Thunderstorms cause heavy rain, flash floods, damaging winds, and often scare dogs and small children. Thunderstorms also do lots of complicated things like reflect sunlight back to space, warm the atmosphere, and help change wind patterns across the entire planet. Many people think thunderstorms are amazing and lots of scientists spend their lives researching them. Therefore, it might be surprising that we don’t fully understand how thunderstorms form.
Also, the complicated computer models that we use to make weather forecasts and tell us about future climate make weird-looking thunderstorms that are the wrong size and shape and normally rain too much or too little.
We know the things that are important for making clouds grow, like the way the temperature changes with altitude and the amount of water vapour in the air. But, to make clouds become big thunderstorms that last a long time something else has to happen. The explanation we typically teach at university is that cold air falls out of the bottom of clouds, which then pushes the air near the surface upwards to make new clouds. When the wind near the surface changes with altitude in just the right way, these new clouds will form thunderstorms that grow bigger and last longer than single clouds.
Most meteorologists are happy with this theory, but it doesn’t work all the time. Something else has to make thunderstorms grow when the wind near the surface doesn’t change very much with altitude or the cold air that comes out of the bottom of the storm isn’t that cold.
CLEX researcher, Prof Todd Lane, used a simple computer model of clouds to investigate another explanation for thunderstorm formation. The model assumes the Earth is flat, doesn’t spin, and doesn’t have land, but the clouds and thunderstorms in the model are actually quite realistic. Using this model, he showed the way the wind varies with altitude above the clouds can change the way thunderstorms grow and how long they live. This is a new idea and what is important is that these wind changes are in the stratosphere, which is well above the top of the thunderstorms.
The way this works is that storms cause ripples in the wind that travel upwards and away from the clouds, much like a stone causes ripples when it is thrown in a pond. Meteorologists call these ripples atmospheric gravity waves or buoyancy waves. Changes in the wind above the clouds reflect the ripples back downwards, which can then affect the temperature and winds around the storms and make them grow, last longer, or die earlier than they otherwise would. This is a complicated process, and to be honest, the researchers are not sure how often it occurs in the real world.
We need to spend more time searching for it in real data from satellites, or in more complicated computer models that look more like Earth. If this mechanism is important, it will make us think carefully about how we make thunderstorms look and behave more realistically in the computer models used for predicting weather and climate. The results should also remind us that the world is a complicated place and we still have things to learn about how storms actually work, which is a prerequisite for understanding how they will change in the future.
- Paper: Lane, T. P. (2021). Does lower‐stratospheric shear influence the mesoscale organization of convection? Geophysical Research Letters, 48, e2020GL091025. https://doi.org/10.1029/2020GL091025