Direct Measurements of Gravity Waves Radiating from Hurricane Irma (2017) Obtained By the Univ. of Miami Helicopter Observing Platform (HOP)

In a previous study, Nolan and Zhang (2017) documented in-situ observations of atmospheric gravity waves radiating from tropical cyclones, showing evidence for the waves in both flight-level data from NOAA P3 aircraft and in surface observations from the ITOP buoy in the Pacific. As Hurricane Irma approached Florida, we made direct observations of gravity waves using the Helicopter Observing Platform (HOP). This new research aircraft is designed to carry in-situ sensors and instrument inlets in the undisturbed air in front of its nose at low airspeed at altitudes from a few feet above the surface and up through the mid troposphere. The HOP, with its hovering capability, is also ideal for conducting various types of remote-sensing observations. Fully fueled, the HOP can carry a scientific payload of up to about 1,000 lbs internally (about 2,000 lbs externally) and fly for nearly 4 hours without refueling at an airspeed of 65 knots that is ideal for in-situ observations. Its fastest cruising speed is about 140 knots and its range at that speed is about 350 nautical miles. The specific helicopter chosen for this platform is the Airbus H125 because of its flat floor design, which is particularly convenient for installing scientific payload and also because of its high-altitude capability (it is the only helicopter that was ever capable of landing at the top of Mt Everest).

Measurements were made during a single flight on September 7, 2017, at which time the center of Irma was over 1000 km ESE of Miami. Four different sensors capable of measuring the three components of the wind at a frequency of 20 Hz (two Campbell Scientific Irgason and two Aventech AIMMS-30) were mounted ahead of the HOP nose. Three 75-km legs were flown, each directly toward, away from, and then toward Irma again, at altitudes of 7500, 8500, and 7500 feet, respectively. Despite the long distance from the storm, vertical motions consistent with prior observations of gravity waves radiating from hurricanes were observed, with amplitudes of up to 1 m/s, wavelengths of 5-10 km, and phase speeds around 20 m/s. Comparisons to “null cases” on days with little or no convection in the region will be shown.