Research Papers Issue RP0224 Analysis of a meteorological wind July 2014 event over Verbania-Cusio-Ossola Impacts on Soil and Coasts Division province By Valeria Rillo SUMMARY The present research paper investigates a meteorological event Impacts on Soil and Coasts Division, CMCC occurred on the 11th May 2014 over Verbania-Cusio-Ossola province, [email protected] characterized by the presence of strong winds, causing numerous damages Maria Paola Manzi and diseases. The aim of the work is to study the exceptionality of such Impacts on Soil and Coasts event by comparing daily wind speeds (mean and maximum values) with a Division, CMCC long time series (30 years) of winds data. For this reason a definition of [email protected] extreme events is proposed, followed by a meteorological analysis and a Nicola Ciro Zollo climatological study of wind speeds over the area of interest. Such Impacts on Soil and Coasts Division, CMCC climatological study has been carried out using outputs of COSMO-CLM [email protected] model driven by ERA-Interim. In order to roughly evaluate the error carried Luigi Cattaneo out, a comparison between the model downscaled results and the available Impacts on Soil and Coasts observed data is proposed. In addition a study of the evolution of the Division, CMCC [email protected] weather situation in the area of interest using numerical weather prediction models, at different resolutions, has been realized. Finally the impacts on and Paola Mercogliano Impacts on Soil and Coasts soil caused by such meteorological event have been described. Division, CMCC Meteo Systems and Instrumentation Keywords: Extreme winds, Meteorological events, Impacts Laboratory, CIRA (Italian Aerospace Research Centre) [email protected] The research has received funding from the Italian Ministry of Education, University and Research and the Italian Ministry of Environment, Land and Sea under GEMINA project. Observed data are property of Arpa Piemonte. CMCC Research Papers INTRODUCTION The main activity of the Impacts on Soil and Coasts division (ISC) regards the study of 02 hydrogeological hazards connected with cli- mate change. In such framework it has de- veloped expertises and tools [34, 33] to deal with extremes of observed and simulated cli- mate datasets at different spatial scales, from international (Mediterrean area, [9, 18]), to national (China and Italy [28, 29, 6, 5]), re- gional (North-Center Italy, Alps, and Sardinia [7, 30, 17, 28, 27]), local (Orvieto [28]). In the last years, CMCC ISC has also collected re- gional high resolution observed precipitations and temperature data, thanks to the collab- oration with various Italian ARPAs (Regional Agencies for Environment and Protection). Fur- thermore international databases (e.g. Map, EOBS, HISTALP, [12, 13, 3]) are available. All Figure 1: these data are used to obtain a wide reference The area of interest: Verbania-Cusio-Ossola province. database to evaluate the outputs of climate Centro Euro-Mediterraneo sui Cambiamenti Climatici simulations performed at ISC division and/or defined as the occurrence of a value of a to analyse specific weather events [21]. The weather or climate variable above (or below) a aim of the present research paper is to study threshold value near the upper (or lower) ends the rarity of the meteorological event occurred ("tails") of the range of observed values of the over Verbania-Cusio-Ossola province (reported variable [15]. Unfortunately, this definition is in grey Figure 1) on the 11 May 2014 and char- more general so, from an operative point of acterized by the presence of strong winds that view, it is not applicable because it does not fix caused numerous damages and diseases. the threshold value. DEFINITION OF EXTREME EVENTS FROM A CLIMATE PERSPECTIVE Many other definitions are available in the liter- ature ([24, 23] [14]; in this paper the following As reported in [25] "Extreme events are gen- definitions are considered: erally easy to recognize but difficult to define [. ]", it is not simple to provide a specific Exceptional: an event with values ex- definition of extreme weather event. ceeding the 99th percentile; A general definition can be "an event that is rare within its statistical reference distribution Extreme: an event with values exceeding at a particular place", specifying that the char- the 95th percentile; acteristics of what is called extreme weather may vary from place to place. Rare: an event with values exceeding the However, the extreme climate events can be 90th percentile. 