Session 1

Though tornadoes commonly form in quasi-linear convective systems (QLCSs), predicting their development remains a challenge, due to their rapid formation, relatively short lifetimes as compared to supercell tornadoes, and the likelihood of formation along any segment of the line. Rapid changes in near-surface vertical vorticity associated with QLCS tornadogenesis can result from environmental heterogeneities, including those associated with mountainous landscapes. Mountains can induce variations in stability and vertical wind shear, for example, due to changes in elevation and the configuration of the valleys. Such heterogeneities can influence the evolution of QLCS structures, and thus vertical vorticity. Sloping mountain surfaces can also induce variations in cold pool characteristics and advancement which may further alter storm structural evolutions. Potential systematic linkages between QLCS vertical vorticity and mountains may offer hope toward improved QLCS tornado prediction.

This study leverages idealized numerical simulations to quantify the potential role of mountainous landscapes in the development and evolution of QLCS vertical vorticity. Two different base-state environments are tested, including a profile observed during a QLCS-tornadic event and an analytic profile with several different vertical wind shear configurations. For each base-state environment, we evaluate the sensitivity of QLCS vertical vorticity evolution to a suite of simplified mountain configurations, including a plateau with either a downward or upward sloping surface, each with or without a localized valley of different widths within the sloping surface. Preliminary results indicate that as QLCSs move over a mountain slope with a valley, line echo wave patterns form, which have been associated with QLCS tornado damage. Thus, terrain may be important for the development of QLCS tornadoes through the modification of the linear structure by the underlying irregular surface landscape. Localized valleys also favor CI and the development of intense convective cells, which may increase tornado potential within and near the valley.

How to cite: Lombardo, K., Wu, F., and Fenrich, A.: How Mountains Alter Quasi-linear Convective System Dynamics and Tornado Potential, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-13, https://doi.org/10.5194/ecss2023-13, 2023.

Why is the Po Valley a hot spot in Europe for tornadoes? In this study the authors propose an explanation to this issue, studying a tornado outbreak that affected the Po Valley on 19 September 2021. During that event seven tornadoes (four of them ranked as F2 according to the Fujita scale) developed between Lombardia and Emilia-Romagna regions in a few hours. Although tornadoes are not rare in Italy, so many tornadoes in such a short time are an unusual event. 

The case study was analysed by means of observations and numerical simulations obtained with the convection permitting MOLOCH model. Observations showed that during the event there were two low-level boundaries in the Po Valley: a cold front coming from the Alps and a dry line generated by the downslope winds from the Apennines. These two boundaries created a triple point, like those observed during tornado outbreaks in the US MidWest, but on a smaller scale. Observations proved a strong correlation between tornado developments and low-level boundaries.

Numerical simulations with 500 m grid spacing showed that a warm and moist air tongue from the Adriatic Sea played a fundamental role in generating the supercells, causing an advection of vorticity and favouring instability conditions. Moreover, through numerical experiments, it has been proved that this moist air tongue was sensitive to the Froude number of the south-westerly flow from the Apennines: the greater the Froude number, the further north and narrower was the tongue of air, with impacts on the development of supercells. Along the cold front large amounts of streamwise vorticity were generated by the buoyancy gradient. Furthermore, the dry line played a key role in the generation of tornadoes, creating locally large amounts of instability and strong wind veering near the surface: kinematic and windshear parameters were comparable to those observed in US-tornado events only along a narrow path near the dry line. 

Comparing these results with previous papers, the presence of thermal boundaries and dry lines represents a typical pattern during tornado-events in the region. Then, in conclusion, a conceptual model also useful for forecasting applications is proposed for the development of tornadoes in the Po Valley, which explains why tornadoes are relatively common in Northern Italy.

How to cite: De Martin, F., Davolio, S., Miglietta, M. M., and Levizzani, V.: A conceptual model for the development of tornado in the Po Valley, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-17, https://doi.org/10.5194/ecss2023-17, 2023.

In this presentation, we will recap the historical use of storm-relative helicity in supercell environments, share updated climatioglical distributions from both the United States and Europe, and use simulations of supercells (described below) to explain why near-ground streamwise horizontal vorticity within the environment is such a powerful tool for forecasting supercell tornadogenesis across multiple continents.

