A Merger-Formation Bow Echo Caused by Low-Level Mesovortex in South China

Based on operational radar observations and high-resolution analyses from the Variational Doppler Radar Analysis System (VDRAS), a bow echo producing high-winds and heavy rainfall that occurred over South China in the pre-rainy season is studied. Results show that this bow echo developed from a quasi-linear convective system (QLCS) and acquired a well-defined bow shape after merging with a pre-line convective cell (CC). Interestingly, the rear-inflow jet (RIJ), which has been well recognized to play a key role in the formation of a bow echo, was absent in this merger-formation bow echo (MFBE). This is ascribed to the weak cold pool and line-end vortices generated within the QLCS as it developed in the monsoon environment of high humidity and weak low-level vertical wind shear.

A new pathway of bow echo formation was proposed instead, which highlighted the importance of the low-level mesovortex (MV) on the leading edge of the QLCS. The MV originated from a weak vertical vorticity band ahead of the QLCS. Vertical vorticity budget analyses revealed that the enhanced stretching effect during the QLCS-CC merger was the main cause of the growth of the MV. The well-developed MV thereby provided a RIJ-like flow wrapping cyclonically from north of the QLCS, forcing the QLCS to distorted into a bow echo. This MV contributed foremost to the near surface gales as well.

Combined with the well-resolved dynamical processes aforementioned, observations from an S band polarimetric radar are employed, aiming to uncover the microphysical and dynamical structures and their interaction processes accounting for the heavy rainfall. The precipitation was shown to be featured of high concentration of large hydrometeors, with maxima basically limited within the intensified MV. The deep QLCS developing far above the freezing level favored for significant ice-phase processes, further enhancing rain rate through melted graupel and hail. High spatiotemporal correlation between the precipitation extremes and the MV suggests the non-negligible role the MV played to determine the microphysics process of this precipitation, which required more detailed researches next.