Mesocyclone Research Articles

Mobile, Phased-Array, Doppler Radar Observations of Tornadoes at X Band

Abstract A mobile, phased-array Doppler radar, the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP), has been used since 2007 to obtain data in supercells and tornadoes. Rapidly updating, volumetric data of tornadic vortex signatures (TVSs) associated with four tornadoes are used to investigate the time–height evolution of TVS intensity, position, and dissipation up through storm midlevels. Both TVS intensity and position were highly variable in time and height even during tornado mature phases. In one case, a TVS associated with a tornado dissipated aloft and a second TVS formed shortly thereafter while there was one continuous TVS near the ground. In a second case, the TVS associated with a long-lived, violent tornado merged with a second TVS (likely a second cyclonic tornado) causing the original TVS to strengthen. TVS dissipation occurred first at a height of ~1.5 km AGL and then at progressively higher levels in two cases; TVS dissipation occurred last in the lowest 1 km in three cases examined. Possible explanations are provided for the unsteady nature of TVS intensity and a conceptual model is presented for the initial dissipation of TVSs at ~1.5 km AGL.

Factors contributing to tornadogenesis in landfalling Gulf of Mexico tropical cyclones

ABSTRACTTropical cyclone tornadoes (TCTs) are brief and often unpredictable events that can produce fatalities and create considerable economic loss. Given these uncertainties, it is important to understand the characteristics and factors that contribute to tornado formation within tropical cyclones. This research analyses this hazardous phenomenon, examining the relationships among tropical cyclone intensity, size and tornado output. Furthermore, the influences of severe weather parameters on tornado output near the time of tornado formation were assessed between two phases of a tropical cyclone's life cycle: during hurricane and tropical storm intensity, termed tropical cyclone tornadoes (TCTs), and during tropical depression and remnant low intensity, termed tropical low tornadoes (TLTs). Results show that tornado output is significantly influenced by tropical cyclone intensity. Values for storm relative helicity, energy helicity index, and severe weather threat index are significantly higher within TCT environments, thus resulting in more tornadoes.

Numerical Prediction of the 8 May 2003 Oklahoma City Tornadic Supercell and Embedded Tornado Using ARPS with the Assimilation of WSR-88D Data

Abstract The 8 May 2003 Oklahoma City, Oklahoma, tornadic supercell is predicted with the Advanced Regional Prediction System (ARPS) model using four nested grids with 9-km, 1-km, 100-m, and 50-m grid spacings. The Oklahoma City Weather Surveillance Radar-1988 Doppler (WSR-88D) radial velocity and reflectivity data are assimilated through the ARPS three-dimensional variational data assimilation (3DVAR) and cloud analysis on the 1-km grid to generate a set of initial conditions that includes a well-analyzed supercell and associated low-level mesocyclone. Additional 1-km experiments show that the use of radial velocity and the proper use of a divergence constraint in the 3DVAR play an important role in the establishment of the low-level mesocyclone during the assimilation and forecast. Assimilating reflectivity data alone failed to predict the mesocyclone intensification. The 100-m grid starts from the interpolated 1-km control initial conditions, while the further nested 50-m grid starts from the 20-min forecast on the 100-m grid. The forecasts on both grids cover the entire period of the observed tornado outbreak and successfully capture the development of tornadic vortices. The intensity of a tornado on the 50-m grid reaches the high end of category 3 on the Fujita scale (F3), while the corresponding simulated tornado on the 100-m grid reaches F2 intensity. The timing of the tornadogenesis on both grids agrees with the observations very well, although the predicted tornado was slightly weaker and somewhat shorter lived. The predicted tornado track parallels the observed damage track although it is displaced northward by about 8 km. The predicted tornado vortices have realistic structures similar to those documented in previous theoretical, idealized modeling and some observational studies. The prediction of an observed tornado in a supercell with a similar degree of realism has not been achieved before.

