What’s in a name?
An introduction to European windstorms

European windstorms, or extra-tropical cyclones to give them their correct meteorological name, can cause significant damage to property and infrastructure over a large area. For example, Storms Ciara and Dennis caused widespread flooding and disruption during the 2019/2020 windstorm season, and the UK recently experienced Storm Christoph during the current 2020/2021 season. In this blog we look at how they form, where they occur and what impacts they can have.

Forming windstorms

Extra-tropical cyclones (ETCs, so called because they form outside the tropics) develop on the polar front jet stream, the fast-moving ribbon of winds that encircles the northern hemisphere at a height of 10 to 15km in the atmosphere. ETCs form within the strong temperature and humidity gradients associated with the jet stream. In this very unstable environment, small ripples or perturbations at higher levels in the atmosphere trigger the development of areas of low pressure that can then intensify rapidly into large storms in a process called cyclogenesis. As atmospheric pressure falls within these storms, winds may increase extremely quickly.

A simple model of the formation and development of storms, known as the Norwegian Cyclone Model because of its development by Jacob Bjerknes and colleagues at the University of Bergen in the late 1910s, is shown below.

Figure 1: Norwegian model of the stages of cyclogenesis (after Bjerknes, 1919). Illustration: NOAA

The polar front is a region of strong temperature gradients and wind shear (1). Small perturbations can lead to the formation of a wave along this boundary (2), which gradually intensifies as warm air pushes north and cold air pushes south (3). At the same time, vertical movement of air contributes to the development process, eventually resulting in a mature ETC (4). In the final stage of the lifecycle, warm and cold air mix within the storm, which is said to be occluded (5).

Location matters

The location of ETCs is determined by the position of the jet stream as it meanders in the middle latitudes between roughly 30 and 40 degrees north, from southern Spain to the north of Scotland (a similar situation occurs in the southern hemisphere.) In winter, the jet stream tends to be stronger and located further south, meaning that ETCs are more intense and present more of a risk to continental Europe.

The westward-facing Atlantic coasts bear the brunt of the winter battering but exceptional storms can track inland through the centre of Europe. The sheer size of ETCs – sometimes over 1000km in diameter – means that their devastating impacts are more widespread than any other natural hazard in Europe.

An additional mechanism for ETC formation is the northward movement of hurricanes into higher latitudes, known as extra-tropical transition, whereupon hurricanes lose their tropical characteristics as they pull in colder air from the north. The results can be exceptional late summer storms.

Notable European windstorms of the past

Historical storms have become notorious for their impacts. Unlike hurricanes, ETCs do not have a categorisation system although they are named by various European weather agencies and universities when they are deemed a significant threat. On top of damaging winds, the rain bands that accompany the storms can lead to heavy rainfall and flooding, and storm surges can result from the high winds and low pressure. These can combine to produced huge economic and insured losses (Table 1).

Table 1: Selected European windstorms from recent history (data source: Swiss Re, 2000, 2008, 2010, 2013, 2019; PERILS.)

The Great Storm of 1987 was estimated to have felled 15 million trees in the UK alone, including six of Sevenoaks’ eponymous landmarks (Guardian, 2017). Windstorm Lothar (December 1999) resulted in the largest insurance loss of the 20th century in Europe, estimated at £7.2bn (Swiss Re, 2019).

However, even these pale into insignificance compared with the impacts of historical storms from past centuries. In the late 11th century a succession of major storms led to the demise of Dunwich on the Suffolk coast. At the time, Dunwich was one of the ten largest towns in England with a population of over 4,000 inhabitants and an important port (Sear et al., 2013) – today, it is a village of around 60 (Dunwich Parish, 2020).

More recently, in 2019 storms Ciara, Dennis and Jorge produced both damaging winds and flooding from intense rainfall across the UK and continental Europe, with an estimated 4,000-5,000 properties affected.

European windstorms cause widespread damage through their strong winds, storm surges and heavy rainfall, often over large areas, resulting in significant financial implications for insurance companies and national economies. It’s vital for re/insurers to understand the potential impacts of an event like this prior to it making landfall. JBA’s Event Response team and Flood Foresight produce forecast and real-time flood footprints to help you understand the potential flood risk.

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References

Bjerknes, J. 1919. On the Structure of Moving Cyclones. Bergen.

Dunwich Parish 2020. http://dunwichpm.onesuffolk.net/ [accessed 17/02/2020]

Guardian 2017. https://www.theguardian.com/environment/2017/oct/15/british-woodlands-30-years-after-great-storm [accessed 14/01/2020]

PERILS, 2020. https://www.perils.org/losses [accessed 17/02/2020]

Sear, D. A., Murdock, A. A., LeBas, T., Baggaley, P. P. and Gubbins, G. 2013. Dunwich Project 5883 Final Report. University of Southampton [accessed 17/02/2020]

Swiss Re, 2000. Storm over Europe: An underestimated risk https://media.swissre.com/documents/storm_over_europe_en.pdf [accessed 17/02/2020]

Swiss Re, 2008. New Swiss Re sigma study: Catastrophe losses in 2007 were highest in Europe. https://www.swissre.com/dam/jcr:1f915abd-983d-4406-857c-be02100971dd/pr_20080311_sigma1_en.pdf [accessed 17/02/2020]

Swiss Re, 2010. https://www.swissre.com/dam/jcr:f98840ac-15d4-4f2d-91a8-55268a713d29/media_information_cata_30_11_2010.pdf [accessed 17/02/2020]

Swiss Re, 2019. https://www.swissre.com/risk-knowledge/mitigating-climate-risk/winter-storms-in-europe/storms-lothar-martin-wreak-havoc-across-europe.html [accessed 14/01/2020]