Radar Observations of the Structure and Evolution of a Severe Bow Echo in East China

The structure and evolution of a severe bow echo, which occurred in the Henan Province of eastern-central China on 3 June 2009, is investigated using Doppler radar observations. Strong northwesterly winds behind the 500 mb trough provided favorable large-scale conditions for the formation of the bow echo, including the provision of enhanced westerly momentum that can be transported to the lower levels to enhance the rear inflow jet behind the bow echo. Results show that the bow echo grew out of a dissipating supercell and became organized through mergers with nearby convective cells at its north. Dual-Doppler wind analyses were formed to study the internal circulation and structures within this bow echo system. At the mature stage, the bow echo was oriented northeast-southwest with 80 km in length. The storm-relative rear-inflow extended up to about 5 km and exhibited a jet like structure with the core near 2 km and the maximum inflow strength of 15~18 m/s. The jet descended from the back of the convective region, and accelerated to about 20 m/s near the leading edge of the system. Cyclonic and anticyclonic vortices existed at the mid-level at the northern and southern ends of the system, respectively, and their circulations were responsible for about 20% of the rear flow behind the apex of the bow. At low-level, there was a cyclonic mesovortex near the apex at the leading edge of the outflow boundary. The ground-relative maximum wind with a magnitude of more than 32 m/s was produced by the superposition of this low-level vortex and the rear-inflow jet. Different from typical northern hemisphere bow echo with bookend vortices, the anticyclonic vortex at the southern end was stronger in this case. During the decaying stage, the low-level cold outflow surged ahead of the bow echo,cutting off warm and moist inflow to the main convection.