Short-Term Convective Mode Evolution along Synoptic Boundaries

Abstract

This paper investigates the relationships between short-term convective mode evolution, the orientations of vertical shear and mean wind vectors with respect to the initiating synoptic boundary, the motion of the boundary, and the role of forcing for ascent. The dominant mode of storms (linear, mixed mode, and discrete) was noted 3 h after convective initiation along cold fronts, drylines, or prefrontal troughs. Various shear and mean wind vector orientations relative to the boundary were calculated near the time of initiation. Results indicate a statistical correlation between storm mode at 3 h, the normal components of cloud-layer and deeplayer shear vectors, the boundary-relative mean cloud-layer wind vector, and the type of initiating boundary. Thunderstorms, most of which were initially discrete, tended to evolve more quickly into lines or mixed modes when the normal components of the shear vectors and boundary-relative mean cloud-layer wind vectors were small. There was a tendency for storms to remain discrete for larger normal shear and mean wind components. Smaller normal components of mean cloud-layer wind were associated with a greater likelihood that storms would remain within the zone of linear forcing along the boundary for longer time periods, thereby increasing the potential for upscale linear growth. The residence time of storms along the boundary is also dependent on the speed of the boundary. It was found that the boundary-relative normal component of the mean cloud-layer wind better discriminates between mode types than does simply the ground-relative normal component. The influence of mesoscale forcing for ascent and type of boundary on mode evolution was also investigated. As expected, it was found that the magnitude and nature of the forcing play a role in how storms evolve. For instance, strong linear low-level convergence often contributes to rapid upscale linear growth, especially if the boundary motion relative to the mean cloud-layer wind prevents storms from moving away from the boundary shortly after initiation. In summary, results from this study indicate that, for storms initiated along a synoptic boundary, convective mode evolution is modulated primarily by the residence time of storms within the zone of linear forcing, the nature and magnitude of linear forcing, and secondarily by the normal component of the cloud-layer shear.

The Initiation and Evolution of Multiple Modes of Convection within a Meso-Alpha-Scale Region

“…It becomes complicated further in cases where two or more of these modes are present at the same time. Past climatological studies (e.g., Dial and Racy 2004;Bunkers et al 2006) have indicated that while not as common as single-mode events, multimode events do happen frequently enough to be of concern. One such event occurred on 30 March 2006 and is the focus of this study.…”

The Dependence of Storm Longevity on the Pattern of Deep Convection Initiation in a Low-Shear Environment

“…By extension, these results also illustrate that a quasi-linear mechanism, such as a preexisting airmass boundary, that initiates multiple deep convective cells in close proximity can lead to longerlived storms than a mechanism that initiates isolated deep convection. While it is most certainly true that 1) not all airmass boundaries that initiate deep convection support upscale growth and (presumably) storm longevity (Dial et al 2010) and 2) preexisting airmass boundaries can also initiate isolated deep convection, the results from these experiments indicate that, all else being equal, isolated initiation is less favorable for storm longevity in this weak-shear environment than quasi-linear initiation.…”

Tornadoes in the Central United States and the “Clash of Air Masses”

“…1 If anything, there is some indication that squall lines, not supercells, are more likely when the temperature gradient associated with an airmass boundary is intense (e.g., Roebber et al 2002;Arnott et al 2006;Stonitsch and Markowski 2007;Dial et al 2010;Duda and Gallus 2010;Schumann and Roebber 2010). In other words, strong horizontal temperature gradients may actually pose a decreased risk of significant tornadoes (EF2 or greater tornadoes; Hales 1988), given that squall lines are less likely to produce significant tornadoes than are discrete supercells (Trapp et al 2005b;Thompson et al 2012;Smith et al 2012).…”