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November 12, 2020 01:00 pm

Hurricanes Might Not Be Losing Steam As Fast As They Used To

An anonymous reader quotes a report from Ars Technica: [A] new study [published in the journal Nature] by Lin Li and Pinaki Chakraborty at the Okinawa Institute of Science and Technology Graduate University focuses on a less-than-obvious question: what happens to hurricanes after landfall in a warming world? Once a storm moves over land, it loses the water vapor from warm ocean waters that fuel it, so it rapidly weakens. The total damage done depends in part on how quickly it weakens. The researchers examined a data set of all North Atlantic landfalling hurricanes between 1967 and 2018. The primary metric they were interested in was the rate the hurricane lost strength over the first 24 hours after landfall. Strength "decays" on an exponential curve, so they boiled this down to a mathematical parameter for decay time. This parameter varies a fair bit from storm to storm depending on weather conditions and terrain, so the researchers compared averages for each half of the 50-year period. They found a pretty strong trend. In the earlier 25-year period, the average storm lost about 75 percent of its strength over the first day. In the latter half, the average storm lost only half of its strength. The researchers also analyzed sea surface temperatures in this area, which have obviously increased over the last 50 years. That means there's a rough correlation between warmer ocean temperatures and hurricanes retaining strength after landfall. But is there a physical reason to believe the former caused the latter? To test this, the researchers used a computer-model simulation of an idealized hurricane -- that is, a hurricane in a homogenous virtual setting rather than above a specific location on the Earth. They simulated a series of hurricanes over increasingly warm water, with intensity capped at Category 4, and had each one make landfall at exactly the same strength. Landfall was simulated by suddenly cutting off the supply of water vapor at the bottom of the storm. Sure enough, the storms that had grown over a warmer ocean took longer to weaken. That means this isn't a matter of, say, the back half of a storm still feeding on warm water while the front half crosses onto land. Instead, it appears that increased water vapor entrained within the storm itself helped sustain it. Another set of simulations confirmed this by also removing the water vapor at landfall -- in this case, the storms all weakened identically.

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