A one-two punch inside intense Hurricane Felix in 2007 turned a NOAA hurricane hunter flight into a harrowing rollercoaster ride, causing the mission to be aborted. A study of the extreme event, scheduled for publication in the next issue of Monthly Weather Review, determined a small-scale vortex known as a misocyclone rotating within the Category 5 hurricane’s eyewall is likely what bucked the plane upward nearly a thousand feet before sending it plunging back to its original altitude in less than a minute. The feature is similar to what nearly crashed the same plane inside Hurricane Hugo in 1989.
According to the study, a “routine penetration” into the eye of the hurricane via the northeast eyewall on September 2, 2007 quickly became anything but. First, the horizontal wind speed at the plane’s altitude of about 10,000 feet jumped from 140 mph to nearly 200 mph. At the same time, a standard descent into the eye at a constant 700 mb pressure height quickly steepened and the plane lost more than 700 feet in altitude in 40 seconds. Then a 70 mph updraft punched the plane up 900 feet immediately followed by a 16 mph downdraft that hammered the plane downward 980 feet, in seconds. The on-board radar quit. And gravitational stresses on the aircraft exceeded safety specifications. The mission was scrubbed and the plane then settled into Felix’s calm 12-mile-wide eye at about 8900 feet, circling five times until it could find a safe pass through the southwest eyewall and out of the hurricane.
They were lucky.
The hurricane hunters have unknowingly flown into these updraft-downdraft combinations before. They seem to only encounter them in monster Category 5 hurricanes, which have sustained winds greater than 156 mph. Besides Felix, researchers have documented the extreme events in Hurricanes Patricia (2015), Isabel (2003), and Hugo (1989). The encounter in Hurricane Hugo took place with the same hurricane hunter plane (NOAA42) flying at just 1500 feet, which was typical back then. Not any more. Fists of wind smashed the aircraft downward more than a thousand feet and then back upward, knocking out three of the its four turboprop engines and crippling the plane. It barely made it out, and afterward the rules for hurricane eye penetrations were rewritten.
Back it 1989, researchers thought they had perhaps flown into a tornado in the eyewall. But in Hurricane Isabel, data revealed a vortex a bit larger but no less intense was encountered. Similar in scope but smaller in size to the rotating 5-10-mile-wide updrafts of supercell thunderstorms, which have become known as mesocyclones, the hurricane eyewall vortexes were only a fraction of that—hence the name misocyclones, or small-scale cyclones.
In Felix, a bit of serendipity: just as the plane encountered the misocyclone, researchers released a commonly used tube of instruments called a dropwindsonde into the eyewall to measure temperature, pressure, humidity, and with onboard GPS tracking, wind speed and direction. The dropwindsonde measured details of the wind within the misocyclone, including a shift in the horizontal direction and a speed that jumped to more than 230 mph at about 400 feet decreasing to just 41 mph near the water. The tremendous shear—change in the wind speed in such a short distance—is “8 orders of magnitude larger than those known to lead to […] horizontal shearing instabilities and misocyclone development,” the study noted based on prior research.
It’s only the second time details of a misocyclone have been measured, making them largely mysterious events. For example, researchers aren’t certain how common or unusual they are. “Many very intense tropical cyclones have been sampled with aircraft without encountering these extreme events,” the study states, adding, “It is unknown whether they have been missed by the relatively sparse observations available, because aircraft tend to deviate around the most intense eyewall convection, or if they are truly rare.”
Deepening the mystery is the timing of the extreme event inside Felix—it occurred as with Hugo at the end of a period of rapid intensification, which is when a hurricane’s central pressure drops precipitously ramping up its sustained winds very quickly. Winds in Hurricane Felix increased by 90 mph to 165 mph sustained that day in 2007. It has been thought that extreme features in hurricanes such as lightning, graupel, and eyewall vortices likely occur during periods of rapid intensification, as occurred with Hurricanes Isabel and Patricia. But Felix is the second intense hurricane where such an extreme event took place at the end of a rapid intensity cycle, and learning why while keeping the hurricane hunters safe will require further study.
“The frequency of these features and their ultimate importance in the structural evolution [of hurricanes] remain research questions. It is clear, though, that improved understanding of these features would enhance the safety of flights into very intense tropical cyclones.”