Speed matters, but so does size
Ida struck Louisiana on Aug. 29 as a strong Category 4 on the Saffir-Simpson scale, with maximum winds of about 150 mph — much higher than the maximum winds of 125 mph from Katrina, a Category 3.
But wind speed tells only one part of a hurricane’s destructive story.
A fuller picture comes from looking at the entire breadth of its wind field — and the energy it contains. Using data from satellites, ground monitors and military hurricane-hunter planes, analysts can measure the size of a storm and calculate its “integrated kinetic energy,” which is basically a way to quantify its total power rather than just the speed of the wind around its eye.
“Each of these wind fields is like a fingerprint,” said Michael Kozar, a meteorologist who models and analyzes storm data for risk-analysis company RMS. “Each one is unique to the storm, and it is why each storm produces a unique amount of loss and has unique impacts.”
“They move at different speeds, they strike the coastline at different angles, and every single one of those seemingly little details can make a huge difference when it comes to people on the ground,” he said. A very large storm with moderate winds may contain more integrated kinetic energy than an intense but small storm, and it may create havoc for people on land in a different way.
For example, Katrina’s wind field reveals that it contained more than twice the energy of Ida. This energy is expressed in terajoules. Katrina contained 116 terajoules, whereas Ida had 47 terajoules. One of the most powerful storms by this measure to hit the United States was Sandy in 2012, Kozar said. His team calculated its energy at 330 terajoules.
Behavior and trajectory count as well
Ida was not even a hurricane until late on Aug. 27, but it quickly intensified in the warm water before making landfall a day and a half later in Port Fourchon in southern Louisiana.
Katrina, on the other hand, hit Florida as a Category 1 hurricane, then spent three full days gaining power over the Gulf of Mexico, becoming a Category 5, with winds up to 175 mph. Even though it weakened as it churned toward land, the storm had plenty of time to generate the massive 19-foot storm surge that it pushed onto the coast. It was that surge that did the most damage, overrunning levees and inundating coastal communities, including much of New Orleans. More than 1,800 people died.
Typically, Kozar said, the worst hazard occurs in places that are on the right sides of storms, where the strongest winds and most water tend to be. That would be the eastern side if a storm is traveling north. Ida’s destruction is still being assessed, but extensive damage to the power grid and major flooding have occurred in areas that were just east of its path.
A storm’s behavior is yet another factor.
Harvey caused so much flooding in the Houston area in 2017 because it stalled just offshore, dumping several feet of rain in many places. By contrast, Andrew was a much smaller and faster hurricane that took mere hours to speed through southern Florida in 1992, but its Category 5 winds leveled entire neighborhoods.
Katrina moved relatively slowly in the gulf but sped up after landfall. Ida did the opposite.
About this article: Wind field analysis by RMS is based on measurements from the National Oceanic and Atmospheric Administration, Air Force Hurricane Hunters, Texas Tech Hurricane Research Team and the University of Florida’s Wind Damage Group. Additional information came from the National Weather Service’s pages on hurricanes Katrina and Andrew. Sentinel 1 imagery via Copernicus EU.
Jason Samenow contributed to this report.