Sting jets and explosive cyclones

A sting jet is a mesoscale (20–100 km wide) airstream of air that descends rapidly within an extratropical cyclone, reaching the surface and causing a narrow corridor of extremely strong winds. The term 'sting jet' was coined by researchers at the University of Reading in 2003, drawing an analogy to the 'scorpion's tail' shape often seen in satellite imagery of the cloud head. They are a feature of some, but not all, rapidly deepening cyclones.

The primary challenge with sting jets is their small spatial scale and transient nature. Conventional observation networks, such as surface weather stations and radiosondes, are often too sparse to definitively capture their presence and full intensity. Evidence for sting jets often relies on a combination of:

• Satellite imagery: The characteristic 'cloud head' feature, often with a hooked shape, can indicate the potential for a sting jet.
• Radar data: High-resolution radar can sometimes resolve the intense precipitation bands associated with the descending air.
• Atmospheric reanalysis: Datasets like ERA5, which combine observations with model simulations, can reconstruct the atmospheric structure and infer sting jet presence.
• Damage surveys: Post-event analysis of wind damage patterns can often align with the narrow, intense corridors expected from a sting jet. This was notably the case for the 'Great Storm of 1987' in the UK, which is now widely believed to have featured a sting jet.

Despite ongoing research, the exact frequency of sting jets and the precise conditions for their formation remain subjects of scientific inquiry. They are commonly cited by meteorologists as one of the most difficult phenomena to forecast with high confidence due to their small scale and sensitivity to initial atmospheric conditions.

Sting jets are typically, though not exclusively, associated with 'bomb cyclones' or 'explosive cyclogenesis'. A bomb cyclone is defined as an extratropical cyclone where the central pressure falls by at least 24 hPa in 24 hours, adjusted for latitude. The adjustment factor is sin(latitude) / sin(60°).

For Ireland, at approximately 53° N latitude, the threshold for explosive cyclogenesis is:

24 hPa * (sin(53°) / sin(60°)) = 24 hPa * (0.7986 / 0.8660) ≈ 22.1 hPa in 24 hours

This rapid deepening indicates an intense system with strong pressure gradients, which provides the dynamic environment conducive to sting jet formation. However, not all bomb cyclones produce sting jets, and some sting jets may occur in systems that do not strictly meet the 24 hPa criterion.

The Wind Agent's pressure tendency chart can help identify potential bomb cyclones by showing the rate of pressure change. A sustained drop exceeding this threshold over 24 hours, particularly when combined with strong wind forecasts, warrants close attention. These events are often associated with significant impacts, including coastal flooding, high seas, and widespread wind damage.

The formation of a sting jet is linked to the complex interaction of airstreams within a rapidly deepening cyclone. As the cyclone intensifies, a dry airstream from the mid-troposphere wraps around the developing low-pressure centre. This dry air, often originating from behind the cold front, descends rapidly.

Simultaneously, a moist airstream, often referred to as the 'warm conveyor belt', ascends ahead of the cold front. The descending dry air, cooler than its surroundings due to evaporative cooling from precipitation, accelerates downwards. This cooling increases its density, enhancing the downward momentum.

As this descending air reaches the surface, it brings high momentum from aloft, resulting in a narrow band of extremely strong winds. The 'cloud head' visible in satellite imagery is often the signature of this process, with the 'sting' appearing where the cloud mass narrows and curves, indicating the trajectory of the descending air. The air within the sting jet is typically dry, which can lead to a 'clear slot' or 'dry slot' on the southern flank of the cloud head, as seen in visible satellite imagery.

The vertical profile of wind within a sting jet shows a strong increase in speed with height in the upper part of the jet, followed by a rapid descent to the surface. This high momentum transfer is what differentiates sting jets from other strong wind events.

