An Investigation of Spiral Gravity Waves Radiating from Tropical Cyclones Using a Linear, Nonhydrostatic Model

A recent study showed observational and numerical evidence for small-scale gravity waves that radiate outward from tropical cyclones. These waves are wrapped into tight spirals by the radial and vertical shears of the tangential wind field. Re-examination of the previously studied tropical cyclone simulations suggests that the dominant source for these waves are convective asymmetries rotating along the eyewall, modulated in intensity by the preferred convection region on the left side of the environmental wind shear vector. A linearized, nonhydrostatic model for perturbations to a balanced vortex is used to study the waves. Forcing the linear model with rotating and pulsing asymmetric heat sources generates radiating gravity waves with multiple vertical and horizontal structures. The pulsation of the rotating heat source generates two types of waves: fast, deep waves with larger radial wavelengths, and slower, secondary waves with shorter radial and vertical wavelengths. The deeper waves produce surface pressure oscillations that have time scales consistent with surface observations, whereas the shorter waves have little surface indication but produce oscillations in vertical velocity with shorter radial wavelengths that are consistent with aircraft observations. Convective forcing that is either not pulsing or not rotating produce gravity waves but they are not as similar to the observed or simulated waves. The effects of varying the intensity of the cyclone, the asymmetry of the forcing, and the static stability of the surrounding atmosphere are explored.