Dynamics Governing a T-initiation Mesoscale Convective System

Bow echoes, identifiable by their distinct bow-like shape on radar, are often associated with intense specific convective processes. The T-initiation Mesoscale Convective System (MCS) exemplifies this phenomenon, appearing as a quasi-linear bow echo with downstream nascent leading convection (LC), forming a characteristic 'T' shape on radar displays. The LC presents significant challenges for operational forecasts and can lead to severe weather conditions, including damaging straight-line winds and extreme precipitation, particularly when they rapidly merge with the parent bow echoes. Focusing on a high-impact T-initiation MCS over the Bohai Sea in China, this study investigates the underlying physical mechanisms governing LC initiation in a coastal environment. Utilizing high-resolution, cloud-resolving simulations from the WRF-ARW model, backward trajectory tracking within a Lagrangian framework and detailed vertical momentum budget analysis are employed to explore the dynamic factors driving the uplift of air parcels within the LC cells from their pre-convective state. Results reveal that a mid-level mesoscale anticyclonic vortex, associated with the bow echo’s upper-level outflow, significantly modulated the local environmental wind field. This resulted in stronger vertical wind shear between 2.5–5 km AGL downstream of the bow-echo. Interactions between this locally enhanced shear and pre-existing vertical motion led to low perturbation pressures at midlevels. This effect subsequently generated an upward-directed nonhydrostatic vertical perturbation pressure gradient force that aided parcel ascent, directly contributing to the initiation of new LC cells. Thus, this study helps elucidate the complex self-organizing and sustaining processes of bow echoes by identifying a novel, dynamically driven initiation mechanism for LC cells in a T-initiation MCS.