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15/6/20

Forecasting tropical cyclones in a changing climate

Written by Dr. Jennifer Saxby
Researcher at the School of Earth and Environment , University of Leeds
@jennifer_saxby

Most of us engage with weather forecasts when we’re trying to plan our weekends, but they can also help us understand and cope with our rapidly changing climate. Data gathered for forecasts is a vital tool in tracking the progress of the current climate emergency; forecasts are also necessary to predict extreme weather events, such as tropical cyclones (also called hurricanes or typhoons), which have claimed about 10,000 lives per year since 1971. Changes to storm intensity, combined with growing coastal populations and the challenge of predicting weather in a changing climate, makes forecasting cyclones a vital area of research.

To understand precisely how tropical cyclone activity will change in a warmer climate, we can use weather and climate models to produce projections. So far, estimates by different scientists agree well: with 2°C global warming, cyclones will occur slightly less frequently, but they will increase in strength, especially for those events that are already most intense. The proportion of high-intensity Category 4-5 cyclones has a projected increase of  13%, based on a median from 11 separate studies.
 
The world has already warmed by 1.1°C relative to pre-industrial times, so can we detect these predicted changes in cyclone activity yet? We don’t have very many years of reliable measurements, making it difficult to understand the most extreme and damaging events, which occur infrequently. There isn’t enough evidence to suggest whether climate change is already impacting cyclone frequency : there was no noticeable increase in the number of tropical cyclones occurring globally between 1970 and 2010. However, there is already evidence to suggest that cyclone intensity is increasing, as predicted; the study only looks at North Atlantic cyclones, so more work is needed to determine whether this is a global trend. 
The increase in cyclone intensity is a worry for forecasters. Track forecasts, which predict the path a cyclone will travel, have steadily advanced: modern 72-hour track forecasts are more accurate than 24-hour forecasts were 40 years ago. However, predictions of storm strength are improving at a much slower pace. A particular worry is that cyclones could intensify more rapidly just before hitting land; rapid intensification is one of the hardest things to forecast. In 2015, Hurricane Patricia in the North Pacific grew from a Category 1 storm to a Category 5 hurricane in 24 hours. In one day, wind speed increased to 207 mph (333 km/hr), an increase of 121 mph from the previous day, but forecasters only predicted an increase of 35 mph. Patricia weakened slightly before making landfall in Mexico, but still caused extensive damage. The effects could have been much worse if the storm had hit land at the end of its rapid intensification, because of poor prediction of the peak intensity it would reach.

Accurately forecasting tropical cyclones is an essential area of research because not only are the worst storms getting worse, at-risk coastal populations are expanding. Risk of tropical cyclone damage is not just a function of the increased hazard (more intense storms), but also exposure: the number of people, or assets such as crops, in hazard-prone areas, and the vulnerability of those people and assets. The population of the world is expected to increase by  2 billion in the next 30 years, and coastal regions generally are more heavily populated and also undergo faster population growth. Between 1970 and 2010, the global population exposed to tropical cyclone hazards almost tripled, with 150 million people estimated to be at risk by 2030. Therefore, we need to offset the effect of increasing coastal populations by better predicting coastal hazards such as cyclones.
This means improving weather forecasts, but it will be a challenge to keep producing better forecasts when our atmosphere is fundamentally changing. There are always uncertainties in weather forecasts because of the chaotic nature of the atmosphere. Changes to the environment in a warmer world have the potential to either increase or decrease the uncertainty in forecasts. We can measure certainty using ensemble forecasts, a recent advance in weather prediction which uses multiple projections with different initial weather states and examines the differences in the results. These studies have shown how climate change could affect day-to-day weather forecasting as well as the prediction of extreme events. For example, it may be easier to forecast temperature and pressure in the northern hemisphere, but we will have less confidence in rainfall predictions. Rainfall depends on myriad atmospheric variables that are complex and difficult to predict, and it can be unevenly distributed in the atmosphere and change rapidly.
 
Changes in the certainty of weather forecasts will have significant implications for future emergency response planners. To maintain confidence in forecasts, we can continue to develop more accurate numerical weather prediction models; this means improving understanding of atmospheric physics, together with increasing computing capability. Another critical area for development is observations such as satellite remote sensing, as we require an accurate picture of the current state of the weather to predict it several days into the future. However, observational data will always have limitations. For example, it’s difficult for satellites to measure sea surface temperature if it’s very cloudy, such as during a tropical cyclone. Still, cyclone prediction relies on this data as storm intensity and sea temperature are linked. Uncertain initial conditions create forecast uncertainties that grow as the forecast runs. The technique of data assimilation, where models are run over time and continuously adjusted with new observations, goes a long way to reduce these uncertainties and has been a recent revolution in forecasting. Assimilating newly available ocean data during the forecasts has a considerable potential to improve the prediction of tropical cyclones.
 
Given the challenges of improving weather forecasts in an uncertain future, understanding and predicting extreme weather is a major area of current research. The World Climate Research Program selected the topic as one of its Grand Challenges, allowing meteorologists and climate scientists worldwide to connect and tackle the challenge of better documenting, understanding, and simulating extreme weather events. For tropical cyclones, the focus is on improving storm intensity predictions; a particular priority is to increase forecasting capability in developing countries, which are hit hardest in terms of risk. By improving our understanding of the complex systems driving storm intensification, we can better forecast hazards such as high winds, rainfall, and storm surges, and increase our capacity to cope with a changing climate.

References

Global trends in tropical cyclone risk scientific paper, Nature Climate Change
Climate change indicators: tropical cyclone activity website, United States Environmental Protection Agency

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