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

Abstract

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. Reexamination 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 produces 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.

Origin of outer tropical cyclone rainbands

“…4a indicates the lack of consistency between the observational TCR radial propagation velocities and theoretical GW phase velocities. Most of the GW phase velocities are confined to a narrower range at approximately ±20 m s −1 , which is the same order of magnitude as the typical radial propagation speeds of several meters per second for GWs excited within TCs 42 , 44 – 47 . These theoretical propagation estimates are distinctly separated from the primary velocity distributions of the observed radial propagation, which are within the range of ±10 m s −1 (Figs.…”

“…In this view, the moist convection occurring in outer TCRs may be triggered by gravity-wave-induced upward motions 42 , 43 . Despite the potential discrepancy in the propagation speed between theoretical GWs and observed outer TCRs 3 , 30 , 42 – 47 , what we learned from these previous works is still unable to adequately address whether outer TCRs would be initially triggered by the GWs within the inner-core region and then propagate outward to the outer region of TCs.…”

The Representation of Spiral Gravity Waves in a Mesoscale Model With Increasing Horizontal and Vertical Resolution

“…Nolan (2020; hereafter N20) proposed that the primary source of SGWs is the pulsation of convective maxima in the TC eyewall as they are advected through the convectively enhanced, downshear‐left quadrant of a TC. N20 tested this hypothesis by forcing a linear dynamical model of perturbations to a TC‐like vortex with idealized heat sources that were rotating and pulsing.…”

“…The most prominent p oscillations had periods of 2,000-3,000 s, consistent with the simulated p waves, but their radial scales or speeds could not be independently deduced from solitary observations at fixed points. Nolan (2020;hereafter N20) proposed that the primary source of SGWs is the pulsation of convective maxima in the TC eyewall as they are advected through the convectively enhanced, downshear-left quadrant of a TC. N20 tested this hypothesis by forcing a linear dynamical model of perturbations to a TC-like vortex with idealized heat sources that were rotating and pulsing.…”