Cloud-resolving simulations of mesoscale vortex intensification and its effect on a serial mesoscale convective system

The authors performed numerical simulations of the 27-28 May 1998 mesoscale convective vortex (MCV) and mesoscale convective system (MCS) to understand how the MCV modulated the convection, and how the convection intensified the vortex. The fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model (MM5) was used, initialized with the analysis from the Rapid Update Cycle version 2 (RUC-2) and nested to achieve 1.5-km grid spacing covering the MCS. The model successfully simulated a north-south convective band that formed near the center of the MCV in the early evening. The simulation then correctly reoriented the convection along a roughly east-west line in response to northward transport of warm, conditionally unstable air within the nocturnal low-level jet. Balanced vertical motion was found to contribute substantially to mesoscale lifting and thermodynamic destabilization, which localized the convection. Horizontal transport of moist, unstable air into the nocturnal convection was significantly modulated by the MCV. Intensification of the MCV began overnight when a lower-tropospheric mesoscale vortex formed on the northern end of the north-south-oriented convective line. Intensification of the midtropospheric vortex followed, occurring in response to the development of a stratiform precipitation region. Melting of hydrometeors appeared to contribute substantially to the development of the midlevel circulation. The result was a deep column of enhanced vorticity and potential vorticity.