Low-level mesocyclones in supercell thunderstorms has historically been associated with the development of storm-generated streamwise vorticity along a baroclinic gradient in the forward flank of supercells. However, the ambient streamwise vorticity of the environment (often quantified via storm-relative helicity), especially near the ground, is particularly skillful at discriminating between nontornadic and tornadic supercells. This study investigates whether the origins of the inflow air into supercell low-level mesocyclones, both horizontally and vertically, can help explain the dynamical role of environmental versus storm-generated vorticity in low-level mesocyclone intensification. Simulations of supercells, initialized with wind profiles common to supercell environments observed in nature, show that the air bound for the low-level mesocyclone primarily originates from the undisturbed, ambient environment, rather than from along the forward flank, and from very close to the ground, often in the lowest 200 - 400 m of the atmosphere. Given that the near-ground environmental air comprises the bulk of the inflow into low-level mesocyclones, this likely explains the forecast skill of environmental streamwise vorticity in the lowest few hundred meters of the atmosphere. Contrary to prior conceptual models of low-level mesocyclones, intensification does not appear to require the development of additional horizontal vorticity in the forward flank. Instead, the dominant contributor to vertical vorticity within the low-level mesocyclone is from the environmental horizontal vorticity. This study therefore supports a revised view of low-level mesocyclones in supercells.

How to cite: Coffer, B., Parker, M., Peters, J., and Wade, A.: Supercell low-level mesocyclones: Origins of inflow and vorticity, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-14, https://doi.org/10.5194/ecss2023-14, 2023.

Nine tornado outbreaks (days with three or more tornadoes) have occurred in the United Kingdom from quasi-linear convective systems in the 16 years between 2004 and 2019. Of the nine outbreaks, eight can be classified into two archetypes: type 1 and type 2. Simulations of each archetype were performed to determine vortex genesis mechanisms. In a type 1 event, a vortex sheet broke down into near-equally spaced misovortices, whereas in a type 2 event, disorganized, elongated cyclonic–anticyclonic couplets evolved into a small number of misovortices. The assessment of Rayleigh's and Fjörtoft's instability criteria, along with misovortices having a wavelength of about 7.5 times the width of the shear zone, implied horizontal shearing instability (HSI) was the initial mechanism for the amplification of perturbations along the vortex sheet in the type 1 event. Vorticity-tendency analysis also revealed near-equally spaced localized horizontal advection maxima prior to the growth of perturbations in the type 1 event, further reinforcing HSI as a plausible mechanism. Deformation was not strong enough to suppress vortex growth. In contrast, the type 2 event did not satisfy either criteria. Parcels acquired vertical vorticity differently dependent on the location and the height that they entered the misovortex. In the type 1 event, parcels entering the misovortex at lower heights experienced varying magnitudes of stretching and tilting, whereas tilting was considerably stronger at greater heights. In the type 2 event, parcels entering the misovortex from the leading edge of the misovortex acquired the majority of their vertical vorticity via both tilting and stretching, and parcels entering the misovortex from the trailing edge acquired the majority of their vertical vorticity via stretching. These results suggest two possible mechanisms for misovortex genesis in UK tornado outbreaks: type 1 where HSI initiates the vortices and type 2 that form from vorticity couplets along the front not formed from HSI.

How to cite: J. Buckingham, T., M. Schultz, D., M. Markowski, P., and J. Trapp, R.: Two Archetypes of Tornadic Quasilinear Convective Systems in the United Kingdom: Vortex Genesis and Maintenance, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-100, https://doi.org/10.5194/ecss2023-100, 2023.

Squall lines are mesoscale lines consisting of numerous convective cells with an along-line extent of at least 100 km and a smaller across-line extent on the order of tenths of kilometers. Typically, squall lines form in a sheared environment by interactions of convective cells that are near to each other. Over time, the convective cells form a continuous cold pool that drives the motion of the squall line.

Initiating single convective cells in a sheared environment leads to the production of vertical vorticity due to tilting. Hence, these cells are connected to positive and negative vertical vorticity patches. While a squall line forms a band of positive and negative vorticity patches along the line, single cells will form a vorticity dipole.

