Clear-Air Turbulence and Climate Change

Europe_turbulence_blog
Figure 1: Percentage change of clear-air turbulence over Europe and the North Atlantic

Clear-Air Turbulence (CAT) is a major hazard to the aviation industry. If you have ever been on a plane you have probably heard the pilots warn that clear-air turbulence could occur at any time so always wear your seatbelt. Most people will have experienced it for themselves and wanted to grip their seat. However, severe turbulence capable of causing serious passenger injuries is rare. It is defined as the vertical motion of the aircraft being strong enough to force anyone not seat belted to leave the chair or floor if they are standing. In the United States alone, it costs over 200 million US dollars in compensation for injuries, with people being hospitalised with broken bones and head injuries. Besides passengers suffering serious injuries, the cabin crew are most vulnerable as they spend most of the time on their feet serving customers. This results in an additional cost if they are injured and unable to work.

Clear-air turbulence is defined as high altitude inflight bumpiness away from thunderstorm activity. It can appear out of nowhere at any time and is particularly dangerous because pilots can’t see or detect it using on-board instruments.  Usually the first time a pilot is aware of the turbulence is when they are already flying through it. Because it is a major hazard, we need to know how it might change in the future, so that the industry can prepare if necessary. This could be done by trying to improve forecasts so that pilots can avoid regions likely to contain severe turbulence or making sure the aircraft can withstand more frequent and severe turbulence.

Our new paper published in Geophysical Research Letters named ‘Global Response of Clear-Air Turbulence to Climate Change’ aims at understanding how clear-air turbulence will change in the future around the world and throughout the year. What our study found was that, the busiest flight routes around the world would see the largest increase in turbulence. For example, the North Atlantic, North America, North Pacific and Europe (see Figure 1) will see a significant increase in severe turbulence which could cause more problems in the future. These regions see the largest increase because of the Jet Stream. The Jet Stream is a fast flowing river of air that is found in the mid-latitudes. Clear-air turbulence is predominantly caused by the wind traveling at different speeds around the Jet Stream. Climate change is expected to increase the Jet Stream speed and therefore increase the vertical wind shear, causing more turbulence.

To put these findings in context, severe turbulence in the future will be as frequent as moderate turbulence historically. Anyone who is a frequent flyer will have likely experienced moderate turbulence at some point, but fewer people have experienced severe turbulence. Therefore, this study suggests this will change in the future with most frequent flyers experiencing severe turbulence on some flight routes as well as even more moderate turbulence. Our study also found moderate turbulence will become as frequent in the summer as it has done historically in winter. This is significant because although clear-air turbulence is more likely in winter, it will however now become much more of a year round phenomenon (see Figure 2).

Seasonal_turbulence_blog
Figure 2: Percentage change of clear-air turbulence around the world in all four seasons. No Stipling (stipling) indicates it is (is not) significant at the 90% confidence level.

This increase in clear-air turbulence highlights the importance for improving turbulence forecasting. Current research has shown that using ensemble forecasts (many forecasts of the same event) and also using more turbulence diagnostics than the one we used in this study can improve the forecast skill. By improving the forecasts, we could consistently avoid the areas of severe turbulence or make sure passengers and crew are seat-belted before the turbulence event occurs. Unfortunately, as these improvements are not yet fully operational, you can still reduce your own risk of injury by making sure you wear your seat belt as much as possible so that, if the aircraft does hit unexpected turbulence, you would avoid serious injuries.

Storer, L. N., Williams, P. D., & Joshi, M. M. (2017). Global response of clear-air turbulence to climate change. Geophysical Research Letters, 44, 99769984. https://doi.org/10.1002/2017GL074618

This blog was originaly writen for EGU Blogs

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