Tropical thunderstorms are set to grow stronger as the world warms

By Martin Singh
16 October 2017
(The Conversation) – Thunderstorms are set to become more intense throughout the tropics and subtropics this century as a result of climate change, according to new research.Thunderstorms are among nature’s most spectacular phenomena, producing lightning, heavy rainfall, and sometimes awe-inspiring cloud formations. But they also have a range of important impacts on humans and ecosystems.For instance, lightning produced by thunderstorms is an important trigger for bushfires globally, while the hailstorm that hit Sydney in April 1999 remains Australia’s costliest ever natural disaster.Given the damage caused by thunderstorms in Australia and around the world, it is important to ask whether they will grow in frequency and intensity as the planet warms.Our main tools for answering such questions are global climate models – mathematical descriptions of the Earth system that attempt to account for the important physical processes governing the climate. But global climate models are not fine-scaled enough to simulate individual thunderstorms, which are typically only a few kilometres across.But the models can tell us about the ingredients that increase or decrease the power of thunderstorms. Thunderstorms represent the dramatic release of energy stored in the atmosphere. One measure of this stored energy is called “convective available potential energy”, or CAPE. The higher the CAPE, the more energy is available to power updrafts in clouds. Fast updrafts move ice particles in the cold, upper regions of a thunderstorm rapidly upward and downward through the storm. This helps to separate negatively and positively charged particles in the cloud and eventually leads to lightning strikes.To create thunderstorms that cause damaging wind or hail, often referred to as severe thunderstorms, a second factor is also required. This is called “vertical wind shear”, and it is a measure of the changes in wind speed and direction as you rise through the atmosphere. Vertical wind shear helps to organise thunderstorms so that their updrafts and downdrafts become physically separated. This prevents the downdraft from cutting off the energy source of the thunderstorm, allowing the storm to persist for longer.By estimating the effect of climate change on these environmental properties, we can estimate the likely effects of climate change on severe thunderstorms.My research, carried out with US colleagues and published today in Proceedings of the National Academy of Sciences, does just that. We examined changes in the energy available to thunderstorms across the tropics and subtropics in 12 global climate models under a “business as usual” scenario for greenhouse gas emissions.In every model, days with high values of CAPE grew more frequent, and CAPE values rose in response to global warming. This was the case for almost every region of the tropics and subtropics.These simulations predict that this century will bring a marked increase in the frequency of conditions that favour severe thunderstorms, unless greenhouse emissions can be significantly reduced. [more]

ABSTRACT: Intense thunderstorms produce rapid cloud updrafts and may be associated with a range of destructive weather events. An important ingredient in measures of the potential for intense thunderstorms is the convective available potential energy (CAPE). Climate models project increases in summertime mean CAPE in the tropics and subtropics in response to global warming, but the physical mechanisms responsible for such increases and the implications for future thunderstorm activity remain uncertain. Here, we show that high percentiles of the CAPE distribution (CAPE extremes) also increase robustly with warming across the tropics and subtropics in an ensemble of state-of-the-art climate models, implying strong increases in the frequency of occurrence of environments conducive to intense thunderstorms in future climate projections. The increase in CAPE extremes is consistent with a recently proposed theoretical model in which CAPE depends on the influence of convective entrainment on the tropospheric lapse rate, and we demonstrate the importance of this influence for simulated CAPE extremes using a climate model in which the convective entrainment rate is varied. We further show that the theoretical model is able to account for the climatological relationship between CAPE and a measure of lower-tropospheric humidity in simulations and in observations. Our results provide a physical basis on which to understand projected future increases in intense thunderstorm potential, and they suggest that an important mechanism that contributes to such increases may be present in Earth’s atmosphere.SIGNIFICANCE: A substantial fraction of the world’s most intense thunderstorms occur in the tropics and subtropics, but the response of such storms to climate change remains uncertain. Here, we show that, in simulations of global warming, a measure of the energy available to thunderstorms increases robustly across the tropics and subtropics. Furthermore, we elucidate an important mechanism contributing to such increases in available energy, and we present observational evidence that this mechanism is present in Earth’s atmosphere. By combining theory, observations, and models, our results provide confidence in climate model projections of future intense thunderstorm potential; such model projections are shown to imply large future increases in the frequency of damaging thunderstorm environments in tropical and subtropical regions.

Increasing potential for intense tropical and subtropical thunderstorms under global warming