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A Mediterranean episode is a meteorological phenomenon specific to the Mediterranean region, characterized by intense thunderstorms and, in particular, heavy convective rainfall. It can even be described as a singular storm sequence, during which a series of more or less severe thunderstorms follow one another over a given area for 12 to 36 hours, with very high daily rainfall totals, often equal to four or six months' rainfall in just 12 or 36 hours. In the most violent episodes, the equivalent of a year's rainfall can even be reached in just 24 h.

Because of its seasonality, frequency and virulence, it can be compared to monsoons and tropical cyclones, since they are regularly observed at the same time of year, with highly variable inter-annual frequency. However, while interannual cycles are clearly identifiable for tropical cyclones and monsoons, this is not at all the case for Mediterranean episodes. In recent years, however, an interesting indicator has been identified: the water temperature in the north-western Mediterranean. The warmer it is, the greater the number and intensity of Mediterranean episodes.

When they mainly affect the relief of the Cevennes, they are more commonly referred to as Cevenol episodes. In fact, this was the term used in the past. However, as the formation mechanisms are similar all along the Mediterranean coast, as in Provence, Roussillon, Corsica, Spanish Catalonia, Italy and North Africa, the overly localized term was gradually replaced by "Mediterranean Episode".

They became particularly famous for the catastrophic consequences of certain episodes in France and Italy. Most of them, however, do not cause disasters, but regularly result in localized flash floods that are often spectacular. They are, however, necessary to replenish soil water in Mediterranean coastal regions.

Regions affected

On the Mediterranean rim, the regions most affected are practically all located around the western basin in autumn, with Italy, southern France, eastern Spain and the Maghreb. In spring, Turkey and Greece are more affected. However, the majority of Mediterranean episodes occur between northeastern Spain and northern Italy, including all French Mediterranean and Rhône departments.

North-western Mediterranean basin

France

Mediterranean episodes hit Gard, Hérault, Ardèche, Lozère and Aude in particular. The rest of France's Mediterranean coastline is less frequently affected than the Cévennes, but cyclically some regions outside the Cévennes can be affected for several consecutive years. The record is the Côte d'Azur, which was particularly hard hit by a series of deadly episodes between 2010 and 2019. Tarn, Aveyron and Drôme are also more or less regularly affected, as these Mediterranean episodes spill over onto the Languedoc coast.

This table, based on data supplied by Météo-France, shows the total number of days between 1958 and 2017 on which rainfall of 120, 160, 200 and 300 mm or more in 24 hours was recorded. The 24-hour rainfall records observed are also for the period 1958-2017. However, these rainfall totals are daily totals recorded between 6 a.m. and 6 a.m., and not between the wettest 24 hours of the episode. It is therefore often the case that these days do not record the entire episode, straddling two climatological days.

The further away visitors are from the Mediterranean arc, the less frequent are intense waves, even during summer thunderstorms. The Cevennes departments are once again in the top three, with the most intense waves and the shortest return periods, while the northern Alpine departments are the least often affected by heavy rain, despite their more accentuated relief:

Italy

The Italian provinces most affected are those on the country's west coast, including Liguria, Tuscany, Piedmont, Lazio, Campania, Basilicata and Calabria. Sardinia and Sicily are less affected, but are occasionally hit by Mediterranean tropical-like cyclone (also known as medicanes). Like France, Italy has major problems with urbanization in flood-prone areas, and regularly suffers hydrological disasters as a result of these Mediterranean episodes, such as the one in November 1994 in Piedmont. ^[4], which killed 70 people, or the one in November 1966, which led to exceptional flooding of the Arno, devastating Florence's historic heritage. The north-east of the country also suffers from torrential rains, as in the historic flooding of the Po in November 1951, or in November 2010 in the Veneto region ^[5].

Spain

The Spanish provinces most affected are Catalonia and Andalusia, as well as Murcia and Valencia. Remember the Aiguat of 1940, which claimed over 300 lives in Catalonia, with rainfall totals approaching 900 mm in 24 hours on the French side. More recently, a dozen people died in Andalusia during heavy rains in autumn 2013. In early December 2016, Murcia also suffered torrential flooding, killing two people. The deadliest episode in Spanish history, along with the Aiguat of 1940, was the Great Flood of Valencia (1957). With cumulative rainfall reaching 361 mm in 24 hours at Bejís on October 13, 1957, and a further 100 mm on the 14th, the Turia, the river that flows through Valencia at its mouth, produced an exceptional flood that devastated the city. Several dozen people died.

Rest of the Mediterranean basin

Maghreb

The Maghreb region is regularly affected by episodes of heavy rainfall, when flows turn north in autumn. The rivers in this region have a wadi-like regime, similar to that of the Cevennes, but more extreme. In the semi-desert climate, most of these rivers are dry all year round, but during the wet season, in autumn in this case, their dried-up beds prevent any rain from infiltrating, resulting in devastating and deadly flash floods. From November 9 to 11, 2001, during the catastrophic flooding of Bab El Oued in Algeria ^[6], around 800 people died and dozens were reported missing. This deadly catastrophe came as a shock to the local community ^[7], and the Algerian state bore a heavy responsibility for the lack of flood risk prevention.

Turkey

Turkey is also regularly affected by very wet autumn storms, which regularly cause torrential flooding. This is a period when warm, humid air from the Mediterranean and Black Sea rises over the coastal mountainous reliefs, generating heavy precipitation in a matter of hours through orographic forcing. High levels of soil sealing due to uncontrolled urbanization are also a factor in these deadly floods. Between September 8 and 9, 2009, heavy torrential rains hit Istanbul, killing around forty people ^[8].

