The gravity wave field radiating from tropical cyclones

Due to the very frequent generation of strongly buoyant updrafts, the hurricane eyewall emits a steady stream of gravity waves that radiate outward, downward, and upward. High resolution numerical simulations of hurricanes with 1 km grid spacing show that, due to the strong inertial stability and the large radial shear of the rapidly rotating flow, these waves are wrapped into tight spirals with radial wavelengths of 5 to 10 km and vertical velocity amplitudes of 0.5 to 1 m/s. As they leave the inner core, the waves appear to become decoupled from both the dynamics and the embedded convection, propagating outward at speeds of 20 to 30 m/s.

The signal of these passing gravity waves can be seen in flight level data from NOAA research aircraft approaching hurricane eyewalls. Preliminary analyses of the data show a peak in the spectrum around 5 km wavelength, suggesting that 1 km grid spacing may be just about sufficient to resolve the dominant frequency.

Numerical simulations of rapid intensification also show that bursts of strong convection generate pulses of higher-amplitude, higher-frequency waves. We will explore the possibility that observations of radiating gravity waves could be correlated with structure and intensity change in the hurricane core.