In-situ Observations Over Tropical Cyclone Hinnamnor Show Large-Amplitude Gravity Wave Disturbances That Are Underestimated in Reanalysis

Abstract

Tropical cyclones (TCs) induce gravity waves which radiate outwards and upwards in concentric rings from the core, or spiral formations. These waves deposit momentum at higher altitudes and create temperature fluctuations that can control local cirrus formation. The induced cold temperature fluctuation from the cold phase of the gravity wave can induce relative humidities above the threshold needed for condensate formation. However, TC-induced gravity waves are mainly known from simulations and satellite observations, which are unable to fully capture the fine scale structure of the perturbations. An overflight of TC Hinnamnor during the 2022 ACCLIP campaign provides new, high resolution data. We find a gravity wave amplitude of 4.7 K (centerline to peak) above the TC at 87 mb (~17.6 km). ERA5 reanalysis underestimates the amplitude of the TC-induced gravity waves: amplitudes in ERA5 at 70 mb are more than three times smaller (1.3 K), and the frequency is two times larger, than in-situ measurements. The in-situ-measured amplitude over Hinnamnor is large but not unprecedented for stratospheric gravity waves. We show that in ERA5, TCs regularly produce gravity waves, albeit their amplitudes are underestimated, and that TC Hinnamnor has temperature variations representative of other TCs. Underestimates of gravity wave amplitude can result in an underestimate of cirrus formation in the cold phase of the wave under appropriate conditions.