A long-lived convectively generated mesoscale vortex associated with heavy rainfall

Midtropospheric vortices (MCVs) generated by mesoscale convective systems are an important facet of the warm-season quantitative precipitation forecast (QPF) problem since they can focus the redevelopment of organized convection for up to several diurnal cycles. In the current study we investigate, using observations and a mesoscale numerical model, an MCV-event that occurred over the central Plains and southeastern United States during a several-day period (27-29 May 1998) and produced multiple episodes of flash flooding.

This MCV case persisted within a moist and conditionally unstable environment with weak vertical wind shear. Deep convection followed a distinct diurnal cycle; occurring along the downshear periphery of the vortex during the late afternoon and early evening, while being more highly organized and situated closer to the remnant circulation center overnight. The nocturnal convection near the center of the MCV is hypothesized to be crucial in reinvigorating the circulation, thus allowing it to persist in the presence of differential advection by the ambient vertical shear.

The preference for deep convection along the downshear flank of the vortex is consistent with the balanced lifting hypothesis advanced by Raymond and Jiang. However, variations in convective organization and in the location of the convection relative to the MCV center suggest important modulation by the diurnal cycle. The vortex is well resolved by analyses from the Rapid Update Cycle (RUC). The RUC analyses are used to initialize simulations with the Penn State/NCAR Mesoscale Model (MM5) that investigate the interplay of the MCV, its associated convection, and the diurnal cycle.