Nasa expert warns of stalling hurricanes
If you’re going to face boxing legend Mike Tyson, you only want to be in the ring for one round rather than ten.
That was the analogical advice of Nasa’s Timothy Hall, who spoke to insurance and reinsurance executives about a trend for hurricanes and tropical storms to slow down and stall over coastal areas, consequently causing greater damage.
“A stalling storm is going to unleash more accumulated rainfall. That’s one of the primary hazards,” he said.
“If a major storm is going to pass by, you want it to pass quickly, not slowly.
“You want to be in the ring with Mike Tyson for only one round as opposed to ten.
“Surge and wind are also exacerbated in a stalling storm. But rainfall is really the most obvious hazard of stalling.”
Dr Hall, a senior research scientist at Nasa’s Goddard Institute for Space Sciences, was a guest speaker at the ILS Convergence event, at the Hamilton Princess & Beach Club. He was unable to attend in person due to a “bureaucratic situation”, so gave his presentation via a video link from New York.
He focused on hurricanes and storms that slow down and stall near the US coast, and used as examples hurricanes Harvey in 2017, Florence last year, and Dorian and Tropical Storm Imelda this year.
“All these storms stalled. Their trajectories slowed dramatically and they lingered over the region in harm’s way for long periods of time,” he said.
He pulled information from the research paper titled Hurricane stalling along the North American coast and implications for rainfall, which he and James Kossin, an atmospheric research scientist, published this year.
He shared a graph that showed the tropical translation rate, that is the speed of storms, globally has slowed from 19km per hour to 17 km/h on average since the 1950s.
A closer look at the data shows that slow-moving storms are the ones that are slowing the most. Not only are they slowing, they are becoming more prone to meandering.
So what are the culprits causing this to happen?
Dr Hall said the large-scale [wind field] flow that is pushing storms along is itself slowing, a consequence of a slowdown in the atmospheric tropical circulation in a warming climate.
“The bottom line is, we have a strong suspect for what might be causing the reduction in the translation of the tropical cyclone, and that is a reduction in the speed of the large-scale tropical flow,” Dr Hall said, explaining that slowing and meandering storms are the ingredients that can cause a storm to stall.
A stalled storm was defined as one that spent 48 hours or more within a 200km radius circle.
A video clip from a geostationary satellite showed the dramatic journey of Hurricane Dorian in August as it quickly grew to a Category 5 and then slowed and stalled for about 48 hours above Grand Bahamas before moving towards the coast of Florida.
Dr Hall said: “Our expectation of increased stalling storms is more rain per stall, and [therefore] more rain catastrophes on coastal regions.
“This is yet another hazard variable of hurricanes that needs to be considered.”
Following Dr Hall’s presentation was Dr Kossin. He was present at the conference, and spoke about the impact on hurricanes of warmer ocean temperatures and reduced wind shear.
Data shows the North Atlantic Ocean temperature has oscillated from a warm period in the 1940s and into the 1960s, followed by about 30 years of cooler ocean temperature, and then another period of warmer temperatures from 2000 to 2015.
In the cooler decades, vertical wind shear increased and helped reduce the number of hurricanes. However, wind shear generally reduces when the ocean temperature is warmer, resulting in more active hurricane periods.
Dr Kossin, of the National Oceanic and Atmospheric Administration, said: “Major hurricanes are much more volatile when they are near or approaching the coast, and are much more likely to rapidly intensify. This is a problem in many ways.”
He said for weather forecasters it is a nightmare scenario, because “you do not want hurricanes to rapidly do anything when they are near or approaching the coast”.
He added: “For rapid intensifiers it is a real problem because you can be unprepared, or evacuations haven’t happened well enough.”
The probability that a US major hurricane will rapidly intensify is about three times less during active hurricane periods, and three times more likely during the quintessence periods when the ocean temperature is cooler.
Dr Kossin wondered what would happen if greenhouse gases in the atmosphere continue to rise. He said the US East Coast had benefited from the wind-shear barrier that has been around for decades.
“The more conducive the tropics are for increasing hurricane peril, the stronger the barrier is going to be, so the hurricanes have to run that gauntlet before they make landfall — on average.”
He said the answer to the question about whether greenhouse gas erodes or enhances the wind-shear barrier was that it degrades it.
“It is predicted to erode this in a very substantial way,” he added. “All other things being equal, this trend is going to substantially increase US hurricane risk, certainly on an individual hurricane basis.
“Overall risk may increase at a lesser rate, because where you have a decrease in shear off the US area, you have an increase in shear offsetting hurricane activity in the tropics.
“But all other things being equal, this is probably going to increase risk [for the US coast]in a fairly substantial way in the somewhat immediate future.”
• The ILS Convergence event continues today.
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