A Numerical Study of the Genesis of Extratropical Convective Mesovortices. Part I: Evolution and Dynamics

Abstract

The purpose of this study is to understand the genesis of extratropical convective mesovortices and the large-scale environmental features that influence the vortex formation. A hypothesis is proposed that mesovortices form in the stratiform rain regions of mesoscale convective systems (MCSs) due to the reduction of static stability that reduces the effective local Rossby radius in such regions. A conceptual model of the mesoscale convective cyclogenesis is introduced, which describes the three stages of the mesovortex formation. A modified version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional hydrostatic mesoscale model is used to simulate mesovortex genesis in analytically generated pre-MCS large-scale environments. The model simultaneously incorporates parameterized convection and a grid-resolvable convective scheme containing the effects of hydrostatic water loading, condensation (evaporation), freezing (melting), and sublimation. A control simulation is performed with a specified pre-MCS environment that is characterized by a midtro-pospheric short wave, a low-level jet ahead of the short-wave trough, a large area of conditionally unstable air, a deep layer of moisture, and small vertical wind shear. A mesovortex forms within a stratiform region behind a leading convective line. The evolution and structure of the mesovortex are similar to observations of the mesovortices associated with MCSs over land at midlatitudes. The results show that the mesovortex is produced by localized warming in a region of locally reduced Rossby radius, which induces convergence and, hence, creates rotational momentum via geostrophic adjustment.