Atmospheric Waves Can Be Used to Detect Hurricane Intensity, Researchers Say

Jonathan Belles
Published: May 17, 2017

A new way to measure hurricane intensity from hundreds of miles away has been found radiating from the eyewall of powerful tropical systems.

Direct observations taken by NOAA Hurricane Hunter aircraft and buoys in the Pacific Ocean have shown gravity waves may be an indicator of changes in hurricane strength, according to a new study from the University of Miami's Rosenstiel School of Marine and Atmospheric Science (RSMAS) and the Hurricane Research Division of NOAA.

Gravity waves – atmospheric ripples that try to balance the upward thrust of thunderstorms, sometimes into the stratosphere, and the force of gravity – have been monitored for many years through satellite imagery like the loop below, but this study also suggests they might be more common near the surface than previously thought.

The loop above shows a visible-image animation of Typhoon Meranti that was taken every two and a half minutes over the course of three hours from the Himawari satellite Sept. 12, 2016. The video is from David Nolan and Brian McNoldy.

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"Of course, hurricanes are very well observed by satellites. But these waves can reveal processes occurring in the eyewall of a hurricane that are obscured from the view of satellites by thick clouds," said David Nolan, professor in the Department of Atmospheric Sciences at UM and lead author of the study. "Any additional measurements, even if they provide similar information as satellites, can lead to better forecasts."

The observed gravity waves, which were reproduced on computer simulations at the UM Center for Computational Science, were shown to be significant enough that they might be useful indicators for maximum wind speed in the eyewall of the storm – even hundreds of miles away from the storm. The amplitude, or height, of these waves is thought to be connected to the intensity of hurricanes.

The study "suggests that, perhaps, in some geographic regions, networks of surface instruments could monitor the intensity of approaching or passing TCs, much as the worldwide network of seismometers continuously monitors earthquakes."

This may provide a less dangerous way to diagnose tropical cyclone intensity in comparison to aircraft reconnaissance.

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The gravity waves were measured using hurricane hunter data and data from weather-observing sites near the surface, including buoys.

On the ground, gravity waves "would have no noticeable effect on the weather that you experience," said Nolan. Gravity waves are very subtle near the surface as they pass and data will need to be filtered in order to see the effects of these waves.

"The buoy data show clear evidence that gravity waves generated by [tropical cyclones] can be detected by surface instruments" including changes in both pressure and wind speed, according to the study. Gravity waves were detected by research buoys surrounding typhoons in the Pacific at distances of 100 to 400 miles from the center of the storm.

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Similar research is being performed with regards to Hurricane Matthew, which traversed the waters off the coast of Florida last October. The authors "are in the process of analyzing data collected by anemometers" located in Miami, said Jun Zhang, co-author of the study.

Gravity waves occur in two layers of the atmosphere: near the top of thunderstorm clouds in the tropics at around 40,000 feet, and just above the surface, where they can be picked up by observing stations and buoys.

How much these waves correlate to the intensity of tropical cyclones is still being researched, and it will be a while, perhaps a decade, before this research could be used in forecasting. The relationship between hurricane intensity and the amplitude of gravity waves at the two levels where they are most prominent may also differ when it comes to hurricane intensity.

At the top of thunderstorm clouds, these gravity waves radiate at more than 45 mph away from the eyewall of hurricanes and are circular stripes in the northern semicircle in the satellite image below.

Radiating spiral waves, as seen in visible satellite images from the Himawari satellite on Sept. 12, 2016. The red line is drawn to show the leading edge of outward-moving gravity waves.
(Brian McNoldy, University of Miami Rosenstiel School of Marine and Atmospheric Science)

Near the surface, these gravity waves pass observing stations a few times an hour after they radiate outward from the hurricane's center.

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