Mediterranean episodes

Trigger conditions

The table below shows the total number of episodes over the period 1958-2016, in which at least 120 mm was recorded in 24 h per département. The majority of these episodes occur between September and January, and even as late as March for some Cevennes departments and Haute-Corse. The peak is often between September and November, depending on the region. Only the Loire and Isère departments experience more heavy rainfall during the summer period (May to August) than during Mediterranean episodes (September to January), which is explained by the importance of summer thunderstorm sequences, which are more frequent than Mediterranean episodes.

It's often in autumn that these phenomena are triggered by the climatic configuration, with Mediterranean surface water temperatures at their highest, the first descents of air masses from the poles, the maintenance of tropical air masses in the Mediterranean, and the arrival of the first winter storms from the Atlantic. The combination of all these factors makes this the most unstable season on the Mediterranean coast, where atmospheric configurations are the most active and can degenerate very quickly. However, for a Mediterranean episode to be triggered, a cold drop must arrive on the Iberian Peninsula. A cold drop is the isolation of a west-to-east low-pressure system, a few hundred km in diameter, moving further south than the polar vortex. When this occurs in autumn or spring, or even winter, between the Bay of Biscay and the north of the Western Mediterranean Basin, it tilts the air flow towards the south of France, propelling a tropical air mass over the region.

This air mass, which crosses the Mediterranean, takes on moisture and closes in on the main mountain ranges of southern France (Pyrenees, Montagne Noire, Cévennes, Corsica, Alpine foothills). At the same time, the tropical air mass is lifted by altitude forcing, further destabilizing the air mass and triggering thunderstorms. Relief, which paralyzes the movement of thunderstorms, and south-flowing air, lead to a permanent regeneration of thunderstorms in moisture over a given area. These retrograde multicellular thunderstorms, constantly regenerating as long as the flows at their origins persist, are major producers of heavy precipitation in a very short space of time. Their apparent immobility is in fact false, since thunderstorms form one after the other, following one another for several hours at a time in the same place.

Observed phenomena and consequences

Heavy rain and runoff

Heavy rainfall generally lasts in the same place for an average of 24 to 36 hours, but can sometimes last for more than 72 hours. While rainfall totals over the whole episode can be spectacular, it is above all the highest hourly intensities that cause the most violent flooding. On October 3, 2015 in Cannes, only 200 mm were recorded in 24 hours, making it a classic Mediterranean episode. However, almost all the cumulative rainfall occurred in 2 hours, and such hourly intensities are exceptional ^[9].

The larger the surface area that receives 100 mm, 200 mm or even more, the greater the mass of water collected. 1 mm of rain represents 1 L of water per m², or 0.001m³ per m². So if a 1,000 Km² catchment area receives 100 mm of rain over 70% of its surface area, and 200 mm over 30% of its surface area, no less than 100 million m³ of water will have fallen. Here are three striking examples of such a mass of water between 2002 and 2010:

• On September 8 and 9, 2002, at least 200 mm of rain fell over a surface area of 5,301 km² in the Gard (out of a total surface area of 5,853 km²), resulting in a water mass of 1.9 billion m^{3 [10]}.
• From December 1 to 4, 2003, with 100 mm of rainfall over an area of 28,126 km² in the Rhône valley (a total surface area of 95,500 km²), no less than 4.9 billion m³ of water fell ^[11].
• On June 15, 2010, over an area of 1,621 km² in the Var (a total surface area of 5,973 km²), at least 200 mm of rain was recorded, giving a total of 425 million m^{3 [12]}

Such a mass of water in such a short space of time (episodes often lasting between 12 and 36 hours) inevitably has major, even catastrophic, hydrological consequences. To get an idea of the amount of water flowing, compare it to the reservoir of Europe's largest dam, the Serre-Ponçon dam (Hautes-Alpes), which can hold a maximum of 1.3 billion m^{3 of} water. In addition to these enormous quantities of rainfall, it is important to take into account the water saturation of the soil, which, if it is very high, can no longer absorb the slightest drop of water, and conversely, if this saturation is very low, the very dry soil does not allow any water to penetrate, causing it to run off in large quantities in both cases. However, if in one case the rivers are able to absorb the very high mass of water due to their high low-water levels (as in November 2011 in the Cévennes), in the other case, the rivers are already at their highest, and can no longer withstand the slightest additional inflow, resulting in massive flooding (as in December 2003 with the Rhône).

These episodes of heavy rainfall are particularly problematic, since the intensity of the rainfall is such that neither the land nor the rainwater system can absorb the floodwaters, resulting in major torrential flooding, exacerbated by soil sealing. Urban areas on hillsides are particularly prone to this problem, with spectacular examples such as the Nîmes disaster on October 3, 1988, the September 2000 episode in Marseille, and the Cannes disaster on October 3, 2015. Heavy rainfall is also a problem in lowland areas, due to accumulation or stagnation of precipitation on naturally water-saturated soils, such as those of a delta or marshy river plain. On September 22, 2003, Arles, located on the marshy Rhône plain, saw its road and rainwater network saturated by torrential rains, which broke a record that day, with no less than 265 mm of rain recorded in just 24 hours, most of it in less than 10 hours. Many motorists were caught by surprise on the roads, and dozens of cars were abandoned on the drowned road network. A less intense episode hit Arles in November 2011, again bringing part of the road network to a standstill, and leading to the evacuation of seventy motorists by boat ^[13]. In 2009, Keraunos developed an R scale to measure the intensity of convective waves in France ^[14].

