String stability of atmospheric sensor balloon swarms on zero-radial-velocity manifolds within hurricanes

Thomas Bewley
University of California, San Diego (UCSD)

Real-time measurements within hurricanes are essential to improve forecasts, protect property and save lives. Current methods for obtaining in-situ data, including radar and satellite imagery as well as drop-sondes deployed from repeated aircraft flights above or even within the hurricane itself, are costly, dangerous and limited in duration or resolution. We demonstrate how a swarm of inexpensive, buoyancy-controlled, sensor-laden balloons can be deployed from altitude or from sea-level within a hurricane flow field, and coordinated autonomously in an energetically-efficient fashion to persistently and continuously monitor relevant properties (pressure, humidity, temperature, windspeed) of a hurricane for days at a time. Rather than fighting the gale-force winds in the storm, the strong, predictable stratification of these winds is leveraged to disperse the balloons into a favorable, time-evolving distribution and to follow the hurricane track as it moves. Certain target orbits of interest in the hurricane can be continuously sampled by some balloons, and can be maintained at nearly uniform separation over these orbits via low-energy control feedback, while other balloons make continuous sweeps between the eye and the spiral rain bands. We expect the acquired data to complement current measurement methods and to be instrumental in improving the numerical models’ forecast skills.

It is noted that the problem of (nearly-) uniformly distributing sensor balloons over a target orbit of interest within a hurricane, in which the average radial velocity is zero, is closely related to the string stability problem considered 20 years ago by PATH researchers considering the coordination of fleets of vehicles moving at small separations and high speeds within automated highways. The present problem has the added complication that the vehicles in this case are moving in 3D manner and are highly underactuated (that is, they can only move up and down), so they must leverage the velocity stratification of the hurricane flowfield to achieve the desired azimuthal separations between balloons. We show that the control authority in this setting is in fact quite significant and, when controlled appropriately, a string of vehicles distributed in a ring on a zero-radial-velocity manifold within a real (non-axisymmetric) hurricane can be kept at nearly constant azimuthal separation with very low-energy control feedback.

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