J, Vinet F, Boissier L. 2013. Towards a database on societal impact of Mediterranean floods within the framework of the HYMEX project. *Nat. Hazards Earth Syst. Sci.* **13**: 1337–1350, doi: 10.5194/nhess-13-1337-2013.

Llasat-Botija M, Llasat MC, Lopez L. 2007. Natural hazards and the press in the western Mediterranean region. *Adv. Geosci.* **12**: 81–85.

McTaggart-Cowan R, Galarneau TJ, Bosart LF, Milbrandt JA. 2010. Development and tropical transition of an Alpine Lee Cyclone. Part I: Case analysis and evaluation of numerical guidance. *Mon. Weather Rev.* **138**: 2281–2307.

Madonna E, Wernli H, Joos H, Martius O. 2014. Warm conveyor belts in the ERA-Interim dataset (1979–2010). Part I: Climatology and potential vorticity evolution. *J. Clim.* **27**: 3–26.

Martin JE. 1999. Quasigeostrophic forcing of ascent in the occluded sector of cyclones and the trowal airstream. *Mon. Weather Rev.* **127**: 70–88, doi: 10.1175/1520-0493(1999)127<0070:QFOAIT>2.0.CO;2.

Miglietta MM, Laviola S, Malvaldi A, Conte D, Levizzani V, Price C. 2013. Analysis of tropical-like cyclones over the Mediterranean Sea through a combined modeling and satellite approach. *Geophys. Res. Lett.* **40**: 2400–2405, doi: 10.1002/grl.50432.

Nuissier O, Ducrocq V, Ricard D, Lebeaupin C, Anquetin S. 2008. A numerical study of three catastrophic precipitating events over southern France. I: Numerical framework and synoptic ingredients. *Q. J. R. Meteorol. Soc.* **134**: 111–130.

Papagiannaki K, Lagouvardos K, Kotroni V. 2013. A database of high impact weather related incidents in Greece: A descriptive impact analysis for the period 2001–2011. *Nat. Hazards Earth Syst. Sci.* **13**: 727–736.

Pfahl S, Madonna E, Boettcher M, Joos H, Wernli H. 2014. Warm conveyor belts in the ERA-Interim dataset (1979–2010). Part II: Moisture origin and relevance for precipitation. *J. Clim.* **27**: 27–40.

Plana-Fattori A, Protat A, Delanoë J. 2010. Observing ice clouds with a Doppler cloud radar. *C. R. Phys.* **11**: 96–103, doi: 10.1016/j.crhy.2009.11.004.

Protat A, Pelon J, Grand N, Delville P, Laborie P, Vinson J-P, Bouniol D, Bruneau D, Chepfer H, Delanoë J, Haeffelin M, Noël V, Tinel C. 2004. Le projet RALI: Combinaison d'un radar et d'un lidar pour l'étude des nuages faiblement précipitants. *La Météorologie* **47**: 23–33, doi: 10.4267/2042/36076.

Rauber RM, Macomber MK, Plummer DM, Rosenow AA, McFarquhar GM, Jewett BF, Leon D, Keeler JM. 2014. Finescale radar and airmass structure of the comma head of a continental winter cyclone: The role of three airstreams. *Mon. Weather Rev.* **142**: 4207–4229, doi: 10.1175/MWR-D-14-00057.1.

Raveh-Rubin S, Wernli H. 2015. Large-scale wind and precipitation extremes in the Mediterranean: A climatological analysis for 1979–2012. *Q. J. R. Meteorol. Soc.* doi: 10.1002/qj.2531.

Romilly TG, Gebremichael M. 2011. Evaluation of satellite rainfall estimates over Ethiopian river basins. *Hydrol. Earth Syst. Sci.* **15**: 1505–1514, doi: 10.5194/hess-15-1505-2011.

Schäfler A, Harnisch F. 2015. Impact of the inflow moisture on the evolution of a warm conveyor belt. *Q. J. R. Meteorol. Soc.* **141**: 299–310.

Schäfler A, Dörnbrack A, Wernli H, Kiemle C, Pfahl S. 2011. Airborne lidar observations in the inflow region of a warm conveyor belt. *Q. J. R. Meteorol. Soc.* **137**: 1257–1272.

Spichtinger P, Gierens K, Wernli H. 2005. A case study on the formation and evolution of ice supersaturation in the vicinity of a warm conveyor belt's outflow region. *Atmos. Chem. Phys.* **5**: 973–987, doi: 10.5194/acp-5-973-2005.

Thorncroft CD, Hoskins BJ, McIntyre ME. 1993. Two paradigms of baroclinic-wave life-cycle behaviour. *Q. J. R. Meteorol. Soc.* **119**: 17–55, doi: 10.1002/qj.49711950903.

Tinel C, Testud J, Hogan RJ, Protat A, Delanoë J, Bouniol D. 2005. The retrieval of ice cloud properties from cloud radar and lidar synergy. *J. Appl. Meteorol.* **44**: 860–875.

Trigo I. 2006. Climatology and interannual variability of storm-tracks in the Euro-Atlantic sector: A comparison between ERA-40 and NCEP/NCAR reanalyses. *Clim. Dyn.* **26**: 127–143.

Trigo IF, Davies TD, Bigg GR. 1999. Objective climatology of cyclones in the Mediterranean region. *J. Clim.* **12**: 1685–1696, doi: 10.1175/1520-0442(1999)012<1685:OCOCIT>2.0.CO;2.

Tous M, Romero R. 2013. Meteorological environments associated with medicane development. *Int. J. Climatol.* **33**: 1–14, doi: 10.1002/joc.3428.

Ziv B, Saaroni H, Romem M, Heifetz E, Harnik N, Baharad A. 2010. Analysis of conveyor belts in winter Mediterranean cyclones. *Theor. Appl. Climatol.* **99**: 441–455.