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Jeffrey Beck and Christopher Weiss

Abstract

Idealized supercell modeling has provided a wealth of information regarding the evolution and dynamics within supercell thunderstorms. However, discrepancies in conceptual models exist, including uncertainty regarding the existence, placement, and forcing of low-level boundaries in these storms, as well as their importance in low-level vorticity development. This study offers analysis of the origins of low-level boundaries and vertical vorticity within the low-level mesocyclone of a simulated supercell. Low-level boundary location shares similarities with previous modeling studies; however, the development and evolution of these boundaries differ from previous conceptual models. The rear-flank gust front develops first, whereas the formation of a boundary extending north of the mesocyclone undergoes numerous iterations caused by competing outflow and inflow before a steady-state boundary is produced. A third boundary extending northeast of the mesocyclone is produced through evaporative cooling of inflow air and develops last. Conceptual models for the simulation were created to demonstrate the evolution and structure of the low-level boundaries. Only the rear-flank gust front may be classified as a “gust front,” defined as having a strong wind shift, delineation between inflow and outflow air, and a strong pressure gradient across the boundary. Trajectory analyses show that parcels traversing the boundary north of the mesocyclone and the rear-flank gust front play a strong role in the development of vertical vorticity existing within the low-level mesocyclone. In addition, baroclinity near the rear-flank downdraft proves to be key in producing horizontal vorticity that is eventually tilted, providing a majority of the positive vertical vorticity within the low-level mesocyclone.

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Christopher C. Weiss and Peter J. Sousounis

Abstract

This study examines the frequency and intensity with which collective lake disturbances (COLDs) develop. These disturbances develop when cold air overspreads the Great Lakes region in winter. The heat and moisture that is transferred from the Great Lakes aggregate into the lower atmosphere, and that spreads across a large region, allows eventually for the development of a meso-α-scale pressure perturbation and circulation.

Cases from the period 1980–90 were identified based on the existence of a surface trough or closed low over the Great Lakes region in the presence of cold air. Output from the Limited-Area Fine Mesh (LFM) model was used rather than performing numerous with-lake and no-lake numerical simulations to determine whether the feature was indeed the result of aggregate heating by the lakes. The LFM did not include the lakes in its simulations, so the 24-h forecast served as an optimal no-lakes simulation. Subtracting the initialization sea level pressure (SLP) field valid at the same time allowed for an assessment of the COLD events in terms of the SLP perturbation.

An average of 33 events per year with an average SLP perturbation of 3–4 hPa was found for the 10-yr period. The synoptic-scale conditions for weak events with SLP perturbations less than 3 hPa differed significantly from those for strong events with SLP perturbations greater than 9 hPa. The weak scenario was characterized by a weak trough over the Great Lakes with high static stability and weak cold advection below 500 hPa and weak vorticity advection at 500 hPa. The strong scenario was characterized by a nearly closed low over the Great Lakes with low static stability and strong cold advection below 500 hPa and strong positive vorticity advection at 500 hPa.

The current study is the first attempt to measure the frequency and intensity with which the Great Lakes collectively generate meso-α-scale disturbances in winter. The LFM-based technique provides a result that cannot likely be obtained without a herculean effort from a numerical modeling standpoint. Future numerical studies using the identified scenarios, however, will be extremely useful to better understand the sensitivities of COLD events to the large-scale conditions.

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Christopher C. Weiss and Howard B. Bluestein

Abstract

On 3 June 1995, as part of the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), the Electra Doppler Radar (ELDORA) onboard the National Center for Atmospheric Research (NCAR) Electra aircraft made possible a high-resolution examination of clear-air motions in the Texas panhandle in and around the intersection of the dryline and a surface baroclinic boundary, a location commonly referred to as the “triple point.” The ELDORA observations, as well as conclusions drawn from analyses of these data, are presented and discussed.

A transverse secondary circulation associated with the dryline is visualized through analyses of the ELDORA data. Typical values of rising and sinking air are found to be 2 m s−1 and 2–3 m s−1, respectively. These vertical velocities are approximately the same as those indicated by in situ data collected onboard the Electra. Because the maximum in rising motion is found at the western edge of the dewpoint gradient and because the low-level relative airflow was from the west, it is suggested that the source region for ascending updraft parcels was primarily from the dry side. The existence of a tilted circulation is also confirmed by dryline-normal cross sections of horizontal divergence, which exhibited a shift of the dryline convergence maximum to the east with height.

Based on vertical cross sections of analyses of ELDORA data just to the north of the triple point, it is shown that there is a residual dryline secondary circulation (RDSC) elevated above the cold pool. Composite hodographs representative of either side of the RDSC identify a distinct difference in the wind profile. The possible roles of the RDSC and outflow boundary to convective initiation are also discussed.

