Ensemble cloud-resolving modelling of a historic back-building mesoscale convective system over Liguria: the San Fruttuoso case of 1915

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.

Role of SST Patterns on Surface Wind Modulation of a Heavy Midlatitude Precipitation Event

“…Fiori et al (), by means of very high horizontal resolution numerical simulations (250‐m grid spacing), explain in details how a cold dry continental air mass plays the role of a virtual orography in lifting the moist warm low‐level jet that produces the HPE considered. The same mechanism occurred also in historical events (Parodi et al, ).…”

Historical weather data for climate risk assessment

“…The second example used dynamical downscaling, which is also a widely used method. It has been applied for a number of historical weather extremes including blizzards, flash floods, or storms . Fortunato et al .…”

Possible impacts of a changing climate on intense Ligurian Sea rainfall events

“…The convergence criterion was based on the threshold used in Parodi et al . () to determine which ensemble members in a WRF‐ARW (Advanced Research WRF) ensemble initialized with 56 ensemble members from the 20 th Century Reanalysis Project had a signal for the MCS associated with the San Fruttuoso flood event of 1915. However, that threshold was adjusted linearly to account for the difference between the 1 km grid spacing used in that study and the 12 km spacing of the EXPRESS‐Hydro output.…”

“…Parodi et al . () required at least four consecutive hours of strong convergence; thus, in the present study where output is limited to every 3 h, two consecutive 3‐h time periods were used. Finally, additional tests were performed using the thresholds described in (2) and (4) but also adding the requirement that the wind direction at either of the two grid points nearest the mountain passes northwest of Savona and north of Genova, respectively, be between 270° and 45° (west through northeast).…”

“…This region is focused over the Ligurian Sea and the nearby coastal regions of northwestern Italy where intense flash-flood producing rainfall has frequently occurred (e.g. Parodi et al, 2012;Rebora et al, 2013;Fiori et al, 2014;Davolio et al, 2015;Fiori et al, 2017;Lagasio et al, 2017;Parodi et al, 2017aParodi et al, , 2017b.…”