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<i>Analyzing the Climatology of Tornadoes </i><i>Relative to Extratropical Cyclones</i>Lauren Ann Kiefer (19192885) 22 July 2024 (has links)
<p dir="ltr">Tornadoes have caused billions of dollars in damage and are one of the leading causes of
weather-related deaths in the United States each year. Recent studies have suggested spatial
shifts in tornado activity, though the reason is unclear. Extratropical cyclones (ETCs), which
are strongly associated with the jet stream, are known to produce an environment favorable
for tornadoes in their warm sector. However, little recent research has been done on the
spatiotemporal relationship between tornadoes and ETCs, so there is a poor understanding
of whether or not the changes in tornado activity are affected by ETC patterns. ERA5
reanalysis, ETC tracking, and historical tornado data from 1980-2022 are used to analyze
the climatology of tornadoes relative to ETCs. We found that 73% of F/EF1+ tornadoes
occurred within 2000km of an ETC and are likely associated with the ETC. Most of those
tornadoes occurred near the median position around 465km away from and to the southeast of
the ETC center. Of those tornadoes, 68% occurred in large outbreaks of 6 or more tornadoes,
where most tornadoes formed closer to and to the southeast of an ETC track as compared
to small outbreak and isolated tornadoes. The spatial and relative distributions were similar
across all intensity levels, though stronger tornadoes tended to have more tornadoes directly
to the southeast of an ETC. Seasonal variances in tornadoes strongly corresponded with
seasonal changes in the jet stream. Summer tornadoes occurred in northern portions of
the US when the jet stream shifts poleward. The jet stream and ETCs are also weakest
in the summer, and the weakest association was found in summer tornadoes based on their
distribution relative to ETCs being more uniform towards the northeast and north-southeast
directions. Winter tornadoes occurred in more southern portions of the US when the jet
stream shifts equatorward, and they had a stronger association with most of the tornadoes
occurring to the southeast and closer to the ETC center, aligning with a strong ETC and jet
stream in the winter. Finally, tornadoes and ETCs had strong spatial covariance and showed
similar linear trends, including a similar rate of change in the eastward shift, providing
strong evidence that a shift in ETCs may be driving the shift in tornadoes. Furthering our
understanding of the relationship between tornadoes and ETCs will help to better predict
how tornadoes will change in the future based on changes in ETCs.</p>
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ZDR Arc Area and Intensity as a Precursor to Low Level Rotation in SupercellsAllison Lafleur (15353692) 26 April 2023 (has links)
<p> It has been hypothesized that some measurable properties of $Z_{DR}$ arcs in supercells may change in the minutes prior to tornadogenesis and tornadogenesis failure, and that $Z_{DR}$ arc area will change with SRH and can be used as a real-time proxy to estimate SRH. Output form the Cloud Model 1 (CM1) along with a polarimetric emulator is used to simulate $Z_{DR}$ arcs in 9 tornadic and 9 non-tornadic supercells. A random forest algorithm is used to automatically identify the $Z_{DR}$ arcs. Finally the inflow sector SRH is calculated at times when $Z_{DR}$ arcs are identified. To analyze the change in intensity and area a comparison between the average $Z_{DR}$ value inside and outside of the arc, as well as the spatial size of the arc and storm was done. Model calculated SRH is then compared to these metrics.</p>
<p> </p>
<p> It has also been observed that hail fallout complicates the automatic identification of $Z_{DR}$ arcs. In this study, three experiments are run where the simulated $Z_{DR}$ arcs are produced. One using all categories of hydrometeors, one where wet growth and melting of hail is excluded, and one excluding the contribution to $Z_{DR}$ from the hail hydrometeor category. The same analysis as above is repeated for all three experiments. Finally observed $Z_{DR}$ arcs are analyzed to see if these results are applicable to the real world. </p>
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<sub><strong>THE EFFECTS OF SURFACE CHARACTERISTICS AND SYNOPTIC PATTERNS ON TORNADIC STORMS IN THE UNITED STATES</strong></sub>Qin Jiang (19183822) 21 July 2024 (has links)
<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>
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