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Nolan T. Atkins
and
Michael St. Laurent

Abstract

This two-part study examines the damaging potential and genesis of low-level, meso-γ-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). This bow echo produced both damaging and nondamaging mesovortices. A series of sensitivity simulations were performed to assess the impact of low- and midlevel shear, cold-pool strength, and Coriolis forcing on mesovortex strength. By analyzing the amount of circulation, maximum vertical vorticity, and number of mesovortices produced at the lowest grid level, it was observed that more numerous and stronger mesovortices were formed when the low-level environmental shear nearly balanced the horizontal shear produced by the cold pool. As the magnitude of deeper layer shear increased, the number and strength of mesovortices increased. Larger Coriolis forcing and stronger cold pools also produced stronger mesovortices. Variability of ground-relative wind speeds produced by mesovortices was noted in many of the experiments. It was observed that the strongest ground-relative wind speeds were produced by mesovortices that formed near the descending rear-inflow jet (RIJ). The strongest surface winds were located on the southern periphery of the mesovortex and were created by the superposition of the RIJ and mesovortex flows. Mesovortices formed prior to RIJ genesis or north and south of the RIJ core produced weaker ground-relative wind speeds. The forecast implications of these results are discussed. The genesis of the mesovortices is discussed in Part II.

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Nolan T. Atkins
and
Michael St. Laurent

Abstract

This two-part study examines the damaging potential and genesis of low-level, meso-γ-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). In Part II of this study, mesovortex genesis was investigated for vortices formed at different stages of convective system evolution. During the early “cellular” stage, cyclonic mesovortices were observed. The cyclonic mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. As the convective system evolved into a bow echo, cyclonic–anticyclonic mesovortex pairs were also observed. The vortex couplet was produced by a local updraft maximum that tilted baroclinically generated vortex lines upward into arches. The local updraft maximum was created by a convective-scale downdraft that produced an outward bulge in the gust front position. Cyclonic-only mesovortices were predominantly observed as the convective system evolved into the mature bow echo stage. Similar to the early cellular stage, these mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. The downdraft parcels descended within the rear-inflow jet. The generality of the mesovortex genesis mechanisms was assessed by examining the structure of observed mesovortices in Doppler radar data. The mesovortex genesis mechanisms were also compared to others reported in the literature and the genesis of low-level mesocyclones in supercell thunderstorms.

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Nolan T. Atkins
and
Roger M. Wakimoto

Abstract

Mean sea-breeze characteristics were determined by analyzing a number of sea-breeze events during offshore, parallel, and onshore flow regimes during the Convection and Precipitation/Electrification Experiment (CaPE). It was observed that offshore flow cases exhibited the widest, and relatively strongest, radar-detected thin lines. The thin-line reflectivity values steadily increased during the day. In contrast, a thin line was detected only during late afternoon on parallel flow days while no easily identifiable thin line was observed during onshore flow days.

The gradients of temperature and moisture, as measured by a surface meteorological station during sea-breeze passage, were strongest and weakest during offshore and onshore flow days, respectively. In addition, the moisture and temperature gradients across the leading edge of the sea breeze steadily increased during the day and were strongest during late afternoon.

Using dual-Doppler techniques, the detailed kinematic structure of the sea-breeze circulation for offshore and onshore flow regimes is presented. In particular, detailed measurements of the sea-breeze return flow at upper levels are presented for both offshore and onshore flow events for the first time.

The observed inland propagation speed for offshore and parallel flow events is consistent with calculated values for density currents. Onshore flow events, however, are observed to move inland at a rate that is slower than what is expected for a density current.

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Roger M. Wakimoto
and
Nolan T. Atkins

Abstract

Observations of a strong (F3) tornado near Newcastle, Texas, on 29 May 1994 during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) are presented. The visual characteristics and intensity of the tornado were revealed by a photogrammmetric analysis of pictures taken by chase teams and by a detailed damage survey. The tornado developed from a low-level shear feature along the flanking line of a supercell. Vortex stretching of this feature to tornadic intensity occurred under the influence of an intense updraft from a rapidly growing storm along the flanking line. No apparent midlevel mesocyclone accompanied this tornado. In contrast, the supercell was characterized by a well-defined mid- and low-level mesocyclone; however, no tornado was observed by ground chase teams. The implications of these findings on the current understanding of tornadogenesis are discussed.

