An Observational Study of Stratospheric Gravity Waves Associated with Rapidly Intensifying Tropical Cyclones

Abstract:

Rapid intensification (RI) of tropical cyclones is often preceded by the formation of Vortical Hot Towers (VHTs) near the eyewall. VHTs are deep tropical cumulonimbus clouds that can penetrate the tropopause and are considered an important source of non-stationary gravity waves in the stratosphere. Statistically significant differences in the observed gravity wave characteristics between RI and non-RI events can significantly improve the technique for estimating the probability of RI in tropical cyclones from satellite observations. The characteristics of convectively generated gravity waves in the vicinity of tropical cyclones are examined using observations from high-altitude balloons deployed by Project Loon. The balloon observations, with a high sampling rate (~60s) can resolve the entire gravity wave spectrum and can be used to directly infer the intrinsic frequency of gravity wave packets. The observations collocated with the best track data for 45 tropical cyclones during 2018-19 are analyzed in a composite framework. Time series of vertical wind perturbations exhibit disturbances with intrinsic wave periods of 20-60 minutes indicative of gravity waves. Momentum fluxes are estimated in order to quantify the intensity of the gravity waves. Wavelet analysis is used to systematically examine the distribution of momentum fluxes as a function of time relative to the RI onset. The wavelet analysis also provides estimates of the waves phase speeds and wavelengths. Even though these medium frequency waves are observed for all the systems, their intensities and duration are different depending on their respective developmental stage (viz. genesis, mature, intensification or weakening). A majority of non-RI events are characterized by short-lived episodes whereas rapid intensification or rapid weakening events are often preceded by intense long-lived episodes of gravity wave activity.