Night at the Museum!

On Friday November 30th, Prof. Paul Williams and I ran a ‘pop-up science’ station at the Natural History Museum’s “Lates” event (these are held on the last Friday of each month; the museum is open for all until 10pm, with additional events and activities). Our station was entitled “Turbulence Ahead”, and focused on communicating research under two themes:

  1.  Improving the predictability of clear-air turbulence (CAT) for aviation
  2.  The impact of climate change on aviation, particularly in terms of increasing CAT

There were several other stations, all run by NERC-funded researchers. Our stall went ‘live’ at 6 PM, and from that point on we were speaking almost constantly for the next 3.5 hours – with hundreds (not an exaggeration!) of people coming to our stall to find out more. Neither of us were able to take much of a break, and I’ve never had quite such a sore voice!

IMG_1769
Turbulence ahead? Not on this Friday evening!

Our discussions covered:

  • What is clear-air turbulence (CAT) and why is it hazardous to aviation?
  • How do we predict CAT? How has Paul’s work improved this?
  • How is CAT predicted to change in the future? Why?
  • What other ways does climate change affect aviation?

Those who came to our stall asked some very intelligent questions, and neither of us encountered a ‘climate denier’ – since we were speaking about a very applied impact of climate change, this was heartening. This impact of climate change is not often considered – it’s not as obvious as heatwaves or melting ice, but is a very real threat as shown in recent studies (e.g. Storer et al. 2017). It was a challenge to explain some of these concepts to the general public – some had heard of the jet stream, others had not, whilst some were physicists… and even the director of the British Geological Survey, John Ludden, turned up! It was interesting to hear from so many people who were self-titled “nervous flyers” and deeply concerned about the future potential for more unpleasant journeys.

I found the evening very rewarding; it was interesting to gauge a perspective of how the public perceive a scientist and their work, and it was amazing to see so many curious minds wanting to find out more about subjects with which they are not so familiar.

My involvement with this event stems from my MMet dissertation work with Paul and Tom Frame looking at the North Atlantic jet stream. Changes in the jet stream have large impacts on transatlantic flights (Williams 2016) and the frequency and intensity of CAT. Meanwhile, Paul was a finalist for the 2018 NERC Impact Awards in the Societal Impact category for his work on improving turbulence forecasts – he finished as runner-up in the ceremony which was held on Monday December 3rd.

So, yes, there may indeed be turbulent times ahead – but this Friday evening certainly went smoothly!

Email: s.h.lee@pgr.reading.ac.uk

Twitter: @SimonLeeWx

References

Storer, L. N., P. D. Williams, and M. M. Joshi, 2017: Global Response of Clear-Air Turbulence to Climate Change. Geophys. Res. Lett., 44, 9979-9984, https://doi.org/10.1002/2017GL074618

Williams, P. D., 2016: Transatlantic flight times and climate change. Environ. Res. Lett., 11, 024008, https://doi.org/10.1088/1748-9326/11/2/024008.

A New Aviation Turbulence Forecasting Technique

Anyone that has ever been on a plane will probably have experienced turbulence at some point. Most of the time it is not likely to cause injury, but during severe turbulence unsecured objects (including people) can be thrown around the cabin, costing the airline industry millions of dollars every year in compensation (Sharman and Lane, 2016). Recent research has also indicated that in the future the frequency of clear-air turbulence will increase with climate change. Forecasting turbulence is one of the best ways to reduce the number of injuries by giving pilots and flight planners ample warning, so they can put on the seat-belt sign or avoid the turbulent region altogether. The current method used in creating a turbulence forecast is a single ‘deterministic’ forecast – one forecast model, with one forecast output. This shows the region where they suspect turbulence to be, but because the forecast is not perfect, it would be more ideal to show how certain we are that there is turbulence in that region.

