A First Look at Turbulence Intensity at Low Altitude in the Eyewall of a Hurricane from Coyote small Uncrewed Aircraft Systems (sUAS)

Small drone aircraft launched from a NOAA P3 hurricane hunter aircraft successfully executed powered flight through the eyewall of Hurricane María within 1000 m of the ocean surface with horizontal wind measurements reported nominally at 2 / s but having short segments up to 10 / s. These sample rates are sufficiently high to enable some level of estimation of the strength of the turbulence. Six Coyote aircraft were deployed into Hurricane María on three days, September 22-24. Four flew under power for their entire half-hour flights, during which they were guided across the eyewall in steps at successively lower elevation down as low as 100 m above the sea before control was lost. Two drones’ motors failed to start, but they were still controllable as gliders. One of these measured at 10 / s all the way down to the sea, the longest usable interval comprising a descent from 600 m to below 50 m.

Although the vertical wind could not be measured from these aircraft, the horizontal winds available at 10 / s allowed spectra to be calculated from which the frequency band of the inertial subrange of turbulence could be determined. It was found to extend as low as 0.1 Hz, well within the resolution of the 2 / s readings that comprise the bulk of the measurements during these flights. Thus the short segments at 10 / s demonstrated that longer segments from the entire flights could be used to obtain at least a crude estimate of the profile of turbulent strength in the eyewall at altitudes below 1500 m above the sea where crewed aircraft fly at their peril.

We present a first look at the turbulence intensity patterns from the six flights. Though measurement of turbulence properly requires full three-component wind, temperature, and moisture content, these flights represent the first set of measurements from which any sort of characterization of the turbulence in a hurricane eyewall could be made at low altitude. Additional data from the full set of navigation information used to control the drone promise some further improvement. From these first simple characterizations of turbulence can begin the development of a robust observation system to sample turbulence from a previously inaccessible part of a hurricane. These results join those from dropsondes, radar, and other systems to inform large-eddy simulation of the marine boundary layer around the eyewall. Other presentations in this conference report conterminous measurements made during these same flights in María along with large-eddy simulation of the marine boundary layer. From these measurements and simulations a mutually beneficial improvement in understanding of the marine boundary layer under the eyewall of a hurricane can develop.