Diurnal Gravity Waves as a Probe of Hurricanes' Internal Structure
Department of the Geophysical Sciences
University of Chicago
TCS Building 240
Satellite observations of cloudy hurricane canopies have shown a universal, daily, wave-like feature that propagates radially outward, as far as 600 km. Daytime solar heating of a hurricane's upper eyewall is surely responsible, but the mechanism for the wave was previously unknown.
Dr. O'Neill will discuss numerical experiments that suggest these waves are internal inertia-gravity waves, and in fact propagate through (almost!) the entire depth of the hurricane. Their structure is similar to the classical "St. Andrew's cross" pattern response to a bobbing cylinder in a quiescent fluid. Due to the hurricane's flow field, diurnal waves can only begin to propagate well beyond the storm core, though the anticyclonic outflow region is more receptive to near-core diurnal propagation. The prohibited region is highly sensitive to a disruptive process called an eyewall replacement cycle (ERC), in which the original eyewall breaks down and is replaced by a larger, contracting eyewall.
Hurricanes remain elusive at both the smallest and largest scales: the fastest wind speeds are enduringly difficult to forecast, and the rather steady global annual count of hurricanes (~90) is completely unbounded theoretically. Only in the last five years has numerical modelling become affordable enough to resolve hurricane-like storms on a full globe. Dr. O'Neill will conclude with an overview of how these new tools will drive a renaissance in hurricane research for a warming world, and outline open questions that have suddenly become tractable.