Storm-relative Environmental Helicity Research Articles

Tornadogenesis in a Quasi-Linear Convective System over Kanto Plain in Japan: A Numerical Case Study

Abstract The environmental characteristics and formation process of a tornado spawned by a quasi-linear convective system (QLCS) over Kanto Plain, Japan, are examined using observations, a reanalysis dataset, and a high-resolution numerical simulation with a horizontal grid spacing of 50 m. The QLCS environment responsible for tornadogenesis was characterized by small convective available potential energy and large storm-relative environmental helicity due to strong vertical shear associated with a low-level jet. The strong low-level jet was associated with a large zonal pressure gradient between two meridionally aligned extratropical cyclones and a synoptic-scale high pressure system to the east. The numerical simulation reproduced the tornado in the central part of the QLCS. Before the tornadogenesis, three mesovortices developed that were meridionally aligned at 500-m height, and a rear inflow jet (RIJ) associated with relatively cold air originated from aloft and developed on the west side of the QLCS, while descending from rear to front. Tornadogenesis occurred in the southernmost mesovortex at the northern tip of the RIJ. This mesovortex induces strong low-level updrafts through vertical pressure gradient force. A circulation analysis and vorticity budget analysis for the mesovortex show that environmental crosswise vorticity in the forward inflow region east of the QLCS played a significant role in the formation of the mesovortex. The circulation analysis for the tornado shows that frictional effects contribute to the increase of circulation associated with the tornado. Moreover, environmental shear associated with horizontal and vertical shear of the horizontal wind also contribute to the circulation of the tornado.

Examining Relationships between Environmental Conditions and Supercell Motion in Time

AbstractAlthough environmental controls on bulk supercell potential and hazards have been studied extensively, relationships between environmental conditions and temporal changes to storm morphology remain less explored. These relationships are examined in this study using a compilation of sounding data collected during field campaigns from 1994 to 2019 in the vicinity of 216 supercells. Environmental parameters are calculated from the soundings and related to storm-track characteristics like initial cell motion and the time of the right turn (i.e., the time elapsed between the cell initiation and the first time when the supercell obtains a quasi-steady motion that is directed clockwise from its initial motion.). We do not find any significant associations between environmental parameters and the time of the right turn. Somewhat surprisingly, no relationship is found between storm-relative environmental helicity and the time elapsed between cell initiation and the onset of deviant motion. Initial cell motion is best approximated by the direction of the 0–6-km mean wind at two-thirds the speed. This is a result of advection and propagation in the 0–4- and 0–2-km layers, respectively. Unsurprisingly, Bunkers-right storm motion is a good estimate of post-turn motion, but storms that exhibit a post-turn motion left of Bunkers-right are less likely to be tornadic. These findings are relevant for real-time forecasting efforts in predicting the path and tornado potential of supercells up to hours in advance.

Exploring Inland Tropical Cyclone Rainfall and Tornadoes under Future Climate Conditions through a Case Study of Hurricane Ivan

AbstractThe overarching purpose of this study is to investigate the impacts of anthropogenic climate change both on the rainfall and tornadoes associated with tropical cyclones (TCs) making landfall in the U.S. Atlantic basin. The “pseudo–global warming” (PGW) approach is applied to Hurricane Ivan (2004), a historically prolific tropical cyclone tornado (TCT)-producing storm. Hurricane Ivan is simulated under its current climate forcings using the Weather Research and Forecasting Model. This control simulation (CTRL) is then compared with PGW simulations in which the current forcings are modified by climate-change differences obtained from the Community Climate System Model, version 4 (NCAR); Model for Interdisciplinary Research on Climate, version 5 (MIROC); and Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL). Changes in TC intensity, TC rainfall, and TCT production, identified for the PGW-modified Ivan, are documented and analyzed. Relative to CTRL, all three PGW simulations show an increase in TC intensity and generate substantially more accumulated rainfall over the course of Ivan’s progression over land. However, only one of the TCs under PGW (MIROC) produced more TCTs than CTRL. Evidence is provided that, in addition to favorable environmental conditions, TCT production is related to the TC track length and to the strength of the interaction between the TC and an environmental midlevel trough. Enhanced TCT generation at landfall for MIROC and GFDL is attributed to increased values of convective available potential energy, low-level shear, and storm-relative environmental helicity.

