The Merger of a Supercell and Squall Line in the Great Plains. 1: Initiation of the Supercell

Using a four-dimensional variational data assimilation system that incorporates radar and surface observations, we investigated the initiation mechanism of a nocturnal supercell in the central Great Plains on 23 May 2020. This pristine convection occurred ahead of an eastward-propagating squall line, despite the absence of discernible surface convergence, and later evolved into a supercell that eventually merged with the approaching squall line. The atmospheric environment, which featured a low-level inversion layer with considerable convective inhibition, was generally unfavorable for surface-based convective initiation (CI). However, our analysis indicated that the spatial heterogeneous nocturnal southerly low-level jet (LLJ) played a key role in the elevated CI. As the LLJ advected moisture and warm air, neighboring regions experienced substantial low-level destabilization. Multiple convergence bands associated with the heterogeneous LLJ were identified, and the timing and location of CI were governed by these bands. CI occurred when one of the convergence bands moved northward to a position beneath a divergence band at the entrance of the mid-tropospheric jet. This overlapping pattern of convergence-divergence persisted for over an hour, facilitating both the initiation and development of the supercell. Additionally, we observed two other episodes of CI ahead of the squall line, each with different characteristics. The first, occurring earlier than the supercell initiation, was triggered by topographic convergence in a valley but quickly dissipated. Later, as the LLJ rotated clockwise, the most prominent convergence band shifted eastward, favoring the continuous development of forward-building cells ahead of the squall line, resulting in a T-shaped system.