Damaging Convective and Non-Convective Winds in Southwestern Iberia during Windstorm Xola

Paulo Pinto,Margarida Belo-Pereira

doi:10.3390/atmos11070692

Paulo Pinto, Margarida Belo-Pereira

Open Access

https://doi.org/10.3390/atmos11070692

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Abstract

On 23/12/2009, windstorm Xola struck mainland Portugal, causing serious damage in a small area north of Lisbon (Oeste region) and in the south region, inflicting economic losses of over EUR 100 million. In both areas, several power towers, designed to withstand up to 46 m s−1 winds, were destroyed. The causes of these two distinct damaging wind events were investigated. Xola was revealed to have a prominent cloud head and a split cold front structure. In the southern region, the damages were due to downburst winds, associated with a mesovortex, observed in a bow echo line triggered by an upper cold front. The cloud head presented several dry air intrusion signatures, co-located with tops progressively lowering towards the hooked tip. This tip revealed features consistent with the presence of slantwise convection, the descending branches of which may have been strengthened by evaporating cooling. At the reflectivity cloud head tip, a jet streak pattern was identified on weather radar, with Doppler velocities exceeding 55 m s−1, just 400 m above ground. This signature is coherent with the presence of a Sting jet, and this phenomenon was associated with the strongest wind gusts (over 40 m s−1) and the largest damages in the Oeste region.

Highlights

Extratropical cyclones, and in particular rapidly deepening extratropical cyclones, are known to cause damaging winds
This study intends to understand the cause of these damaging winds
Windstorm Xola formed over the warm front of an occluding extratropical cyclone, and ahead of a

Summary

Introduction

Extratropical cyclones, and in particular rapidly deepening extratropical cyclones, are known to cause damaging winds. Shapiro and Keyser [1] proposed a new conceptual life-cycle model of extratropical cyclones, associated with damaging winds, characterized by four stages: (I) incipient frontal cyclone; (II) frontal fracture, when the cold front loses contact with the warm front, (III) frontal T-bone (cold and warm fronts nearly perpendicular) and bent-back front; and (IV) warm core seclusion (Figure S1). At stage IV, the cold air in the dry slot encircles the cyclone center, leading to the seclusion of warm air surrounded by the tail of the bent-back front and the associated tip of the cloud head [3]. The cloud head and the dry slot are characteristic features of cyclones that cause damaging winds [3]

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