Abstract
Abstract. Highly localized and persistent back-building mesoscale convective systems represent one of the most dangerous flash-flood-producing storms in the north-western Mediterranean area. Substantial warming of the Mediterranean Sea in recent decades raises concerns over possible increases in frequency or intensity of these types of events as increased atmospheric temperatures generally support increases in water vapour content. However, analyses of the historical record do not provide a univocal answer, but these are likely affected by a lack of detailed observations for older events. In the present study, 20th Century Reanalysis Project initial and boundary condition data in ensemble mode are used to address the feasibility of performing cloud-resolving simulations with 1 km horizontal grid spacing of a historic extreme event that occurred over Liguria: the San Fruttuoso case of 1915. The proposed approach focuses on the ensemble Weather Research and Forecasting (WRF) model runs that show strong convergence over the Ligurian Sea (17 out of 56 members) as these runs are the ones most likely to best simulate the event. It is found that these WRF runs generally do show wind and precipitation fields that are consistent with the occurrence of highly localized and persistent back-building mesoscale convective systems, although precipitation peak amounts are underestimated. Systematic small north-westward position errors with regard to the heaviest rain and strongest convergence areas imply that the reanalysis members may not be adequately representing the amount of cool air over the Po Plain outflowing into the Ligurian Sea through the Apennines gap. Regarding the role of historical data sources, this study shows that in addition to reanalysis products, unconventional data, such as historical meteorological bulletins, newspapers, and even photographs, can be very valuable sources of knowledge in the reconstruction of past extreme events.
Highlights
Flash floods are phenomena very common to most Mediterranean coastal cities, accountable for millions of euros of damage and tens to hundreds of victims every year (Gaume et al, 2009)
It is important to highlight that the availability of the 56 members ensemble is a key strength in the present study, which enables estimates of uncertainties associated with dynamical downscaling down to the ARF-Weather Research and Forecasting (WRF) 1 km d03 domain
A historic extreme precipitation event occurring over Liguria on September 1915, which seems to be due to one of these systems, was investigated in this paper by both means of a large collection of observational data and means of atmospheric simulations performed using the ARF-WRF model forced by an ensemble of reanalysis fields from the 20th Century Reanalysis Project
Summary
Flash floods are phenomena very common to most Mediterranean coastal cities, accountable for millions of euros of damage and tens to hundreds of victims every year (Gaume et al, 2009). This paper focuses on a case study with the aim of investigating the ability of cloud-resolving grid-spacing atmospheric simulations to capture the main features of an event causing a very severe flash flood These simulations are performed using the Weather Research and Forecasting (WRF; Skamarock et al, 2008) numerical meteorological model forced by an ensemble of reanalysis fields from the 20th Century Reanalysis Project (Compo et al, 2006, 2011). Based on the newspapers of the time and documentary sources, after relatively light rain during the night between 24 and 25 September, in the early morning of 25 September, the area was hit for a few hours (07:00–11:00 UTC) by violent rain that triggered widespread flash flooding and a devastating debris flow This landslide half demolished the San Fruttuoso 1000-year-old abbey and laid down a thick layer of sand and rocks to form a stillexisting 20 m wide 2 m deep beach (Faccini et al, 2009), Clim.
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