To do this, a probabilistic forecast can be created using an ensemble (a collection of forecast model outputs with slightly different model physics or initial conditions). A probabilistic forecast essentially shows model confidence in the forecast, and therefore how likely it is that there will be turbulence in a given region. For example, if all 10 out of 10 forecast outputs predict turbulence in the same location, the pilots would be confident in taking action (such as avoiding the region altogether). However, if only 1 out of 10 models predict turbulence, then the pilot may choose to turn on the seat-belt sign because there is still a chance of turbulence, but not enough to warrant spending time and fuel to fly around the region. A probabilistic forecast not only provides more information in the certainty of the forecast, but it also increases the chances of forecasting turbulence that a single model might miss.

Gill and Buchanan (2014) showed this ensemble forecast method does improve the forecast skill. In my project we have taken this one step further and created a multi-model ensemble, which is combining two different ensembles, each with their own strengths and weaknesses (Storer et al., 2018). We combine the Met Office Global and Regional Ensemble Prediction System (MOGREPS-G), with the European Centre for Medium Range Weather Forecasting (ECMWF) Ensemble Prediction System (EPS).

Analysis_plot_1_new
Figure 1: Plot of a moderate-or-greater turbulence event over the possible sources of turbulence: top left: orography, shear turbulence (bottom left: MOGREPS-G and bottom right: ECMWF EPS probability forecast), and top right: convection from satellite data (colour shading indicates deep convection). Both the MOGREPS-G and ECMWF-EPS ensembles forecast the shear turbulence event. The circles indicate turbulence observations with grey indicating no turbulence, orange indicating light turbulence and red indicating moderate or greater turbulence. The convective classification can be found in Francis and Batstone (2013).

There are three main sources of turbulence. The first is mountain wave turbulence, where gravity waves are produced from mountains that ultimately lead to turbulence. The second is convectively-induced turbulence, which includes in-cloud turbulence and also gravity waves produced as a result of deep convection that also lead to turbulence. The third is shear-induced turbulence, which is the one we are trying to forecast in this example. Figure 1 is an example plot showing orography and thus mountain wave turbulence (top left), convection and thus convectively induced turbulence (top right), the MOGREPS-G ensemble forecast of shear turbulence (bottom left) and the ECMWF ensemble forecast of shear turbulence (bottom right). The red circle indicates a ‘moderate or greater’ turbulence event, and we can see that because it is over the North Atlantic it is not a mountain wave turbulence event, and there is no convection nearby, but both the ensemble forecasts correctly predict the location of the shear-induced turbulence. This shows that there is high confidence in the forecast, and action (such as putting the seat-belt sign on) can be taken.

Value_1
Figure 2: Value plot with a log scale x-axis of the global turbulence with the 98 convective turbulence cases removed showing the forecast skill of the MOGREPS-G (dot-dash), ECMWF (dot), combined multi-model ensemble (dash) and the maximum value using every threshold of the combined multi-model ensemble (solid). The data used has a forecast lead time between +24 hours and +33 hours between May 2016 and April 2017.

To understand the usefulness of the forecast, Figure 2 is a relative economic value plot. It shows the value of the forecast for a given cost/loss ratio (which will vary depending on the end user). The multi-model ensemble is more valuable than both of the single model ensembles for all cost/loss ratios, showing that every end user will benefit from this forecast. Although our results do show an improvement in forecast skill, it is not statistically significant. However, by combining ensemble forecasts we gain consistency and more operational resilience (i.e., we are still able to produce a forecast if one ensemble is not available), and is therefore still worth implementing in the future.

Email: luke.storer@pgr.reading.ac.uk

References

Gill PG, Buchanan P. 2014. An ensemble based turbulence forecasting system. Meteorol. Appl. 21(1): 12–19.

Sharman R, Lane T. 2016. Aviation Turbulence: Processes, Detection, Prediction. Springer.

Storer, L.N., Gill, P.G. and Williams, P.D., 2018. Multi-Model Ensemble Predictions of Aviation Turbulence. Meteorol. Appl., (Accepted for publication).