Dr. O’Neill presented a seminar on “Diurnal gravity waves as a probe of hurricanes’ internal structure“.
Satellite observations of cloudy hurricane canopies have shown a universal, daily, wave-like feature that propagates radially outward, as far as 600 km (Dunion et al. 2014). Daytime solar heating of a hurricane’s upper eyewall is surely responsible, but the mechanism for the wave was previously unknown. I 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 disruptions to the wind field that resemble an eyewall replacement cycle.
A recording of the presentation is available on the anonymous ftp site: ftp://ftp.aoml.noaa.gov/hrd/pub/blog/seminars/2017/ONeill_HRD_Seminar_20171114.mp4