Characteristics and importance of horizontal momentum surges in supercells
Mahalik, Matthew C.
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While recent advances have improved our understanding of how tornadoes form, the full picture remains lacking in some details. It is generally understood that tornadogenesis culminates in stretching of existing near-ground vertical vorticity, but a full comprehension of the sources of this vorticity remains elusive. Because fluctuations of momentum within supercellular outflow are known to be associated with near-ground vertical vorticity extrema, it is of value to understand their characteristics. The purpose of this study is twofold: (1) develop a simple, efficient method to identify and measure basic statistics of horizontal momentum surges within the cold pool of a supercell across a specified location, and (2) apply it to a variety of thunderstorm simulations to analyze their relationships with vertical velocity, vertical vorticity, and microphysics parameterizations. The algorithm developed here objectively identifies momentum surges by defining them as storm-relative horizontal wind perturbations. During initial testing, it accurately identified prescribed surges and their frequencies and intensities in isolated, idealized cases, lending confidence in its skill. Applied to full-storm simulations, its utility was demonstrated by identifying interesting storm characteristics. A direct, physical link between downdraft pulsing intensity and surge intensity supports the notion that outflow surges are generated by pulses originating within the main downdraft. An interval of roughly 500-750 s between surge generation was observed in nearly all cases, with little variability between different storm environments an microphysics; updraft pulses of similar frequency have been inferred from lightning measurements, implying the updraft may ultimately drive cold pool unsteadiness. There was a general dependence between surge strength and microphysics, with surges simulated using single moment parameterization schemes having a higher intensity on average. Surges generally were favored to move northwestward from its parent downdraft, although they were observed in every direction; this initial movement of surges appeared to be influenced somewhat by the base-state wind direction. While surges were most common northwest of the parent downdraft, the surges traveling southeastward were most likely to interact with the primary region of surface vorticity and increase near-ground rotation. As described here, the algorithm shows promise for future use in a myriad of studies and provides the framework for a tool to be applied to improve our understanding of the characteristics of surges and their role in tornadogenesis.