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Abby Hutson, Christopher Weiss, and George Bryan

Abstract

This study investigates whether the thermodynamics of supercell rear-flank outflow can be inferred from the propagation speed and vertical structure of the rear-flank gust front. To quantify the relationship between outflow thermodynamic deficit and gust front structure, CM1 is applied as a two-dimensional cold pool model to assess the vertical slope of cold pools of varying strength in different configurations of ambient shear. The model was run with both free-slip and semislip lower boundary conditions and the results were compared to observations of severe thunderstorm outflow captured by the Texas Tech University Ka-band mobile radars. Simulated cold pools in the free-slip model achieve the propagation speeds predicted by cold pool theory, while cold pool speeds in the semislip model propagate slower. Density current theory is applied to the observed cold pools and predicts the cold pool speed to within about 2 m s−1. Both the free-slip and semislip model results reveal that, in the same sheared flow, the edge of a strong cold pool is less inclined than that of a weaker cold pool. Also, a cold pool in weak ambient shear has a steeper slope than the same cold pool in stronger ambient shear. Nonlinear regressions performed on data from both models capture the proper dependence of slope on buoyancy and shear, but the free-slip model does not predict observed slopes within acceptable error, and the semislip model overpredicts the cold pool slope for all observed cases, but with uncertainty due to shear estimation.

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Jessica M. McDonald and Christopher C. Weiss

Abstract

Many numerical studies have focused on the importance of baroclinically generated vorticity at the edge of cold pools in supercellular tornadogenesis, and observational work has consistently found that strongly tornadic supercells have less dense, more buoyant cold pools than weakly or nontornadic supercells. However, there is a lack of observational studies that consider potential relationships between cold pool characteristics (e.g., density) and tornado production within linear systems, such as MCS or QLCS events. This study presents two tornadic QLCS events that were observed during the Verification of the Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) field project in 2016 and 2017. Supercell and hybrid modes were also observed and compared to the observations from the linear systems. No obvious differences in the thermodynamic deficits of the tornadic and nontornadic samples were found, likely due to the weakness of the produced tornadoes (≤EF1) and the small tornadic sample size (five cold pools). Comparison across storm mode did find some differences, with QLCS cold pools producing larger θv and θe deficits than those observed in supercells. More importantly, our findings suggest that, in a QLCS, the magnitude of density gradients along the leading edge of the cold pool may be related to tornadogenesis by virtue of the implied baroclinic vorticity generation.

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Alex Schueth, Christopher Weiss, and Johannes M.L. Dahl

Abstract

The forward-flank convergence boundary (FFCB) in supercells has been well documented in many observational and modeling studies. It is theorized that the FFCB is a focal point fore baroclinic generation of vorticity. This vorticity is generally horizontal and streamwise in nature, which can then be tilted and converted to mid-level (3-6 km AGL) vertical vorticity. Previous modeling studies of supercells often show horizontal streamwise vorticity present behind the FFCB, with higher resolution simulations resolving larger magnitudes of horizontal vorticity. Recently, studies have shown a particularly strong realization of this vorticity called the streamwise vorticity current (SVC). In this study, a tornadic supercell is simulated with the Bryan Cloud Model at 125-m horizontal grid spacing, and a coherent SVC is shown to be present. Simulated range-height indicator (RHI) data show the strongest horizontal vorticity is located on the periphery of a steady-state Kelvin-Helmholtz billow in the FFCB head. Additionally, similar structure is found in two separate observed cases with the Texas Tech University Ka-band (TTUKa) mobile radar RHIs. Analyzing vorticity budgets for parcels in the vicinity of the FFCB head in the simulation, stretching of vorticity is the primary contributor to the strong streamwise vorticity, while baroclinic generation of vorticity plays a smaller role.

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Christopher C. Weiss, Howard B. Bluestein, Robert Conzemius, and Evgeni Fedorovich

Abstract

A variational procedure is developed that utilizes mobile ground-based range–height indicator (RHI) Doppler radar velocity data for the synthesis of two-dimensional, RHI plane wind vectors. The radial component winds are obtained with the radar platform in motion, a data collection strategy referred to as the rolling RHI technique. Using the assumption of stationarity—standard to any pseudo-multiple-Doppler processing technique—individual radial velocity values at a given point in space will contribute a varying amount of independent information to the two components of wind velocity in the RHI plane, depending strongly on the difference in radar viewing angles amongst the looks.

