Atmospheric tug-of-war and why turbulence is winning more often

Canadian Science Writers' Association blog, Nicola Temple, 18 March 2014

As a Canadian currently living in the UK, I am no stranger to the trans-Atlantic flight. As I prepare for a flight to the US later this week I find myself thinking of a time when seat belts seemed more optional as we cruised above the clouds. Now we are ardently reminded to fasten our seat belts when seated and it's likely that the seatbelt sign may be on for more of the journey in the future.

Aircraft encounter air turbulence at cruising altitudes and although most incidents are rarely enough to spill your coffee, others can be significant - causing injury to passengers and cabin crew as well as structural damage to the aircraft itself. This type of turbulence can't be seen by the pilots and isn't visible to on-board radar or satellites. It's known as clear-air turbulence and research suggests that trans-Atlantic flights will encounter it more frequently as atmospheric carbon dioxide levels increase.

Clear-air turbulence is caused by instabilities in the atmosphere. The Earth's atmosphere is stratified - the air is the most dense close to the ground; as you move up in the atmosphere the air becomes less dense. This layering has a stabilizing effect because it inhibits parcels of air from travelling vertically up or down. Wind, however, travels at different speeds at different altitudes, generally getting faster the higher up you go. This is known as wind shear and this can create a lifting mechanism that allows air to travel vertically, which has a destabilizing effect.

These two processes - stratification and wind shear - are in a constant state of tug-of-war and 99 percent of the time it is the stabilzing stratification that wins out and there's no turbulence. The remaining one percent of the time, however, wind shear is strong enough to offset the balance, creating turbulence.

Now introduce climate change into this game of tug-of-war. Carbon dioxide has a warming effect in the lower part of our atmosphere, but higher in the stratosphere it is having a cooling effect. The stratosphere is the upper atmosphere, about 14 to 22 kilometres above the surface, and contains the protective UV absorbing ozone layer. Depleted ozone means less UV radiation is absorbed and less heat is generated, having an overall cooling effect. It is the temperature difference between this upper atmosphere and lower atmosphere that drives the jet stream and rising CO2 levels means that this difference is going to get bigger. This will lead to a stronger jet stream and wind shear will begin to win at atmospheric tug-of-war more often; our atmosphere will be more unstable more of the time.

A study in 2013 out of the UK looked at how clear-air turbulence would be affected by increased CO2 levels.  They ran 20 years of simulations using as many turbulence diagnostics as they could find and in all cases, clear-air turbulence increased in both strength and frequency of occurrence with increasing CO2. Under a doubled CO2 scenario, predicted to occur around 2050 if we continue on our current trajectory, the simulations predicted a 10 to 40 percent increase in the strength of the turbulence and a 40 to 170 percent increase in the frequency of occurrence for flights within the trans-Atlantic corridor.

Does this mean that flights of the future will have passengers strapped in for eight hours or more as they cross the Atlantic? Can we kiss what's left of in-flight service goodbye? Not likely. The European Commission has funded research collaborations between academics and industry that is developing technology that might help. For example, the DELICAT project (Demonstration of LIdar based Clear Air Turbulence detection) has demonstrated that it's possible to detect clear-air turbulence accurately and reliably at a distance of about 10 nautical miles ahead of the aircraft. This means that with further development a device could be installed on future aircraft that would detect the turbulence about one minute before the plane reaches it. Not exactly a lot of warning, but perhaps just enough time to get trolleys secured and passengers and cabin crew seated with seat belts secured. The new technology could significantly reduce the number of injuries, but it isn't likely to be part of on-board instrumentation for at least another five years.

Advancements in predictive models may also help improve forecasts as to where clear-air turbulence is likely to be on a flight path. These predictions may result in route alterations to avoid turbulent areas, leading to longer flights and increased fuel costs.

So, until we are better able to predict and detect clear-air turbulence, I will be buckled up on my flight this week, avoiding hot drinks and out-of-control trolleys. I will also be wallowing in my own guilt that my transportation choice is of course contributing to the situation that will likely make flights bumpier in the future. Ah. Yes. The irony.