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Abstract
The first tornado climatology for Romania is presented based on datasets attained from three periods between 1822 and 2013. The historical period (1822–1944) contains 33 tornado reports originating from historical newspaper archives and publications of the Romanian Meteorological Institute. Evidence of tornado observations in Romania before the nineteenth century is found in the representation of tornadoes in the Romania folk mythology. The socialist period (1945–89) contains only seven tornado reports, likely because during this period it was believed that tornadoes did not occur in Romania. The recent period (1990–2013) contains 89 tornado reports that came from mass-media sources and eyewitness reports. Of the 129 tornadoes from the Romanian tornado database, 98 were reported between May and July with a peak in May (36 reports). Most of the tornadoes (28 reports) occurred during the afternoon hours 1500–1659 local time. Tornadoes were more frequently reported over eastern Romania compared with other regions of the country, with a maximum over southeastern Romania [0.37–0.45 (105 km2)−1 yr−1].
Abstract
The first tornado climatology for Romania is presented based on datasets attained from three periods between 1822 and 2013. The historical period (1822–1944) contains 33 tornado reports originating from historical newspaper archives and publications of the Romanian Meteorological Institute. Evidence of tornado observations in Romania before the nineteenth century is found in the representation of tornadoes in the Romania folk mythology. The socialist period (1945–89) contains only seven tornado reports, likely because during this period it was believed that tornadoes did not occur in Romania. The recent period (1990–2013) contains 89 tornado reports that came from mass-media sources and eyewitness reports. Of the 129 tornadoes from the Romanian tornado database, 98 were reported between May and July with a peak in May (36 reports). Most of the tornadoes (28 reports) occurred during the afternoon hours 1500–1659 local time. Tornadoes were more frequently reported over eastern Romania compared with other regions of the country, with a maximum over southeastern Romania [0.37–0.45 (105 km2)−1 yr−1].
Abstract
The first study of the characteristics of cloud-to-ground (CG) lightning in Romania, based on the data recorded by the Romanian National Lightning Detection Network (RNLDN), is presented. The data, more than 1.75 million CG flashes, covers the entirety of Romania and were recorded between January 2003 and December 2005 and January and December 2007. The spatial analyses (total and positive flash density, the percentage of positive flashes, and negative and positive peak currents) were done with a resolution of 20 km. The average spatial distribution shows a maximum (3.06 flashes km−2 yr−1) over the south slopes of the central meridional Carpathians possibly associated with the Romanian Plain convergence zone. The mean monthly variation shows maximum CG lightning between May and September (98%) and minimum values in December and January. High values (>0.028 km−2 yr−1) for positive CG lightning density are observed in southwestern and central Romania. The monthly distribution of positive flashes shows a main maximum in May (25%) and a secondary maximum in August (23%), suggesting that positive flashes tend to occur earlier in the year than total flashes. The mean annual percentage of positive flashes has lower values at 1.3% in the central parts of the country. The percentage of positive CG flashes changes over the year from 1% in June to 19% in January. The monthly variation of the median first-strike peak currents has a maximum in winter and reaches a minimum in July, for both negative and positive currents. The mean diurnal cycle for total CG lightning flashes peaks between 1230 and 1430 UTC (2.2%) and shows a minimum between 0600 and 0800 UTC (0.3%).
Abstract
The first study of the characteristics of cloud-to-ground (CG) lightning in Romania, based on the data recorded by the Romanian National Lightning Detection Network (RNLDN), is presented. The data, more than 1.75 million CG flashes, covers the entirety of Romania and were recorded between January 2003 and December 2005 and January and December 2007. The spatial analyses (total and positive flash density, the percentage of positive flashes, and negative and positive peak currents) were done with a resolution of 20 km. The average spatial distribution shows a maximum (3.06 flashes km−2 yr−1) over the south slopes of the central meridional Carpathians possibly associated with the Romanian Plain convergence zone. The mean monthly variation shows maximum CG lightning between May and September (98%) and minimum values in December and January. High values (>0.028 km−2 yr−1) for positive CG lightning density are observed in southwestern and central Romania. The monthly distribution of positive flashes shows a main maximum in May (25%) and a secondary maximum in August (23%), suggesting that positive flashes tend to occur earlier in the year than total flashes. The mean annual percentage of positive flashes has lower values at 1.3% in the central parts of the country. The percentage of positive CG flashes changes over the year from 1% in June to 19% in January. The monthly variation of the median first-strike peak currents has a maximum in winter and reaches a minimum in July, for both negative and positive currents. The mean diurnal cycle for total CG lightning flashes peaks between 1230 and 1430 UTC (2.2%) and shows a minimum between 0600 and 0800 UTC (0.3%).
