Mesovortices within the 8 May 2009 Bow Echo over Central United States: Analyses of the Characteristics and Evolution based on Doppler Radar Observations and a High-Resolution Model Simulation

A derecho-producing bow echo event over the Central US on 8 May 2009 is analyzed based on radar observations and a quite successful real-data WRF simulation at 0.8 km grid spacing. Emphasis is placed on documenting the existence, evolution and characteristics of low-level mesovortices (MVs) that form along the leading edge of the bow echo. Relationships between the MVs and high surface winds are also examined. Significant MVs are detected from the radar radial velocity using a linear least squares derivatives method, and from the model simulation based on calculated vorticity. Despite of some discrepancies, the simulated bow echo and embedded MVs share many commonalities with the observed counterparts, which thus provides a basis for a detailed diagnostic study on their genesis processes. These bow-echo MVs are mainly produced during the early stage of the convective system and predominantly form north of the bow echo apex. MVs that develop on the southern bow tend to be weaker and shorter-lived than their northern counterparts. The most long-lived MVs are found to be located at or near the bow apex, propagating consistently with the system. Vortex merger is commonly seen between MVs during their forward movement, which causes remarkable redevelopment of some MVs in the decaying stage of the bow echo system. In the model simulation, damaging straight-line winds are primarily found in the mature and weakening stage of the bow echo system. Similar to observations, not all mesovortices are damaging and the strongest surface winds in the simulation are found in association with a meosvortex at the bow apex which is embedded in, and promoted by the system rear-inflow jet (RIJ). Descending of the RIJ is suggested to have played an important, or dominant, role in producing the strongest damaging winds at the bow apex. Evolution and structural features of this bow-apex mesovortex are also examined. The results show that this mesovortex arises from a pre-existing shear region ahead of the system and acquires significant vertical vorticity near the surface via vertical stretching when the RIJ descends toward the surface. The genesis mechanism for this mesovortex is studied in detail using both the Eulerian and Lagrangian budget analyses of the absolute vertical vorticity.