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Roger M. Wakimoto
and
Nolan T. Atkins

Abstract

An analysis of two sea-breeze events on 6 August (an onshore flow event) and 12 August (an offshore flow event) 1991 is presented using single-Doppler observations, satellite images, and cloud pictures collected during the Convection and Precipitation/Electrification (CaPE) Experiment. Documentation of the alongfrontal variability at the leading edge of the sea-breeze circulation is presented for the first time. The horizontal structure of the front was strongly modulated by the near-perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 August. These intersection points also appeared to be preferred locations for cloud development along the front. Horizontal convective rolls were also documented on 6 August; however, their orientation was nearly parallel to the sea-breeze front. As a result, extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection points. First cloud development along the front occurred at periodic locations and only along the sections where the rolls and front had merged. Further elucidation of the frontal characteristics under synoptic flow that is onshore versus offshore is discussed. The absence of a sharp frontal discontinuity and nonuniform propagation speed for the 6 August case were noted. There were substantial differences in the character of the radar-detected fine line on these two days. When compared with the 6 August line, the 12 August case was wider, easily identifiable, and characterized by higher reflectivities. Indeed, there were times on 6 August when identifying the location of the sea-breeze front based on radar reflectivity in the clear air was difficult.

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Nolan T. Atkins
and
Roger M. Wakimoto

Abstract

The thermodynamic properties of wet-microburst-producing days, as observed during the 1986 MIST (MIcroburst and Severe Thunderstorm) field project, conducted in northern Alabama, have been examined and are shown to exhibit common characteristics. The parent storms and environment for this microburst type are substantially different than those documented over the High Plains in that the cloud bases are warmer, the subcloud layer is shallower, the radar reflectivities are greater, and the thermal environment is more moist and stable. Analyses of the rawinsonde data, launched in the morning and afternoon, show that low-level moisture is present and is capped by a midlevel dry layer. This midlevel dry air is generally advected from the northwest, where a large area of dry air exists over the central United States.

In addition, it appears to be possible to differentiate between microburst days and thunderstorm days producing no wet microbursts by plotting the vertical profile of the equivalent potential temperature (o e ). The strong wind-shear days are potentially more unstable. The difference between the surface value of o e and the minimum value aloft (in the afternoon) is greater than 20 K for the microburst days, whereas it is less than 13 K for the thunderstorm days with no microbursts. Consequently, these results suggest that they may be used by the forecaster to issue, in a timely manner (2–12 hours), a “wind-shear alert” to the general population and, more importantly, to the aviation community.

Analyses of microburst storm structures indicate that they are vertically deeper than those storms developing during days with no microbursts, and that the precipitation core is largely composed of ice. Convergence, or inflow of environmental air into the microburst storms, was also commonly observed new the level of minimum o e .

These wet microburst soundings and o e profiles were compared to other well-documented events. In each case, the soundings and o e profiles were similar to those derived here.

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Roger M. Wakimoto
,
Nolan T. Atkins
, and
Joshua Wurman

Abstract

This study presents a single-Doppler radar analysis combined with cloud photography of the LaGrange, Wyoming, tornado on 5 June 2009 in an attempt to relate the radar-observed hook echo, weak-echo hole (WEH), and rotational couplet to the visual characteristics of the tornado. The tornado was rated EF2. The circulation at low levels went through two intensification periods based on azimuthal shear measurements. The first intensification was followed by the appearance of a brief funnel cloud. The second intensification was coincident with the appearance of a second funnel cloud that remained in contact with the ground until the tornado dissipated.

A deep WEH rapidly formed within the hook echo after damaging wind was identified at the ground and before the appearance of a funnel cloud. The echo pattern through the hook echo on 5 June undergoes a dramatic evolution. Initially, the minimum radar reflectivities are near the surface (<15 dBZ) and the WEH does not suggest a tapered structure near the ground. Subsequently, higher reflectivities appear at low levels when the funnel cloud makes contact with the ground. During one analysis time, the increase of the echo within the WEH at low levels results in a couplet of high/low radar reflectivity in the vertical. This increase in echo at low levels is believed to be associated with lofted debris although none was visibly apparent until the last analysis time. The WEH was nominally wider than the visible funnel cloud. The dataset provides the first detailed analysis of the double-ring structure within a hook echo that has been reported in several studies. The inner high-reflectivity region is believed to be a result of lofted debris. At higher-elevation angles, a small secondary WEH formed within the first WEH when debris was lofted and centrifuged.