03 Figure 2: Bracknell chart valid at 00 UTC of the 10th May 2014. Available at http://www.wetterzentrale.de/topkarten/tkfaxbraar.htm. Centro Euro-Mediterraneo sui Cambiamenti Climatici Figure 3: Bracknell chart valid at 00 UTC of the 11th May 2014. Available at http://www.wetterzentrale.de/topkarten/tkfaxbraar.htm. In the present work, the exceptionality of the METEOROLOGICAL ANALYSIS OF THE event has been evaluated, comparing the ob- EVENT served data to a 30 years dataset (1981-2010) that is considered the standard reference cli- In order to describe the synoptic situation oc- mate period. The Standard Reference Period in curred during the day investigated in this paper, climatology is a time interval of 30 years which a brief synoptic analysis is reported. averages are characterising the present ’nor- On the 10th May 2014 (Figure 2), a cold mal’ climate conditions over a certain area [22]. front coming from the North Atlantic area ap- CMCC Research Papers WIND SPEEDS CLIMATOLOGICAL ANALYSIS In order to understand if the meteorological 04 wind event over Verbania-Cusio-Ossola has been exceptional, extreme, rare or none of them, it is necessary to study the climatologi- cal features of wind speeds over such province. Such analysis will be carried out by computing the percentiles of wind speed and comparing them with the observed values. The observed Figure 4: values is defined exceptional if it exceeds the COSMO LM forecast of winds at 925 hPa over Italy on the 11th May 2014. 99th percentile, extreme if it exceeds the 95th percentile and rare if it exceeds the 90th per- centile. In particular, will be investigated not only daily mean wind speed values but also daily maximum wind speeds that can better rep- proached the Alpine region, reaching it on the resent the exceptionality of the event. In fact, 11th (Figure 3) and carrying thunderstorms and during thunderstorms can occur wind gusts that strong winds over Northern Italy. In Figure 4 can significantly exceed the daily mean values. it is shown the COSMO LM forecast of winds over Italy at 18:00 UTC: it can be noticed that The dataset used to perform the climatological the Northern part of Italy has been interested analysis is the output of COSMO-CLM model Centro Euro-Mediterraneo sui Cambiamenti Climatici by strong winds. In fact as reported in [1, 20] [4] with a spatial resolution of 8 km, forced the cold front crossed the Alps leading to ex- by ERA-Interim reanalysis (global atmospheric tended Foehn conditions over the entire Pied- reanalysis produced by the European Centre mont region. Foehn is a dry wind descending in for Medium range Weather Forecasts) [11] and the lee of a mountain range producing strong covering a 30 years period from 1981 to 2010. wind gusts, reduction of relative humidity and Reanalysis data are frequently used as surro- increase of temperatures [19]. Such conditions gates for observational data for validation of caused the presence of wind gusts over the en- large-scale processes. Such data are basi- tire region [1, 20]: in the Alps were registered cally interpolations of observational data based wind gusts up to 150 km/h, in Val di Susa up to on a dynamical model (so called data assimila- 80-85 km/h and in Cuneo and Turin areas they tion), and are therefore complete and physically reached speeds of 60-70 km/h. Such phenom- consistent, even if they are subjected to model ena interested also lowland areas of Piedmont bias [16]. The choice of ERA Interim reanal- region. ysis dynamically downscaled using COSMO- CLM model, is due to the lack of the avail- ability of a long period observational dataset Moreover in order to evaluate the evolution of for the variable investigated. Anyway, in order the weather situation in the area of interest an to roughly quantify the error committed using analysis through the use of numerical weather COSMO model, a comparison with the avail- prediction models, at different resolutions, has able observed dataset is reported in the follow- been realized in the following. ing section. Such gridded data have been interpolated of wind gusts occurred. In Table 2 the compari- over Verbania-Cusio-Ossola province using the son among the observed maximum values and Natural Neighbour technique through software the 99th, 95th and 90th percentiles of the dis- Clime [8]. In fact, all the analyses presented tribution of wind gusts is reported, showing that 05 in this work has been obtained using CLIME, the event has been exceptional for all stations a special purpose GIS software integrated in apart from Larecchio one for which it has been ESRI ArcGIS Desktop 10.X, and developed extreme. In addition the PDFs of maximum by CMCC-ISC Division in the frame of Project daily wind speeds for the entire year and spring GEMINA, in order to better evaluate the impact season are reported in Figure 6. The results on soil of climate changes. CLIME has been proposed prove that the meteorological event designed to bridge the usually existing gap be- has been exceptional or extreme for most of tween atmospherical data gathered from differ- the area investigated not only in terms of wind ent sources and impact communities. For more gusts but also in terms of daily mean values.