Insights into Supercells and Their Environments from Three Decades of Targeted Radiosonde Observations

AbstractHundreds of supercell proximity soundings obtained for field programs over the central United States are analyzed to reconcile differences in recent studies and to refine our knowledge of supercell environments. The large, storm-centric observation-based dataset and high vertical resolution of the sounding data provide an unprecedented look at supercell environments. Not surprisingly, storm-relative environmental helicity (SRH) is found to be larger in tornadic soundings than in nontornadic soundings. The primary finding that departs from previous studies is that storm-relative winds contribute substantially to the larger SRH. Stronger ground-relative winds and more rightward-deviant storm motions contribute to the larger storm-relative winds for the tornadic soundings. Spatial analyses of the soundings reveal lower near-ground pressure perturbations and stronger low- to midlevel cyclonic flow for the tornadic soundings, which suggests stronger mesocyclones, perhaps explaining the more rightward-deviant motions. Differences in the mean critical angle between the tornadic and nontornadic soundings are small and do not contribute to the larger mean SRH, but the tornadic soundings do have fewer instances of smaller (<60°) critical angles. Furthermore, the critical angle is shown to be a function of azimuth from the updraft. Other results include a low-to-the-ground (~250 m on average) hodograph kink for both the tornadic and nontornadic soundings and few notable differences in thermodynamic quantities, except for the expected lower LCLs related to higher RH for the tornadic soundings, somewhat smaller 0–3 km lapse rates in tornadic environments related to weaker/shallower capping inversions, and larger 0–3 km CAPE in near-field environments.

The Historic 2 April 2017 Louisiana Tornado Outbreak

On 2 April 2017 Louisiana saw one of its largest tornado outbreaks since 1950. The day saw a total of 21 tornadoes, six of which were classified as significant (EF2+). This extreme event resulted in an estimated 4.2 million dollars in damage and two fatalities. A climatology of Louisiana individual tornado days dating back to 1950 ranked this event second for total number of tornadoes (21) in a day and second for number of significant (EF2+) tornadoes (6) in a day. To assess the overall impact of this outbreak, the Destruction Potential Index (DPI) was used to compare both the overall number and strength of tornadoes to previous events. The 2 April 2017 event had the highest DPI for the state of Louisiana since 1950. Comparisons made to tornado days in nearby states in the region also highlight the significance of this event. The synoptic conditions of 2 April 2017 were compared to a 25-member synoptic scale composite of past Louisiana tornado days with six or more tornadoes. The composite highlighted anomalies in synoptic ingredients that contribute to tornado outbreak environments. Most notably, the synoptic structure consisted of a more meridional flow pattern, which allowed for stronger moisture transport, low level shear, deep layer shear, and increased thermal advection. This produced an optimal mesoscale environment with high instability and storm-relative environmental helicity. The meteorological significance of this event with respect to Louisiana stems from the uncommon combination of a high shear/high CAPE near-storm environment in a southern tornado outbreak. The high shear/high CAPE environment largely responsible for the 2 April 2017 Louisiana tornado outbreak is more commonly observed in Great Plains tornado outbreaks.

