Near-Surface Vortex Formation in Supercells from the Perspective of Vortex Patch Dynamics
Johannes M. L. Dahl
https://doi.org/10.1175/MWR-D-20-0080.1
Abstract
In many supercell simulations, near-ground vortex formation results from the collapse of an elongated region of enhanced vertical vorticity. In this study, this “roll-up” mechanism is analyzed by investigating the behavior of several 2D elliptic vortex patches. The problem is treated as a nonlinear initial value problem, which is better suited to describe vortex-patch axisymmetrization than the more commonly employed normal-mode analysis. Using the Bryan Cloud Model 1, it is demonstrated that the condition for vortex formation is an initial finite-amplitude nonuniformity within the vortex patch. Vortex formation results from differential self-advection due to the flow induced by the patch itself. Background straining motion may either aid or suppress vortex patch axisymmetrization depending on the initial orientation of the patch relative to the deformation axis. It is also found that in some cases numerical dispersion may lead to nonuniformities that serve as seed for axisymmetrization, thus resulting in unphysical vortex development.
Dynamical Insights into Extreme Short-Term Precipitation Associated with Supercells and Mesovortices
Erik R. Nielsen and Russ S. Schumacher
https://doi.org/10.1175/JAS-D-17-0385.1
Abstract
In some prominent extreme precipitation and flash flood events, radar and rain gauge observations have suggested that the heaviest short-term rainfall accumulations (up to 177 mm h−1) were associated with supercells or mesovortices embedded within larger convective systems. In this research, we aim to identify the influence that rotation has on the storm-scale processes associated with heavy precipitation. Numerical model simulations conducted herein were inspired by a rainfall event that occurred in central Texas in October 2015 where the most extreme rainfall accumulations were collocated with meso-β-scale vortices. Five total simulations were performed to test the sensitivity of precipitation processes to rotation. A control simulation, based on a wind profile from the aforementioned event, was compared with two experiments with successively weaker low-level shear. With greater environmental low-level shear, more precipitation fell, in both a point-maximum and an area-averaged sense. Intense, rotationally induced low-level vertical accelerations associated with the dynamic nonlinear perturbation vertical pressure gradient force were found to enhance the low- to midlevel updraft strength and total vertical mass flux and allowed access to otherwise inhibited sources of moisture and CAPE in the higher-shear simulations. The dynamical accelerations, which increased with the intensity of the low-level shear, dominated over buoyant accelerations in the low levels and were responsible for inducing more intense low-level updrafts that were sustained despite a stable boundary layer.
Dynamical Mechanisms Supporting Extreme Rainfall Accumulations in the Houston “Tax Day” 2016 Flood
Erik R. Nielsen and Russ S. Schumacher
https://doi.org/10.1175/MWR-D-19-0206.1
Abstract
This research examines the environmental and storm-scale characteristics of the extreme rainfall and flooding in the Houston, Texas, area on 18 April 2016, known as the “Tax Day” flood. Radar and local mesonet rain gauge observations were used to identify the locations and structures of extreme rain-rate-producing cells, with special attention given to rotating updrafts. To supplement this observation-based analysis, a WRF-ARW simulation of the Tax Day storm in 2016 was examined for the influence of any attendant rotation on both the dynamics and microphysics of the cells producing the most intense short-term (i.e., subhourly to hourly) rainfall accumulations. Results show that the most intense rainfall accumulations in the model analysis, as in the observational analysis, are associated with rotating convective elements. A lowering of the updraft base, enhancement of the low-level vertical velocities, and increased low-level rainwater production is seen in rotating updrafts, compared to those without rotation. These differences are also maintained despite increased hydrometeor loading. The results agree with the findings of previous idealized model simulations that show dynamical accelerations associated with meso-γ-scale rotation can enhance convective rainfall rates.
The Development of Severe Vortices within Simulated High-Shear, Low-CAPE Convection
Keith D. Sherburn and Matthew D. Parker
https://doi.org/10.1175/MWR-D-18-0246.1
Abstract
Environments characterized by large values of vertical wind shear and modest convective available potential energy (CAPE) are colloquially referred to as high-shear, low-CAPE (HSLC) environments. Convection within these environments represents a considerable operational forecasting challenge. Generally, it has been determined that large low-level wind shear and steep low-level lapse rates—along with synoptic-scale forcing for ascent—are common ingredients supporting severe HSLC convection. This work studies the specific processes that lead to the development of strong surface vortices in HSLC convection, particularly associated with supercells embedded within a quasi-linear convective system (QLCS), and how these processes are affected by varying low-level shear vector magnitudes and lapse rates. Analysis of a control simulation, conducted with a base state similar to a typical HSLC severe environment, reveals that the key factors in the development of a strong surface vortex in HSLC embedded supercells are (i) a strong low- to midlevel mesocyclone, and (ii) a subsequent strong low-level updraft that results from the intense, upward-pointing dynamic perturbation pressure gradient acceleration. Through a matrix of high-resolution, idealized simulations, it is determined that sufficient low-level shear vector magnitudes are necessary for the development of low- to midlevel vertical vorticity [factor (i)], while steeper low-level lapse rates provide stronger initial low-level updrafts [factor (ii)]. This work shows why increased low-level lapse rates and low-level shear vector magnitudes are important to HSLC convection on the storm scale, while also revealing similarities between surface vortexgenesis in HSLC embedded supercells and higher-CAPE supercells.
