A case study of mesospheric frontal interaction and associated processes over the western Himalaya

Abstract

An interesting event of interaction between a mesospheric bore and a front is observed in O(1S) 557.7 nm airglow images over the western Himalayan region on the night of 25 April 2022. The event is unique as it is the first report of a mesospheric bore interacting with a typical mesospheric front. The vertical profiles of temperature and Brunt Vaisala frequency indicate presence of a strong mesospheric inversion layer (MIL) which acts as a stable thermal duct for propagation of the mesospheric bore. Chemical heating is believed to be a causative mechanism for sustaining such strong MIL/thermal duct, although the presence of Doppler duct could not be verified due to a lack of wind measurement. The bore front shows an anti-clockwise rotation with time which is attributed to the differential phase speed between the extreme parts of the bore due to variation of duct depth. The bore propagation above is believed to push the underlying OH layer downward, resulting in a maximum horizontal slope of the peak height of the OH volume emission rate (VER) on the event day. The results highlight the characteristics of bore-front interaction, bore propagation supported by the thermal duct, and its effect on the vertical displacement of the airglow emission layers.