Abstract
Lightning-related fatalities in Romania are analyzed and presented for the first time using data from the Romanian National Institute of Statistics. The database contains 724 lightning fatalities that occurred between 1999 and 2015 in Romania, corresponding to an average of 42.6 fatalities per year. The annual number of lightning fatalities decreased from 65 fatalities per year between 1999 and 2003 to 23.2 fatalities per year between 2011 and 2015. The majority of fatalities occurred in May–August (42% of all fatalities) with a peak in June (31%) and July (28%). The highest fatality rates (>2.6 fatalities per million inhabitants per year) are observed over southwestern Romania, a region characterized by high values of cloud-to-ground lightning density (>2 flashes per square kilometer per year) and by a relatively high percentage (>40%) of the population living in rural areas. The majority of fatalities (78%) were reported in rural areas. Approximately 78% of the victims were male. The most vulnerable group was males between the ages of 10–39 living in rural areas. To further reduce the lightning fatality rate in Romania, currently one of the highest in Europe, the authors argue that lightning mitigation activities and information campaigns about the risks associated with lightning should be initiated in Romania.
Abstract
Lightning-related fatalities in Romania are analyzed and presented for the first time using data from the Romanian National Institute of Statistics. The database contains 724 lightning fatalities that occurred between 1999 and 2015 in Romania, corresponding to an average of 42.6 fatalities per year. The annual number of lightning fatalities decreased from 65 fatalities per year between 1999 and 2003 to 23.2 fatalities per year between 2011 and 2015. The majority of fatalities occurred in May–August (42% of all fatalities) with a peak in June (31%) and July (28%). The highest fatality rates (>2.6 fatalities per million inhabitants per year) are observed over southwestern Romania, a region characterized by high values of cloud-to-ground lightning density (>2 flashes per square kilometer per year) and by a relatively high percentage (>40%) of the population living in rural areas. The majority of fatalities (78%) were reported in rural areas. Approximately 78% of the victims were male. The most vulnerable group was males between the ages of 10–39 living in rural areas. To further reduce the lightning fatality rate in Romania, currently one of the highest in Europe, the authors argue that lightning mitigation activities and information campaigns about the risks associated with lightning should be initiated in Romania.
Abstract
According to the Norwegian cyclone model, whether a warm-type or cold-type occluded front forms depends upon which cold air mass is colder: the prewarm-frontal air mass or the postcold-frontal air mass. For example, a cold-type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm-frontal air mass is warmer than the postcold-frontal air mass. This temperature difference and the resulting occluded-frontal structure in the Norwegian cyclone model is part of what is called the temperature rule. Paradoxically, no clear example of a rearward-sloping, cold-type occluded front has been found in the literature, even though the required temperature difference has been documented in several cases. This article presents the first documented, rearward-sloping, cold-type occluded front. This occluded front forms in a cyclone over the North Atlantic Ocean on 3–5 January 2003 and is documented in model output from the European Centre for Medium-Range Weather Forecasts. Cross sections through the evolving cyclone show the occluded front forms as the less statically stable warm-frontal zone ascends over the more stable cold-frontal zone. Such a stability difference between the cold- and warm-frontal zones is consistent with a previously published hypothesis that the less stable air is lifted by the more stable air to form occluded fronts, in disagreement with the temperature rule. Because warm-frontal zones and the cold air underneath tend to be more stable than cold-frontal zones and the postcold-frontal air, warm-type occluded fronts are much more common than cold-type occluded fronts, explaining why well-defined, rearward-sloping, cold-type occluded fronts are not common in the meteorological literature.
Abstract
According to the Norwegian cyclone model, whether a warm-type or cold-type occluded front forms depends upon which cold air mass is colder: the prewarm-frontal air mass or the postcold-frontal air mass. For example, a cold-type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm-frontal air mass is warmer than the postcold-frontal air mass. This temperature difference and the resulting occluded-frontal structure in the Norwegian cyclone model is part of what is called the temperature rule. Paradoxically, no clear example of a rearward-sloping, cold-type occluded front has been found in the literature, even though the required temperature difference has been documented in several cases. This article presents the first documented, rearward-sloping, cold-type occluded front. This occluded front forms in a cyclone over the North Atlantic Ocean on 3–5 January 2003 and is documented in model output from the European Centre for Medium-Range Weather Forecasts. Cross sections through the evolving cyclone show the occluded front forms as the less statically stable warm-frontal zone ascends over the more stable cold-frontal zone. Such a stability difference between the cold- and warm-frontal zones is consistent with a previously published hypothesis that the less stable air is lifted by the more stable air to form occluded fronts, in disagreement with the temperature rule. Because warm-frontal zones and the cold air underneath tend to be more stable than cold-frontal zones and the postcold-frontal air, warm-type occluded fronts are much more common than cold-type occluded fronts, explaining why well-defined, rearward-sloping, cold-type occluded fronts are not common in the meteorological literature.