In this work, we investigate the interaction between squall lines and convective cells initiated in the vicinity of the squall line. This interaction can be understood as an interaction between different vortices. We study different configurations using (1) idealized simulations with the non-hydrostatic, convection-permitting Cloud Model 1 (CM1) and (2) a theoretical point vortex model representative of the vorticity ensembles.

Squall line simulations with CM1 are initially run for 3 hours until a mature squall line forms. After 3 hours, convective cells are initiated at different positions near the squall line. The impact of the convection on the squall line is studied by an analysis of the squall line intensity and motion. In addition, the setup is studied using a point vortex model of a comparable vortex ensemble. A point vortex model is an idealized, mathematical model that describes vortex dynamics of a two-dimensional, non-divergent, inviscid atmosphere. Despite these strong limitations, it can be used to understand vortex dynamics and interactions in a relatively simple, but descriptive way. Moreover, the model describes well the vortex dynamics on the larger, synoptic scale. For example, a vortex couple of zero total circulation with the positive vortex south of the negative one starts to move towards the west which counteracts the typical westerly flow of the midlatitudes and, hence, leads to a slowed-down motion of the couplet compared to the westerlies.

The general aim of this work is to better understand the interaction between single cells and convective lines. The results will contribute towards improved squall line forecasting.

How to cite: Schielicke, L., Pacey, G., Gatzen, C., and Pfahl, S.: Idealized study of the interaction between squall lines and convective cells, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-108, https://doi.org/10.5194/ecss2023-108, 2023.

While convection commonly initiates along synoptic-scale cold fronts during the European warm-season, rarely does convection initiate along the entirety of the boundary. This indicates smaller-scale processes, e.g., solar heating, outflow boundaries and topography, are likely responsible for the spatial and temporal onset of convective initiation. Interactions between the frontal circulation and such smaller-scale processes are not currently well-understood. Our cold-frontal convective cell climatology highlighted convective cells are most frequent marginally ahead of the surface front. On the other hand, pre-surface-frontal cells have the largest fraction associated with mesocyclones, intense convective cores and lightning. Pre-surface-frontal convergence lines, which have been observed to be linked to the western edge of elevated mixed layer plumes, could play a role in these larger cell intensity fractions.

Here we train a logistic regression model for cold-frontal convective cells based on thermodynamic and dynamic variables derived from ERA5 and assess its performance based on the area under the ROC curve (AUC). The logistic regression model exhibits high skill (AUC~0.90) but the model performance is not consistent across the cold-frontal environment. The highest model skill is for cells in the drier and cooler post-frontal airmass, whereas the lowest is for cells near the surface front. This highlights the complexity of smaller-scale processes favouring the development of convective cells near to the surface front. To further understand the underlying processes and multi-scale interactions responsible for convection initiation and cell evolution in cold-frontal environments we analyse case studies of interest in the Central European domain. We will also use these case studies to study up-scale feedbacks on the frontal structure caused by convective cells. This is an important step towards a more comprehensive understanding of scale interactions between cold fronts and convection.

How to cite: Pacey, G., Pfahl, S., and Schielicke, L.: Towards a deeper understanding of the scale interactions between cold fronts and convection during the European warm-season, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-122, https://doi.org/10.5194/ecss2023-122, 2023.

Our knowledge of tornadogenesis, evolution, and structure has advanced substantially with the increase in computing resources and the availability of fine temporal- and spatial- scale observations.  Even so, many questions/hypotheses remain observationally unconfirmed/untested.  These include: the role of secondary rear-flank downdraft surges in the tornado genesis/maintenance process, the mechanisms of the generation of horizontal vorticity (ωh) and its transformation into tornado-strength near-surface vertical vorticity (ζ), how the evolution of the low-level mesocyclone relates to tornadogenesis, evolution and dissipation, the variability of tornado wind speeds with height and time, and how tornado intensity, propagation speed and structure, including variations in wind with height and time, affect human-impacting damage.  The degree to which the answers to these questions vary depending on the strength, size, structure, and persistence/duration of tornadoes is unknown.  