A feature noted in past studies showing high-resolution vertical cross sections of single-Doppler velocity normal to the radar beam is an intense rotational couplet of negative and positive values in the lowest few hundred meters. This couplet was also evident in the analysis of the LaGrange tornado. The couplet was asymmetric with stronger negative velocities owing to the motion of the tornado toward the radar. The damaging wind observed by radar extended well beyond the condensation funnel in the lowest few hundred meters. However, another couplet indicating strong rotation was also noted aloft in a number of volume scans. The decrease in rotational velocities between the low-and upper-level couplets may be related to air being forced radially outward from the tornado center at a location above the intense inflow.

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Dustan M. Wheatley
,
Robert J. Trapp
, and
Nolan T. Atkins

Abstract

This study examines damaging-wind production by bow-shaped convective systems, commonly referred to as bow echoes. Recent idealized numerical simulations suggest that, in addition to descending rear inflow at the bow echo apex, low-level mesovortices within bow echoes can induce damaging straight-line surface winds. In light of these findings, detailed aerial and ground surveys of wind damage were conducted immediately following five bow echo events observed during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX) field phase. These damage locations were overlaid directly onto Weather Surveillance Radar-1988 Doppler (WSR-88D) images to (i) elucidate where damaging surface winds occurred within the bow-shaped convective system (in proximity to the apex, north of the apex, etc.), and then (ii) explain the existence of these winds in the context of the possible damaging-wind mechanisms.

The results of this study provide clear observational evidence that low-level mesovortices within bow echoes can produce damaging straight-line winds at the ground. When present in the BAMEX dataset, mesovortex winds produced the most significant wind damage. Also in the BAMEX dataset, it was observed that smaller-scale bow echoes—those with horizontal scales of tens of kilometers or less—produced more significant wind damage than mature, extensive bow echoes (except when mesovortices were present within the larger-scale systems).

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Nolan T. Atkins
,
Eva M. Glidden
, and
Timothy M. Nicholson

Abstract

This study presents an integrated analysis of dual-Doppler, cloud photogrammetry, surface mobile mesonet, and sounding data to examine wall cloud formation in two supercells observed during the Verification of the Origins of Rotation in Tornadoes Experiment II (VORTEX2). One of the wall clouds contained significant rotation and spawned an (enhanced Fujita) EF2 tornado, while the other was clearly displaced horizontally from the mesocyclone and exhibited little rotation at the time of data collection. Backward parcel trajectories show that the majority of the air entering the wall cloud base originates in the forward-flank region. A small fraction of the parcels enter the wall cloud base from the inflow. Some rear-flank downdraft parcels descend into the strongly rotating wall cloud. For both wall clouds, much of the observed wall cloud lowering is attributed to evaporatively cooled parcels in the forward-flank region being ingested into the low-level updraft. Additional wall cloud-base lowering is observed near the circulation center of the strongly rotating wall cloud. This localized lowering is created by the pressure deficit and associated cooling. The observational results presented herein are compared to long-standing wall cloud formation conceptual models published in the refereed literature.

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Nolan T. Atkins
,
Morris L. Weisman
, and
Louis J. Wicker

Abstract

A three-dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms, with emphasis on the low-level mesocyclone, interacting with preexisting boundaries. The impacts of low-level environmental shear, storm motion relative to boundary orientation, and boundary strength are assessed. In the low-level shear experiments, significant low-level rotation is consistently observed earlier, tends to be stronger, and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment. Low-level rotation is weaker in storms crossing the boundary and moving into the colder air. In contrast, all storms moving along or into the warm air ahead of the boundary develop significant low-level rotation. Increasing the temperature gradient and shear across the boundary has little impact on the low-level mesocyclone evolution. Storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those created when a temperature gradient also exists across the boundary.

It will be shown that the mechanisms generating the low-level mesocyclone appear to be different for storms interacting with boundaries than those initiated in a homogeneous environment. Consistent with previous studies, storms initiated in a homogeneous environment derive their low-level rotation from tilting of streamwise horizontal vorticity generated along the storm’s forward flank region. In contrast, for storms interacting with a boundary, a significant fraction of the air composing the low-level mesocyclone originates at low levels from the cool air side of the boundary. These parcels contain significant streamwise vorticity, which is tilted and stretched by the storms updraft. Vertical vorticity along the preexisting boundary may also have contributed to mesocyclogenesis. The forward-flank region appears to play a minor role in generating low-level rotation when a preexisting boundary is present.

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