The variational technique is tested successfully with observation system simulation experiments, using both a homogeneous flow field and large eddy simulation (LES) output from a highly sheared convective boundary layer simulation. Pseudoradar data are collected in these tests in a manner consistent with the specifications of the University of Massachusetts mobile W-band radar, which was used in a separate study to resolve the finescale structure of a dryline during the International H2O Project (IHOP_2002). The results of these tests indicate clearly that the technique performs well in regions of adequate “look” angle separation. Observation error contributes significantly to the analysis when the radar looks become more collinear.

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Brian D. Hirth, John L. Schroeder, and Christopher C. Weiss

Abstract

The rear-flank downdraft regions of two tornadic supercells were sampled on 12 June 2004 and 9 June 2005 using four “mobile mesonet” probes. These rear-flank downdraft outflows were sampled employing two different data collection routines; therefore, each case is described from a different perspective. The data samples were examined to identify variations in measured surface equivalent potential temperature, virtual potential temperature, and kinematics. In the 12 June 2004 case, the tornadic circulation was accompanied by small equivalent potential temperature deficits within the rear-flank downdraft outflow early in its life followed by increasing deficits with time. Virtual potential temperature deficits modestly increased through the duration of the sample as well. The 9 June 2005 case was highlighted by heavy precipitation near the tornado itself and relatively small negative, or even positive, equivalent and virtual potential temperature perturbations. Large horizontal variations of surface thermodynamic properties were also noted within several regions of this rear-flank downdraft outflow.

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Christopher C. Weiss, Howard B. Bluestein, and Andrew L. Pazmany

Abstract

The dryline has long been associated with the development of severe thunderstorms in the southern plains during the spring and early summer months. The propagation and structure of the dryline are closely tied to surface processes that are neither well understood nor well resolved with current observational capabilities. As a result, there are often large errors in forecasts of dryline position and structure.

Improvements in radar technology have allowed for better observations of the dryline in recent years. Here, very finescale radar observations taken with the University of Massachusetts—Amherst (UMass) mobile W-band radar during an International H2O Project (IHOP) double-dryline event on 22 May 2002 in the Oklahoma panhandle are presented. The observations are placed in the context of the dryline secondary circulation, which describes flow in a plane normal to the dryline. The narrow, half-power beamwidth of the antenna on the W band (0.18°) permitted the measurements of channels of upward (8–9 m s−1 over a horizontal distance of 50–100 m) and downward vertical velocity, greater in absolute magnitude than that previously reported in dryline field studies.

A ground-based variational pseudo-multiple-Doppler processing technique is introduced, which is used to decompose time series of RHI velocity data into horizontal and vertical wind components. The technique is applied to a retrograding dryline from 22 May 2002. Finescale structure of the retreating dryline interface is presented.

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Howard B. Bluestein, Christopher C. Weiss, and Andrew L. Pazmany

Abstract

Analyses of a dust-devil dataset collected in northwest Texas are presented. The data were collected just above the ground at close range with a mobile, W-band (3-mm wavelength) Doppler radar having an azimuthal (radial) resolution of 3–5 m (30 m) at the range of the dust devils. Most dust devils appeared as quasi-circular rings of relatively high radar reflectivity. Four dust-devil vortices were probed, three of which were cyclonic and one anticyclonic. Documentation was obtained of a pair of adjacent cyclonic vortices rotating cyclonically around each other.

Approximate radial profiles of azimuthal and radial wind components and of radar reflectivity are detailed and discussed. The diameters of the core of the dust devils ranged from 30 to 130 m; the latter diameters are much wider than that of typical dust devils in a homogeneous environment. The widest vortex was cyclonic and exhibited evidence of a two-cell structure (i.e., sinking motion near the center and rising motion just outside the radius of maximum wind), a broad, calm eye, and an annulus of maximum vorticity just inside the radius of maximum wind. As the vortex widened, it developed an asymmetry, and some evidence was found that two waves propagated cyclonically around it. The narrowest dust devil had the structure of a Rankine combined vortex, that is, a central core of constant vorticity surrounded by potential flow. Owing to very strong radial shear of the azimuthal wind, the vorticity in the dust-devil cores ranged from 0.5 to 1 s−1, which is as high as the vorticity in some tornadoes. However, the maximum ground-relative wind speeds in each dust devil were only 6.5–13.5 m s−1. The location of the highest radar reflectivity was located at or within the radius of maximum wind. In the widest dust devil, the vorticity estimated from the Doppler shear associated with its vortex signature was much less than the smaller-scale vorticity ring estimated from the azimuthal wind profile. It is therefore suggested that the vorticity estimated from the Doppler shear in tornadoes may be underestimated significantly when the tornado vortex exhibits a two-cell structure and that Doppler shear alone may not be a good indicator of vortex intensity.

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