Answering these critical questions relating to the evolution of ζ, the role of downdrafts, updrafts, and baroclinic zones depends on the diagnosis of kinematically important quantities involving vector wind fields such as Div(V), vertical velocity (w), ωh and ζ, and tilting/stretching of ωh and ζ.  In tandem with DOW radar deployments, surface weather stations (Pods) have been utilized in many DOW tornado missions for well over a decade, obtaining not only surface wind data, but surface thermodynamic data, as well.  These integrated observations have been collected over a wide range of tornado structures and at various times in tornado evolution.  Results will be presented from analyses of multiple cases with both low-level DOW radar and surface thermodynamic data.  Preliminary observations suggest that there is a distinct and localized change in the near-field thermodynamics during tornado evolution over short time and spatial scales, which suggest changes in storm processes.     

Plans for the proposed 2023-2024 BEST (Boundary-layer Evolution and Structure of Tornadoes) project, focusing on characterizing the thermodynamic and kinematic structure of tornadoes below 100 m AGL, especially below 50 m AGL, using proximate DOW radars, PODNet, and Driftersondes from the University of Illinois Flexible Array of Radars and Mesonets (FARM) will be presented.

How to cite: Kosiba, K. and Wurman, J.: Tornado Thermodynamics Observations and the Boundary-layer Evolution and Structure of Tornadoes (BEST) Study, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-111, https://doi.org/10.5194/ecss2023-111, 2023.

How to cite: Feldmann, M., Rotunno, R., Germann, U., and Berne, A.: Supercell thunderstorms in complex topography - how lakes in mountain valleys can increase occurrence frequency, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-51, https://doi.org/10.5194/ecss2023-51, 2023.

Convective available potential energy (CAPE), when considered by itself, is not a skillful discriminator of tornadic supercells from their nontornadic counterparts.  However, there is a longstanding notion that large CAPE may allow strong-to-violent tornadoes to occur in environments with otherwise weak vertical wind shear.  There is also anecdotal evidence that the largest tornadoes often occur on days with moderate-to-extreme CAPE.  These notions and anecdotes prompt us to ask whether large CAPE is conditionally supportive for significant tornado formation?

We use a matrix of large-eddy simulations of supercells to address this question, wherein the vertical wind shear and the magnitude and vertical distribution of CAPE are independently varied.  Our analysis of these simulations identifies two influences of CAPE on storm evolution that potentially facilitate both tornadogenesis, and the formation of large and intense tornadoes.  Large CAPE generally fostered more negatively buoyant and expansive cold pools.  In environments with strong shear and low-level storm-relative flow, a more negatively buoyant rear-flank downdraft amid large CAPE fortified the rear-flank convergence zone beneath the updraft, leading to a more efficient projection of initially horizontal streamwise vorticity into the vertical.  This led to more intense mesocyclones and tornadoes, when compared to simulations with the same wind profile but smaller CAPE.  In environments with weak vertical wind shear and large CAPE, storms were more resistant to the negative effects of entrainment and thereby more prone to sustaining mesocyclones and producing tornadoes, when compared to environments with both weak shear and CAPE.  We argue that these processes explain past anecdotes about the role of CAPE in tornadogenesis.

How to cite: Peters, J. M.: Does big CAPE equate to big tornado potential in supercell thunderstorms?, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-94, https://doi.org/10.5194/ecss2023-94, 2023.

An accurate assessment of atmospheric water vapour is useful for the nowcasting of thunderstorms and heavy precipitation. Meteosat Third Generation (MTG) will offer new possibilities with the Flexible Combined Imager (FCI) channel at 0.91 µm for low level moisture and geostationary sounding using the Infra-Red Sounder (IRS), in addition to the "classic" water vapour imaging channels at 6.3 µm and 7.3 µm. Water vapour profiles offer better utility for nowcasting than integrated or single-level products. Profiles allow for the calculation of stability parameters such as Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN). They are regularly derived from radiosondes, satellite sounding, and numerical weather prediction (NWP). However, radiosonde measurements are expensive and sparse; satellite soundings have a limited vertical resolution and are relatively poor in the boundary layer; and forecasts have inherent uncertainties compared to measurements.