Toward better assessment of tornado potential in typhoons: Significance of considering entrainment effects for CAPE

AbstractThe characteristics of typhoons that spawned tornadoes (tornadic typhoons: TTs) in Japan from 1991 to 2013 were investigated by composite analysis using the Japanese 55 year Reanalysis and compared with those of typhoons that did not spawn tornadoes (nontornadic typhoons: NTs). We found that convective available potential energy (CAPE), which considers the effects of entrainment (entraining CAPE: E‐CAPE), and storm‐relative environmental helicity (SREH) are significantly large in the northeast quadrant of TTs where tornadoes frequently occur and that E‐CAPE and SREH in that quadrant for TTs are larger than those for NTs. On the other hand, ordinary CAPE without entrainment does not account for the spatial distribution of tornado occurrences nor does it distinguish TTs from NTs. E‐CAPE is sensitive to humidity in the midtroposphere; thus, it is effective for detecting a conditionally unstable layer up to about 550 hPa, which is distinctive of TTs.

Structural and Environmental Characteristics of Extratropical Cyclones Associated with Tornado Outbreaks in the Warm Sector: An Idealized Numerical Study

Abstract To clarify the effects of the horizontal shear of the jet stream on the structure and environment of extratropical cyclones that are accompanied by tornado outbreaks (OCs) and those that are not (NOCs), two idealized numerical experiments are performed. The experiments (OC-CTL and NOC-CTL) adopt the basic states taken from the corresponding composites of reanalysis data (JRA-55), except that the humidity field in both cases is taken from the OC composite. The simulated cyclone in OC-CTL exhibits a more meridionally elongated structure and stronger low-level wind in the southeast quadrant of the cyclone center, resulting in larger values of storm relative environmental helicity (SREH) than those in NOC-CTL. These results are consistent with the characteristics of the cyclones found for OCs and NOCs in the authors’ composite study. The distributions of surface-based convective available potential energy (SBCAPE) show no notable differences between OC-CTL and NOC-CTL, while those of CAPE based on the most unstable air parcel (MUCAPE) show some differences. A sensitivity experiment without moist processes such as condensation heating and evaporative cooling shows that the differences in the cyclone structure and environmental parameters between OCs and NOCs can be qualitatively explained by the dry dynamics. However, inclusion of moist processes results in notably larger differences.

Structural and Environmental Characteristics of Extratropical Cyclones that Cause Tornado Outbreaks in the Warm Sector: A Composite Study

Abstract The structural and environmental characteristics of extratropical cyclones that cause tornado outbreaks [outbreak cyclones (OCs)] and that do not [nonoutbreak cyclones (NOCs)] are examined using the Japanese 55-year Reanalysis dataset (JRA-55). Composite analyses show differences between OCs and NOCs: for OCs, storm relative environmental helicity (SREH) and convective available potential energy (CAPE) are notably larger, and the areas in which these parameters have significant values are wider in the warm sector than they are for NOCs. The larger CAPE in OCs is due to larger amounts of low-level water vapor, while the greater SREH is due to stronger low-level southerly wind. The composite analyses for environmental fields defined by 20-day means suggest that environmental meridional flows have the potential to advect large amounts of warm and moist air northward, creating atmospheric instability in the troposphere that contributes to the occurrence of a tornado outbreak. A piecewise potential vorticity (PV) diagnosis shows that low- to midlevel PV anomalies are the main contributor to the difference in the low-level winds between OCs and NOCs, whereas upper-level PV anomalies make only a minor contribution. An examination of the structures of the extratropical cyclones and the upper-level jet stream suggests that the difference in the low-level winds between OCs and NOCs is due to differences in the structure of the jet stream. The OCs develop when the jet stream displays larger anticyclonic shear. This causes a more meridionally elongated structure of OCs, resulting in stronger low-level winds in the southeastern quadrant of the cyclones.