Detection of hail through the three-body scattering signatures and its effects on radar algorithms observed in Romania

The Romanian National Meteorological Administration (NMA) radar network consists of five S-band and four C-band radars. Observation of convection in Romania through the Doppler radar network offered a new perspective in understanding the climatologic risk of certain regions and mesoscale environments. Highly organized convective systems, such as supercells, are better observed and their subsequent threat can be better anticipated during the nowcasting process using Doppler velocity fields and detection algorithms such as mesocyclones (MESO) and tornadic vortex signature (TVS). However, for warning purposes, these tools cannot be used without a subjective validation because of the associated errors and limitations of radar observations. In this paper several cases are presented where the presence of large hail inside the storm produced a radar artifact named three-body scatter signature (TBSS) that disturbed the Doppler velocity field. The cases presented were observed with S-band radars and were associated with hail reports on the ground. The first case shows a TBSS whose radial Doppler velocities are negative due to the falling hydrometeors. The second case is a less frequent event; there the Doppler velocities in the TBSS region are positive due to the updraft. The third case has both positive and negative values in the TBSS region; it ocurred in a supercell that affected the city of Varna in Bulgaria with large hail. The positive values were associated with the overhang region in the rotational updraft at upper heights, while the negative values in the regions outside the rotational updraft at lower heights, were associated with the downdrafts. Features produced by the TBSS have perturbed the output of the MESO and TVS algorithms by introducing false strong values of wind shear that have been interpreted as rotation. Thus false mesocyclonic and tornadic vortex signatures were generated. In some of the cases large hail and weak tornadoes were reported, so the presence of a TBSS was a challenge for the nowcasters. In the last part of the paper we analyze the link between the TBSS appearance with reports of large hail at the ground in 2009 within the coverage area of the Romanian S-band radars, which also cover parts of Hungary, Bulgaria, the Republic of Moldova and Serbia. The results show that the TBSS artifact is a strong indicator of large-size hail.

The Influence of Environmental Low-Level Shear and Cold Pools on Tornadogenesis: Insights from Idealized Simulations

Abstract Idealized, dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near-surface vortices in supercell-like “pseudostorms.” A cyclonically rotating updraft is produced by a stationary, cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity. Once a nearly steady state is achieved, a heat sink, which emulates the effects of latent cooling associated with precipitation, is activated on the northeastern flank of the updraft at low levels. Cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near-surface vertical vorticity via the “baroclinic mechanism,” as has been diagnosed or inferred in actual supercells that have been simulated and observed. An intense cyclonic vortex forms in the simulations in which the environmental low-level wind shear is strong and the heat sink is of intermediate strength relative to the other heat sinks tested. Intermediate heat sinks result in the development (baroclinically) of substantial near-surface circulation, yet the cold pools are not excessively strong. Moreover, the strong environmental low-level shear lowers the base of the midlevel mesocyclone, which promotes strong dynamic lifting of near-surface air that previously resided in the heat sink. The superpositioning of the dynamic lifting and circulation-rich, near-surface air having only weak negative buoyancy facilitates near-surface vorticity stretching and vortex genesis. An intense cyclonic vortex fails to form in simulations in which the heat sink is excessively strong or weak or if the low-level environmental shear is weak.

A Parametric Wind–Pressure Relationship for Rankine versus Non-Rankine Cyclostrophic Vortices

