This thesis was funded by a scholarship from the Faculty of Engineering and Physical Sciences, University of Manchester and is presented in an alternative thesis format. The thesis consists of three separate journal articles which form a coherent research project. Paper 1 is a climatology of tornadoes in the British Isles from 1980-2012. The climatology included interannual variability, seasonality, diurnal cycle, intensity, location of occurrence, sounding-derived environmental parameters, and parent storm types of tornadoes. One finding from Paper 1 was that the most common parent storm type in the British Isles was linear storms, for example, storms forming along cold fronts. This finding motivated Papers 2 and 3, which studied vortexgenesis in a tornadic narrow cold-frontal rainband (NCFR), a storm type common to the British Isles, which occurred 29 November 2011. This NCFR caused seven tornadoes across Wales and England. Paper 2 compares the differ- ences in WRF simulation runs of the NCFR based on initialization time, planetary boundary layer scheme, microphysics scheme, and land surface scheme. Out of 96 simulations, the most realistic (most similar to observed radar reflectivity) run was chosen for a case study in Paper 3. Paper 3 analyzes vortices along the NCFR to determine mechanisms dominating their formation and maturation.

No description available.

This thesis explores the relationship between surface elevation roughness and tornado frequency throughout the eastern United States. It builds upon previous studies which demonstrated a negative relationship between roughness and tornado frequency for the Great Plains and Arkansas. A generalized linear model with tornado frequency as the response variable and roughness and population density as the predictors is generated. This model demonstrates that increased roughness is associated with decreased tornado frequency at the scale of the entire eastern United States, especially where roughness is greater than 20 meters. The methods are also performed for 13 smaller regions within the eastern US, but an effect of roughness is only confirmed for the regions encompassing the Great Lakes and central Appalachians. From these results, it is concluded that mountain ranges, where roughness exceeds 20 meters, clearly inhibit tornado activity, but there is little evidence that smaller terrain variations have the same effect.

No description available.

Tropical cyclone (TC)-spawned tornadoes in Florida were analyzed to determine patterns of occurrence based on storm and geographic features. Tornadoes were determined to be associated with a landfalling or nearby TC if a tornado occurred within 800 km of the TC’s center of circulation. TC-tornadoes were analyzed for patterns based on distance and angle from TC’s center, topographic influences, population biases, and influence based on time of landfall. Most TC-Tornadoes tend to occur more often before landfall than after. It was discovered that tornadoes have occurred in different areas with respect to the bearing from the center depending on the landfall location and time of landfall. It was also discovered that land use type, and elevation had little to do with TC-Tornado occurrence. The results do suggest some population bias. The findings will be a guide for operational meteorologists to aid in forecasting likely tornadogenesis from TCs.

<p dir="ltr">It is known that tornadic storms favor environments characteristic of high values of thermal instability, adequate vertical wind shear, abundant near-surface moisture supply, and strong storm-relative helicity at the lowest 1-km boundary layer. These mesoscale environmental conditions and associated storm behaviors are strongly governed by large-scale synoptic patterns and sensitive to variabilities in near-surface characteristics, which are less known in the current research community. This study aims to advance the relatively underexplored area regarding the interaction between surface characteristics, mesoscale environmental conditions, and large-scale synoptic patterns driving tornadic storms in the U.S. </p><p dir="ltr">We first investigate the impact of surface drag on the structure and evolution of these boundaries, their associated distribution of near-surface vorticity, and tornadogenesis and maintenance. Comparisons between idealized simulations without and with drag introduced in the mature stage of the storm prior to tornadogenesis reveal that the inclusion of surface drag substantially alters the low-level structure, particularly with respect to the number, location, and intensity of surface convergence boundaries. Substantial drag-generated horizontal vorticity induces rotor structures near the surface associated with the convergence boundaries in both the forward and rear flanks of the storm. Stretching of horizontal vorticity and subsequent tilting into the vertical along the convergence boundaries lead to elongated positive vertical vorticity sheets on the ascending branch of the rotors and the opposite on the descending branch. The larger near-surface pressure deficit associated with the faster development of the near-surface cyclone when drag is active creates a downward dynamic vertical pressure gradient force that suppresses vertical growth, leading to a weaker and wider tornado detached from the surrounding convergence boundaries. A conceptual model of the low-level structure of the tornadic supercell is presented that focuses on the contribution of surface drag, with the aim of adding more insight and complexity to previous conceptual models.</p><p dir="ltr">We then examine the behaviors and dynamics of TLVs in response to a range of surface drag strengths in idealized simulations and explore their sensitivities to different storm environments. We find that the contribution of surface drag on TLV development is strongly governed by the interaction between surface rotation, surface convergence boundaries, and the low-level mesocyclone. Surface drag facilitates TLV formation by enhancing near-surface vortices and low-level lifting, mitigating the need for an intense updraft gradient developing close to the ground. As surface drag increases, a wider circulation near the surface blocks the inflow from directly reaching the rotating core, leading to a less tilted structure that allows the TLV position beneath the pressure minima aloft. Further increase in drag strength discourages TLV intensification by suppressing vertical stretching due to a negative vertical pressure perturbation gradient force, and it stops benefiting from the support of surrounding convergence boundaries and the overlying low-level updraft, instead becoming detached from them. We hence propose a favorable condition for TLV formation and duration where a TLV forms a less tilted structure directly beneath the low-level mesocyclone but also evolves near surrounding surface boundaries, which scenario strongly depends on underlying surface drag strength. </p><p dir="ltr">Beyond near-surface characteristics, we further explore how these storm-favorable environmental conditions may interact with the larger-scale synoptic patterns and how these interactions may affect the tornadic storm potential in the current warming climate. We employ hierarchical clustering analysis to classify the leading synoptic patterns driving tornadic storms across different geographic regions in the U.S. We find that the primary synoptic patterns are distinguishable across geographic regions and seasonalities. The intense upper-level jet streak described by the high values of eddy kinetic energy (EKE) associated with the dense distribution of Z500 contours dominates the tornado events in the southeast U.S. in the cold season (November-March). Late Spring and early Summer Tornado events in the central and south Great Plains are dominated by deep trough systems to the west axes of the tornado genesis position, while more summer events associated with weak synoptic forcing are positioned closer to the lee side of Rocky Mountain. Moreover, the increasing trend in tornado frequency in the southeastern U.S. is mainly driven by synoptic patterns with intense forcing, and the decreasing trends in portions of the Great Plains are associated with weak synoptic forcing. This finding indicates that the physical mechanisms driving the spatial trends of tornado occurrences differ across regions in the U.S.</p>