Electrical activity

Electrical activity is particularly high under V-shaped thunderstorms. On September 8, 2002, the Gard region was hit by the most lightning in 20 years, following the blocking of a V-shaped thunderstorm over the southern Cévennes, ahead of September 7, 2010, also during an intense Mediterranean episode ^[15]. Particularly intense electrical activity was observable from over a hundred kilometers away, with reports of it recorded in Béziers (Hérault), Mende (Lozère), Carpentras (Vaucluse), and even Salon-de-Provence (Bouches-du-Rhône), where witnesses spoke of glimmers on the horizon without suspecting their true nature. On September 8, 2002, lightning struck the control tower at Marseille-Marignane airport, paralyzing air traffic for over an hour ^[16]. Three people were also struck by lightning in Vaucluse.

This intense electrical activity can be explained by the number of thunderstorm cells that followed one another throughout the duration of the mesoscale convective system. The polarity of lightning strikes evolves throughout its lifetime, as Goodman demonstrated in 1986. He determined a conceptual model that is generally observed, although in the field, when several MCS interact with each other, electrical activity will behave differently. MCSs have three electrical phases during their life cycle ^[17]:

• the formation phase: each MCS cell has its own single-cell electrical signature. When the cells begin to organize, there is a sharp rise in the number of negative impacts, with a very low number of positive impacts, all located in the zone of strong upward currents.
• Mature phase: the number of negative impacts decreases, as cells become organized. An increase in the number of positive impacts is observed in the stratiform trail of the MCS, as it increases in size. A few negative impacts also appear in this zone.
• dissipation phase: during this phase, thunderstorm cells are no longer renewed, and the number of positive impacts exceeds the number of negative impacts. The rate of decrease in the number of negative impacts remains lower than that of growth observed during the formation period.

It should be remembered that positive impacts are much more powerful and destructive than negative impacts, due to their more damaging polarity for power grids. The electrical activity observed in real time also makes it easier to pinpoint the location of the MCSs, and thus to identify the areas with the greatest precipitation, which are always immediately behind the area most affected by lightning. This very high level of electrical activity, typical of multicellular thunderstorm organizations, also makes it possible to follow the chronology of the episode, and to predict how it will evolve.

Wind phenomena

Mediterranean storms cause considerable damage to coastlines, as in November 2011 on the Var coast, or in November 2008 on Corsica. These stormy winds are accompanied by heavy swells, which cause major damage to coastal infrastructures through the mechanical effects of the waves. In addition, a surge is often observed, adding to the swell and hindering the flow of flooded rivers at their mouths, exacerbating the effects of flooding on coastal plains. In December 2013, during the passage of Cyclone Dirk, a major Mediterranean episode hit the Côte d'Azur, accompanied by a marine surge, partially drowning Nice-Côte d'Azur airport and severely disrupting air traffic in the region at the height of the Christmas season ^[18]. In November 1999, high winds and swell caused three cargo ships in the Aude to break their moorings and run aground on the beaches of Port-la-Nouvelle.

The instability at the heart of Mediterranean episodes generates not only stationary thunderstorms, but also highly mobile thunderstorm organizations, which sometimes tend to provoke convective wind phenomena. During the November 2016 episode, downbursts accompanying a tornado were recorded in the Tarn and Hérault regions ^[19]. However, these are the phenomena least frequently observed during a Mediterranean episode. Convective gusts are much more frequent during summer thunderstorms, as in the summer of 2014, when a derecho was observed in Languedoc-Roussillon ^[20].

Tornadoes

Mediterranean episodes regularly generate tornadoes on the coast and even in the coastal hinterland. The latest example of a major tornado in the South-East, linked to a Mediterranean episode, is the EF2 tornado that hit Pont-de-Crau, in Arles in October 2019, damaging more than 190 buildings, including a dozen rendered uninhabitable, and injuring 6 people, one of whom is temporarily in a coma. The most intense case linked to the passage of a Mediterranean episode was that of October 22, 1844 in Sète, Hérault, with an EF4 tornado, which killed 20 people ^[21]. This is one of the three deadliest tornadoes to have occurred in France since the nineteenth century.

All the cases presented are available in detail on the website of the French Observatory of Violent Storms and Tornadoes, Keraunos ^[22]

Other phenomena

Hail

The mobile thunderstorms that accompany Mediterranean episodes sometimes produce hail. Less frequently observed than tornadoes, these hailstorms can prove just as formidable. Among the most notable cases was that of June 26, 1994, when pigeon-sized hailstones were observed after a localized V-shaped thunderstorm stalled for 2 h over the Loup valley in the Alpes-Maritimes. ^[23]. On September 19, 2014, a supercellular V-shaped thunderstorm produced hailstones 3 to 5 cm in diameter over the southern Var region ^[24].

Snowy weather

In winter, under certain conditions, a Mediterranean episode can produce heavy snow, paralyzing these unaccustomed regions in a matter of hours ^[25]. In March 2010, 10 cm of snow fell in Sainte-Marie-de-la-Mer, 20 cm in Arles and Avignon, 30 cm in Nîmes and the surrounding area, and 40 cm in the Alpilles. But the most remarkable episode in this area was that of January 30 and 31, 1986 ^{[26] [27]}, which affected the whole of southern France. Rainfall totals of 100 to 150 mm over 36 hours were recorded ^[26], while snowfall totals exceeded 25 cm over large parts of the Aude, Pyrénées-Orientales and Lozère regions, and 200 cm in the mountains. The Côte d'Azur and Provence were hit by a full-blown storm, with wind gusts reaching 150 km/h on the coast in places, and 130 km/h inland ^[26]. The surge caused considerable damage to the Provencal coastline ^[26]. It is also known that on January 2009 the storm in the Marseille metropolitan area, when 40 cm of rainfall around the Étang de Berre and on the Côte Bleue, as well as 15 to 20 cm in downtown Marseille, paralyzed the entire city and the Marseille-Provence airport in the space of an hour ^[28].