Analysis of mesovortex characteristics, behavior, and interactions during the second 30 June‒1 July 2014 midwestern derecho event
Anthony Lyza, Adam Clayton, Kevin Knupp, Eric Lenning, Matthew Friedlein, Richard Castro, Evan Bentley
https://ejssm.org/ojs/index.php/ejssm/article/view/162/111
Abstract
A pair of intense, derecho-producing quasi-linear convective systems (QLCSs) impacted northern Illinois and northern Indiana during the evening hours of 30 June through the predawn hours of 1 July 2014. The second QLCS trailed the first one by only 250 km and approximately 3 h, yet produced 29 confirmed tornadoes and numerous areas of nontornadic wind damage estimated to be caused by 30‒40 m s‒1 flow. Much of the damage from the second QLCS was associated with a series of 38 mesovortices, with up to 15 mesovortices ongoing simultaneously. Many complex behaviors were documented in the mesovortices, including: a binary (Fujiwhara) interaction, the splitting of a large mesovortex in two followed by prolific tornado production, cyclic mesovortexgenesis in the remains of a large mesovortex, and a satellite interaction of three small mesovortices around a larger parent mesovortex. A detailed radar analysis indicates no definitive differences between tornadic and nontornadic mesovortices. All observed mesovortices were cyclonic, indicating that either the vertical tilting of streamwise vorticity, generation of vortices via the release of horizontal shearing instability, or both were involved in mesovortex genesis. This paper examines the environment ahead of the second QLCS, the characteristics of the mesovortices produced, and the aforementioned complex interactions. It also discusses implications for mesovortex genesis and dynamics as well as operational considerations.
The Dynamical Coupling of Convective Updrafts, Downdrafts, and Cold Pools in Simulated Supercell Thunderstorms
G. R. Marion, R. J. Trapp
https://doi.org/10.1175/MWR-D-18-0246.1
Abstract
The initiation of second‐generation convective storms by the cold pools of first‐generation storms is known to depend on cold pool characteristics such as depth and speed. It is not clear, however, how these characteristics relate back to the convective storm components and, in turn, to the environment. Here we investigate the hypothesis that wider updraft cores result in wider downdraft cores, which subsequently lead to the development of deeper cold pools that are more likely to initiate new convection. This hypothesis is addressed using a large set of idealized numerical simulations of highly organized convective storms, specifically, supercells. Quantifications of the convective components show strong interrelationships between updraft area, downdraft area, and cold pool depth, thus supporting our primary hypothesis. These interrelationships are highly sensitive to the parcel buoyancy and vertical wind shear, with large convective available potential energy, strong wind shear, and a deep mixed layer ultimately contributing to the deepest (and strongest) cold pools. Diagnoses of the forcings of vertical accelerations show that draft width, and therefore cold pool depth, are initially influenced by the buoyancy and linear dynamic forcings, but thereafter are controlled mostly by the nonlinear dynamic forcings. These results, overall, have implications on the development (or improvement) of cold pool parameterizations in weather and climate models.
Doppler Radar Observations of Horizontal Shearing Instability in Quasi-Linear Convective Systems
Dustin Conrad, Kevin Knupp
https://doi.org/10.1175/MWR-D-18-0257.1
Abstract
Dual-Doppler radar observations of two cold-season, wave-propagating quasi-linear convective systems (QLCS), which evolved in high-shear, low-CAPE (HSLC) environments, are analyzed to determine the role that horizontal shearing instability (HSI) plays in the formation of mesovortices. One QLCS occurred on 4 January 2015 and produced two mesovortices within the dual-Doppler region, one of which was associated with an EF-1 tornado with a pathlength of 10 km. The second QLCS occurred on 28 November 2016 and did not produce any mesovortices. Storm characteristics such as the low-level wind speed and wind shift angle are investigated. Rayleigh and Fjørtoft instability criteria, which are required but insufficient for HSI, are also examined. The Rayleigh and Fjørtoft instability criteria are satisfied for the 4 January 2015 QLCS and the 28 November 2016 QLCS, highlighting one of the issues of the “required, but insufficient” characteristic of the criteria. Analysis of the wind shift angle and wind speed agree with previous studies that pronounced wind shifts close to 90° and strong wind speeds were conducive to the formation of mesovortices, while weak wind shift angles and weaker wind speeds were not. It was found that for the 4 January 2015 case, HSI was the likely formation mechanism of the vortices as other features associated with preexisting mesovortexgenesis theories were not observed.
Analysis of mesovortex characteristics, behavior, and interactions during the second 30 June‒1 July 2014 midwestern derecho event
Anthony Lyza, Richard Castro, Eric Lenning, Matthew Friedlein, Brett Borchardt, Adam Clayton, Kevin Knupp
https://ejssm.org/ojs/index.php/ejssm/article/view/170/120
Abstract
Two derecho-producing quasi-linear convective systems moved across northeast Illinois and northwest Indiana on the evening of 30 June 2014. Both produced damage across a large area, including a concentrated cluster in the Kankakee River Valley associated with two long-lived and adjacent
mesovortices in the second derecho. Post-event surveys of this region revealed a widespread, complex damage pattern and initially documented 14 tornadoes of EF1 rating on the Enhanced Fujita (EF) Scale, along with a swath of damaging nontornadic winds. Later, a number of uncertainties from the original surveys were examined in conjunction with high-resolution satellite imagery not available immediately after the event. Inconsistencies noted during this process led to an intensive reanalysis of the entire cluster using radar data, the original ground and aerial surveys, and the newly available satellite imagery. This new analysis concluded that at least 18 tornadoes impacted the Kankakee Valley, most with a path orientation different from the initial results. This paper details the reassessment process from initial motivation to final results, addresses potential sources of error, and discusses operational considerations stemming from such a prolifically tornadic event.