Investigating Vortexgenesis from Quasi-Linear Convective Systems in the United Kingdom

Abstract

This thesis consists of two separate journal articles that together form a coherent research project. The first journal article documented nine tornado outbreaks (days with three or more tornadoes) from quasi-linear convective systems in the United Kingdom between 2004--2019. Assessment of their synoptic-scale environments revealed that eight can be classified into two synoptic categories: type 1 and type 2. Type 1 events are typified by a meridional surface trough extending from a low pressure centre located to the north of the United Kingdom, moving from west to east. Type 2 events are typified by a zonally oriented surface trough extending from a low pressure centre located to the northeast of the United Kingdom, moving from north to south. Analysis of Met Office radar reflectivity mosaics revealed type 1 events were all characterised by a narrow cold-frontal rainband and subsequently displayed precipitation core-and-gap structures. In contrast, type 2 events were characterised by disorganised precipitation cores embedded within a wider frontal rainband. The development of precipitation cores was speculated to be a consequence of vortexgenesis, yet the contrasting precipitation morphologies and magnitudes of low-level (0--1-km) prefrontal bulk shear (15--20 m/s and 10 m/s for type 1 and 2 events, respectively) hinted at differences in the vortexgenesis mechanisms. The second journal article was motivated by the results of the first, with the aim to determine the vortexgenesis mechanisms. Real-data, high-resolution simulations were performed to understand the evolution of vorticity in both types. Analysis of Rayleigh's and Fjortoft's instability criteria, the along-front stretching-deformation rate, and the ratio of the misovortex wavelengths to the width of the shear zone revealed that the type 1 event was unstable to horizontal shearing instability (HSI) prior to vortexgenesis, yet the type 2 event was not. Assessing the time-integrated vertical vorticity tendency terms along trajectories revealed misovortices acquired vertical vorticity differently in each type. The type 1 event misovortices intensified by low-level tilting, whereas the type 2 event acquired their vertical vorticity from both tilting and stretching. The implication was that HSI and subsequently tilting was the vortexgenesis mechanism in the type 1 event and the tilting of horizontal vorticity which is subsequently stretched the mechanism in the type 2 event.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Geraint Vaughan (Supervisor) & David Schultz (Supervisor)