Mediterranean subtropical cyclone

Occasionally, during a Mediterranean episode, a Mediterranean subtropical cyclone, also known as a Medicane, forms. However, it is very rare for the atmospheric situation triggering a Mediterranean episode to also trigger such a phenomenon. During the memorable episode of early November 2011, a subtropical cyclone was observed at the end of the episode on the Var coast, which was named Rolf, and generated wind gusts of over 150 km/h ^[29]. Combined with the Mediterranean episode, this led to major flooding of the Argens for the second time in two years ^[30]. The 1947 cyclone on the same coast and the 1983 cyclone Tino Rossi in Corsica also fall into the same category.

Forecasting history

1854-1920: beginning of modern meteorology

In France, modern meteorology made its appearance after a catastrophic event for the French Navy ^[31]. On November 14, 1854, while a joint military operation between France and Great Britain was en route to the Crimea, a violent storm hit the fleet in the Black Sea, while it was stationed in Kamiesch Bay. The surprise storm sank 38 French, British and Turkish ships, claiming the lives of several hundred soldiers and sailors. This major event enabled Urbain Le Verrier to prove the usefulness of a network of weather observatories. In 1854, at the instigation of Napoleon III, the first French weather observatory network was created. This network comprised 24 stations, 13 of which were linked by telegraph, and by 1865 had expanded to 59 observatories across Europe. In 1863, the first weather forecast (24-hour forecasts based on maps and daily weather bulletins) was produced for the port of Hamburg.

Two years later, in May 1856, another catastrophic event occurred in the Rhone valley, this time the river's worst flood in a century. After the catastrophic flood of November 1840 ^[32], this was the second time in twenty years that the entire Rhone valley was devastated, from Lyon to the sea. In May 1856 ^[33], the entire Saône and Loire valleys were also hit by exceptional floods. Around a hundred people lost their lives, and the devastation caused to the region's main cities (Lyon, Valence, Avignon, Arles) prompted the public authorities to dam the river in order to fix its course once and for all. This event led Maurice Champion, a French historian interested in flooding, to take stock of these disasters. Between 1858 and 1864, he published the six volumes of his monumental work, which describes the floods that struck the country between the middle ages and the 19th century. This work, for which he was awarded the Légion d'honneur in 1865, describes the most important floods on each of the country's rivers over a period of almost 1,300 years.

The Bureau Central de la Météorologie was created by decree on May 14, 1878, to provide a new meteorological service for the whole country, and to collect data from all national stations. The first synthetic maps were produced, enabling the first weather forecasts to be made on a European scale. In the wake of torrential flooding and massive deforestation in the Cévennes, the project to build a weather observatory covering a state-owned forest was launched on Mont Aigoual in 1887. Construction was a challenge due to the conditions prevailing on the summit, and it took seven years to complete the work, which ruined the contractor. The summit station was inaugurated on August 18, 1894. It was to play a major role in the study of heavy rainfall in the Cévennes. In 2017, the weather station is still in operation, enabling new measuring instruments to be tested under extreme conditions, as well as fine forecasts to be made for the entire massif.

At the very end of the 19th century, Mont Aigoual recorded its most remarkable rainfall episode in history... In September 1900, 950 mm of rain in 24 hours was recorded at the foot of the massif, in Valleraugue. The flood was titanic, devastating the village and killing around thirty people. This deadly flood, which followed a series of catastrophic floods in the Cévennes, marked the start of a pivotal decade in the study of natural disasters in France. Two years later, Mount Pelée exploded, killing 30,000 people in Martinique; in 1907, a deadly autumn was recorded in the Cévennes ^[34]; in 1909, Lambesc and the Aix-en-Provence area suffered the most powerful and deadly earthquake of the 20th century in France; finally, in 1910, Paris and its suburbs were flooded by a 100-year flood of the Seine, as was Besançon by the flood of the Doubs. This decade of deadly and devastating disasters increased the interest of many French scientists and engineers in understanding and predicting them.

1921-1959: first studies and understanding of the phenomenon

In March 1921, the Bureau central de la Météorologie became the Office national météorologique, which published its very first radio weather bulletin, broadcast from the Eiffel Tower on July 15, 1922. From then on, three daily bulletins were issued to the population and the armed forces. It was a revolution for the country.

Three years later, Maurice Pardé, the future great specialist in Mediterranean and mountain flooding, graduated from Grenoble University with a doctorate published in the Revue de géographie alpine, thanks to his thesis on the hydrological regime of France's most powerful and turbulent river, the Rhône. Fascinated by the study of floods, he tried unsuccessfully to found a river hydrology organization. In 1930, Maurice Pardé was recruited as an assistant professor at the ENSH in Grenoble, then two years later was appointed lecturer at the Faculté des Lettres in Grenoble, and finally full professor of physical geography three years later. He studied in detail previous floods in southern France, such as the historic Ardèche flood of 1890, the paroxysmal episode of September 1900 at Valleraugue, and the torrential floods regularly observed in the mountainous regions of the Alps and Pyrenees. These studies enabled him to learn more about flooding mechanisms and their consequences, giving him an international reputation in the field of potamology. While the study of floods provides a wealth of knowledge, our understanding of the atmospheric phenomena behind them remains almost non-existent. This little-known phenomenon, despite a few leads already known to "old-timers", saw its first major dynamic study in the midst of the Second World War.

