An observational and numerical study of a flash-flood event over south-eastern Italy

Abstract. Tornadoes have been reported in Greece over recent decades in specific sub-geographical areas and have been associated with strong synoptic forcing. While it has been established that meteorological conditions over Greece are affected at various scales by the significant variability of topography, the Ionian Sea to the west and the Aegean Sea to the east, there is still uncertainty regarding topography's importance on tornadic generation and development.The aim of this study is to investigate the role of topography in significant tornadogenesis events that were triggered under strong synoptic scale forcing over Greece. Three tornado events that occurred over the last years in Thebes (Boeotia, 17 November 2007), Vrastema (Chalkidiki, 12 February 2010) and Vlychos (Lefkada, 20 September 2011) were selected for numerical experiments. These events were associated with synoptic scale forcing, while their intensities were T4-T5 (on the TORRO scale), causing significant damage. The simulations were performed using the non-hydrostatic weather research and forecasting model (WRF), initialized by European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses, with telescoping nested grids that allow for the representation of atmospheric circulations ranging from the synoptic scale down to the mesoscale. In the experiments, the topography of the inner grid was modified by: (a) 0 % (actual topography) and (b) −100 % (without topography), making an effort to determine whether the occurrence of tornadoes -mainly identified by various severe weather instability indices -could be indicated by modifying topography. The principal instability variables employed consisted of the bulk Richardson number (BRN) shear, the energy helicity index (EHI), the storm-relative environmental helicity (SRH), and the maximum convective available potential energy (MCAPE, for parcels with maximum θ e ). Additionally, a model verification was conducted for every sensitivity experiment accompanied by analysis of the absolute vorticity budget.Numerical simulations revealed that the complex topography constituted an important factor during the

Section: T Matsangouras Et Al: Effect Of Topography On Tornadogementioning

confidence: 87%

Numerical modeling and analysis of the effect of complex Greek topography on tornadogenesis

Matsangouras

Pytharoulis

2014

“…During the fall season the sea surface temperature is still warm and provides ample moisture which can give rise to torrential rainfall events which account for a substantial fraction of the annual rainfall (Romero et al, 1999;Homar et al, 2002;Lionello et al, 2006;Martin-Vide, 2004;Lebeaupin et al, 2006;Miglietta and Regano, 2008). This is to say that the precipitation regime in such areas seems to concentrate a significant portion of the annual rainfall into very short periods of time in the form of what often turns out to be an extreme event with deep convection playing a central role.…”

Section: Climatological Features Of the Veneto Coastal Areamentioning

confidence: 99%

Recurring features of extreme autumnall rainfall events on the Veneto coastal area

Barbi

Monai

Racca

et al. 2012

Abstract. Recent recurring episodes of heavy flash floodproducing rainfall events on the Veneto coastal area have renewed the interest in documenting the frequency and key dynamical ingredients of such events. A climatological analysis of the precipitation in Veneto reveals that, in comparison with the rest of the region, the coastal area is characterized by fewer rain days, lower rainfall accumulations, yet more days with heavy precipitation. If set in relation to the yearly rainfall, daily accumulation can reach values as high as 40 % of the yearly total rainfall, more regularly between 15 % and 30 %, often in periods of 12 h or less.Four such heavy rainfall events were analyzed and synthetically described to highlight key ingredients which appear instrumental in producing the high rainfall accumulations. These comprise an upper-level trough elongating or cutting off into the Western Mediterranean basin after a period of one to two weeks of anticyclonic fair weather conditions with temperatures above normal. The moisture supply over the Adriatic onto north-eastern Italy is favoured by above normal sea surface temperatures, enhanced advection by a surface low in the Gulf of Genoa, and in three of the four cases, an additional surface low over southern Italy. The air flows associated with the upper-level trough for the cases discussed were of moderate to weak intensity, and convectively conditionally unstable. The flow intensity was such that the lower tropospheric portion was blocked by and forced to flow around the Alpine barrier, i.e. manifesting as a north-easterly, low-level flow over much of the north-eastern Italian plains. This blocked flow seemed to interact with the larger-scale synoptic flow to form a distinct and persistent low-level convergence in the area of the Veneto coast.It is suggested that these low-level convergence patterns are key in releasing the convective instability present in the larger-scale flow just on the Veneto coastal area. Hereby, it is the synoptic rather than the convective setting which dictated the observed timescales of intense rainfall. Therefore, the convective rainfall rates paired with the synoptic durations combine to produce the exceptionally high rainfall accumulations observed. Cases like these are significant contributors to forming the coastal precipitation climatology, which for this area is found to be distinctly different than for the rest of the region in terms of precipitation concentration.

“…The forecast range is 72 h for all the three models. With a similar implementation, the modelling systems were able to simulate in an accurate way some extreme weather events over Puglia region Miglietta and Regano, 2008). iii) For the application in the Malta area, the MARIA Malta Atmospheric forecasting system -consisting of a chain of meteo-marine models with downscaling to high resolution sub-domains -has been set up for the region north of 34 • latitude in the Sicilian Channel, and comprising of the Maltese Islands.…”

Section: Methodologiesmentioning

confidence: 99%

The RISKMED project: philosophy, methods and products

Bartzokas

Azzopardi

Bertotti

et al. 2010

Abstract. This paper presents RISKMED, a project targeted to create an Early Warning System (EWS) in case of severe or extreme weather events in the central and eastern Mediterranean and specifically in southern Italy, northwestern Greece, Malta and Cyprus. As severe or extreme weather events are considered, cases when the values of some meteorological parameters (temperature, wind, precipitation) exceed certain thresholds, and/or a severe weather phenomenon (thunderstorm, snowfall) occurs. For an accurate weather forecast, selected meteorological models have been operated daily, based on a nesting strategy using two or three domains, providing detailed forecasts over the above mentioned areas. The forecast results are further exploited for the evaluation and prediction of human discomfort and fire weather indices. Finally, sea wave models have also been operating daily over the central and eastern Mediterranean Sea. In case a severe or extreme weather event is forecasted within the next 48 or 72 h for selected target areas (sub-regions defined by their morphological and population characteristics), the local authorities and the public are informed via a user-friendly graphic system, the so-called RISK MAP. On the web page of the Project (http://www.riskmed.net), additional information is provided about the real-time values of some meteorological parameters, the latest satellite picture and the time and space distribution of lightning during the last 24 h.