Study of the tornado event in Greece on March 25, 2009: Synoptic analysis and numerical modeling using modified topography

Recent research revealed that western Greece and NW Peloponnese are regions that favor prefrontal tornadic incidence. On March 25, 2009 a tornado developed approximately at 10:30 UTC near Varda village (NW Peloponnese). Tornado intensity was T4–T5 (TORRO scale) and consequently caused an economic impact of 350,000 € over the local society. The goals of this study are: (i) to analyze synoptic and remote sensing features regarding the tornado event over NW Peloponnese and (ii) to investigate the role of topography in tornadogenesis triggered under strong synoptic scale forcing over that area.Synoptic analysis was based on the European Centre for Medium-Range Weather Forecasts (ECMWF) data sets. The analysis of daily anomaly of synoptic conditions with respect to 30 years' climatology (1981–2010), was based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data sets. In addition, numerous remote sensing data sets were derived by the Hellenic National Meteorological Service (HNMS) weather station network in order to better interpret the examined tornado event.Finally, numerical modeling was performed using the non-hydrostatic Weather Research and Forecasting model (WRF), initialized by ECMWF gridded analyses, with telescoping nested grids that allow the representation of atmospheric circulations ranging from the synoptic scale down to the meso-scale. The two numerical experiments were performed on the basis of: (a) the presence and (b) the absence of topography (landscape), so as to determine whether the occurrence of a tornado – identified by diagnostic instability indices – could be indicated by modifying topography.The energy helicity index (EHI), the bulk Richardson number (BRN) shear, the storm-relative environmental helicity (SRH), and the maximum convective available potential energy (MCAPE, for parcels with maximum θe) were considered as principal diagnostic instability variables and employed in both numerical experiments. Furthermore, model verification was conducted, accompanied by analysis of the absolute vorticity budget.Synoptic analysis revealed that the synoptic weather conditions on March 25, 2009 are in agreement with the composite synoptic climatology for tornado days over western Greece. In addition, maximum daily anomalies at the barometric levels of 500, 700, 850 and 925hPa were found, compared to the climatology of composite mean anomalies for tornado days over western Greece. Numerical simulations revealed that the topography of NW Peloponnese did not constitute an important factor during the tornado event on March 25, 2009, based on EHI, SRH, BRN, and MCAPE analyses.

Practical Predictability of the 20 May 2013 Tornadic Thunderstorm Event in Oklahoma: Sensitivity to Synoptic Timing and Topographical Influence

Abstract The practical predictability of severe convective thunderstorms during the 20 May 2013 severe weather event that produced the catastrophic enhanced Fujita scale 5 (EF-5) tornado in Moore, Oklahoma, was explored using ensembles of convective-permitting model simulations. The sensitivity of initiation and the subsequent organization and intensity of the thunderstorms to small yet realistic uncertainties in boundary layer and topographical influence within a few hours preceding the thunderstorm event was examined. It was found that small shifts in either simulation time or terrain configuration led to considerable differences in the atmospheric conditions within the boundary layer. Small shifts in simulation time led to changes in low-level moisture and instability, primarily through the vertical distribution of moisture within the boundary layer due to vertical mixing during the diurnal cycle as well as advection by low-level jets, thereby influencing convection initiation. Small shifts in terrain led to changes in the wind field, low-level vertical wind shear, and storm-relative environmental helicity, altering locally enhanced convergence that may trigger convection. After initiation, an upscale growth of errors resulting from deep moist convection led to large forecast uncertainties in the timing, intensity, structure, and organization of the developing mesoscale convective system and its embedded supercells.

A Bayesian modelling framework for tornado occurrences in North America.

Tornadoes represent one of nature's most hazardous phenomena that have been responsible for significant destruction and devastating fatalities. Here we present a Bayesian modelling approach for elucidating the spatiotemporal patterns of tornado activity in North America. Our analysis shows a significant increase in the Canadian Prairies and the Northern Great Plains during the summer, indicating a clear transition of tornado activity from the United States to Canada. The linkage between monthly-averaged atmospheric variables and likelihood of tornado events is characterized by distinct seasonality; the convective available potential energy is the predominant factor in the summer; vertical wind shear appears to have a strong signature primarily in the winter and secondarily in the summer; and storm relative environmental helicity is most influential in the spring. The present probabilistic mapping can be used to draw inference on the likelihood of tornado occurrence in any location in North America within a selected time period of the year.

