An Atmospheric Bore in North China Plain and Its Impact on Nocturnal Convection

Atmospheric bores (a special type of large-amplitude gravity wave) are generated when density currents intrude into the nocturnal stable boundary layer, and they are proven to play a significant role in triggering and maintaining nocturnal mesoscale convective systems. Potential role of atmospheric bores in nocturnal convection over North China Plain is investigated through looking into the generation and impact mechanisms. Atmospheric bores are identified using surface station data and Doppler radar base data at Weifang Station. A nocturnal convection event originating in the northern Taihang Mountains and propagating eastward to the eastern North China Plain on 17 June 2022, is diagnosed and simulated using CMA-MESO operational model (1 km). Observational analysis reveals that convection-generated density currents intrude into the nocturnal stable boundary layer, triggering nonundular bore, and as the environmental flow regime transitions from supercritical to subcritical state, large-amplitude internal gravity waves are generated and trapped, transforming into undular bore. CMA-MESO operational model successfully simulates the evolution of the entire convective system and the southward-propagating atmospheric bore ahead of the convection, as well as the convective cells triggered by the atmospheric bore on the southern side of the convection. The bore propagates at approximately 13.2 m·s^-1, which is typical for gravity wave propagation speed and consistent with observations. The depth ration between near-surface light fluid and density current and Froude number of the selected profiles indicate that convection-generated density current intrudes into the nocturnal stable boundary layer, causing a complete blocking flow regime that triggers a type C bore ahead of the convective system. As the density current weakens, the bore gradually transitions into a type B bore. According to linear wave theory, Scorer parameter, defined by both stability and wind speed curvature terms, characterizes the atmosphere's ability to trap gravity waves. The vertical shear generated by the nocturnal low-level jet reduces Scorer parameter, providing a suitable waveguide for the maintenance and propagation of atmospheric bore, which persists for approximately 3 h. After the passage of atmospheric bore, the nocturnal convective environment improves, with convective available potential energy increasing by about 270 J·kg^-1 and convective inhibition decreasing by about 100 J·kg^-1. Additionally, the lifting condensation level decreases by about 1800 m. Atmospheric bore propagates faster than the convective system and induces strong vertical lifting, peaking above the nocturnal stable boundary layer. This process weakens the inversion layer through vertical lifting and transports water vapor from the lower layers upward, providing water vapor for the development of the convective system and maintaining existing convective cells. As water vapor is lifted to higher levels, the cloud water mixing ratio increases, triggering new convective cells. These new convective cells merge with the original convective system, thereby enhancing and sustaining the nocturnal convection.