Extratropical cyclones – forecasting the sting in the tail

What is a sting jet?

A sting jet is a core of extremely strong winds that occurs within an extratropical cyclone and extends towards the ground. Although it can cause severe damage at the Earth's surface, it affects a much smaller area than the main storm - typically around 100 km in the maximum width. Additionally, it has a far shorter duration, usually three to four hours.

Most extratropical cyclones don't have a sting jet. For example, it's estimated that they occur in around 39-49% of the strongest extratropical cyclones over the North Atlantic.

What causes sting jets to form?

Sting jets tend to occur in a particular type of extratropical cyclone known as a Shapiro-Keyser cyclone. This type of weather system features low pressure at its centre and forms outside the tropics, causing various weather events such as rain and storms.

On a surface pressure chart, sting jets have a distinct appearance that differs significantly from a typical low-pressure system. These lows develop much more rapidly through a process known as explosive cyclogenesis, during which the associated warm and cold fronts do not merge to form an occlusion. The point at which the cold and warm fronts fail to 'attach' is known as the frontal fracture region.

Like all extratropical cyclones, the Shapiro-Keyser cyclone has two conveyor belts: warm and cold. The warm conveyor belt moves parallel to the surface cold front, rising above the surface warn front. As it rises, the moisture it carries condenses, resulting in a cloud head. The cold conveyor belt runs parallel to the surface warm front, then wraps around behind the surface cold front.

The sting jet forms inside the cloud head, in the mid-levels of the troposphere. which is the lowest layer of the Earth's atmosphere, around three to four kilometres above the ground. Evaporative cooling then causes the jet to accelerate downwards towards the frontal fracture zone and surface. As the jet descends, snow and rain fall into it (the snow turns into rain). The rain evaporates, and the jet becomes colder and denser, causing it to sink to the surface. When the jet hits the surface ahead of the cold conveyor belt, it generates winds that are far stronger than those normally associated with an extratropical cyclone. The wind speed depends on the stability of the atmosphere.

Additional mechanisms may also be involved in generating a sting jet, such as weakened weather fronts and some types of convective instability - the relative importance of these processes remains an active area of research. It's a complex topic, not least because an extratropical cyclone can produce several sting jets - and a sting jet can create damaging winds in many areas.

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