The Aiguat of 1940, which struck the Roussillon region between October 16 and 21, 1940, wreaked exceptional havoc in the Pyrénées-Orientales and Spanish Catalonia, killing 350 people. Recorded rainfall totals were extraordinary, with an unofficial total of 1,000 mm in 24 hours measured by Guillaume Julia at Saint-Laurent-de-Cerdans. Maurice Pardé, despite the disorganization of the meteorological services (France being occupied), managed to study this episode of exceptional rainfall in detail. His study was published in the first-half 1941 issue of "La Météorologie". However, due to German censorship, it could not be supplemented by the dynamic study of the atmosphere during the episode, carried out by R. Tasseel and A. Viaut, which they submitted to the Société Météorologique de France in 1942. In fact, the sum total of this work was not published until 1944. It confirmed what the "old-timers" of the region had observed in the past, namely the presence of a moderately unstable air mass, and an advection of cold, dry air aloft. The study uncovered important indicators of the occurrence of these rainstorms. His description of the phenomenon reveals the surprising nature of these rainy episodes, compared to the idea we have of the Mediterranean climate:

"The Mediterranean climate, so pleasant for its warmth or tepidness, the predominant purity of its azure and the absence or rarity of fog or long-lasting fine rain, nonetheless contains real scourges.

The most damaging of these is the possibility of heavy rain accompanied by electric flares and sometimes tempestuous squalls, which often combine the disadvantages of widespread showers with those of the much more localized thunderstorms of oceanic climates. In a single day, they can pour down waterspouts equal to the average annual rainfall in many regions that are already well-watered.

They are concentrated in single or repeated paroxysms of brief duration, of unimaginable fury, say witnesses, for those who have not witnessed them.

Following Maurice Pardé's detailed study, many other scientists around the Mediterranean began to study these heavy rainfall events more systematically, and in the 1950s established a set of key elements in their formation, which were enshrined at the Rome symposium in 1958:

• the Mediterranean relief, which is very important near the coast, disturbs and deflects air flows. Most of the depressions affecting this sea have formed over its waters;
• the sea's geographical position in the subtropics, ensuring a constant supply of humidity throughout the year, with a peak in autumn;
• the presence of the Sahara nearby, positioning a large mass of dry tropical air to the south of the basin, with very high evaporative power when it passes over the Mediterranean;
• the presence of a Cold drop, an isolated disturbance in the polar flow circulating at low latitude, propelling the tropical air mass towards the north of the basin;
• the essential discovery of mesoscale mechanisms, at the heart of the formation process of Mediterranean episodes, and still largely unknown at the time.

In 1958, the Rome Colloquium on the Meteorology of the Mediterranean Basin, attended by scientists of all nationalities, took stock of the current state of knowledge and defined new directions for research. Italy, marked by the historic flooding of the Po in 1951, and France by the Aiguat of 1940 and the cyclone of 1947 on the Côte d'Azur, made Mediterranean episodes their priorities. In his opening speech, Giuseppe Caron summed up the forecasting situation in the Mediterranean as follows:

"The Mediterranean, surrounded by three continents with profoundly different climatic conditions constitutes, from a meteorological point of view, one of the most difficult regions in the world."

1960-1979: the technological and digital revolution

During this period, weather forecasting underwent a major revolution, if not its most important revolution in history, with the arrival of satellites, computers and weather radar. On April 1, 1960, the Americans successfully launched their second weather satellite, TIROS-1. It preceded a major space program by NASA and NOAA, the Nimbus Program, whose mission was to launch a series of satellites of all types to observe the Earth's atmosphere from every angle. In the same year, France acquired its very first computer, the KL 901, which was used to carry out the first studies on computer modeling of the state of the atmosphere. In 1963, at the newly-created Centre de météorologie Spatial in Lannion, the very first TIROS-8 satellite image was received. This was to prove a veritable revolution, enabling us to study the atmosphere and its phenomena in unprecedented detail. One of the greatest advances was the systematic observation of cyclonic phenomena in the North Atlantic, something impossible less than ten years ago.

In France, however, the first computer models did not yet take into account the Mediterranean rim, which remained an enclave in the process of globalizing world forecasts. And for good reason: during this period, computer models and the first satellites launched did not yet allow for precise mesoscale studies. Yet the phenomena that occur there are essential to understanding severe thunderstorms, which are much more localized than tropical cyclones and storms, which are synoptic in scale. But in the 1970s, the relentless progress of computer technology enabled us to create ever more precise models, in order to open up the Mediterranean. And it's not the only part of the world where this need is vital, as the Great Plains of North America, regularly devastated by severe thunderstorms, also require more refined forecasts.

On April 3, 1974, the Super Outbreak, which ravaged 13 American states, brought to light the world's leading specialist in severe thunderstorm phenomena and mesoscale forecasting, Tetsuya Theodore Fujita. During this episode, which he studied in detail, he developed a completely new method of studying tornadoes, still in use today, and put an end to a whole series of myths about them. He discovered multi-vortex tornadoes and downbursts, but was unable to prove the latter before 1978. All the studies he carried out over the years led to a significant improvement in our knowledge of the mesoscale.

In the late 1970s, improvements in computer models made it possible to study thunderstorm organization in much greater detail. In 1976, in the USA, Browning identified supercell thunderstorms, recognized as producers of particularly violent storm phenomena. In 1977, Europe, and France in particular, launched their own Météosat weather satellite programs, providing specific forecasts for the old continent. It was at this pivotal time that meteorologists learned more about the synoptic environment favorable to Mediterranean episodes. This is thanks to the increasing power and finesse of computer models and satellite imagery. The Mediterranean region is well on the way to being liberated from global numerical weather forecasts.

