The Vorticity Structure of Bowing Segments Embedded withing a Squall Line

There have been a number of numerical simulations that have shown the importance of tilting of horizontal vorticity in generating mesoscale vortices within quasi-linear convective systems. These vortices lead to the bow-shaped structure in radar reflectivity and their associated counter-rotating circulations contribute to the rear-inflow jet. However, there have been only a few observational studies that have had both the spatial and temporal resolution to examine these processes in detail, especially for the bowing segments of the convective line that are 20-40 km in length (sometimes referred to as subsystem-scale vortices) that are common in many squall lines. On 2 June 2003, a quasi-linear convective line with a trailing stratiform region developed over Mississippi while being sampled by two airborne Doppler radars during BAMEX (Bow Echo and MCV Experiment). ELDORA flew a total of 7 legs ~100 km in length out ahead of the convective line while collecting finescale reflectivity and dual-Doppler data. The NOAA P-3 was able to coordinate with ELDORA on 4 of these legs which produced quad-Doppler wind syntheses for these times. The subsequent Doppler radar analysis documented the entire evolution of the convective line while also capturing the development of a ~40 km bowing segment within the line (Fig. 1). The counter-rotating vortices associated with the bow echo are shown to be a result of tilting of horizontal vorticity generated by the cold pool. The horizontal scale of the bowing segment was related to the variability in the depth of the cold pool along the convective line as hypothesized by numerical simulations. The bowing segment developed at the location where the cold pool was deep and, accordingly, associated with strong horizontal vorticity that was baroclinically produced. Time-height cross sections show the upward growth of the vortices from the boundary layer into the midlevels of the convective system. The counter-rotating circulations are also shown to be the primary forcing mechanism for the rear-inflow jet. Also apparent in the dual-Doppler wind synthesis of the quasi-linear convective line were prominent horizontal bands of cyclonic and anticyclonic vertical vorticity separated by 15-20 km and oriented parallel to the line. This type of banded structure has been rarely reported in the literature. Indeed, the current study was able to document the structure of these vorticity bands with significantly higher spatial resolution than past studies. A large component of the ambient wind shear was aligned parallel to the convective line. Accordingly, the horizontal vorticity vector at low levels pointed across the convective line and into the stratiform region. Alternating regions of updrafts and downdrafts appear to tilt the vorticity vector and resulted in the banded structure. The reasons why this type of vorticity structure has not been well-documented in past studies is discussed.