While models do not explicitly label 'sting jets', their presence can be inferred from specific patterns in high-resolution numerical weather prediction (NWP) model output. Key indicators include:

• High wind speeds at low levels: Look for narrow bands of extreme surface wind speeds (10 m wind) that are significantly higher than the surrounding areas.
• Strong vertical wind shear: Examine the wind profile for a rapid increase in wind speed with height in the lower troposphere, particularly in the mid-troposphere, followed by strong winds extending to the surface. The Shear Glass can help visualise this.
• Descending air: Look for strong downward vertical velocities (omega, or pressure velocity) in the mid-troposphere, especially on the poleward side of the cold front.
• Dry intrusions: Identify regions of low relative humidity in the mid-troposphere descending towards the surface, often associated with the dry slot.
• Characteristic cloud features: While models simulate clouds, visual satellite imagery remains a primary tool for identifying the 'cloud head' signature.

Forecasters often examine multiple model runs and ensemble members to assess the likelihood and potential location of a sting jet. The small scale means that slight differences in initial conditions can lead to significant variations in the forecast sting jet's position and intensity.

Storm Eunice, which impacted Ireland and the UK on 18 February 2022, is a well-documented example where a sting jet was strongly suspected and subsequently confirmed by post-analysis. The storm exhibited classic characteristics: rapid deepening, a distinct cloud head on satellite imagery, and a narrow corridor of exceptionally damaging winds.

In Ireland, while the most extreme winds from Eunice affected the south-west coast, the system demonstrated how quickly conditions can escalate. Met Éireann issued Status Red wind warnings for several counties, indicating the potential for severe and destructive gusts. The highest gust recorded was 130 km/h at Roche's Point, Co. Cork, with widespread gusts exceeding 100 km/h in affected areas.

Another historical example is the 'Great Storm of 1987', which caused widespread damage across southern England and northern France. Although the term 'sting jet' had not yet been coined, reanalysis and research have since indicated that a sting jet was a key factor in the extreme winds observed. These events underscore the destructive potential of sting jets and the importance of understanding their dynamics, even if precise forecasting remains challenging. The Wind Agent's evidence records can help track measured conditions against forecasts during such events.

Forecasting sting jets is highly sensitive to the initial conditions fed into numerical weather prediction models. Small errors or uncertainties in the observations used to initialise a model run can lead to significant differences in whether a sting jet is forecast to develop, and if so, its exact location, timing, and intensity. This sensitivity is a major reason why ensemble forecasting systems are particularly valuable for these types of events.

Ensemble forecasts run the same model multiple times with slightly perturbed initial conditions, creating a range of possible future scenarios. When multiple ensemble members show the development of a sting jet or a narrow corridor of extreme winds, even if their exact positions differ, it increases confidence that such an event is possible. Conversely, if only a few members show a sting jet, the confidence in its occurrence is lower.

For users of The Wind Agent, examining the ensemble plume for gust speeds is crucial during periods of potential explosive cyclogenesis. A wide spread in the plume, particularly for gust forecasts, indicates high uncertainty and suggests that extreme conditions are possible but their precise manifestation is not yet certain. This uncertainty should be factored into operational decision-making, favouring more conservative limits.

When model output suggests the potential for a sting jet, even if uncertain, a heightened level of vigilance is required. Given their destructive potential, the following steps are commonly advised:

1. Review official warnings: Always prioritise official Met Éireann warnings. If a Status Orange or Red wind warning is issued, it indicates a significant threat.
2. Monitor high-resolution models: If available, consult higher-resolution models (e.g., AROME, HARMONIE) which are better equipped to resolve mesoscale features like sting jets.
3. Examine ensemble forecasts: Pay close attention to the spread in ensemble members for gust speeds and consider the most extreme outcomes shown by a reasonable fraction of members.
4. Use The Wind Agent's Shear Glass: Observe the wind profile at your working height. A sting jet would manifest as exceptionally strong winds at all levels, with high momentum transfer to the surface.
5. Prepare for rapid changes: Sting jets can develop and move quickly. Be prepared for a sudden onset of extreme winds and rapid changes in direction.
6. Secure equipment: Ensure all loose equipment, structures, and vessels are secured or removed from exposed areas.
7. Consult local expertise: Local knowledge of terrain effects can be invaluable in understanding how extreme winds might behave in specific locations.

Remember, the goal is not to predict a sting jet with absolute certainty, but to recognise the atmospheric conditions that could produce one and to prepare for the associated hazards. The Wind Agent's exceedance fan can help quantify the probability of exceeding your operational limits under such conditions.