1980-1999: mesoscale studies

Identification of thunderstorm organizations

In 1980, using satellite infrared imagery, Maddox identified MCCs or Mesoscale Convective Complexes. These large, circular thunderstorm organizations cause large quantities of water to fall over a large area in just a few hours, regularly generating flash floods in the Great Plains of North America. Two years later, a similar storm pattern struck the Valencia region of Spain, as described by Rivera and Riosalido in 1986. The study of these thunderstorms and the synoptic environment that gave rise to them led to major advances in the forecasting of thunderstorms that generate extreme precipitation.

One year later, T. Fujita identified a new extreme precipitation thunderstorm organization, smaller in size and different in shape from the MCCs, the V-shaped MCS or V-shaped Mesoscale Convective System. Its study was completed by Scofield and Purdom in 1986. V-shaped thunderstorms have the disadvantage of being retrograde, meaning that they constantly regenerate as long as the conditions that gave rise to them remain in place. Cells form one after the other, forming a kind of feather duster on radar and satellite imagery, where the tip is always towards the mid-troposphere wind. They can last for several hours, bringing down very large quantities of water over a very localized area, regularly resulting in catastrophic flooding. This V-shape concentrates the power of the storm in a given area, which concentrates the fury of the storms, making V-shaped thunderstorms among the most intense to occur.

French meteorologists have been able to identify these storms and assume that they are the ones regularly at work in the south of France, although they do not yet have the scientific proof. In the mid-1980s, the French meteorological services launched the Radome project, with the aim of creating a network of 13 weather radars, supplemented by CASTOR software, in order to control them all at the same time, so as to have a mosaic of all the radars, and monitor precipitation over the whole country. Between 1990 and 2000, this network was continually expanded to better monitor heavy rainfall events in the south of France, and is now known as the ARAMIS network.

In 1987, Nîmes saw the completion of its brand-new weather radar, covering a large part of the lower Rhône valley and the Cévennes, as well as part of the coastal plains of the Hérault and Gard departments. The next Mediterranean episode will finally be able to be studied with radar images, which will be a revolution in our understanding of them. Unfortunately for Nîmes, this will be to its detriment, since on October 3 1988, one of the worst rainy episodes in the history of the Gard, was concentrated on Nîmes. The radar images were clear: a V-shaped thunderstorm came to rest over the town, bringing down no less than 420 mm of rain in 9 hours (or more, as the rain gauge overflowed four times). Nîmes was devastated by torrential flooding, and residents were shocked by the intensity of this exceptional episode. This episode and those to follow will be studied in detail, enabling a better understanding of these phenomena, and greatly improving the forecasting of these diluvial storms. On September 26, 1992, an MCC was identified on the Roussillon, responsible for the catastrophic floods at Rennes-les-Bains.

In the 1990s, with the expansion of the weather radar network and the arrival of more powerful supercomputers, the Arpège global model was developed and commissioned in 1994, replacing the old Peridot model. It was supplemented by the Aladin mesoscale model in 1996, which greatly improved forecasting of Mediterranean episodes, thanks to finer meshes capable of better predicting mesoscale processes. On November 12 and 13, 1999, Arpège successfully predicted 24 hours in advance the exceptional episode that led to the Corbières disaster in the Aude and Tarn regions.

Cold drops

In 1986, Chiari published a study on the position of cold drops and the location of Mediterranean episodes, enabling better forecasting of these torrential downpours. The correlation between Mediterranean episodes and cold drops has existed since the 1940s, but with the development of mesoscale models and a better understanding of synoptic environments, it is now possible to study them in great detail and thus better understand their role in the occurrence of such phenomena.

If the cold drop is positioned over the Bay of Biscay, then the regions affected are eastern Languedoc and Provence (the lower Rhône valley); if it is in the vicinity of Spanish Catalonia, then the eastern Pyrenees and western Languedoc are affected; finally, if it is east of the Balearic Islands, then Corsica, the Côte d'Azur and the Alps are affected.

Between 1988 and 1996, an initial inventory of heavy rainfall events was carried out in the south of France, revealing some fifteen Mediterranean episodes that gave rise to a V-shaped MCS. It also enables us to classify the situations giving rise to these Mediterranean episodes into two main types, depending on whether the Cold Drop is located in the Atlantic or over the Bay of Biscay. In reality, the situations encountered are not exactly those described, but a variant between the two, closer to one or the other:

Class 1 situation

• Atmospheric configuration:
  • Cold drop between the Bay of Biscay and the Balearic Islands;
  • Threatened area depending on where the cold drop is located;
  • Threatened area at the outlet of the Jet Stream flow;
  • Short-wavelength trough off the Balearic Islands moving upstream in a south-westerly/north-easterly direction;
  • Type of forcing: synoptic forcing
• Forecast elements:
  • Clearly identifiable 12 to 24 h in advance;
  • Intense, long-lasting episode, with several severe thunderstorms expected;
  • Warning triggered 12 to 24 h in advance and confirmed 6 h before the start of the episode, over 4 to 5 départements.

As a cold drop is an isolated low-pressure system, it is surrounded by a zone of high pressure. In a cold drop, air masses circulate in an anti-clockwise direction, propelling south and south-westerly air masses northwards. In fact, when a cold drop circulates between the Bay of Biscay and the Balearic Islands, it propels a tropical air mass from the Sahara, loaded with moisture over the warm waters of the Mediterranean, towards south-east France. At the same time, the anticyclonic zones surrounding this cold drop see the air masses circulating in a clockwise direction, so it's the air masses to the north and north-west that are propelled southwards. Cold, dry continental air masses meet these tropical air masses.