AbstractA parametric tangential wind profile model is presented for depicting representative pressure deficit profiles corresponding to varying tangential wind profiles of a cyclostrophic, axisymmetric vortex. The model employs five key parameters per wind profile: tangential velocity maximum, radius of the maximum, and three shape parameters that control different portions of the profile. The model coupled with the cyclostrophic balance assumption offers a diagnostic tool for estimating and examining a radial profile of pressure deficit deduced from a theoretical superimposing tangential wind profile in the vortex. Analytical results show that the shape parameters for a given tangential wind maximum of a non-Rankine vortex have an important modulating influence on the behavior of realistic tangential wind and corresponding pressure deficit profiles. The first parameter designed for changing the wind profile from sharply to broadly peaked produces the corresponding central pressure fall. An increase in the second (third) parameter yields the pressure rise by lowering the inner (outer) wind profile inside (outside) the radius of the maximum. Compared to the Rankine vortex, the parametrically constructed non-Rankine vortices have a larger central pressure deficit. It is suggested that the parametric model of non-Rankine vortex tangential winds has good potential for diagnosing the pressure features arising in dust devils, waterspouts, tornadoes, tornado cyclones, and mesocyclones. Finally, presented are two examples in which the parametric model is fitted to a tangential velocity profile, one derived from an idealized numerical simulation and the other derived from high-resolution Doppler radar data collected in a real tornado.

Workshop On Polar Lows

What: Twenty-five scientists from universities, weather forecasting and space centers, and national research institutions in France, Germany, Norway, Russia, Sweden, the United Kingdom, Canada, and Japan presented and discussed recent developments in polar low research. When: 21–22 May 2012 Where: Oslo, Norway T he International Polar Year (IPY) has brought new momentum to polar low (PL) research and a huge amount of new data on, and insight to, mesoscale processes in polar regions. The Oslo polar low workshop was the 12th workshop of the European Polar Low Working Group (EPLWG1; www.eplwg .uni-trier.de). The EPLWG focuses its work on polar mesocyclones (MCs) in both hemispheres, but research includes other mesoscale weather phenomena such as katabatic winds, tip jets, boundary layer fronts, cold air outbreaks, and Arctic fronts. This workshop2 was convened by EPLWG to summarize recent experimental, climatological, modeling, and remote sensing studies on PLs and polar mesoscale processes. The workshop started with an overview lecture on the influence of low-frequency variability on the life cycles of high-impact weather. Subsequent presentations addressed observational studies, the climatology of PLs, modeling and theoretical aspects, and related and future projects (see www.eplwg.uni-trier .de/index.php?id=46884L extended abstracts of the presentations can be found at www.eplwg.uni-trier.de /index .php?id=46883&L=2).

Assessing Ensemble Forecasts of Low-Level Supercell Rotation within an OSSE Framework

Abstract Under the envisioned warn-on-forecast (WoF) paradigm, ensemble model guidance will play an increasingly critical role in the tornado warning process. While computational constraints will likely preclude explicit tornado prediction in initial WoF systems, real-time forecasts of low-level mesocyclone-scale rotation appear achievable within the next decade. Given that low-level mesocyclones are significantly more likely than higher-based mesocyclones to be tornadic, intensity and trajectory forecasts of low-level supercell rotation could provide valuable guidance to tornado warning and nowcasting operations. The efficacy of such forecasts is explored using three simulated supercells having weak, moderate, or strong low-level rotation. The results suggest early WoF systems may provide useful probabilistic 30–60-min forecasts of low-level supercell rotation, even in cases of large radar–storm distances and/or narrow cross-beam angles. Given the idealized nature of the experiments, however, they are best viewed as providing an upper-limit estimate of the accuracy of early WoF systems.