Numerical modeling and analysis of the effect of complex Greek topography on tornadogenesis

Abstract. Tornadoes have been reported in Greece over recent decades in specific sub-geographical areas and have been associated with strong synoptic forcing. While it has been established that meteorological conditions over Greece are affected at various scales by the significant variability of topography, the Ionian Sea to the west and the Aegean Sea to the east, there is still uncertainty regarding topography's importance on tornadic generation and development. The aim of this study is to investigate the role of topography in significant tornadogenesis events that were triggered under strong synoptic scale forcing over Greece. Three tornado events that occurred over the last years in Thebes (Boeotia, 17 November 2007), Vrastema (Chalkidiki, 12 February 2010) and Vlychos (Lefkada, 20 September 2011) were selected for numerical experiments. These events were associated with synoptic scale forcing, while their intensities were T4–T5 (on the TORRO scale), causing significant damage. The simulations were performed using the non-hydrostatic weather research and forecasting model (WRF), initialized by European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses, with telescoping nested grids that allow for the representation of atmospheric circulations ranging from the synoptic scale down to the mesoscale. In the experiments, the topography of the inner grid was modified by: (a) 0% (actual topography) and (b) −100% (without topography), making an effort to determine whether the occurrence of tornadoes – mainly identified by various severe weather instability indices – could be indicated by modifying topography. The principal instability variables employed consisted of the bulk Richardson number (BRN) shear, the energy helicity index (EHI), the storm-relative environmental helicity (SRH), and the maximum convective available potential energy (MCAPE, for parcels with maximum θe). Additionally, a model verification was conducted for every sensitivity experiment accompanied by analysis of the absolute vorticity budget. Numerical simulations revealed that the complex topography constituted an important factor during the 17 November 2007 and 12 February 2010 events, based on EHI, SRH, BRN, and MCAPE analyses. Conversely, topography around the 20 September 2011 event was characterized as the least significant factor based on EHI, SRH, BRN, and MCAPE analyses.

Vorticity Evolution Leading to Tornadogenesis and Tornadogenesis Failure in Simulated Supercells

Abstract A three-dimensional idealized cloud model was used to study the storm-scale differences between simulated supercells that produce tornado-like vortices and those that do not. Each simulation was initialized with a different Rapid Update Cycle, version 2 (RUC-2), sounding that was associated with tornadic and nontornadic supercells in nature. The focus is an analysis of vorticity along backward-integrated trajectories leading up to tornadogenesis (19 simulations) and tornadogenesis failure (14 simulations). In so doing, the differences between the nontornadic and tornadic cases can be explored in relation to their associated environmental sounding. Backward-integrated trajectories seeded in the near-surface circulation indicate that the largest differences in vertical vorticity production between the tornadic and nontornadic simulations occur in parcels that descend to the surface from aloft (i.e., descending). Thus, the results from this study support the hypothesis that descending air in the rear of the storm is crucial to tornadogenesis. In the tornadic simulations, the descending parcels experience more negative vertical vorticity production during descent and larger tilting of horizontal vorticity into positive vertical vorticity after reaching the surface, owing to stronger horizontal gradients of vertical velocity. The larger vertical velocities experienced by the trajectories just prior to tornadogenesis in the tornadic simulations are associated with environmental soundings of larger CAPE, smaller convective inhibition (CIN), and larger 0–1-km storm-relative environmental helicity. Furthermore, in contrast with what might be expected from previous works, trajectories entering the incipient tornadic circulations are more negatively buoyant than those entering the nontornadic circulations.

Environmental factors influential to the duration and intensity of tornadoes in simulated supercells

Simulations were performed using an idealized cloud model to investigate the environmental conditions influential to tornado duration and intensity. The results from twenty‐one tornadic simulations are presented—far more than any previous study. The results show a nearly linear relationship between storm relative environmental helicity (SREH) and simulated tornado duration. A multiple linear regression analysis was performed showing that, in addition to SREH, convective available potential energy (CAPE), convective inhibition (CIN), and precipitable water (Pwat) are useful predictor variables for simulated tornado duration. Similar results were found for simulated tornado intensity, as represented by the maximum central pressure drop.

