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Extra-Tropical Cyclones, Tropical Cyclones and Convective Storms

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3.7 Meteorological risk: extra-tropical cyclones, tropical cyclones and convective storms Thomas Frame, Giles Harrison, Tim Hewson, Nigel Roberts 3.7.1 scale and their typical lifetime. Other extra-tropical cyclone is closely relat- types of storm system do exist, but ed to the strength of this jet stream. Storm types these can be considered subtypes of The strongest extra-tropical cyclones and associated the three systems listed above. occur in winter months when the jet hazardous stream is at its strongest. 3.7.1.2 phenomena Extra-tropical cyclones Storm systems can be 3.7.1.1 Extra-tropical cyclones are large ro- distinguished from each Storms tating weather systems that occur in other by their mechanism the extra-tropics (more than 30° lat- Conceptually, there are two types of itude away from the equator). They of development (growth), storm in meteorology: (1) the hazard- consist of an approximately circular structure, geographic ous weather phenomena themselves region of low surface pressure, of a location, spatial scale and (such as windstorms, rainstorms, radius of 100-2 000 km, accompa- typical lifetime. snowstorms, hailstorms, thunder- nied by cold and warm fronts. They storms and ice storms (freezing rain)), typically develop in regions of strong and (2) the meteorological features in horizontal temperature gradients, the atmosphere — the ‘storm sys- which are commonly denoted on a Periods when the jet stream is unusu- tems’ — that can be said to be re- weather chart as a cold or quasi-sta- ally strong can lead to two or more sponsible for this adverse weather tionary front. In turn, such fronts of- strong cyclones occurring within days (notably tropical cyclones, extra-trop- ten connect to a pre-existing decaying of each other. The total lifecycle of ical cyclones and convective systems). extra-tropical cyclone, which itself is an extra-tropical cyclone from birth These storm systems, which are a situated some way downstream (typ- (genesis) through to development focal point in the following discus- ically to the north-east). At the same and on to decay (lysis) can occasion- sion, can be distinguished from one time, high up in the atmosphere ally be more than 10 days; however, another by their mechanism of de- (around 10 km altitude) one com- somewhere in the range of 2-5 days velopment (growth), their structure, monly finds a jet stream relatively is much more typical (Ulbrich, 2009). their geographic location, their spatial close by. Indeed, the intensity of an 246 CHAPTER 3 UNDERSTANDING DISASTER RISK: HAZARD RELATED RISK ISSUES - SECTION III The major hazards associated with coastal regions as well as for small is- ing from around a few hundred me- extra-tropical cyclones are high winds lands. tres up to several kilometres. How- and precipitation (rain and snow). ever, severe convective systems can Precipitation occurs primarily along For historical and cultural reasons, comprise many cells organised into fronts and, on average, is not particu- the strongest tropical cyclones are a larger coherent structure with di- larly intense relative to that delivered assigned different terminology in dif- ameters of up to a few hundred kilo- by tropical cyclones and convective ferent regions of the globe. In the metres. These can persist for much storms. However, when a cyclone is North Atlantic and North-East Pacif- longer than the individual cells, as developing, some very heavy pre- ic, they are called hurricanes; in the new cells tend to replace old ones cipitation can occur, particularly in a North-West Pacific they are called ty- within the structure. For example, narrow band just to the left (north) of phoons, and in the Indian Ocean and convective cells may be organised in a the cyclone track. The band is ordi- southern hemisphere they are simply linear fashion into squall lines or dere- narily between about 20 and 200 km called cyclones. The term hurricane is cho systems. They may also form part wide, depending on the scale of the also sometimes used erroneously by of a rotating system such as a super- cyclone. In addition, fronts connected the media to refer to extra-tropical cell or a large meso-scale convective to cyclones can sometimes become cyclones that have hurricane-strength system. Convective storms mostly very slow-moving, remaining over the winds. Tropical cyclones lead to very occur in the tropics and over land in same location for many hours, and intense surface winds (notably in a summer or over the sea in winter in potentially up to 2 days, leading to small annulus around the eye), as well the extra-tropics. large rainfall accumulations and po- as heavy rain and lightning. The most tential flooding. significant threat that they pose is coastal flooding from the associated 3.7.2 3.7.1.3 storm surge. Frequency and Tropical cyclones 3.7.1.4 geographical A tropical