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Todd A. Murphy
and
Kevin R. Knupp

Abstract

Cold season tornadic outbreaks occur with regularity in the southeastern United States; however, detailed analyses of parent supercell storms in the cold season environment (often low CAPE, high shear) are scarce. This is often because storms do not always move close enough to radars for a comprehensive single-Doppler analysis and significant topography or trees in the Southeast make it difficult for mobile radars to operate, thus limiting dual-Doppler coverage. However, during the Super Tuesday tornado outbreak of 5–6 February 2008, two tornadic supercell storms passed within 30–40 km of the Weather Surveillance Radar-1988 Doppler (WSR-88D) sites in Memphis and Nashville, Tennessee (KNQA and KOHX, respectively). The relative steadiness of these storms during passage, along with the large motion vector (from the southwest at 20–25 m s−1), allowed the application of a synthetic dual-Doppler (SDD) analysis. As such, a detailed analysis of these storms was completed, including examinations of low-level circulations, updraft strength and location, as well as retrievals and evaluations of perturbation pressure and the vertical pressure gradient. This study presents one of the first comprehensive analyses of cold season supercells using only one Doppler radar. Additionally, the relative success and failures of using the SDD technique on supercell storms are discussed. Major findings for the primary case include the updraft maximizing at a very low height (3.0 km AGL), and weak pressure forcing within the rear flank resulting in a nonexistent rear-flank downdraft (RFD).

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Timothy A. Coleman
and
Kevin R. Knupp

Abstract

Ducted gravity waves and wake lows have been associated with numerous documented cases of “severe” winds (>25 m s−1) and wind damage. These winds are associated with the pressure perturbations and transient mesoscale pressure gradients occurring in many gravity waves and wake lows. However, not all wake lows and gravity waves produce significant winds nor wind damage. In this paper, the factors that affect the surface winds produced by ducted gravity waves and wake lows are reviewed and examined. It is shown theoretically that the factors most conducive to high surface winds include a large-amplitude pressure disturbance, a slow intrinsic speed of propagation, and an ambient wind with the same sign as the pressure perturbation (i.e., a headwind for a pressure trough). Multiple case studies are presented, contrasting gravity waves and wake lows with varying amplitudes, intrinsic speeds, and background winds. In some cases high winds occurred, while in others they did not. In each case, the factor(s) responsible for significant winds, or the lack thereof, are discussed. It is hoped that operational forecasters will be able to, in some cases, compute these factors in real time, to ascertain in more detail the threat of damaging wind from an approaching ducted gravity wave or wake low.

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Dustin M. Conrad
and
Kevin R. Knupp

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.

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James R. Stalker
and
Kevin R. Knupp

Abstract

Using high-resolution three-dimensional numerical experiments, this paper shows that the cell separation distance scales as 0.75 times the planetary boundary layer (PBL) depth for successful cell mergers between constructively interacting cells within multicell thunderstorms. This boundary layer scaling is determined from several simulations of convective cell pairs with a fixed PBL depth and is shown to be valid for other sensitivity simulations with larger PBL depths. This research establishes a robust and quantitative relation between prestorm ambient conditions and cell merger potential useful for research efforts on the multifaceted cell merger process of multicell thunderstorms. The weakly sheared ambient prestorm conditions of the 9 August 1991 Convection and Precipitation/Electrification Experiment (CaPE) multicell thunderstorm are used to initialize the cell pair simulations.

Since ambient wind and wind shear are assumed to be zero, only simple cell mergers, defined in this study as those between cell updraft cores joined but not overlapping in the convective stage, are shown to be possible. The coarse-resolution simulations of Stalker suggest that ambient wind shear may be necessary for forced cell mergers, defined in this study as those in which the initial updraft cores are found apart. The scenarios of overlapping initial updraft cores for cell merger are considered physically invalid in this study.

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Timothy A. Coleman
and
Kevin R. Knupp

Abstract

This study uses data from a microwave profiling radiometer (MPR), along with 915-MHz wind profiler, Doppler radar, and surface data to quantify the kinematic and thermodynamic effects of two wave features, an undular bore and a soliton, on the nocturnal boundary layer (NBL) at high temporal resolution. Both wave features passed directly over the MPR and the wind profiler, allowing for detailed analyses. The effects of the wave features on the convective environment are examined, and convective initiation (CI) associated with the wave features is discussed.

The undular bore was illustrated well in Doppler velocity data, and profiler measurements indicated that it produced four wavelengths of upward and downward motion. MPR-derived time–height sections of potential temperature and mixing ratio showed an increase in the depth of the stable boundary layer, along with a decrease in stability, partially associated with mixing of the NBL. The soliton produced a temporary decrease in the depth of the NBL, and also produced destabilization. Trajectory analyses were performed assuming the wave features were two-dimensional, allowing a time-to-space conversion of profiler data. Trajectory analyses, in addition to propagation speed, confirm that the wave features were indeed a bore and a soliton, and that there was vertical divergence in the NBL, likely associated with the decrease in static stability.

MPR data were also used to produce time series of convective parameters, including CAPE, convective inhibition (CIN), and the level of free convection (LFC). The CIN was initially too large for free convection despite sufficient CAPE, but MPR data showed that the CIN decreased by more than 50% upon passage of the bore, and again with the soliton. The waves also decreased the LFC due to cooling above the NBL and slight warming near the surface in the bore. Both the reduction in CIN and the lowering of the LFC made convection more likely. Convective initiation occurred behind both wave features, and the vertical motion provided by the waves may have also aided in this CI.