Synoptic Composites of Tornadic and Nontornadic Outbreaks

Abstract Tornadic and nontornadic outbreaks occur within the United States and elsewhere around the world each year with devastating effect. However, few studies have considered the physical differences between these two outbreak types. To address this issue, synoptic-scale pattern composites of tornadic and nontornadic outbreaks are formulated over North America using a rotated principal component analysis (RPCA). A cluster analysis of the RPC loadings group similar outbreak events, and the resulting map types represent an idealized composite of the constituent cases in each cluster. These composites are used to initialize a Weather Research and Forecasting Model (WRF) simulation of each hypothetical composite outbreak type in an effort to determine the WRF’s capability to distinguish the outbreak type each composite represents. Synoptic-scale pattern analyses of the composites reveal strikingly different characteristics within each outbreak type, particularly in the wind fields. The tornado outbreak composites reveal a strong low- and midlevel cyclone over the eastern Rockies, which is likely responsible for the observed surface low pressure system in the plains. Composite soundings from the hypothetical outbreak centroids reveal significantly greater bulk shear and storm-relative environmental helicity values in the tornado outbreak environment, whereas instability fields are similar between the two outbreak types. The WRF simulations of the map types confirm results observed in the composite soundings.

Case study of the 20 May 2008 tornadic storm in Hungary — Remote sensing features and NWP simulation

This paper discusses a case study of a tornadic thunderstorm event that occurred over Hungary on 20 May 2008. The paper presents both remote sensing measurements and numerical simulations of the storm system, and the synoptic situation is also described. The pre-convective environment was analyzed using satellite retrieved instability indices and total precipitable water content. The evolution of the convective system was described using 5-minute satellite imagery, 15-minute radar data, and continuous lightning observations. A high resolution MODIS image was also examined. The analysis of cloud top characteristics using satellite imagery revealed numerous cloud top features typically associated with severe storms: cold-ring and cold-U shapes in the infrared images, as well as longer lasting series of strong overshooting tops, gravity waves, above-anvil plumes, and radial cirrus. The cloud top microphysical properties indicated severe updraft — the imagery showed many small ice crystals within the cloud tops. The analysis of retrieved effective radius and brightness temperature profiles also pointed to a strong updraft. Some of the radar images displayed bow echo features often associated with severe storms. Some radar cross-sections showed WER echoes. The total lightning rate was extremely high. The event was simulated using the MM5 non-hydrostatic numeric model, with two goals. First, we wanted to test the model's ability to predict the event. Second, we hoped to improve our knowledge of dynamic processes related to mesocyclogenesis and increased vorticity within this type of thunderstorms. An analysis of the vorticity equation terms indicate that at mid-tropospheric levels vorticity was generated mainly by the tilting of streamwise horizontal vorticity in the thunderstorm updraft. The ambient horizontal vorticity substantially increased near intense thunderstorm cells. At low levels, rotation intensified as a result of vertical acceleration and the stretching of the air column. Overall, the results suggest that in similar conditions (moderate values of CAPE and Storm-Relative Environmental Helicity of the ambient air), explicit numerical forecasts of thunderstorms can warn about the likely development of mesocyclones and other severe weather features such as rear inflow jets.

