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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-54
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-54
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 Oct 2020

29 Oct 2020

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This preprint is currently under review for the journal WCD.

A numerical study to investigate the roles of former hurricane Leslie, orography, and evaporative cooling in the 2018 Aude heavy precipitation event

Marc Mandement and Olivier Caumont Marc Mandement and Olivier Caumont
  • CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France

Abstract. In southeastern 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. The synoptic situation was perturbed by the former hurricane Leslie, involved in the formation of a Mediterranean surface low that focused the convective activity. At mesoscale, convective cells focused west of a quasi-stationary cold front and downwind of the terrain. To investigate the roles of Leslie, orography and evaporative cooling in the processes that led to the observed rainfall, numerical simulations are run and evaluated with near-surface analyses comprising standard and personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updrafts mainly originate from the Mediterranean Sea, east of 4.5° E, whereas in a second part, an increasing number originates from Leslie's remnants. Air masses from east of 4.5° E appear as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moisten mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department does not play any substantial role in the stationarity of the cold front. Regarding lifting mechanisms, most of the air parcels found inside strong updrafts near the location of the maximum rainfall are lifted above the cold front, attesting its key role in focusing convection. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting seem to be 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 appears to be driven primarily by the location of the quasi-stationary cold front and secondarily by the location of convective bands downwind of the orography.

Marc Mandement and Olivier Caumont

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Marc Mandement and Olivier Caumont

Marc Mandement and Olivier Caumont

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Short summary
On 14–15 October 2018, in the Aude department (France), a heavy precipitation event produced up to about 300 mm of rain in 11 h. Simulations carried out show that the former hurricane Leslie, while involved, is not the first supplier of moisture over the entire event. The location of the highest rainfall appears to be driven primarily by the location of a quasi-stationary cold front and secondarily by the location of precipitation bands downwind of the mountains bordering the Mediterranean Sea.
On 14–15 October 2018, in the Aude department (France), a heavy precipitation event produced up...
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