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John M. Brown
and
Kevin R. Knupp

Abstract

A severe thunderstorm which spawned at least four tornadoes, one of them anticyclonic, formed over central Iowa during the afternoon of 13 June 1976. This storm moved toward the east-northeast, approximately parallel to but slower than the mean tropospheric flow. The anticyclonic tornado (F3) and the most intense (F5) of the cyclonic tornadoes coexisted for 23 min and traveled on nearly parallel, cycloidal-like tracks, with the anticyclonic tornado 3–5 km southeast of the cyclonic. The major emphasis of this paper is on this pair of tornadoes and their relationship to the structure and evolution of the parent thunderstorm.

Radar recorded the development of a hook echo just prior to the genesis of the intense cyclonic tornado. A strengthening mesolow was centered somewhere south of this tornado soon after it formed. The mesolow is believed to have initiated a new updraft; the anticyclonic tornado formed in association with this updraft, south of the cyclonic tornado. It is hypothesized that the mesolow was responsible (through alteration of the storm-scale airflow) for the nearly simultaneous sharp right turns made by these tornadoes. Each of these tornadoes was observed to diminish in intensity soon after becoming associated with heavy rain.

It is argued that the parent thunderstom's distinctive airflow and thermodynamic structure at low levels provided a more favorable setting for the amplification of anticyclonic vorticity than is typical of most severe thunderstorms.

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Charles A. Knight
and
Kevin R. Knupp

Abstract

The growth trajectories of precipitation particles that attain diameters from 0.5 to 2.0 cm are modeled within the wind field of a small, relatively steady-state, southeastern Montana thunderstorm. The trajectories are calculated backwards, from systematic arrays of particles of specified sizes at a level near cloud base. Using a simple set of criteria for rejecting the obviously impossible trajectories, the patterns of accepted trajectory end-points are compared with the radar echo patterns. Good agreement lends credence to the qualitative aspects of the trajectories. For a given size of precipitation particle, the method helps one to assign different trajectory types to specific regions within the horizontal plant on which the calculations were started. The relative importance of the different types of trajectories can thus be estimated. Particle origin mechanisms are discussed in terms of the regions in which the trajectories are found to start. The variety of successful trajectories leading to 1 cm and larger hail in a storm of considerable structural simplicity is noteworthy.

Sensitivity tests indicate that the liquid water content is by far the most important specification in this framework. Ongoing work is directed toward improving this specification and deriving estimates of particle concentration.

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Frank B. Tatom
,
Kevin R. Knupp
, and
Stanley J. Vitton

Abstract

At the present time the only generally accepted method for detecting when a tornado is on the ground is human observation. Based on theoretical considerations combined with eyewitness testimony, there is strong reason to believe that a tornado in contact with the ground transfers a significant amount of energy into the ground. The amount of energy transferred depends upon the intensity of the tornado and the characteristics of the surface. Some portion of this energy takes the form of seismic waves, both body and surface waves. Surface waves (Rayleigh and possibly Love) represent the most likely type of seismic signal to be detected. Based on the existence of such a signal, a seismic tornado detector appears conceptually possible. The major concerns for designing such a detector are range of detection and discrimination between the tornadic signal and other types of surface waves generated by ground transportation equipment, high winds, or other nontornadic sources.

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Dong-Kyun Kim
,
Kevin R. Knupp
, and
Christopher R. Williams

Abstract

Kinematic and microphysical characteristics of a stratiform rainband within Tropical Storm Gabrielle during landfall on 14 September 2001 were investigated using data from a collocated 915-MHz wind profiler and scanning Doppler radar. The curved 60-km-wide rainband was relatively intense with mesoscale updrafts and downdrafts exceeding ±1 m s−1. The bright band is classified as strong, as indicated by reflectivity factors in excess of 50 dBZ and rainfall rates below the bright band peaking at 10–20 mm h−1. The melting layer microphysical processes were examined to understand the relation between brightband processes and precipitation intensity and kinematics (mesoscale downdraft in particular) below the melting layer. The profiler and Doppler radar analyses, designed to maximize vertical resolution of flows within the melting layer, disclose a striking convergence–divergence couplet through the melting layer that implies a prominent cooling-induced finescale circulation. Melting-driven cooling initiates midlevel convergence in the upper part of the melting region, while weak convergence to positive divergence is analyzed within the lower melting layer. A melting-layer parameter study indicates the significance of the level of maximum reflectivity that separates convergence above from divergence below and also reveals a mixture of aggregation and breakup of ice particles, with aggregation being dominant. In this vigorous rainband case, the presence of strong mesoscale downdrafts cannot be ignored for accurate retrievals of raindrop size distribution and precipitation parameters from the Sans Air Motion model. When downdrafts are included, retrieved rainfall estimates were much higher than those under the zero vertical air motion assumption and were slightly less than those from a power-law ZR relation. The rainfall estimates show a positive correlation with reflectivity factor and brightband intensity (i.e., aggregation degree) but less dependence on brightband height.

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Timothy A. Coleman
,
Kevin R. Knupp
, and
John T. Tarvin

Abstract

The electromagnetic pulses (EMPs) associated with two lightning flashes on 22 July 2008 in central Alabama produced audible clicking sounds. These clicks were observed almost simultaneously with the lightning flashes, but a significant period of time before the thunder. The lightning flashes and associated sounds were recorded in digital video and audio by a video camera. Based on theories primarily developed to explain reports of sounds associated with aurora and meteors entering the earth’s atmosphere, it appears that the sounds were associated with transduction of the electromagnetic energy at audible frequencies into vibrations in objects near the camera. Coronal discharges are also possible. Examination of spectrograms of the clicks and the subsequent thunder, and comparison to measurements of the normalized light intensity in each frame of video, show that the clicks must have been associated with sounds in nearby objects. Therefore, the sounds were associated with the lightning EMP.

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