Here’s why hurricanes are rapidly exploding in strength

By Chris Mooney
11 October 2018
(The Washington Post) – The unforgettable thing about record-setting Hurricane Michael will always be how rapidly it became a near-Category 5 storm, perfectly timed for a sneak attack on the Florida Panhandle.On Tuesday morning, Floridians knew a storm was coming but not how strong it would be. As of 5 a.m., Michael was a strong Category 1 hurricane with a minimum pressure of 973 millibars, a measure of atmospheric pressure indicating that air is rising in the storm, pulling winds toward its center. The official forecast took the storm up to mid-Category 3 at landfall.But 24 hours later, Michael was already far stronger: It now had 140 mph winds and a pressure falling sharply. The wind speed increased 45 mph in just 24 hours, representing a leap from Category 1 to Category 4 — and the storm wasn’t done intensifying.Pressure would ultimately fall to 919 millibars, one of the lowest measures of any hurricane at landfall in the United States — and the winds responded by increasing to 155 mph right as the storm struck the coast. This was a borderline Category 5 storm, and it’s clear that the only reason Michael didn’t quite cross that threshold was because it was crossing beaches by that time instead.This process of “rapid intensification” — extremely dangerous near a coastline — is something we keep seeing lately. Technically, it is defined by the National Hurricane Center as an increase in wind speeds of 35 mph or more in 24 hours.All of the worst hurricanes of the past two years — Harvey, Irma, Maria, Florence, and Michael — intensified even more rapidly than this. Maria increased a stunning 80 mph in wind speeds, leaping from a Category 1 to a Category 5 storm in 24 hours, not long before its catastrophic landfall in Puerto Rico that ultimately led to thousands of deaths.Harvey and Michael didn’t strengthen quite so much, so fast, but they rapidly intensified in the crucial hours before making their final continental U.S. landfalls. […]Climate scientists have begun to focus on hurricane rapid intensification as an increasingly prevalent feature in the world we’re entering. Simply put, with warmer seas, storms ought to be able to pull this off more often.In a recent study in the Journal of Climate, researchers found more rapid intensifications in a simulation of a human-warmed world, and also that this would prove a key pathway toward more intense hurricanes in general. [more]

By Chris Mooney
11 September 2018
(The Washington Post) – In little more than a day, Hurricane Florence exploded in strength, jumping from a Category 1 to a Category 4 behemoth with 140 mph winds. This process — hurricanes intensifying fast — is both extremely dangerous and poorly understood. But new research says that as the climate continues to warm, storms will do it faster and more often, and in some extreme cases, grow so powerful that they might arguably be labeled “Category 6.”The new science is a testament to the growing ability of supercomputing to power simulations of the planet that show the future of massive features like the atmosphere and oceans, but still also maintain enough detail to capture smaller ones like Category 4 and 5 hurricanes. That’s how a model created at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory generated the new findings, and identified how fast-intensifying storms could make things a whole bunch worse later in this century.

“The reason there are going to be more major hurricanes is not necessarily there are going to be that many more storms … it’s really the fact that those storms are going to get there faster,” said Kieran Bhatia, lead author of the new research in the Journal of Climate. Bhatia completed the work while a graduate researcher at Princeton University and the nearby NOAA laboratory.

We’ve already seen the signs, in the past several years, of ultra-intense hurricanes that get that way by explosively intensifying. Last year’s Hurricane Maria, for instance, spun up from a mere tropical depression into a Category 5 storm in just over two days; or 2015’s Hurricane Patricia, whose winds in the Eastern Pacific exceeded 210 mph, more than 50 mph stronger than the weakest Category 5 storms.Kerry Emanuel, an expert on hurricanes and climate change at MIT, said in an email that the research “may prove a game-changer in climate-hurricane studies.” Emanuel was not directly involved in the work.The study draws on enhanced computer power to achieve something that until now has been very difficult — capturing a reasonable representation of hurricanes in a global climate change model that simulates both the atmosphere and the oceans.Dividing the globe up into squares roughly 16 miles by 16 miles in size, researchers were able to simulate Category 4 and 5 storms, and where they currently occur around the globe — not perfectly, but in a way that was broadly representative.But when the researchers then moved from simulating the hurricanes of the late 20th century to those of the future under a middle-of-the-road climate change scenario, they found big changes. For the period between 2016 and 2035, there were more hurricanes in general and 11 percent more hurricanes of the Category 3, 4 and 5 classes; by the end of the century, there were 20 percent more of the worst storms.What’s more, the research found that storms of super-extreme intensity, with maximum sustained winds above 190 mph, also became more common. While it only found nine of these storms in a simulation of the late 20th century climate, it found 32 for the period from 2016 to 2035 and 72 for the period from 2081 to 2100. [more]

Category 6? Climate change may cause more hurricanes to rapidly intensify.

ABSTRACT: As one of the first global coupled climate models to simulate and predict category 4 and 5 (Saffir–Simpson scale) tropical cyclones (TCs) and their interannual variations, the High-Resolution Forecast-Oriented Low Ocean Resolution (HiFLOR) model at the Geophysical Fluid Dynamics Laboratory (GFDL) represents a novel source of insight on how the entire TC intensification distribution could be transformed because of climate change. In this study, three 70-yr HiFLOR experiments are performed to identify the effects of climate change on TC intensity and intensification. For each of the experiments, sea surface temperature (SST) is nudged to different climatological targets and atmospheric radiative forcing is specified, allowing us to explore the sensitivity of TCs to these conditions. First, a control experiment, which uses prescribed climatological ocean and radiative forcing based on observations during the years 1986–2005, is compared to two observational records and evaluated for its ability to capture the mean TC behavior during these years. The simulated intensification distributions as well as the percentage of TCs that become major hurricanes show similarities with observations. The control experiment is then compared to two twenty-first-century experiments, in which the climatological SSTs from the control experiment are perturbed by multimodel projected SST anomalies and atmospheric radiative forcing from either 2016–35 or 2081–2100 (RCP4.5 scenario). The frequency, intensity, and intensification distribution of TCs all shift to higher values as the twenty-first century progresses. HiFLOR’s unique response to climate change and fidelity in simulating the present climate lays the groundwork for future studies involving models of this type.

Projected Response of Tropical Cyclone Intensity and Intensification in a Global Climate Model