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

M M Miglietta,A Regano

doi:10.5194/nhess-8-1417-2008

Abstract

Abstract. A flash-flood episode affecting a small area in Apulia (south-eastern Italy) on 22 October 2005 is documented. A rainfall amount of 160 mm was recorded in a 6 h interval in the central part of the region, producing severe damage and causing six fatalities. Synoptic maps, observations from surface stations and remote-sensing data are used here to describe the evolution of the rainfall system. The vertical profiles show features similar to those observed in other orographic heavy-rain events, such as a low-level jet, a conditionally unstable environment, and a nearly saturated warm low-level air mass. The low hills in the centre of the region play an important role in the release of the instability and the localisation of the rainfall, providing the uplift necessary to the air parcels to reach the level of free convection. Numerical simulations are performed in order to understand the mechanisms responsible for the heavy rain event. The Weather Research and Forecasting model (WRF) is setup in a 2-way nesting configuration including two domains. The model is able to realistically simulate the evolution of the precipitation system and to capture fairly well the localisation, the amount and the timing of the rainfall. The simulations suggest the important synergy of low and upper-tropospheric features which act as the triggering mechanism for the development of convection. A sensitivity experiment confirms the importance of the orography for the development of convective cells.

Highlights

Flash flooding is normally defined as a local flood of great volume and short duration due to heavy rainfall in the vicinity
- the pronounced low-level flow directed toward the mountain chain, that is necessary to force the air parcels from near the ground to above the lifting condensation level, removing the convective inhibition and allowing the release of potential instability www.nat-hazards-earth-syst-sci.net/8/1417/2008/
(note that the Murge hills have a maximum peak only 700 m high; the very low lifting condensation level – 965 hPa, about 500 m, in Brindisi sounding at 00:00 UTC, 23 October – allows for even a small ascending motion to generate condensation and eventually convection);

Summary

Introduction

Flash flooding is normally defined as a local flood of great volume and short duration due to heavy rainfall in the vicinity. Quite far from the location of the largest rainfall, the vertical profiles can at least provide a rough description of the temporal evolution of the atmosphere over the southern part of the region, upstream of the Murge hills It emerges from comparing the two soundings that in 12 h the wind from the low levels up to 400 hPa backs from southwest to south, due to the approach of the weak synoptic trough shown, and the wind speed increases at levels below 700 hPa (the wind speed weakens very close to the ground). The warming of the layers around 850 hPa produces a temperature inversion which inhibits the vertical motions from below

Discussion

Comparison with observations

Findings

Discussion and conclusions