Abstract
Abstract. This paper presents a method to identify intense warm season storms with convective character based on intensity thresholds and the presence of lightning, and analyzes their statistical properties. Long records of precipitation and lightning data at 4 stations and 10 min resolution in different climatological regions in Switzerland are used. Our premise is that thunderstorms associated with lightning generate bursts of high rainfall intensity. We divided all recorded storms into those accompanied by lightning and those without lightning and found the threshold I* that separates intense events based on peak 10 min intensity Ip ≥ I* for a chosen misclassification rate α. The performance and robustness of the selection method was tested by investigating the inter-annual variability of I* and its relation to the frequency of lightning strikes. The probability distributions of the main storm properties (rainfall depth R, event duration D, average storm intensity Ia and peak 10 min intensity Ip) for the intense storm subsets show that the event average and peak intensities are significantly different between the stations. Non-parametric correlations between the main storm properties were estimated for intense storms and all storms including stratiform rain. The differences in the correlations between storm subsets are greater than those between stations, which indicates that care must be exercised not to mix events of different origin when they are sampled for multivariate analysis, for example, copula fitting to rainfall data.
Highlights
Rainfall is one of main causes of natural hazards in hydrological systems (e.g. Vörösmarty et al, 2013; Wilhelmi and Morrs, 2013; Kyselý et al, 2013) and is inseparably related to flood risk (e.g. Hlavcova et al, 2005; Blöschl, 2008; Borga et al, 2011; Winsemius et al, 2013)
Formal convective-stratiform separation techniques are inherently limited because they do not directly quantify the origin and type of convection (Zimmer et al, 2011), they have one thing in common – that a locally high rainfall intensity is expected in events where convection is present even if embedded in stratiform systems. It is this extremity of rainfall intensity during a storm which we would like to capture in our selection of intense events, regardless of their exact meteorological origin and genesis
The calibrated thresholds to identify intense warm season storms with convective character at the four studied stations are lower than what is reported in most climatological and engineering hydrology literature for extreme convective rain
Summary
Rainfall is one of main causes of natural hazards in hydrological systems (e.g. Vörösmarty et al, 2013; Wilhelmi and Morrs, 2013; Kyselý et al, 2013) and is inseparably related to flood risk (e.g. Hlavcova et al, 2005; Blöschl, 2008; Borga et al, 2011; Winsemius et al, 2013). Storms associated with high rainfall intensities often lead to severe flooding in catchments and urban areas, accelerated hillslope and channel erosion, triggering of landslides, mud and debris flows The severity of storms and their damage potential is dependent on the total rainfall amount and on other storm characteristics such as peak rainfall intensity and event duration. Convective rain is associated with higher rainfall intensities and is commonly identified from ground synoptic observations of clouds, state of weather and/or rain intensity fluctuations (e.g. Pešice et al, 2003; Chvila et al, 2005; Tremblay, 2005; Llasat et al, 2007; Berg et al, 2013; Ruiz-Leo et al, 2013; Rulfová and Kyselý, 2013). Weather radar provides additional information (e.g. updraft velocities and cloud water content), which aid the identification of convective rain
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