This study documents atmospheric conditions, development, and evolution of a severe weather outbreak that occurred on 11 August 2017 in Poland. The emphasis is on analyzing system morphology and highlighting the importance of a mesovortex in producing the most significant wind damages. A derecho-producing mesoscale convective system (MCS) had a remarkable intensity and was one of the most impactful convective storms in the history of Poland. It destroyed and partially damaged 79 700 ha of forest (9.8 million m3 of wood), 6 people lost their lives, and 58 were injured. The MCS developed in an environment of high 0–3-km wind shear (20–25 m s−1), strong 0–3-km storm relative helicity (200–600 m2 s−2), moderate most-unstable convective available potential energy (1000–2500 J kg−1), and high precipitable water (40–46 mm). Within the support of a midtropospheric jet, the MCS moved northeast with a simultaneous northeastward inflow of warm and very moist air, which contributed to strong downdrafts. A mesocyclone embedded in the convective line induced the rear inflow jet (RIJ) to descend and develop a bow echo. In the mature stage, a supercell evolved into a bookend vortex and later into a mesoscale convective vortex. Swaths of the most significant wind damage followed the aforementioned vortex features. A high-resolution simulation performed with initial conditions derived from GFS and ECMWF global models predicted the possibility of a linear MCS with widespread damaging wind gusts and embedded supercells. Simulations highlighted the importance of cloud cover in the preconvective environment, which influenced the placement and propagation of the resulting MCS.

In this study a 10-year (2008-2017) radar-based mesoscale convective system (MCS) and derecho climatology for Poland is presented. This is one of the first attempt for a European country to investigate morphological and precipitation archetypes of MCSs as prior studies were mostly based on satellite data. Despite its ubiquity and significance for society, economy, agriculture and water availability, little is known about climatological aspects of MCSs over Central Europe. Our results indicate that MCSs are not rare in Poland as an annual mean of 77 MCSs and 49 days with MCS can be depicted for Poland. Their lifetime ranges typically from 3 to 6 hours with initiation time around afternoon hours (12-14 UTC) and dissipation stage in the evening (19-20 UTC). Most frequent morphological type of MCSs is a broken line (58% of cases), then areal / cluster (25%) and quasi-linear convective system (QLCS; 17%), usually associated with a bow-echo (72% of QLCS). QLCS feature also with the longest lifecycle. Among precipitation archetypes of linear MCSs trailing stratiform (73%) and parallel stratiform (25%) are the most common. MCSs are usually observed from April to September with a peak in mid-July. A majority of MCSs travels from W, SW and S sectors. A total of 16 derecho events were identified (1.5% of all MCS and 9.1% of all QLCS), the majority of them were produced by a warm-season QLCS, while only 4 by a cold season narrow cold-front rainbands. Warm season derechos produced a bigger impact compared to cold season events, even though their damage paths were shorter.

<p>In this work, we use 8 years of OPERA radar data, ESWD severe weather reports, and ATDnet lightning detection data to create a climatology of quasi-linear convective systems (QLCS) across Europe. In the first step, 15-minute composite animations of radar reflectivity and lightning data were used to manually identify 2201 QLCS polygons in the period of 2014 to 2021. Severe weather reports, lightning data, and morphological properties (such as the presence of bowing segments) allowed classifying QLCSs according to their intensity into 1844 marginal, 304 moderate, and 53 derecho cases. The manual evaluation also allowed us to identify: the basic archetype of the system, the location of stratiform precipitation relative to the active convective part, propagation with respect to the mean wind, areal coverage, width, length, accompanying hazards, and social impacts associated with each QLCS. Results indicate that QLCSs are the most frequent during summer in Central Europe, while in southern Europe the season of their occurrence is extended to late autumn. QLCSs are the least frequent during winter when they appear mostly in northwestern Europe in the form of narrow cold frontal rainbands (NCFR). In spring they are most common across western Europe. The vast majority of systems move from southwest and west. 22% of the systems were associated with a bow echo signature and 6% produced a mesoscale convective vortex. Among precipitation modes, trailing (53%) and embedded (45%) stratiform types were the most common. The longest classified QLCS had a length of 2200 km (9 Aug 2018), while the widest reached 1635 km (10 Jan 2015). The most frequent hazard accompanying QLCSs was lightning (produced during 95% of the total QLCSs lifetime), followed by severe winds gusts (7.7%), excessive precipitation (5.3%), large hail (2.7%), and tornadoes (0.4%). Derechos had the largest coverage of severe wind reports with respect to their path area (49%), while back-building QLCS had the largest coverage of excessive precipitation events (12%). Large hail was most common with bow-echo complexes (6.5%).</p>

<p>Each year supercell storms in Europe are responsible for significant property damage and cause injury and death to people. Storms that have a deep persistent rotating updraft are capable of generating particularly violent phenomena - flash floods, large hail and strong wind gusts of convective origin. Supercells are also responsible for producing the strongest tornadoes with intensity of even F4-F5 in Fujita scale as evidenced over the recent decades across Europe, including Poland. Despite significant hazards posed by these types of storms, no research on climatological aspects of supercell thunderstorms in Poland has been carried out so far.</p> <p>The goal of this work was to study spatial and temporal characteristics of supercell thunderstorms in Poland between 2008 and 2022. In order to accomplish this task, a vector-tabular database of supercell thunderstorms over Poland was created, based on a manual analysis of 10-minute interval radar data accompanied by severe weather reports from the European Severe Weather Database (ESWD). The typical radar-derived signatures of supercells (e.g. bounded weak echo region, velocity couplet, hook echo) and/or long, continuous paths of high radar reflectivity with deviant motion were one of the main identification criterias. Identified supercells were classified into 3 groups, based on the confidence of their detection from plausible events to those producing significant severe weather. Manual evaluation of 15 years of radar and ESWD data allowed to analyze in the climatological context supercells track widths and lengths, storm duration, spatiotemporal frequency, accompanying hazards and characteristics such as right- or left-moving movement propagation. Moreover, ERA5 reanalysis was used to study accompanying atmospheric environments of identified supercells. An addition of lightning data from the PERUN network enabled also to evaluate non-supercell storm environments to show differences with supercells.</p>

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