Abstract
Abstract. The case of the heavy precipitation event on 14 and 15 October 2018 which has led to severe flash flooding in the Aude watershed in south-western France is studied from a meteorological point of view using deterministic and probabilistic numerical weather prediction systems, as well as a unique combination of observations from both standard and personal weather stations. This case features typical characteristics of Mediterranean heavy precipitation events such as its classic synoptic situation and its quasi-stationary convective precipitation that regenerates continuously, as well as some peculiarities such as the presence of a former hurricane and a pre-existing cold air mass close to the ground. Mediterranean Sea surface temperature and soil moisture anomalies are briefly reviewed, as they are known to play a role in this type of hydrometeorological events. A study of rainfall forecasts shows that the event had limited predictability, in particular given the small size of the watersheds involved. It is shown that the stationarity of precipitation, whose estimation benefits from data from personal stations, is linked to the presence near the ground of a trough and a strong potential virtual temperature gradient, the stationarity of both of which is highlighted by a combination of observations from standard and personal stations. The forecast that comes closest to the rainfall observations contains the warmest, wettest, and fastest low-level jet and also simulates near the ground a trough and a marked boundary between cold air in the west and warm air in the east, both of which are stationary.
Highlights
Heavy precipitation events (HPEs) are common in coastal regions bordering the Mediterranean (e.g. Ricard et al, 2012)
As for the position of the trough, even if AROME-France analyses are most of the time close to Standard and Personal Weather Stations (SPWS) analyses regarding the position of the virtual potential temperature gradient (Fig. 17b), they underestimate the small-scale signal of stationarity of this gradient east of Trèbes seen in SPWS analyses
The case of the deadly flash floods in the Aude catchment area on 14 and 15 October 2018 was studied on several scales using operational numerical weather prediction systems and observations including observations of connected objects
Summary
Heavy precipitation events (HPEs) are common in coastal regions bordering the Mediterranean (e.g. Ricard et al, 2012). A better understanding of heavy rainfall events is necessary to identify the weaknesses of numerical weather prediction systems and focus efforts on the most critical aspects of these systems, be they observations, model parameterizations, resolution or others. This was, for example, the objective of the Hydrological in the Mediterranean Experiment First Special Observation Period (Ducrocq et al, 2014) that took place in the north-western Mediterranean in autumn 2012.
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