Storm-Relative Helicity Revealed from Polarimetric Radar Measurements

Abstract The dual-polarization radar variables are especially sensitive to the microphysical processes of melting and size sorting of precipitation particles. In deep convective storms, polarimetric measurements of such processes can provide information about the airflow in and around the storm that may be used to elucidate storm behavior and evolution. Size sorting mechanisms include differential sedimentation, vertical transport, strong rotation, and wind shear. In particular, winds that veer with increasing height typical of supercell environments cause size sorting that is manifested as an enhancement of differential reflectivity (ZDR) along the right or inflow edge of the forward-flank downdraft precipitation echo, which has been called the ZDR arc signature. In some cases, this shear profile can be augmented by the storm inflow. It is argued that the magnitude of this enhancement is related to the low-level storm-relative environmental helicity (SRH) in the storm inflow. To test this hypothesis, a simple numerical model is constructed that calculates trajectories for raindrops based on their individual sizes, which allows size sorting to occur. The modeling results indicate a strong positive correlation between the maximum ZDR in the arc signature and the low-level SRH, regardless of the initial drop size distribution aloft. Additional observational evidence in support of the conceptual model is presented. Potential changes in the ZDR arc signature as the supercell evolves and the low-level mesocyclone occludes are described.

A Nonlinear Extension of the Helicity Formula for Convective Storms.

There exists a firm relation between the rotational updraught (and/or downdraught) of a convective storm and the storm-relative environmental helicity. The already-known formula, which was derived with linear approximation and on a form-preserving assumption, describes the relation qualitatively well. In this note, a similar formula is derived without the linear approximation, although the formpreserving assumption is retained. The nonlinear terms generate the acceleration of propagation and/or the temporal variation of growth rate of the storm, which are absent from the linear formula. These nonlinear effects enter the derived formula.

Using short-range ensemble forecasts for predicting severe weather events

An ensemble of mesoscale model simulations of the most severe tornadic outbreak of the modern period, 24–25 June 1967, is examined. Results indicate that the fields of convective available potential energy (CAPE), storm-relative environmental helicity (SREH), and bulk Richardson number shear (BRNSHR), are able to provide useful guidance on the region of tornadic supercell thunderstorms for this outbreak. CAPE is used to define the region in which convection is possible, SREH is used to define the region in which thunderstorms are likely to be supercells, and BRNSHR is used to define the region in which low-level mesocyclogenesis is more likely. These results highlight the potential value of analyzing various severe weather parameters in forecasting tornadic thunderstorms, and the value of mesoscale ensemble forecasts for predicting severe, life threatening weather events in Europe. The ensemble also clearly shows the sensitivity and uncertainty of the model simulations to the physical process schemes used. Variations in all three severe weather parameters are significant near regions of convection, which unfortunately are where forecasters need these parameter values to be most reliable. These sensitivities highlight the value of an ensemble system for short-range weather forecasting, since it is clear that if a single model forecast of convective activity goes awry, then it provides little guidance to forecasters concerned with threats to life and property from severe weather.

The Use of Vertical Wind Shear versus Helicity in Interpreting Supercell Dynamics

A series of idealized simulations of supercell storms are presented for environments representing straight through circular hodographs to clarify the character of the storm dynamics over the large spectrum of hodograph shapes commonly observed. The primary emphasis is on comparing and contrasting recent theories of supercell dynamics, based on updraft–shear interactions, storm-relative environmental helicity (SREH), and Beltrami-flow solutions, to help clarify the degree to which each theory can represent the essential storm dynamics. One of the particular questions being addressed is whether storm dynamics are significantly different for straight versus curved hodographs, which has become a point of some controversy over recent years. In agreement with previous studies, the authors find that the physical processes that promote storm maintenance, rotation, and propagation are similar for all hodograph shapes employed, and are due primarily to nonlinear interactions between the updraft and the ambient shear, associated with the localized development of rotation on the storm’s flank. Significant correlations between the updraft and vertical vorticity are also observed across the shear spectrum, and, in agreement with predictions of linear theories associated with SREH, this correlation increases for increasing hodograph curvature. However, storm steadiness and propagation must already be known or inferred for such concepts to be applied, thus limiting the applicability of this theory as a true predictor of storm properties. Tests of the applicability of Beltrami solutions also confirm reasonable agreement for purely circular hodographs, for which the analytical solutions are specifically designed. However, analysis of the model results indicates that the terms ignored for such solutions, representing the nonlinear effects associated with storm rotation, are more significant than those retained over most of the hodograph spectrum, which severely limits the general applicability of such analyses.