The role of preexisting airmass boundaries in the maintenance and rotation of deep convection in a high-cape, low-shear environment.

摘要

A detailed examination of the role played by preexisting airmass boundaries in a prototypical high-CAPE, low-shear environment was undertaken in this work. The first phase of the analysis focused on a long-lived, tornadic storm complex that affected central Texas on 27 May 1997. This event was notable because the storm complex developed in a high-CAPE, low-shear environment and propagated against the mean environmental wind along a dryline near its intersection with a cold front and the storm's gust front. Analysis of this event revealed that storm longevity was controlled by the preexisting boundaries. Analysis also demonstrated that all of the tornadoes that produced damage rated greater than F0 occurred on the storm-generated gust front, not on the preexisting boundaries. However, because the cold front played a significant role in the overall organization of the storm complex, tornadoes may not have developed in this event in its absence. Examples of both mesocyclonic and non-mesocyclonic tornadoes were observed during this event; however, the mesocyclonic tornadoes were found to be larger and more destructive than their non-mesocyclonic counterparts.; In the second phase of this research, numerical experiments were used to examine the open issues that emerged from the observational analysis. Simulations were conducted using an idealized cloud model with a horizontally heterogeneous initialization that included the dominant preexisting airmass boundaries of the 27 May 1997 event. With a localized initialization and no preexisting boundaries, the simulated storm was short-lived and therefore consistent with the high-CAPE, low-shear archetype. When both the dryline and cold front were included in the initialization, the overall behavior of the simulated storm complex and boundaries closely resembled the early stages of the 27 May 1997 event. The quasi-continuous maintenance/propagation of the simulated storm was found to be controlled by the cold front. Moreover, strong, temporally coherent near-surface vertical vorticity required the horizontal shear associated with either preexisting boundary.