The impact of nocturnal low-level jets on the wind industry

Local weather conditions have a significant impact on the progression of the wind industry, with nocturnal low-level jets (NLLJs) posing risks to the safety and efficiency of wind operations globally. That’s why accurately predicting these strong, low-altitude winds is integral to weather forecasting remits.

By Neville Smith, Regional Manager Forecasting and Consultancy, Metocean Science, Europe and Africa, Fugro

Understanding nocturnal low-level jets and their impact

NLLJs are fast moving winds that occur at low levels of the atmosphere during the night.

These jets can significantly influence weather conditions when they break down, generating downdrafts that disrupt the typical pattern of surface winds. These downdrafts can also trigger the emission of large quantities of dust, which can significantly impact offshore and onshore wind operations, making it crucial to understand and accurately forecast them.

Wind turbines are greatly impacted by these jets, which can lead to sudden and unexpected changes in wind speed and direction, affecting the performance and state of wind turbines.

The increased wind speeds associated with NLLJs can cause mechanical stress on turbine components, potentially leading to increased maintenance costs and reduced operational lifespan. These large emissions of dust can also cause wear and tear on mechanical components, reducing the efficiency of air intakes and cooling systems.

These fast-moving ribbons of air at low altitudes have been reported in many geographic regions at certain times of year. In Africa, for example, the weather conditions are favourable for the formation of NLLJs between late November and mid-March; and in the USA’s Great Plains it’s during the spring and summer.

The morning breakdown of NLLJs and subsequent increase in surface wind speed can cause huge quantities of dust to be lifted from the ground. This happens when the wind reaches a critical speed, capable of picking up loose particles. Once airborne, the particles can be transported over long distances.

In some West African countries, the emissions are so extensive that locals refer to this effect as ‘Harmattan haze’.

The haze is rather like a heavy, extremely dry fog which impedes visibility. Its effects are far-reaching. The fine dust poses a serious health risk, especially to the respiratory system. Airlines face financial losses due to cancelled and diverted flights. And when the dust finally settles, it can cause significant crop damage.

The science behind NLLJ formation

NLLJs form due to a combination of atmospheric conditions, such as dry air moving into an area. For example, the cool, dry Harmattan wind in West Africa brings dry air from the Central Sahara, reducing humidity.

This means that there are fewer water molecules to absorb heat from the surface, leading to faster-than-usual drops in overnight air temperature. This increases the pressure gradient in the lower atmosphere which, in turn, increases wind speeds and leads to the formation of NLLJs.

Bridging the gap between models and expertise

When forecasters rely solely on the output of forecast models, the risk of overlooking these jets can be high. There’s a very simple explanation. NLLJs are small-scale, complex weather phenomena, but forecast models typically have a relatively coarse resolution, which prevents them from capturing the development of NLLJs and predicting their impact.

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However, now is a critical time to challenge this trend. By drawing from existing knowledge and practical experience, combined with outputs from observations and models, forecasters can apply their expertise to make informed decisions that add value, even if the models fail to capture the feature in question.

Expert intervention: A case study of NLLJ prediction in the Niger Delta

In February 2024, a forecaster noticed a pattern in the Niger Delta: calm overnight winds that were escalating each day between 09:00 and 12:00. The forecaster recognised that these windspeeds were typical of NLLJ activity.

Dry Harmattan conditions were expected to continue. With this in mind – and despite the absence of the morning wind speed peaks in the model output – the forecaster manually adjusted their predictions for the next few days to reflect the anticipated continuation of the jet breakdown each day.

After comparing the subsequent observations with the manually adjusted forecast, it was apparent that the NLLJ cycle had continued. This example highlights the added value that informed human intervention can provide for operations, above and beyond the limitations of weather forecast models.

An early warning success in North Africa

On the morning of 23 January 2024, the breakdown of NLLJs triggered powerful surface winds that lifted huge quantities of dust into the atmosphere in Central Algeria.

Weather forecast models showed that easterly winds were expected to continue, so a forecaster issued an early warning of the winds moving toward Senegal. The warning was based on the anticipation of westward advection of dust, even though none of the dust forecast models picked this up. The predictions proved to be accurate: the dust plume was dispersed to Senegal, where a significant drop in visibility was recorded in station observations the following day at Dakar Airport.

Dust outbreaks significantly impact wind operations across the world, reflecting the importance of accurately forecasting NLLJs. This successful example reinforces the value of human intervention in helping to improve forecast accuracy and provide information that helps operations work safely and effectively.

Read more: Why advanced time series data is pivotal for enhancing APAC’s offshore wind resources

The path to operational safety

The accurate prediction of nocturnal low-level jets is essential for ensuring the safety and efficiency of wind operations. By combining advanced forecasting models with expert human intervention, reliable and actionable insights can be provided to help mitigate the risks associated with NLLJs.

Commitment to understanding and forecasting NLLJs not only enhances operational safety but also supports the industry in navigating the challenges posed by these unique weather events. Leveraging cutting-edge technology and human expertise offers businesses the opportunity to set new standards in weather forecasting and operational safety.