What's your worst hurricane experience

Hurricanes: The worst comes at last

Delayed effect: hurricanes often cause their greatest damage not from the wind, but from their rain. A study has now shown that these heavy rainfalls are not only becoming more intense - they are also discharging more and more frequently over land. Because the heaviest precipitation now often only occurs when the hurricane has weakened into a tropical storm after landfall.

Studies show that tropical and extra-tropical storms are already getting stronger and more rainy due to climate change. Because with rising temperatures, the storm gains more energy, at the same time warmer air can absorb more moisture, which is then discharged in torrential rain. In addition, tropical cyclones are now moving more slowly than 50 years ago and sometimes even stop over a region, as was the case with Hurricane Harvey in the summer of 2017.

Heavy rain only after it had subsided

But there is still one change in hurricanes and the like, as Danielle Touma from the University of California at Santa Barbara and her colleagues have now found out. For their study, they examined how wind intensity and rainfall from cyclones develop before and after they land. To do this, they evaluated data from weather stations in the southern United States from 1900 to 2017.

The result: The rainiest phase of a hurricane does not threaten when it lands, when it is still at its greatest strength, but afterwards. "You might think that hurricanes are most dangerous when their winds are the fastest, but that's not true," says Touma's colleague Samantha Stevenson. "Our study shows that the risk is greatest when the hurricane has already weakened."

More rain after landfall

This means, however, that the most severe floods are not brought about by a hurricane directly on the coast, but when it has already moved several kilometers into the country. This delayed heavy rain is most evident in strong cyclones, as Touma and her colleagues found. The trend towards late rain is also noticeable in weaker cyclones, even if the total amount of rain is lower in these.

And climate change also seems to play a role: In the past 60 years, the amount of heavy rain falling over land has increased significantly during the cyclones, as the researchers report. The strong hurricanes in particular transport more and more water further inland, where they then rain it down after they have weakened into a tropical storm. "As a result, the risk of flooding in these regions has increased over the past few decades," said Touma and her team.

For regions of the world at risk from hurricanes, this means that areas further inland also have to increasingly adapt to the effects of storms. “Our work now provides more information about what types of storms we need to prepare for in the disaster control plans,” says Stevenson. (Geophysical Research Letters, 2019; doi: 10.1029 / 2019GL083452)

Source: University of California - Santa Barbara

December 9, 2019

- Nadja Podbregar