Echo over Central US: 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 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 bowing system. The genesis of near-surface high winds within the system is also investigated. Significant MVs are detected from the radar radial velocity using a linear least squares derivatives (LLSD) method, and from the model simulation based on calculated vorticity. Both the observed and simulated bow-echo MVs predominantly form north of the bow apex. MVs that develop on the southern bow tend to be weaker and shorter-lived than their northern counterparts. Vortex mergers occur between MVs during their forward movement, which causes redevelopment of some MVs in the decaying stage of the bow echo. MVs located at (or near) the bow apex are found to persist for a notably longer lifetime than the other MVs. Moreover, the model results show that these bow-apex MVs are accompanied with damaging straight-line winds near the surface. These high winds are mainly caused by the descent of the rear inflow jet at the bow apex, but the MV-induced vortical flow also has a considerable contribution. The locally enhanced descent of the rear inflow jet near the mesovortex is forced primarily by the dynamically induced downward vertical pressure gradient force while the buoyancy force only plays a minor role there. 1. Introduction Low-level, meso-γ-scale (2−20 km, Orlanski 1975) mesovortices (MVs) are frequently observed at the leading edge of quasi-linear convective systems (QLCSs) like squall lines and bow echoes (e.g., Funk et al. 1999; Atkins et al. 2004). Recently, MVs have received increasing attention, owing to their propensity to produce damaging straight-line winds (i. The production of damaging winds within QLCSs has long been linked to the descent to the surface of a rear-inflow jet (RIJ) at the bow apex (Fujita 1978, 1979). Various factors have been implicated in the development of RIJs, such as the hydrostatically induced mid-level pressure minimum behind the leading convective updrafts (Lafore and Moncrieff 1989), horizontal buoyancy gradients related to the upshear-tilting convective circulation Additionally, the RIJ strength had been found to be sensitive to ice microphysical processes as well as the environmental humidity (Yang and Houze 1995; Mahoney and Lackmann 2011). However, the strongest straight-line wind damage may …