Hot Towers Intensify Hurricanes

June 1 marks the beginning of hurricane season. Every year, meteorologists strive to predict hurricane behavior to reduce risk to lives and property while also minimizing the costs associated with storm preparations and evacuations. Perfect predictions of hurricane behavior are still a thing of the future, but NASA scientists are combining observations from field campaigns and the Tropical Rainfall Measuring Mission (TRMM) satellite with supercomputing modeling power to shed light on the internal workings of hurricanes and how the eye of the storm itself feeds towering thunderstorm clouds known as hot towers, which cause storms to intensify.

This series of images comes from an animation that illustrates the results of a supercomputer simulation of air movement, moisture, and energy within Hurricane Bonnie, which made landfall in North Carolina on August 26, 1998. The top image shows TRMM observations of Hurricane Bonnie from the satellite’s Visible and Infrared Sensor (VIRS). The second and third images are made from observations from TRMM’s Precipitation Radar (PR), the world’s first space-based instrument to provide three-dimensional maps of storm structure. TRMM’s radar measurements indicated the presence of a towering thunderstorm cloud known as a hot tower in the northwest part of Bonnie’s eyewall (the rings of clouds surrounding the eye.) The bottom image is a computer drawing of the air movement in the hot tower. Red arrows indicate warm air moving upward through the hot tower. Gray arrows indicate air escaping and re-entering the hot tower via the hurricane’s eye.

Hot towers can almost reach the base of Earth’s stratosphere, roughly 10 kilometers (6 miles) above sea level—well above the altitude at which commercial jets typically fly. These towers channel warm, moist air upward inside the hurricane. The turbo-charging of hot towers by warm, humid air from the eye was one of the most important processes verified by the model. Over the past five decades, the classic paradigm for scientists thinking about air flow in hurricanes has been the “in-up-and-out” pattern. This classic pattern says that air parcels flow “in” to the low-pressure area at the heart of the storm at the surface, they rise “up” to form clouds and precipitation, and they transport heat to the upper atmosphere before moving “out” into surrounding environmental air. Air moving between the eyewall and the eye was thought to be trivial to the process.

But according to Scott Braun, research meteorologist at NASA’s Goddard Space Flight Center, the model results showed that spinning vortices that form at the boundary between the calm air of the eye and the intense circulation of the eyewall “carry very warm, moist eye air into the eyewall that acts as a turbocharger for the hurricane heat engine.” This warm, moist air boosts the strength of the updrafts in the hot tower. This energy boost may help explain why some storms remain stronger than expected, particularly when encountering weakening influences, including cooler ocean water temperatures.

NASA images from the Goddard Space Flight Center Scientific Visualization Studio Towers in the Tempest animation. Caption partially adapted from NASA’s Close-Up Look at a Hurricane’s Eye Reveals a New “Fuel” Source.