Dynamical Influences of Anvil Shading on Simulated Supercell Thunderstorms

Abstract Numerical simulations of supercell thunderstorms including parameterized radiative transfer and surface fluxes are performed using the Advanced Regional Prediction System (ARPS) model to investigate how low-level air temperature deficits within anvil shadows affect the simulated storms. The maximum temperature deficits within the modeled cloud shadows are 1.5–2.0 K, which is only about half that previously observed. Within the shadows, the loss of strong solar heating cools and stabilizes the near-surface layer, which suppresses vertical mixing and modifies the near-surface vertical wind shear. In a case of a stationary storm, the enhanced easterly shear present beneath the anvil leads to a thinning of the outflow layer and corresponding acceleration of the rear-flank gust front far ahead of the overlying updraft, weakening the low-level mesocyclone. It is further shown that the direct absorption and emission of radiation by clouds does not significantly affect the simulated supercells. Varying the time of day of model initialization does not prevent the simulated storms from weakening. This behavior is mirrored for storms that slowly move along the major axis of the anvil shadow. If the rear-flank gust front moves into the anvil shadow and the updraft moves normal to the shadow (i.e., northward movement of the updraft), cyclic periods of intensification and decay can result, although this result is likely highly dependent on the storm-relative wind profile. If the gust front does not advance into the shaded region (i.e., southward movement), or if the storm moves rapidly, the storm is relatively unaffected by anvil shading because the rear-flank gust front speed and outflow depth remain relatively unchanged.

On the Predictability of Supercell Thunderstorm Evolution

Abstract Supercell thunderstorms produce a disproportionate amount of the severe weather in the United States, and accurate prediction of their movement and evolution is needed to warn the public of their hazards. This study explores the practical predictability of supercell thunderstorm forecasts in the presence of typical errors in the preconvective environmental conditions. The Advanced Research Weather Research and Forecasting model (ARW-WRF) is run at 1-km grid spacing and a control run of a supercell thunderstorm is produced using a horizontally homogeneous environment. Forecast errors from supercell environments derived from the 13-km Rapid Update Cycle (RUC) valid at 0000 UTC for forecast lead times up to 3 h are used to define the environmental errors, and 100 runs initialized with environmental perturbations characteristic of those errors are produced for each lead time. The simulations are analyzed to determine the spread and practical predictability of supercell thunderstorm forecasts from a storm-scale model, with the control used as truth. Most of the runs perturbed with the environmental forecast errors produce supercell thunderstorms; however, there is much less predictability for storm motion and structure. Results suggest that an upper bound to the practical predictability of storm location with the current environmental uncertainty for a 1-h environmental forecast is about 2 h, with the predictability of the storms decreasing to 1 h as lead time increases. Smaller-scale storm features, such as midlevel mesocyclones and regions of heavy rainfall, display much less predictability than storm location. Mesocyclone location is predictable out to 40 min or less, while heavy 5-min rainfall location is not predictable.

An Assessment of Low-Level Baroclinity and Vorticity within a Simulated Supercell

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.

Analysis and Forecast of a Tornadic Thunderstorm Using Multiple Doppler Radar Data, 3DVAR, and ARPS Model

A three-dimensional variational (3DVAR) assimilation technique developed for a convective-scale NWP model—advanced regional prediction system (ARPS)—is used to analyze the 8 May 2003, Moore/Midwest City, Oklahoma tornadic supercell thunderstorm. Previous studies on this case used only one or two radars that are very close to this storm. However, three other radars observed the upper-level part of the storm. Because these three radars are located far away from the targeted storm, they were overlooked by previous studies. High-frequency intermittent 3DVAR analyses are performed using the data from five radars that together provide a more complete picture of this storm. The analyses capture a well-defined mesocyclone in the midlevels and the wind circulation associated with a hook-shaped echo. The analyses produced through this technique are used as initial conditions for a 40-minute storm-scale forecast. The impact of multiple radars on a short-term NWP forecast is most evident when compared to forecasts using data from only one and two radars. The use of all radars provides the best forecast in which a strong low-level mesocyclone develops and tracks in close proximity to the actual tornado damage path.

