Climate change is making thunderstorm winds much more widespread

Severe thunderstorms produce damaging winds, large hail and tornadoes, but climate scientists have debated how they’re affected by rising temperatures. Now a new study published in Nature Climate Change estimates the central United States has seen a dramatic increase in the coverage of severe thunderstorm winds over the past four decades and that human-caused climate change is contributing. The findings suggest a fivefold increase in the area affected by straight-line winds since the early 1980s. Straight-line winds are different from tornadoes, since they feature downward-moving air that fans out in a straight line, but can cause just as much damage. It’s estimated that thunderstorm winds cause an average of 26 deaths and $2.5 billion in damages each year in the United States. Among the most serious windstorms are derechos — large, arcing thunderstorm complexes that deliver a wall of straight line winds as strong as a hurricane. On rare occasions, such winds can exceed 130 mph. That’s what happened on Aug. 10, 2020, when a violent derecho formed in eastern Nebraska, blasted Iowa and continued into Chicago. Satellite data suggested that 10 million acres of corn, representing 43 percent of Iowa’s crop, was damaged. A staggering $7.5 billion in damage resulted, making it the most costly severe thunderstorm event in U.S. history. Experts fear that similar episodes may become increasingly common in a warmer climate but scientists say understanding the climate change influence on thunderstorm winds has proven challenging. “Wind is something extremely difficult to assess and is the peril we probably know the least about in terms of potential change due to anthropogenic warming,” Victor Gensini, a professor of meteorology and severe storms expert at Northern Illinois University, said in an email. The study’s findings Historically, thunderstorm-scale hazards like tornadoes, downbursts and hail were too small for computerized climate models to simulate. It’s also difficult for scientists to ascertain trends when the coverage of observation networks — composed of weather stations — is irregular and incomplete at best. Because of modern computing technology, however, scientists at the National Center for Atmospheric Research were able to develop models with a fine enough resolution, or grid spacing, to simulate the processes at work within individual thunderstorm cells. The study was led by Andreas Prein, a scientist at the National Center for Atmospheric Research in Boulder, Colo. Prein ran his model over the central U.S., dividing the region into 109,387 grid points. The model depicted a 4.8 fold increase in the area affected by significant severe winds above 85 mph. However, that’s just model output and, like anything from a computer model, needs to be taken with a grain of salt. That’s why Prein compared the results to actual data sampled at 95 weather stations scattered about the Plains. He selected the maximum summertime wind gust every year from each weather station and verified it was “convective,” or thunderstorm-driven, by reviewing satellite cloud data at the time. “I think the nice thing is that the model is reproducing what we see at the stations pretty well,” Prein said in an interview. “The model is giving us a robust but conservative estimate; I have trust in this number. It seems the model is underdoing the change compared to the observations we have.” The results suggest that maximum wind speeds are increasing by 2.58 percent per decade, which complements the expanding areal coverage shown by computer models. Scientists not involved in the research said the results made physical sense but weren’t convinced much confidence could be placed in the magnitude of the changes computed. “I don’t think a suggested increase in spatial coverage of straight-line thunderstorm wind hazards is necessarily surprising given our understanding of projected changes of large-scale environments as a result of anthropogenic [human-caused] warming,” Maria Molina, an atmospheric scientist at the University of Maryland, said in an email. “However, I think the study limitations, caveats, and sources of uncertainty are very important to communicate (by the media) with the public.” Among these uncertainties? Convective initiation, or the actual sparking of thunderstorms. Molina noted that, even though more fuel may be available for storms in a warmer world, other processes at play could actually cut back on how many storms form. Prein acknowledged that. “We see that, on average, you get less storms,” he said. “You suppress a lot of the weak storms. They don’t trigger anymore. But those that do can be more extreme. You get a decrease in moderate or low intensity storms, but an increase in extreme storms.” What’s causing this? It’s believed that human-caused climate change is causing a swifter warming of the lower atmosphere than higher altitudes. That makes for a starker temperature difference between rain-cooled air descending from aloft and the surrounding mild air outside the storm. Air will sink as long as it’s chillier, and therefore denser, than its surroundings. If the temperature difference between air in the downdraft, or downward-moving air in the storm, and the ambient environment increases, the air in the downdraft will be accelerated downward more quickly. That translates to pockets of air hitting the ground and splattering outward more quickly, resulting in greater surface wind gusts. It’s also probable that warmer surface temperatures are increasing the amount of CAPE, or Convective Available Potential Energy — thunderstorm fuel — for storms to work with, exacerbating their severity. Still, more work is needed to confirm the veracity of these findings. “We need more such studies, using different numerical models and associated experiment configurations, so that we can continue to gain insights into future changes of hazards associated with intense thunderstorms,” Molina said.