cyclone is a rotating storm Convective systems distribution of originating in tropical latitudes, with severe storm low surface pressure at its centre. Convective storms are produced by related hazards These develop over warm oceans in a localised rapid ascent of air, which tropical regions, have a radius in the is made buoyant by the heating of air range of about 100-500 km, and have near the Earth’s surface or the cool- 3.7.2.1 a lifetime of between a few days and ing of air higher up, with the ascent High winds associated with a couple of weeks. They also have a of the air maintained by heat supplied extra-tropical structure in wind, rainfall, tempera- by condensation of water vapour cyclones ture, etc., that is relatively axisymmet- within it. The rapid ascent of air in ric (unlike extra-tropical cyclones, the convective storms often produces Extra-tropical cyclones account for structures of which are not generally very heavy but relatively short-lived the majority of recorded high surface axisymmetric). The development and rainfall, thunder and lightning, as well winds in Europe. Their capacity to maintenance of tropical cyclones re- as, potentially, hail, very strong wind travel inland, and the fact that some quires that the ocean surface is very gusts and even tornadoes. At their cyclones are themselves very large, warm relative to the air above, and simplest, convective storms consist means that the winds associated with that the air above has high humidity of a single short-lived convective cell, a single storm system can affect large (Emanuel, 2003). The requirement of comprising one ascending and one areas. For example, as extra-tropical a warm ocean surface beneath means descending column of air (updraft cyclone Kyrill (January 2007) travelled that tropical cyclones will decay as and downdraft). across Europe wind gusts of 25 m/s they move inland. This makes them or more were reported over most of primarily a hazard for oceanic and Individual cells have diameters rang- Ireland, the southern United King- 247 dom, northern France, the Nether- associated predictability levels. WJ is cyclones (leading to the term ‘medi- lands, Belgium, Germany, Switzer- relatively easy to predict, while SJ, the canes’ in the former case), although land, Austria, the Czech Republic, most extreme type, is notoriously dif- such storms are not as long lived and Slovenia, Slovakia and Poland (Fink et ficult. the most extreme cases are much less al., 2009; RAIN, 2016). Cyclone Kyrill severe than the most extreme tropical caused 46 fatalities (EEA, 2011), cre- cyclones (Cavicchia et al., 2014). The ated total estimated insured losses of frequency with which severe cyclones between EUR 4.5 and EUR 4.8 bil- Storm systems lead to occur is difficult to define because lion (EEA, 2011; AIR Worldwide, a variety of hazardous the observational record is not suffi- 2015) and a total estimated damage phenomena, including ciently long (Della-Marta et al., 2009; of EUR 7.7 billion. An example of a Welker et al., 2016) and because cur- much smaller but much more intense high winds, precipitation rent climate models, which could in storm system for which the econom- and lightning, with principle, generate very long synthetic ic losses were about the same was the spatial extent and representations of the current climate Cyclone Lothar, in December 2009 duration of the hazard on which to base an accurate estimate, (Mitchell-Wallace and Mitchell, 2007; being strongly dependent typically lack the resolution needed to Roberts et al., 2014). Lothar affect- on the type of storm. represent severe windstorms (Zappa ed only a relatively narrow swathe of et al., 2013; Donat, 2011). In addition, northern France, south-west Germa- if severe cyclones cluster as has been ny and Switzerland, but wind gusts suggested by Pinto et al. (2013) and widely exceeded 35 m/s. An impor- others, then frequency estimates such tant consideration regarding impacts Extra-tropical cyclones are ubiqui- as return periods need to be interpret- is that damage is typically estimated tous in the extra-tropics, occurring ed carefully. to vary according to gust strength to at all locations and all year round (al- the power of 3 (Leckebusch, 2007). though they are more frequent and, The effect of climate change on Therefore, 35 m/s gusts are much on average, more intense in late au- the intensity and distribution of ex- more destructive than 25 m/s gusts. tumn/winter). Europe is affected by tra-tropical cyclones is still very uncer- about 10 extra-tropical cyclones per tain; however, the IPCC AR5 (IPCC, Understanding of the structure of month (based on Hoskins and Hodg- 2014) states that it is unlikely that the extra-tropical cyclones has increased es, 2002); however, the vast majority number of cyclones will reduce by considerably in recent decades (see, of such cyclones do not lead to dam- more than a few per cent and that for example, Browning, 2004; Hew- aging winds.
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