Convection initiation and misocyclone development: Is there a link?

Convection initiation often occurs non-uniformly along boundary layer convergence lines, such as gust fronts, which leads to difficulties in accurate nowcasting of severe local storms. Misocyclone development induced by horizontal shearing instabilities is one manifestation of kinematic variability along such boundaries that may have an impact on convection initiation. However, a clear link between convection initiation and misocyclone development along boundaries has not yet been verified either through observations or numerical simulations. During the Convection and Precipitation/Electrification (CaPE) experiment conducted in east-central Florida during July-August 1991, a series of misocyclones spaced at 3-5 km intervals were observed along an intense gust front (Kingsmill 1995). Although convergence and vertical velocity maxima were observed at locations adjacent to each of the misocyclones, they were not preferred areas of convection development. In contrast, results from numerical simulations of thunderstorm outflows indicate that misocyclone locations and their associated regions of convergence and vertical velocity maxima were indeed preferred areas of convection development (Lee and Wilhelmson 1997).

Since fundamentally different conclusions regarding the importance of horizontal shearing instabilities in convection initiation exist, this study focuses on a broader analysis of kinematic structures along gust fronts observed during CaPE. Multiple-Doppler radar derived wind fields from eleven cases were analyzed quantifying the intensity and size of misocyclones and their distribution along the gust front. These cases were further analyzed to determine the width of the horizontal shear zone and the character of the pre- and post-frontal wind fields in relation to misocyclone development. Initial results show that in most cases, misocyclone development was weak, with an average maximum vertical vorticity value below 0.002 1/s. Only in two cases did these vorticity values exceed 0.004 1/s. The characteristics of the pre- and post-frontal wind field do not necessarily correlate with the strength of misocyclone development. We are not yet able to link the observed kinematic structures to convection initiation. Further analysis is underway to explore this linkage in greater detail.

Kingsmill, D. E., 1995: Convection initiation associated with a sea-breeze front, a gust front, and their collision. Mon. Wea. Rev., 123, 2913-2933.

Lee, B. D. and R. B. Wilhelmson, 1997: The numerical simulation of non-supercell tornadogenesis. Part I: Initiation and evolution of pretornadic misocyclone circulation along a dry outflow boundary. J. Atmos. Sci., 54, 32-60.