This study looks into the value of total precipitable water (TPW) measurements from ground-based Global Navigational Satellite Systems (GNSS) in combination with satellite soundings and images. Central Europe has a dense network of GNSS stations with hourly measurements. Although GNSS cannot measure profiles, TPW measurements (calculated from bending angles) are highly accurate. When the NWP forecast sounding disagrees with the satellite sounding and no radiosonde sounding is available, GNSS TPW may help to assess which profile is more consistent with TPW and may be more accurate for CAPE or CIN. In addition, the study looks into the added value of the reflectance at 0.91 µm. Note that this study is still in its early stages and any results presented are highly preliminary. While waiting for FCI and IRS measurements, OLCI and IASI are used as proxies.

How to cite: Holl, G.: Can GNSS help satellite measurements for thunderstorm nowcasting?, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-65, https://doi.org/10.5194/ecss2023-65, 2023.

Hail is by far the greatest contributor to insured losses from severe convective storms on an annual basis. Individual severe convective storm outbreaks can cause hail losses well above EUR 1 bn. On 19-22 June 2022 a series of such events impacted France, and in particular the large metropolitan region of Ile-de-France. There were many reports of large hailstones, causing significant damage to property and motor vehicle. Total insured hail loss estimates in France alone exceeded EUR 2.4 bn, of which EUR 1.34 bn of property loss and EUR 1.08 bn of motor vehicle loss. These were the largest hail events in France in terms of losses since Storm Ela’s, which on 9-10 June 2014 resulted in insured hail losses in excess of 900 mn in 2021 EUR.

Common denominator to these two impactful events were persistent meteorological situations conducive to large-scale severe convective storms for several consecutive days. These compounded with local conditions favorable for the development of severe hail. Maximum hailstone sizes of 12 cm in diameter were observed in the administrative regions of Centre-Val-De-Loire (Ela) and Occitanie (June 2022). In this study we present a reconstruction of these events based on eye-witness event reports from ESWD and a local provider cross-referenced with weather radar data. We analyze the synoptic configurations and pre-convective environments that characterized them, with focus on those properties and features that are peculiar to severe hail-forming thunderstorms. We discuss in particular different formulations of CAPE and vertical wind shear, as well as the performance of the Significant Hail Parameter (SHiP) as predictor for severe hail occurrence. These event reconstructions are part of our effort to construct a Realistic Disaster Scenario (RDS) model for France and Belgium to stress test both individual client portfolios and the market as a whole.

How to cite: Panosetti, D., Allen, C., Obaidullah, Y., and Thollon, O.: Severe hail in France: reconstruction of Storm Ela’s and June 2022 hailstorms, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-69, https://doi.org/10.5194/ecss2023-69, 2023.

The Knowledge of the development of the severe thunderstorms is crucial for improvement of weather forecasts which are provided to air traffic control (ATC) units. Intense thunderstorms, strong winds and hazards such as severe turbulence and icing, lightning activity, microbursts, and hail are very important for the aviation safety.

The present work focuses on the study case of atmospheric conditions preceding and during the development of unusual severe convection over southeast Bulgaria on 17^th September 2022. when the squall line passed through the airport of Burgas bringing a lot of damage to the airport and surrounding towns.

On this days the weather was dominated by the presence of a very unstable air mass over Southeast Bulgaria, ahead of the atmospheric frontal zone. As convection continued its development, it moved east/northeast forward Burgas with overshooting cloud top height up to 14 km, cloud top temperature of -70C and maximum radar reflectivity of 60 dBz. In the late afternoon the process led to formation of gust front that reached the Burgas airport with the record for the past 50 years wind speed exceeding 45 m/s that damaged the mast of the airport's instrument landing system. At the same time heavy rain and intense lighting activity are reported. For analysis of the severe weather conditions are used weather satellite, radar and lightning data, Mode-S aircraft wind data and surface observations. In addition, information provided by the ground based Global Navigation Satellite System (GNSS) network is used. Integrated water vapor shows a sharp increase several hours before the convection developed. As moisture is one of the main ingredients for intense thunderstorms formation, GNSS tropospheric products are used together with other surface data to help the process of deep convection monitoring and forecasting.

How to cite: Kostashki, B., Penchev, R., and Guerova, G.: Severe convection event in southeast of Bulgaria on 17th September 2022, 11th European Conference on Severe Storms, Bucharest, Romania, 8–12 May 2023, ECSS2023-40, https://doi.org/10.5194/ecss2023-40, 2023.