Sting Jets

Early on October 15, 1987, an innocuous low-pressure system was moving across the Bay of Biscay off the west coast of France. Within one day, it became one of the most strongest windstorms in European history. Poorly anticipated, the storm produced hurricane-force sustained winds for hours over portions of Great Britain and France as well as absurdly strong gusts. The highest measured during the storm was 135 mph, corresponding to category 4 on the hurricane Saffir-Simpson scale (the cyclone was not tropical, however, so the word "hurricane" did not apply). Subsequently known simply as the "Great Storm of 1987," it prompted further study of the mechanisms by which extratropical storms produce extreme winds.

The above satellite image shows the Great Storm of 1987 with a long frontal "tail" extending all the way down to the Canary Islands.

By the time of the Great Storm, the overall genesis process for extratropical cyclones was well understood. The energy for extratropical cyclone formation ultimately derives from temperature differences: cold air from the polar regions meets warm air from the subtropics, usually between 30 and 60 degrees latitude north and south. At these interfaces, there are differences in air pressure at the same altitude in the atmosphere since cold air is denser than warm. This instability provides the energy to drive cyclone formation.

Under the right circumstances, small perturbations in the flow along a boundary of air masses can trigger the formation of a low-pressure system, as indicated above. The cyclonically rotating boundaries between warm and cold air become the warm and cold fronts that control weather in the mid-latitudes. Note that all diagrams, including that above, are the correct orientations in the Northern Hemisphere: the directions of spin would be reversed south of the equator. Our concern in this post is investigating where the strongest surface winds occur in these extratropical storms.

The above schematic shows major low-level winds associated with an extratropical cyclone at different stages of development. These are typical of a rapidly developing and strong storm, which is assumed to be moving northeast. As the storm ramps up, the dominant feature is the mild and wet Warm Conveyor Belt (WCB). This feeds the center a supply of moisture; indeed, nearly all the precipitation occurs ahead (east) of the advancing cold front boundary. Windy conditions can accompany the WCB, but they are not usually too extreme.

In the wake of the cold front comes the chilly and dry Cold Conveyor Belt (CCB). Most intense in a mature storm, this feature often packs stronger winds than the WCB, though they occur after precipitation has passed. Both of these are large-scale, well-understood features, but could not account for the unusually strong surface winds observed in some rapidly intensifying storms. It is the third feature above that fills in the gap: the so-called Sting Jet (SJ).

Named for the "sting at the end of the tail", the sting jet occurs near the very tip of the cloud head, where the bent-back cold front in the diagram above ends. This feature occurs most commonly in cyclones that explosively intensify or "bomb cyclones". The technical definition for this is a pressure drop of 24 mb or more in a period of 24 hours. As shown in the diagram below, just east of this "tail" of the cloud structure, instability causes dry air to descend from high in the atmosphere. Below this is the sting jet. It is a smaller feature compared to the CCB and WCB, about 100 km wide instead of several hundred. This conveyor belt of air is pushed toward the ground by the intruding dry air above.

Typically, friction with the land (or ocean) keeps winds near the surface lower than the strongest winds a few thousand feet above sea level. However, the descending site jet can transport these strong winds quickly to the surface. Moreover, the sting jet comes just head of the CCB (written CJ in the above picture) out of the south or southwest. In a cyclone moving northeast, these winds align with the storm's direction of motion, boosting them even higher. The result: localized but extremely intense wind gusts at the ground, associated with little to no precipitation.

Nearly all documented examples of sting jets are associated with north Atlantic storms impacting Europe. Since the Great Storm of 1987, roughly a dozen more examples have been positively identified. Satellite data indicate that further events likely occur over water where surface observations are sparse. Few studies have investigated the occurrence of sting jets elsewhere around the world, but explosive intensification of extratropical cyclones also occurs in the northwest Pacific and near Antarctica. Fortunately, comparable events in these regions have far fewer human impacts.

A thorough survey of the causes of sting jets is beyond the scope of this post; however, our understanding of this phenomenon is far from complete. Computer models struggle to resolve the feature, especially its tendency to "fan out" in to many small jets near the surface. As a result, predicting these events is still difficult. There is a lot on the line: the Great Storm of 1987 killed 22 people and caused billions in damages. Hopefully, future advances in advance warning will avert the worst impacts of these powerful storms.

Sources: https://journals.ametsoc.org/jcli/article/30/14/5455/97090/Sting-Jet-Windstorms-over-the-North-Atlantic, https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/wind/sting-jet, https://rmets.onlinelibrary.wiley.com/doi/abs/10.1256/qj.02.143, https://rams.atmos.colostate.edu/at540/fall03/fall03Pt5.pdf, https://www.britannica.com/science/cyclogenesis, https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/qj.3267