The Merger of a Supercell and Squall Line in the Great Plains: 2. Formation of a Bow Echo

Abstract

The merger of squall line and supercell has been commonly observed in the Great Plains of the United States. On 23–24 May 2020, an isolated supercell formed ahead of a squall line and subsequently evolved into a bow echo through their merger. Utilizing a 10-min update four-dimensional variational data assimilation system that integrates radar and surface observations, we investigated the convective-scale processes that led to the formation of this bow echo. Our findings indicated that as the squall line advanced eastward, its inflow was initially obstructed by the supercell, causing the weakening of both the squall line and its associated cold pool. However, during the merger, a strong rear inflow jet (RIJ) rapidly developed ahead of the squall line's leading edge and behind the supercell. Unlike a classic RIJ that typically expands rearward over the cold pool, this RIJ accelerated forward toward the supercell. Diagnostic pressure analysis revealed that the RIJ was primarily driven by the developing mesolow embedded within the supercell. Additionally, the supercell's strengthening prior to bowing generated a deep-tropospheric gravity wave downdraft, which amplified the RIJ by enhancing near-surface westerlies. Despite the weakening of the squall line's cold pool, it still contributed to the RIJ formation by providing negative horizontal vorticity at its leading edge and facilitating the RIJ's descent. Consequently, the intense RIJ and the subsequent replacement of the supercell against the initial leading edge led to the formation of a severe bow echo, producing a broad swath of damaging winds.