Medium‐Scale Traveling Ionospheric Disturbances Induced by Secondary Gravity Waves Associated With Record‐Breaking Heavy Rainfall in August 2021 Over Kyushu, Japan

Abstract

This study reconstructs a multistep gravity wave (GW) pathway, from the generation of primary GWs by deep convection in the troposphere to the generation of secondary GWs that drive semi‐concentric medium‐scale traveling ionospheric disturbances (MSTIDs) in the ionosphere, during the record‐breaking heavy rainfall over Kyushu, Japan, in August 2021. Around 17 UT on 12 August, Atmospheric Infrared Sounder (AIRS) observations detected stratospheric GWs above the heavy rainfall, and ∼six hours later, Global Navigation Satellite System (GNSS) observations revealed semi‐concentric MSTIDs whose apparent epicenter was located near the heavy rainfall region. A convection‐resolving simulation using the Weather Research and Forecasting (WRF) model reproduced both the deep convection and the observed stratospheric GWs, showing that the primary GWs were continuously generated by deep convection associated with the rainfall. The convection developed in an environment of high convective available potential energy (CAPE) and strong low‐tropospheric wind shear. Forward ray tracing indicated that the AIRS‐observed primary GWs in the stratosphere broke near 132°E and 33°N at altitudes of ∼60–110 km, roughly coincident with the convergence region inferred from backward ray tracing of the MSTID‐driven GWs. This convergence of the MSTID‐driven GWs near the estimated breaking altitude indicates that these thermospheric GWs were most likely generated at the breaking altitude as secondary GWs. These results provide strong evidence that the observed MSTIDs were generated by secondary GWs produced by primary GW breaking rather than by directly propagating primary or evanescent GWs. These independent observations and simulations consistently support a multistep vertical coupling mechanism linking the record‐breaking rainfall event to the ionospheric disturbances.