Evolution and Maintenance of the 22-23 June 2003 Nocturnal Convection during BAMEX

Abstract

On 22-23 June 2003 two mesoscale convective systems (MCSs) evolved throughout the evening and night time hours and were observed by the Bow Echo and Mesoscale convective vortex Experiment (BAMEX). These two MCSs were studied by analyzing the observations, and performing both case study and idealized model simulations. The first of these MCSs originated from a group of supercells that had been initiated in a north-south line along a pre-existing outflow boundary in eastern Nebraska. These supercells anchored to the pre-existing outflow boundary leading to large rainfall totals and facilitating cell mergers. These cell mergers increased the depth and strength of the surface cold pool, which became the forcing mechanism for new convection. As this happened, the convection reoriented from a north-south line of isolated supercells into an east-west, southward propagating squall line. While the squall line was developing and reorienting, isolated supercells developed along the dryline in north-central Kansas. These supercells moved northeastward, eventually passing the southward propagating squall line and evolving into a small MCS that continued to move northeastward during the night.

These two modes of convection developed and evolved in a similar nocturnal environment suggesting that each MCS was being forced differently or feeding off of a different source layer. A northeastward mean wind vector explains the motion of all of the cells, including individual cells within the squall line, however, does not account for the differing storm motions of the two resulting MCSs. This can be explained by te presence of a deep cold pool at the surface that was responsible for the maintenance of the southward propagating squall line throughout the nocturnal hours. The nocturnal boundary layer cooled and stabilized, however, convection was able to remain surface-based as long as a mechanism existed to lift air to its level of free convection (LFC). In this study, both cold pool dynamics and supercell dynamics played an important roll in lifting air to the LFC throughout the nocturnal hours.