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How Meteorologists Predict the Next Big Hurricane

Climate
Hurricane Florence, as seen over the Atlantic Ocean on Sept. 9. NOAA / NWS National Hurricane Center

By Mark Bourassa and Vasu Misra

Hurricane Florence is heading toward the U.S. coast, right at the height of hurricane season.


Hurricanes can cause immense damage due to the winds, waves and rain, not to mention the chaos as the general population prepares for severe weather.

The latter is getting more relevant, as the monetary damage from disasters is trending up. The growing coastal population and infrastructure, as well as rising sea level, likely contribute to this increase in costs of damage.

This makes it all the more imperative to get early and accurate forecasts out to the public, something researchers like us are actively contributing to.

Making Predictions

Hurricane forecasts have traditionally focused on predicting a storm's track and intensity. The track and size of the storm determine which areas may be hit. To do so, forecasters use models—essentially software programs, often run on large computers.

Unfortunately, no single forecast model is consistently better than other models at making these predictions. Sometimes these forecasts show dramatically different paths, diverging by hundreds of miles. Other times, the models are in close agreement. In some cases, even when models are in close agreement, the small differences in track have very large differences in storm surge, winds and other factors that impact damage and evacuations.

What's more, several empirical factors in the forecast models are either determined under laboratory conditions or in isolated field experiments. That means that they may not necessarily fully represent the current weather event.

So, forecasters use a collection of models to determine a likely range of tracks and intensities. Such models include the NOAA's Global Forecast System and European Centre for Medium-Range Weather Forecasts global models.

The FSU Superensemble was developed by a group at our university, led by meteorologist T.N. Krishnamurti, in the early 2000s. The Superensemble combines output from a collection of models, giving more weight to the models that showed better predicted past weather events, such Atlantic tropical cyclone events.

A forecaster's collection of models can be made larger by tweaking the models and slightly changing the starting conditions. These perturbations attempt to account for uncertainty. Meteorologists cannot know the exact state of the atmosphere and the ocean at the time of the start of the model. For example, tropical cyclones are not observed well enough to have sufficient detail about winds and rain. For another example, the sea surface temperature is cooled by the passage of a storm, and if the area remains cloud-covered these cooler waters are much less likely to be observed by satellite.

Limited Improvement

Over the past decade, track forecasts have steadily improved. A plethora of observations—from satellites, buoys and aircraft flown into the developing storm—allow scientists to better understand the environment around a storm, and in turn improve their models. Some models have improved by as much as 40 percent for some storms.

A buoy collecting weather data. U.S. National Oceanic and Atmospheric Administration

However, forecasts of intensity have improved little over the last several decades.

That's partly because of the metric chosen to describe the intensity of a tropical cyclone. Intensity is often described in terms of peak wind speed at a height of 10 meters above the surface. To measure it, operational forecasters at the National Hurricane Center in Miami look at the maximum, one-minute average wind speed observed at any given point in the tropical cyclone.

However, it's extremely difficult for a model to estimate the maximum wind speed of a tropical cyclone at any given future time. Models are inexact in their descriptions of the entire state of the atmosphere and ocean at the start time of the model. Small-scale features of tropical cyclones—like sharp gradients in rainfall, surface winds and wave heights within and outside of the tropical cyclones—are not as reliably captured in the forecast models.

Both atmospheric and ocean characteristics can influence storm intensity. Scientists now think that better information about the ocean could offer the the greatest gains in forecast accuracy. Of specific interest is the energy stored in the upper ocean and how this varies with ocean features such as eddies. Current observations are not sufficiently effective at placing ocean eddies in the correct location, nor are they effective in capturing the size of these eddies. For conditions where the atmosphere doesn't severely limit hurricane growth, this oceanic information should be very valuable.

Meanwhile, forecasters are pursuing alternative and complementary metrics, like the size of the tropical cyclones.

Reposted with permission from our media associate The Conversation.

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A volcano erupts on New Zealand's Whakaari/White Island on Dec. 9, 2019. Michael Schade / Twitter

A powerful volcano on Monday rocked an uninhabited island frequented by tourists about 30 miles off New Zealand's coast. Authorities have confirmed that five people died. They expect that number to rise as some are missing and police officials issued a statement that flights around the islands revealed "no signs of life had been seen at any point,", as The Guardian reported.

"Based on the information we have, we do not believe there are any survivors on the island," the police said in their official statement. "Police is working urgently to confirm the exact number of those who have died, further to the five confirmed deceased already."

The eruption happened on New Zealand's Whakaari/White Island, an islet jutting out of the Bay of Plenty, off the country's North Island. The island is privately owned and is typically visited for day-trips by thousands of tourists every year, according to The New York Times.

Michael Schade / Twitter

At the time of the eruption on Monday, about 50 passengers from the Ovation of Seas were on the island, including more than 30 who were part of a Royal Caribbean cruise trip, according to CNN. Twenty-three people, including the five dead, were evacuated from the island.

The eruption occurred at 2:11 pm local time on Monday, as footage from a crater camera owned and operated by GeoNet, New Zealand's geological hazards agency, shows. The camera also shows dozens of people walking near the rim as white smoke billows just before the eruption, according to Reuters.

Police were unable to reach the island because searing white ash posed imminent danger to rescue workers, said John Tims, New Zealand's deputy police commissioner, as he stood next to Prime Minister Jacinda Ardern in a press conference, as The New York Times reported. Tims said rescue workers would assess the safety of approaching the island on Tuesday morning. "We know the urgency to go back to the island," he told reporters.

"The physical environment is unsafe for us to return to the island," Tims added, as CNN reported. "It's important that we consider the health and safety of rescuers, so we're taking advice from experts going forward."

Authorities have had no communication with anyone on the island. They are frantically working to identify how many people remain and who they are, according to CNN.

Geologists said the eruption is not unexpected and some questioned why the island is open to tourism.

"The volcano has been restless for a few weeks, resulting in the raising of the alert level, so that this eruption is not really a surprise," said Bill McGuire, emeritus professor of geophysical and climate hazards at University College London, as The Guardian reported.

"White Island has been a disaster waiting to happen for many years," said Raymond Cas, emeritus professor at Monash University's school of earth, atmosphere and environment, as The Guardian reported. "Having visited it twice, I have always felt that it was too dangerous to allow the daily tour groups that visit the uninhabited island volcano by boat and helicopter."

The prime minister arrived Monday night in Whakatane, the town closest to the eruption, where day boats visiting the island are docked. Whakatane has a large Maori population.

Ardern met with local council leaders on Monday. She is scheduled to meet with search and rescue teams and will speak to the media at 7 a.m. local time (1 p.m. EST), after drones survey the island, as CNN reported.