Abstract
Abstract. In south-eastern France, the Mediterranean coast is regularly affected by heavy-precipitation events. On 14–15 October 2018, in the Aude department, a back-building quasi-stationary mesoscale convective system produced up to about 300 mm of rain in 11 h. At synoptic scale, the former Hurricane Leslie was involved in the formation of a Mediterranean surface low that channelled conditionally unstable air towards the coast. At mesoscale, convective cells focused west of a decaying cold front that became quasi-stationary and downwind of the terrain. To investigate the roles of the moisture provided by Leslie, orography and evaporative cooling among the physical processes that led to the location and intensity of the observed rainfall, numerical simulations are run at 1 km and 500 m horizontal grid spacing and evaluated with independent near-surface analyses including novel crowd-sourced observations of personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updraughts mainly originated from areas east of the Balearic Islands, over the Mediterranean Sea, whereas in a second part, an increasing number originated from Leslie's remnants. Air masses from areas east of the Balearic Islands appeared as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moistening mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department did not play any substantial role in the stationarity of the quasi-stationary front. Regarding lifting mechanisms, the advection of conditionally unstable air by a low-level jet towards the quasi-stationary front, confined to altitudes below 2 km, reactivated convection along and downwind of the front. Most of the air parcels found inside strong updraughts near the location of the maximum rainfall were lifted above the quasi-stationary front. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting were maintained by low-level leeward convergence, mountain lee waves and a favourable directional wind shear; when terrain is flattened, rainfall is substantially reduced. The location of the exceptional precipitation was primarily driven by the location of the quasi-stationary front and secondarily by the location of convective bands downwind of orography.
Highlights
Heavy-precipitation events (HPEs), usually defined as events with daily rainfall exceeding 150 mm (Ricard et al, 2012), affect all the coastal areas of the western Mediterranean region, often producing flash floods (Nuissier et al, 2008)
mesoscale convective systems (MCSs) develop preferentially northwards of a slowly evolving surface low located between the Iberian Peninsula and the Balearic Islands focusing a south-easterly low-level jet (LLJ)
The Mediterranean Sea supplies moisture – up to 60 % of the total air parcel moisture in previous HPEs according to water budgets of Duffourg and Ducrocq (2013) – and heat to this low-level airflow through evaporation and heat exchange, which both depend on the sea surface temperature (SST)
Summary
Heavy-precipitation events (HPEs), usually defined as events with daily rainfall exceeding 150 mm (Ricard et al, 2012), affect all the coastal areas of the western Mediterranean region, often producing flash floods (Nuissier et al, 2008). Caumont: A numerical study of the 2018 Aude heavy-precipitation event year period (2002–2006) With this climatology, synoptic situations favouring HPEs over Languedoc-Roussillon, the maritime part of the Occitanie region in southern France (Fig. 1), are well known (e.g. Fig. 11 of Ricard et al, 2012; Fig. 1 of Ducrocq et al, 2016). The location and the deepening of the slowly evolving surface low is a key ingredient in focusing convection over the same area and continuously initiating convection inside MCSs (Duffourg et al, 2016; Nuissier et al, 2016). (above sea level) (Lebeaupin et al, 2006), modulating the intensity of convective precipitation
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call