The short-wavelength trough propels the tropical air mass onto the continental air mass, creating a large-scale dynamic forcing. The less dense, warm, moist air can only be forced to rise to higher altitudes, condensing as it reaches the troposphere to create thunderstorm clouds. These thunderstorms continually feed on moisture, with the maintenance of a flow of warm, moist air, created by the convergence of surface flows and warm advection. At the same time, mountainous terrain blocks the advance of these thunderstorms, and in turn generates orographic forcing which is coupled with dynamic forcing. It is in this precise zone that V-shaped MCSs are most likely to form.

This situation is regularly observed in autumn, with the Mediterranean Sea and a dry, tropical air mass at maximum temperature at the end of summer. Both of these air masses were either not yet affected by polar air masses, or only to a very limited extent, and were beginning to descend towards southern Europe.

Typical example of a class 1 Mediterranean episode:

• October 3, 1988, leading to the Nîmes disaster;
• September 21-22, 1992, leading to the Vaison-la-Romaine disaster;
• December 1-4, 2003, causing the historic Rhône flood of December 2003.

Class 2 situation

• Atmospheric configuration:
  • Cold drop off the English Channel;
  • Threatened area dependent on warm advection and fine-scale forcing;
  • Influence of a rather "soft", diffluent West-South-West current;
  • Short-wavelength trough approaching;
  • Type of forcing: orographic or frontal forcing.
• Forecast elements:
  • Difficult to identify, even in the very short term;
  • Intense but localized episode, with only one possible thunderstorm organization, forming mainly at night;
  • Alert triggered in one or two départements only if V-shaped thunderstorm observed, to avoid false alarms.

In this situation, with the cold drop a long way off, the south-east of the country is under the influence of a tropical air mass, but with no dynamic forcing to lift it. It does not necessarily trigger a Mediterranean episode, but if one does occur, all the convection will be focused in a very localized area, due to mesoscale forcing. However, at this time of the year, as we still have a very poor understanding of this phenomenon, it is very difficult to predict where this forcing will occur, and false alarms are very common. What's more, it's very often during the night that this situation triggers a V-shaped storm, posing a real threat to populations who may find themselves trapped in the middle of the night by the resulting flash flooding. There are two types of mesoscale forcing: orographic and frontal. Frontal forcing occurs when a quasi-stationary warm front, roughly perpendicular to the surface flow and advection, focuses convection. In the case of orographic forcing, it's a mountain barrier that plays the role of a warm front, and we see this very regularly over eastern Corsica and the Cévennes.

This type of situation tends to occur more frequently in winter or late autumn. If a class 1 situation is triggered over the Languedoc, a class 2 situation may simultaneously affect Liguria, northern Italy, or perhaps eastern Corsica. We can also see a Mediterranean episode begin with a class 2 situation, then with the approach of the cold drop, see it become a class 1 episode, and end again with a class 2 situation.

Typical example of a class 2 Mediterranean episode:

• October 31-November 1, 1993 due to orographic forcing, leading to catastrophic flooding in Corsica;
• January 28, 1996 due to frontal forcing, leading to the Puisserguier disaster.

Isolated minimum and warm advection

An isolated minimum is a low-pressure zone linked to a cold drop, which is a low-temperature zone. The two are not at the same altitude: the cold drop is at altitude, while the isolated minimum is much closer to the ground. The intensity of the Mediterranean episode differs according to whether or not there is an offset between the two. If the isolated minimum is slightly offset from the cold drop, warm advection will be present over southern France, without forcing but with orographic accumulation. The result is a long-lasting episode of continuous rainfall, rarely convective, but with cumulus values over the whole episode that can be very high. This type of situation is often referred to as a Cevennes episode. If the isolated minimum is not offset by the cold drop, then a cold air mass will be present over southern France. This leads to large-scale dynamic forcing, producing localized thunderstorms of short duration, but with very high hourly accumulations.

Mesoscale cold drop

In the 1990s, with the detailed study of V-shaped mesoscale convective systems (MCS), we discovered another aspect of these storms, linked to the entry of cold, dry air at mid-altitude, a key point highlighted in the 1950s. This cold, dry air crosses the zone of heavy precipitation, where it becomes denser and more humid. This cold, moist air descends to the ground and radiates in all directions. This cold, moist air mass creates a meso-anticyclone (or mesoscale cold drop), which forces warm, moist air converging on the thunderstorm. The thunderstorm creates its own dynamic forcing, forcing the warm, moist air to keep rising into the thunderstorm, against the cold, dry air entering at mid-altitude. As the warm, moist air rises, it condenses at the top of the thunderstorm, overhanging the arrival of cold, dry air at mid-level, causing precipitation to fall on this cold air, which in turn becomes denser, creating a self-perpetuating V-shaped SCM cycle.

2000-2009: weather forecasts and vigilance

Weather vigilance

At the end of December 1999, two successive storms, Lothar and Martin, devastated Western Europe, particularly France, which paid the heaviest human and material toll. While Martin's arrival had been foreseen in advance, Lothar's arrival came as a surprise, even though Météo-France had alerted the civil services responsible for this type of crisis, but underestimated the winds involved. The combination of a number of factors led to a communication failure which, on the very day of Lothar's passage, gave rise to heated controversy in the media and among the general public. What's more, the inappropriate behavior of some people, particularly those who had taken to the road in strong winds, led to deaths, particularly from falling trees. This phenomenon, which marked the end of a very deadly decade for meteorological phenomena in France, radically changed the way warnings were communicated, with the creation of an entirely new warning system, intended for both the public and civil security services: weather vigilance. It was introduced between 2000 and 2001, and was used for the first time on October 1, 2001 ^[35]. The very first orange vigilance was triggered on October 6, 2001, for the parameter of violent winds, during the passage of a storm in Brittany. As for Mediterranean episodes, the very first orange alert was issued at 4:00 pm on October 8, 2001, for the whole of the lower Rhône valley (Bouches-du-Rhône, Gard, Vaucluse), Hérault, Lozère and Var.

