The United States just suffered the most intense hurricane season in more than a decade, and possibly the costliest ever. Hurricane Harvey hit Houston in mid-August. Hurricane Irma struck Florida in early September, and just two weeks later, Hurricane Maria devastated Puerto Rico and the Caribbean.
Shuyi Chen, a professor in the atmospheric sciences department at the University of Washington, talks about why the 2017 season was so devastating, the future of forecasting, and why hurricanes have a beef with land.
There were a lot of storms in 2016, too, but we didn’t get a lot of the impact because most of them were just over open ocean, and nobody even hears about them.
The record year for hurricanes was 2005. In 2005, there were 28 named storms and people in the field were saying “we ran out of letters” because we usually only use 23 letters (A-W) for storm names, and then we have to go to other naming systems. That was a really, really busy season.
To better understand and predict hurricane intensity, we fly into hurricanes on research aircraft to collect data. The temperature, pressure, and wind measurements in hurricanes can help us develop better computer models for hurricane forecasting.
Observing a hurricane inside the eye is one of the most amazing experiences I’ve had. My students and I were onboard the NASA DC-8 aircraft flown into Tropical Storm Cindy in June 2017 right before it made landfall near New Orleans.
If you follow a storm in time, the storm actually evolves. A very intense storm often forms multiple eyewalls; we call it “concentric eyewalls.” And eventually the inner one is deprived of energy and dies, and the second one will replace it, and that’s why they go through many lifecycles without dying. I was also on a research aircraft flown inside of Hurricane Rita in 2005 when Rita went through an eyewall replacement cycle.
While the air is subsiding from very high elevation, it is warming up. That warming in the eye creates a low-pressure center near the surface. The pressure outside of the hurricane is higher, so you have a pressure difference that drives the strong winds in hurricanes. These processes help explain how a hurricane can intensify.
The other way they can intensify is that the strong part of the eyewall is actually shrinking with time. Like a figure skater pulling their arms in, when the hurricane eye is shrinking, then it spins faster. That’s when we get very, very strong winds.
The other piece of a hurricane that’s self-regulating is that hurricanes actually have an impact on the ocean. The winds can stir up cold water from the deeper ocean to the surface. After a storm passes through, the ocean gets so much colder, sometimes 3 to 4 degrees colder. The hurricane would not like this cold water.
Finally, once the hurricane hits land it will begin to die. Hurricanes don’t like land because they lose the energy source from the warm ocean and because the land provides such strong friction that the storm can’t survive.
With Sandy, water rose on the right side of the storm with onshore winds and large waves pushing water toward the coast, which led to the major storm surge and flooding in New York, whereas on the left side with offshore winds the water was pulling out of the Chesapeake Bay. Manhattan is one of the places that has the most tide gauges because of its industrial shipping history, so we ended up being able to record that event very well and use it to verify the models.
Nature always throws us a curveball. Apparently Irma was different. It moved along the southwest coast of Florida. On the western side of Florida, in Tampa Bay, the water all disappeared ahead of the storm. The forecasters had predicted 10-15 feet of storm surge. But instead they found themselves in a very different situation.
This is the first time many people have seen the bottom of Tampa Bay. We saw a depression of sea surface on the west coast of Florida because the wind was blowing offshore. On the other hand, water was piling up on the Miami side because the wind was blowing onshore. These are very complex things, and this is at the leading edge of research, where we are developing models that combine atmosphere, ocean and waves to improve the forecasts of hurricane impacts.
If that rain had been a bit farther south, in the rural areas, we wouldn’t have had the huge damage. But it happened in Houston. The other thing is that the storm sat right on the coast, so the warm water and moist air over the Gulf of Mexico could keep resupplying energy and water into the storm. Also with onshore winds the water piles up onto the coast, so the storm surge in the coastal region prevented water from draining out into the ocean.
We are also working on understanding and predicting other high-impact tropical weather systems, such as the Madden-Julian Oscillation. That system forms over the tropical Indian Ocean and propagates eastward over the Pacific. It affects weather globally, including tropical cyclones, heatwaves and rainfall over the US, especially the West Coast and Pacific Northwest regions.
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