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Why scientists had trouble predicting Hurricane Michael’s rapid intensification [sciencemag.org]

Why scientists had trouble predicting Hurricane Michael’s rapid intensification

By Frankie Schembri [sciencemag.org]Oct. 15, 2018 , 4:05 PM

Hurricane Michael roared into Mexico Beach, Florida, on 10 October as the strongest storm ever to strike the Florida Panhandle in terms of wind speed, and the third strongest to make landfall in the continental United States. The storm caused severe damage to several coastal communities, Tyndall Air Force Base, and Florida State University’s Panama City campus. Officials have attributed 18 deaths to the storm and dozens of people have been reported missing.

Although National Hurricane Center (NHC) forecasters were able to predict where and when Michael was likely to make landfall several days in advance, the storm’s rapid intensification—jumping from a Category 2 to just shy of a Category 5 in 24 hours—proved tougher to anticipate. NHC defines “rapid intensification” as a storm’s maximum sustained winds increasing by at least 56 kilometers per hour in 24 hours or less. Michael underwent at least three intensification periods on its 5-day march toward the coast.

“Predicting a hurricane’s track is relatively straightforward because storms are propelled in one direction or another by the large-scale air currents in the atmosphere,” says Robert Rogers, a meteorologist at the National Oceanic and Atmospheric Administration’s (NOAA’s) Hurricane Research Division in Miami, Florida. “We’ve gotten a much better handle on predicting those large-scale currents over the past 20 years.”

But when it comes to predicting changes to a storm’s intensity, the underlying physics becomes much more complicated, says Kerry Emanuel, a professor of atmospheric sciences at the Massachusetts Institute of Technology in Cambridge. That’s because hurricanes are complex, massive rotating heat engines, Emanuel says, fueled by a favorable combination of warm ocean water, moist air, and consistent atmospheric winds.

Hurricanes have a theoretical limit on how intense they can become in terms of wind speed, he notes, but it is never reached because storms are derailed by one of the trio of factors mentioned above. For example, storms lose fuel when they travel over cooler water or onto land. And inconsistent atmospheric winds can be an obstacle to intensification. For instance, when “wind speed varies with height,” Emanuel says, “you get what’s known as wind shear. This can tip over the core of the hurricane and allow dry air to invade, which disrupts the storm like tossing water on a fire.”

Scientists now have an arsenal of tools—including piloted and robotic aircraft that NOAA flies alongside storms—that collect data on a host of variables that can be incorporated into weather models, and help researchers estimate when a hurricane will rev up. Such data help reveal the larger physical processes at play, Rogers says. But there is still considerable work to do in understanding finer-scale microprocesses that also dictate how quickly a hurricane will intensify. “Thunderstorm formation, raindrop formation, ice particle formation, all these things are happening inside each hurricane and can affect its intensity,” he says. “These micro-level processes can be very challenging to model.”

Researchers also know little about a crucial zone where the ocean and atmosphere meet. Most of a hurricane’s heat flux takes place in this transitional layer, where water and air mix into a sort of emulsion. “It’s nearly impossible to get samples from this layer, and it’s tricky to simulate this kind of condition in the lab to study it,” Emanuel says.

Still, researchers are making some progress on improving intensity forecasts. More data are always helpful, Rogers says, and NOAA was able to collect a “substantial amount” from Hurricane Michael. Rogers’s team is also working on developing robust unmanned aircraft to fly directly into storms and collect higher quality data about microprocesses. Several teams are also developing probes and submersibles [sciencemag.org] to monitor the water column during hurricanes—including the international Argo program [argo.net] that distributes instrumented floats that drift with currents and periodically sink below the surface.

Meteorologists are also keeping a close eye on climate change, as warmer oceans and rising sea levels could complicate hurricane intensity predictions. “If you put more carbon dioxide into the atmosphere and warm the climate, the maximum speed limit for hurricane goes up,” Emanuel says. “The rate at which hurricanes can intensify also increases, meaning you could have storms intensify much faster than Michael.” 

If storms rev up more quickly, authorities will have less time to coordinate evacuation efforts, which could prove both deadly and expensive. “It’s a forecaster’s worst nightmare,” Emanuel says, “to go to bed one evening with a tropical storm somewhere in the Gulf of Mexico and wake up the next morning with a Category-4 storm just about to make landfall.”

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