Convective Gravity Waves in the Stratosphere and the Mesosphere: A Case Study of Airglow Over West Africa Using the Weather Research and Forecasting Model

The Weather Research and Forecasting (WRF) Model is employed to explore gravity waves excited by a storm complex over West Africa on 15 August 2020, propagating to the mesosphere and lower thermosphere. Although the simulation portrays gravity waves in the stratosphere in a complex structure, it reveals a concentric-ring pattern of gravity waves in the mesosphere. The waves closely resemble the airglow image acquired by Suomi National Polar-orbiting Partnership satellite near the mesopause. This intriguing phenomenon of the emergence of concentric rings after passing the stratopause is investigated by analyzing wave characteristics, including horizontal wavenumbers, frequency, and phase speed. The analysis reveals that the wind filtering effect directly influences the behavior of gravity waves, leading to variations in their appearance with altitude. The wind filtering effect, which typically breaks symmetric ring waves, could help reveal the ring pattern when the waves are subject to interference. Consequently, the concentric rings, propagating faster than the background winds, are shown to originate from the cloud tops of the storm complex. A wind duct at a specific location is identified through spectral analysis. This mechanism is further elucidated with the assistance of the dispersion relation of a classical linear theory.