The development of surface signatures of mesoscale convective vortices

The dynamical evolution, including vertical penetration of cyclonic circulation to the surface, of two mesoscale cyclonic vortices (MCVs) is examined using numerical simulations and observations from the Bow Echo and MCV Experiment (BAMEX). Although one case evolves in strong vertical wind shear and the other evolves in modest shear, the evolutions are remarkably similar. In addition to the well-known mesoscale convergence that spins up vorticity in the mid-troposphere, the generation of vorticity in the lower troposphere occurs along the convergent outflow boundary of the parent mesoscale convective system (MCS). Lateral transport of this vorticity from the convective region back beneath the mid-tropospheric vorticity center is important for obtaining a deep column of cyclonic vorticity. However, this behavior would be only transient without a secondary phase of vorticity growth in the lower troposphere that results from a radical change in the divergence profile favoring lower-tropospheric convergence. Following the decay of the nocturnal MCS, subsequent convection occurs in a condition of greater relative humidity through the lower troposphere and small conditional instability. Vorticity and potential vorticity are efficiently produced near the top of the boundary layer and a cyclonic circulation appears at the surface. These results imply that the appearance of a surface circulation necessarily lags the development of cyclonic circulation in the middle troposphere. Furthermore this process is facilitated not by intense, tropospheric deep convection, but rather by relatively benign convection in a relatively moist, downdraft-free environment.