Secondary Gravity Wave Propagation in Tropical Thermospheric Region: Role of Varying Kinematic Viscosity.

The current study has investigated the thunderstorm induced atmospheric gravity waves (AGWs) over Indian region based on the perturbation signatures in ionospheric total electron content (TEC) measurement. Robust traveling ionospheric disturbance (TID) signature has been identified along the east side of the thunderstorm affected area. Neutral wind was found to have a favorable impact in this aspect for a certain time duration of the day by modulating the vertical wavelength. The role of temperature was analyzed in terms of kinematic viscosity which is a crucial component, especially over tropical region, for wave dissipation and reflection along its propagation path. Ray tracing algorithm is also applied with varying kinematic viscosity and thermal diffusivity for retrieval of possible ray paths and source location of observed waves. A statistical investigation has been carried out to identify the dissipation altitude of observed waves along the ray paths. It has been found that all waves dissipated at almost a constant altitude for a specific kinematic viscosity and above this altitude vertical wavelength was found to decrease. The ray paths interacted at a common point which was located at about 125 km altitude and was very close to the region of maximum lightning activity. It can also be noted that the observed phase velocities can't be achieved by a wave below the turbopause. It indicates that the observed waves were excited from a secondary source and not directly connected to convective system. The study provides an in‐depth analysis of mesoscale system induced gravity wave propagation and dissipation over tropical region. Plain Language Summary: Mesoscale convective systems in tropical regions are key sources of low‐frequency atmospheric gravity waves (GWs). GWs usually generates above thunderstorm altitudes and propagates to ionospheric heights through dissipation and atmospheric filtering. This is one of the first study about the propagation mechanism of thunderstorm‐generated GWs at thermospheric altitude over the Indian region. The concentric source of the wave was evidenced by concentric rings in traveling ionospheric disturbance and phase velocity Vp $\left({V}_{p}\right)$ trends which coincides with high lightning location. It is also observed that wave with Vp ${V}_{p}$ above 250 m/s were unable to propagate below 100 Km and indicate the secondary source of the gravity wave at thermospheric altitudes which was not directly related with the thunderstorm activity. The effect of background temperature on the propagation mechanism was tested by examining the role of kinematic viscosity on wave dissipation. The maximum momentum fluxes were found around 260 km altitude for a specific kinematic viscosity profile. Interestingly, the ray‐tracing validation fails one scale height above this altitude, aligning with theories of wave dissipation or reflection. Additionally, it was also observed that all waves interacted at a common point in both horizontal and temporal domains. This is one of the key finding of this investigation. Key Points: The study presents signature of thunderstorm induced gravity wave above lower thermospheric altitudePhase velocity and intrinsic period increased linearly from thunderstorm locationAn almost unique wave dissipation altitude found for all waves for a specific kinematic viscosity [ABSTRACT FROM AUTHOR]