Convective Modes for Significant Severe Thunderstorms in the Contiguous United States. Part III: Tropical Cyclone Tornadoes

Abstract A gridded, hourly, three-dimensional environmental mesoanalysis database at the Storm Prediction Center (SPC), based on objectively analyzed surface observations blended with the Rapid Update Cycle (RUC) model-analysis fields and described in Parts I and II of this series, is applied to a 2003–11 subset of the SPC tropical cyclone (TC) tornado records. Distributions of environmental convective parameters, derived from SPC hourly mesoanalysis fields that have been related to supercells and tornadoes in the midlatitudes, are evaluated for their pertinence to TC tornado occurrence. The main factor differentiating TC from non-TC tornado environments is much greater deep-tropospheric moisture, associated with reduced lapse rates, lower CAPE, and smaller and more compressed distributions of parameters derived from CAPE and vertical shear. For weak and strong TC tornado categories (EF0–EF1 and EF2–EF3 on the enhanced Fujita scale, respectively), little distinction is evident across most parameters. Radar reflectivity and velocity data also are examined for the same subset of TC tornadoes, in order to determine parent convective modes (e.g., discrete, linear, clustered, supercellular vs nonsupercellular), and the association of those modes with several mesoanalysis parameters. Supercellular TC tornadoes are accompanied by somewhat greater vertical shear than those occurring from other modes. Tornadoes accompanying nonsupercellular radar echoes tend to occur closer to the TC center, where CAPE and shear tend to weaken relative to the outer TC envelope, though there is considerable overlap of their respective radial distributions and environmental parameter spaces.

The Bowdle, South Dakota, Cyclic Tornadic Supercell of 22 May 2010: Surface Analysis of Rear-Flank Downdraft Evolution and Multiple Internal Surges

Abstract Mobile mesonet sampling in the hook echo/rear-flank downdraft (RFD) region of a tornadic supercell near Bowdle, South Dakota, provided the opportunity to examine RFD thermodynamic and kinematic attributes and evolution. Focused analysis of the fifth low-level mesocyclone cycle that produced two significant tornadoes including a violent tornado, revealed four RFD internal surge (RFDIS) events. RFDISs appeared to influence tornado development, intensity, and demise by altering the thermodynamic and kinematic character of the RFD region bounding the pretornadic and tornadic circulations. Significant tornadoes developed and matured when the RFD, modulated by internal surges, was kinematically strong, only weakly negatively buoyant, and very potentially buoyant. In contrast, the demise of the Bowdle tornado was concurrent with a much cooler RFDIS that replaced more buoyant and far more potentially buoyant RFD air near the tornado. This surge also likely contributed to a displacement of the tornado from the storm updraft. Development of the first tornado and rapid intensification of the Bowdle tornado occurred when an RFDIS boundary convergence zone interacted with the pretornadic and tornadic circulations, respectively. In the latter case, a strong vertical vortex sheet along an RFDIS boundary appeared to be a near-surface cyclonic vorticity source for the tornado. A downdraft closely bounding the right flank of the developing first tornado and intensifying Bowdle tornado provided some of the inflow to these circulations. For the Bowdle tornado, parcels were also streaming toward the tornado from its immediate east and northeast. A cyclonic–anticyclonic vortex couplet was observed during a portion of each significant tornado cycle.

The Pretornadic Phase of the Goshen County, Wyoming, Supercell of 5 June 2009 Intercepted by VORTEX2. Part II: Intensification of Low-Level Rotation

Abstract The dynamical processes responsible for the intensification of low-level rotation prior to tornadogenesis are investigated in the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The circulation of material circuits that converge upon the low-level mesocyclone is principally acquired along the southern periphery of the forward-flank precipitation region, which is a corridor characterized by a horizontal buoyancy gradient; thus, much of the circulation appears to have been baroclinically generated. The descending reflectivity core (DRC) documented in Part I of this paper has an important modulating influence on the circulation of the material circuits. A circuit that converges upon the low-level mesocyclone center prior to the DRC’s arrival at low levels (approximately the arrival of the 55-dBZ reflectivity isosurface in this case) loses some of its previously acquired circulation during the final few minutes of its approach. In contrast, a circuit that approaches the low-level mesocyclone center after the DRC arrives at low levels does not experience the same adversity. An analysis of the evolution of angular momentum within a circular control disk centered on the low-level mesocyclone reveals that the area-averaged angular momentum in the nearby surroundings of the low-level mesocyclone increases while the mesocyclone is occluding and warm-sector air is being displaced from the near surroundings. The occlusion process reduces the overall negative vertical flux of angular momentum into the control disk and enables the area-averaged angular momentum to continue increasing even though the positive radial influx of angular momentum is decreasing in time.

