Polarimetric Radar and VDRAS investigation of a Bow Echo after a Squall Line Merged with a Convective Cell

This study documents the development of a merger-formation bow echo in southeast China evolving from a subtropical squall line merging with a pre-line convective cell using polarimetric radar observations and the four-dimensional Variational Doppler Radar Analysis System (VDRAS) analysis. As the squall line propagated towards the coast, an isolated convective cell was initiated above the sea-breeze front located in front of the squall line, which possessed a rear-inflow jet and book-end vortices but never produced a bow echo until the two convective entities merged together. The bowing segment first emerged on radar reflectivity after merger, yet the leading edge of surface cold pool still fell behind with the original squall line, which was quite different from that in classical squall line bow echoes. The strengthened updraft from the merger enhanced precipitation ahead of the original squall line, resulting in the surface cooling ahead of the surface cold pool. This newly developed cold air enhanced the squall line cold pool to catch up with the bowing reflectivity. The expansion of the surface cold pool enhanced the rear-inflow jet via horizontal vorticity generated by baroclinity. Such an observational case revealed different processes of the evolution of merger-formation bow echo from the classical squall line bow echoes.