Mediterranean episodes have rapidly become the weather phenomenon that triggers the greatest number of "red" warnings, the highest alert level, since this procedure was first introduced. In fact, it was a Mediterranean episode that triggered Météo-France's first red alert, during the exceptional storm of September 8-9, 2002, with a red "thunderstorm" alert for the Gard region. Of the 35 red vigilance procedures triggered since September 2002, 18 have been for a Mediterranean episode (including one with snow in 2018), compared with 10 for winter storms and their consequences, 4 for flooding outside the Mediterranean basin, 2 for avalanches, and 1 for snowy weather. These are also the phenomena that can trigger the greatest number of simultaneous orange and/or red vigilance parameters in the same département, depending on the extent and nature of the episode: Heavy rain or Rain-Flood, Thunderstorms, Flooding, Wave/Submergence, Violent wind, Snow-Ice, and Avalanches (only for Pyrénées-Orientales, Alpes-Maritimes, Alpes de Haute-Provence, Haute-Corse, and Corse du Sud).

HyMex project

It was against this backdrop that Météo-France launched the Hymex project, with the aim of gaining a better understanding of these singular weather phenomena in the Mediterranean region, so as to be able to forecast them better in the long term, and better understand the data observed a few days to a few hours after the episode. Météo-France and CNRS are overseeing this international research program (HyMeX) ^[36] from 2010 to 2020, with the support of 400 scientists and all the latest meteorological observation technologies: research boat, buoys, portable weather station, satellite observation, atmospheric probes, and data collection from all the weather stations in the region (Italy, France and Spain). This large- and small-scale study, at sea, on land, and in the atmosphere, will last until 2020, and initial results from the study conducted in 2012, were released during Autumn 2016 by the CNRS.

The first campaign took place between September 5 and November 6, 2012. It enabled us to gather data, particularly on areas still poorly covered, such as the sea or cloudy, precipitating areas. It also assessed the relevance of using new data in the models, from research instruments such as lidars (Light Detection and Ranging, a remote sensing device that emits laser waves and records the rainfall signal from these pulses), or operational radar networks (for example, data that can distinguish hail from rain or snow). These new data will help refine the representation of processes in climate and forecasting models, and improve the use of observed data in these models.

The campaign was also a test bed for new weather forecasting systems, such as Météo-France's Arome ensemble forecasting system. Météo-France uses two forecasting techniques: deterministic forecasting and ensemble forecasting. Deterministic forecasting involves using observations to produce a numerical simulation of future weather conditions. But the observations and the model are not perfect (measurement errors, data-free zones, modeling assumptions, etc.). The ensemble forecast takes these imperfections into account: the weather forecast is no longer described by a single simulation, but by several. Instead of a single scenario obtained with deterministic forecasting, forecasters now have a range of possible scenarios at their disposal. Today, Météo-France produces deterministic forecasts with its three models, Arpège, Aladin and Arome, and ensemble forecasts with Arpège alone. By 2015, forecasters should also have access to ensemble forecasts with the fine-mesh Arome model. The first intensive campaign enabled us to test and evaluate the contribution of this version of Arome to forecasting intense rainfall (location and intensity). Arome ensemble forecasts will also be fed into hydrological models to estimate their contribution to flood forecasting in the Cévennes and Var basins.

The HyMeX program is funded in France by CNRS, Météo-France, CNES, Irstea, Inra, the ANR white program and the Corsican regional authorities. It also benefits from European and international support ^[36].

Impact of global warming

The latest research in this field has demonstrated a trend towards an increase and intensification of these phenomena towards the end of the ^21st century. However, as yet poorly understood hydrological cycles remain difficult to predict in the long term, as do the interannual cycles of Mediterranean episodes. Year after year, there is no clear trend in long-term trends between 1958 and 2015. On the other hand, several long-term models show a decrease in average annual rainfall in these regions, but a clear increase in the number and intensity of Mediterranean episodes, aggravating forecasts of the consequences of global warming in this part of the world. Hence the importance of the HyMex research program in improving future forecasts ^[37]

Important cases

Bibliography

• Champion, M. Maurice. Les Inondations en France depuis le vie siècle jusqu'à nos jours, recherches et documents contenant les relations contemporaines, les actes administratifs, les pièces officielles, etc. de toutes les époques, avec détails historiques sur les quais, ponts, digues, chaussées, levées, etc., publiés, annotés et mis en ordre. Paris: V. Dalmont. OCLC  17558448.
• Rivrain, J.C. (1997). Phénomènes remarquables no 4: Les épisodes orageux à précipitations extrêmes sur les régions méditerranéennes de la France. Météo-France.
• Aullo, G.; Santurette, P.; Jack, V.; Guillemot, F.; Sénéquier, D.; Bourdette, N.; Bessemoulin, P. (2002). Phénomènes remarquables no 8 : L'épisode de pluies diluviennes du 12 au 13 novembre 1999 sur le Sud de la France. Météo-France.

References

See also

• Climate of France.
• Météo-France

Category:Weather Category:France Category:Storms Category:Floods