The LaGrange Tornado during VORTEX2. Part II: Photogrammetric Analysis of the Tornado Combined with Dual-Doppler Radar Data

Abstract This study presents the synthesis of dual-Doppler and cloud photography data of the 5 June 2009 Goshen County, Wyoming, tornado observed during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Analyses focused on the hook region of the parent supercell. It will be shown that radar-determined tornadogenesis and initial surface wind damage occurred 14 min before the funnel cloud was observed continuously on the ground. In addition to the cyclonic wall cloud, an anticyclonic lowering was also observed on the southern flank of the hook echo near the time of tornadogenesis. The relationship between the intensities of the tornado and its parent circulation, the low-level mesocyclone, will also be discussed. Funnel diameter was not well correlated with the maximum vertical vorticity or circulation associated with the mesocyclone. Furthermore, changes in the minimum reflectivity observed in the tornado-scale weak echo hole (WEH) were weakly correlated with the maximum vertical vorticity of the mesocyclone. The tornado funnel was observed within and was relatively small compared to the WEH diameter. The distribution and evolution of angular momentum were also examined. The radial increase of angular momentum terminated at or beyond the wall cloud edge. Prior to the time that the funnel made continuous contact with the ground, low-level angular momentum increased despite the fact that azimuthally averaged low-level flow within the mesocyclone was divergent, advecting low angular momentum air away from the circulation center. Both the tornado and mesocyclone generally intensified during this time. Thereafter, while the tornado continued to intensify, angular momentum within the low-level mesocyclone weakened.

The Pretornadic Phase of the Goshen County, Wyoming, Supercell of 5 June 2009 Intercepted by VORTEX2. Part I: Evolution of Kinematic and Surface Thermodynamic Fields

Abstract The authors analyze the pretornadic phase (2100–2148 UTC; tornadogenesis began at 2152 UTC) of the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The analysis relies on radar data from the Weather Surveillance Radar-1988 Doppler (WSR-88D) in Cheyenne, Wyoming (KCYS), and a pair of Doppler-on-Wheels (DOW) radars, mobile mesonet observations, and mobile sounding observations. The storm resembles supercells that have been observed in the past. For example, it develops a couplet of counter-rotating vortices that straddle the hook echo within the rear-flank outflow and are joined by arching vortex lines, with the cyclonic vortex becoming increasingly dominant in the time leading up to tornadogenesis. The outflow in the hook echo region, where sampled, has relatively small virtual potential temperature θυ deficits during this stage of evolution. A few kilometers upstream (north) of the location of maximum vertical vorticity, θυ is no more than 3 K colder than the warmest θυ readings in the inflow of the storm. Forward trajectories originating in the outflow within and around the low-level mesocyclone rise rapidly, implying that the upward-directed perturbation pressure gradient force exceeds the negative buoyancy. Low-level rotation intensifies in the 2142–2148 UTC period. The intensification is preceded by the formation of a descending reflectivity core (DRC), similar to others that have been documented in some supercells recently. The DRC is associated with a rapid increase in the vertical vorticity and circulation of the low-level mesocyclone.

