Coherent Regeneration and the Role of Water Vapor and Shear in a Long-Lived Convective Episode

Abstract

In a recent radar-based climatological study of warm-season precipitation over the continental United States, Carbone et al. found a high frequency of long-lived coherent rainfall episodes. Many of the events were of longer duration than normally associated with mesoscale convective complexes and exhibited phase speeds ?10 m s?1 in excess of the phase speed associated with synoptic systems. The observations led to the speculation that cold pool dynamics and wavelike propagation mechanisms were responsible for the longevity of the systems. One of the long-lived episodes included in the statistics of the Carbone et al. study is described here. Occurring on 14?15 July 1998, the system lasted ?50 h and traveled over 2800 km. At its peak intensity the system was a bow echo producing damaging wind, large hail, and local flash flooding. An interesting aspect of the event was an abrupt 90° turn in the storm's orientation and propagation vector midway through its life. The environmental factors that led to the observed behavior are investigated. The episode consisted of two mesoscale convective systems (MCSs) and owed its longevity to a coherent regeneration process, with favorable cold pool?wind shear interactions playing a dominant role. Synoptic-scale forcing indirectly played a role by moistening the environment and creating a favorable wind shear region. An important observation is an E?W spatial displacement (?200 km) between the N?S corridor of maximum low-level moisture/CAPE and the maximum low-level wind shear/system relative inflow, and the fact that the storm followed the most favorable wind shear corridor. High moisture/instability alone were not enough to ensure the longevity of this system.