Observations of the Surface Boundary Structure within the 23 May 2007 Perryton, Texas, Supercell

AbstractIn situ data collected within a weakly tornadic, high-precipitation supercell occurring on 23 May 2007 near Perryton, Texas, are presented. Data were collected using a recently developed fleet of 22 durable, rapidly deployable probes dubbed “StickNet” as well as four mobile mesonet probes. Kinematic and thermodynamic observations of boundaries within the supercell are described in tandem with an analysis of data from the Shared Mobile Atmospheric Research and Teaching Radar.Observations within the rear-flank downdraft of the storm exhibit large deficits of both virtual potential temperature and equivalent potential temperature, with a secondary rear-flank downdraft gust front trailing the mesocyclone. A primarily thermodynamic boundary resided across the forward-flank reflectivity gradient of the supercell. This boundary is characterized by small deficits in virtual potential temperature coupled with positive perturbations of equivalent potential temperature. The opposing thermodynamic perturbations appear to be representative of modified storm inflow, with a flux of water vapor responsible for the positive perturbations of the equivalent potential temperature. Air parcels exhibiting negative perturbations of virtual potential temperature and positive perturbations of equivalent potential temperature have the ability to be a source of both baroclinically generated streamwise horizontal vorticity and greater potential buoyancy if ingested by the low-level mesocyclone.

Mesocyclone Evolution Associated with Varying Shear Profiles during the 24 June 2003 Tornado Outbreak

Abstract The morphology of mesocyclones associated with the regional tornado outbreak on 24 June 2003 is examined to illustrate the effects of changing vertical wind profiles. The large-scale environment supported deep moist convection, with forcing for ascent and convective instability. Postevent analysis indicated there were changes in the shear in space and time across a small geographical area. The event was separated into sectors based on both the synoptic setting and the differing shear profiles. Near the surface warm front, the vertical wind profile and mesocyclone evolution exhibited a classic appearance and produced significant tornadoes. In the warm sector, where no discernible surface boundaries were evident, classic supercells initially were favored but only produced short-lived tornadoes rated as F0 on the Fujita scale. The vertical wind profile changed as a low-level jet intensified after 0000 UTC 25 June. The majority of the vertical wind shear became located below 3 km. Meanwhile, mesocyclone elevation lowered and rotational velocity increased. As the dynamically induced low-level jet and an area of mixed-layer (ML) convective available potential energy (CAPE) became juxtaposed where the boundary layer was uncapped, strong low-level mesocyclones and 32 tornadoes developed in an area with no discernible surface boundaries. The event illustrates the need for warning meteorologists to monitor not only the amount of shear present, but also its distribution in the hodograph owing to its strong correspondence with mesocyclone morphology.

Analysis of three supercell storms with Doppler weather radar data

Three supercell storms on 24 June 2004 (0624), 28 June 2003 (0628), and 27 September 2002 (0927) induced different damages in Shandong Province. Storm 0927 was inferior in size and intensity to storms 0628 and 0624. The structure and evolvement of the three storms were analyzed in detail based on the WSR-98D radar data in combination with weather charts. The results show that mesoscale surface convergence triggered release of instable energy, which resulted in severe convection. During the development stage, storms 0927, 0628, and 0624 displayed multi-cell propagation, single-cell evolution, and multi-cell mergence, respectively. The storm tracks were similar: they were all right-moving supercell storms, i.e., moving at an angle of 30°–70° to the right of the mean wind and at a speed of about 45%-70% of the mean wind speed. In the mature stage, the maximum reflectivity appeared at the low level in storm 0927, mid level in storm 0628, and mid-upper level in storm 0624. These storms possessed almost all typical features of supercell storms: weak echo region (WER), bounded weak echo region (BWER), and mesocyclone. An organized mesocyclone formed at the middle height of an updraft, deepened gradually downward and upward, and became a typical mid-level mesocyclone with strong updrafts. The vertical structures of airflows in the three storms were similar, i.e., significant convergence at low level, nearly pure rotation at mid level, and divergent rotation at upper level. However, signatures of mid-level horizontal airflows in the three storms were different: at mid level, there was a single vortex in storm 0628, but a double-vortex flow pattern was seen in storms 0927 and 0624. The horizontal structure of the double-vortex flow was hard to be blown away by the environmental airflow, and thus the storms could persist for a longer period of time than the single vortex storm.