Abstract
A five-year (2006–10) radar-based climatology of tropopause folds and convective storms was constructed for Wales, United Kingdom, to determine how deep, moist convection is modulated by tropopause folds. Based on the continuous, high-resolution data from a very high frequency (VHF) wind-profiling radar located at Capel Dewi, Wales, 183 tropopause folds were identified. Tropopause folds were most frequent in January with a secondary maximum in July. Based on data from the U.K. weather radar network, a climatology of 685 convective storms was developed. The occurrence of convective storms was relatively high year-round except for an abrupt minimum in February–April. Multicellular lines (43.5%) were the most common morphology with a maximum in October, followed by isolated cells (33.1%) with a maximum in May–September, and nonlinear clusters (23.4%) with a maximum in November–January. Convective storms were associated with 104 (56.8%) of the tropopause folds identified in this study, with the association strongest in December. Of the 55 tropopause folds observed on the eastern side of an upper-level trough, 37 (67.3%) were associated with convective storms, most commonly in the form of multicellular lines. Of the 128 tropopause folds observed on the western side of an upper-level trough, 42 (32.8%) were associated with convective storms, most commonly isolated cells. These results suggest that more organized storms tend to form in environments favorable for synoptic-scale ascent.
Abstract
A five-year (2006–10) radar-based climatology of tropopause folds and convective storms was constructed for Wales, United Kingdom, to determine how deep, moist convection is modulated by tropopause folds. Based on the continuous, high-resolution data from a very high frequency (VHF) wind-profiling radar located at Capel Dewi, Wales, 183 tropopause folds were identified. Tropopause folds were most frequent in January with a secondary maximum in July. Based on data from the U.K. weather radar network, a climatology of 685 convective storms was developed. The occurrence of convective storms was relatively high year-round except for an abrupt minimum in February–April. Multicellular lines (43.5%) were the most common morphology with a maximum in October, followed by isolated cells (33.1%) with a maximum in May–September, and nonlinear clusters (23.4%) with a maximum in November–January. Convective storms were associated with 104 (56.8%) of the tropopause folds identified in this study, with the association strongest in December. Of the 55 tropopause folds observed on the eastern side of an upper-level trough, 37 (67.3%) were associated with convective storms, most commonly in the form of multicellular lines. Of the 128 tropopause folds observed on the western side of an upper-level trough, 42 (32.8%) were associated with convective storms, most commonly isolated cells. These results suggest that more organized storms tend to form in environments favorable for synoptic-scale ascent.
Abstract
The tornado outbreak of 24–25 June 1967 was the most damaging in the history of western Europe, producing 7 F2–F5 tornadoes, 232 injuries, and 15 fatalities across France, Belgium, and the Netherlands. Following tornadoes in France on 24 June, the Royal Netherlands Meteorological Institute (KNMI) issued a tornado forecast for 25 June, which became the first ever—and first verified—tornado forecast in Europe. Fifty-two years later, tornadoes are still not usually forecast by most European national meteorological services, and a pan-European counterpart to the NOAA/NWS/Storm Prediction Center (SPC) does not exist to provide convective outlook guidance; yet, tornadoes remain an extant threat. This article asks, “What would a modern-day forecast of the 24–25 June 1967 outbreak look like?” To answer this question, a model simulation of the event is used in three ways: 20-km grid-spacing output to produce a SPC-style convective outlook provided by the European Storm Forecast Experiment (ESTOFEX), 800-m grid-spacing output to analyze simulated reflectivity and surface winds in a nowcasting analog, and 800-m grid-spacing output to produce storm-total footprints of updraft helicity maxima to compare to observed tornado tracks. The model simulates a large supercell on 24 June and weaker embedded mesocyclones on 25 June forming along a stationary front, allowing the ESTOFEX outlooks to correctly identify the threat. Updraft helicity footprints indicate multiple mesocyclones on both days within 40–50 km and 3–4 h of observed tornado tracks, demonstrating the ability to hindcast a large European tornado outbreak.
Abstract
The tornado outbreak of 24–25 June 1967 was the most damaging in the history of western Europe, producing 7 F2–F5 tornadoes, 232 injuries, and 15 fatalities across France, Belgium, and the Netherlands. Following tornadoes in France on 24 June, the Royal Netherlands Meteorological Institute (KNMI) issued a tornado forecast for 25 June, which became the first ever—and first verified—tornado forecast in Europe. Fifty-two years later, tornadoes are still not usually forecast by most European national meteorological services, and a pan-European counterpart to the NOAA/NWS/Storm Prediction Center (SPC) does not exist to provide convective outlook guidance; yet, tornadoes remain an extant threat. This article asks, “What would a modern-day forecast of the 24–25 June 1967 outbreak look like?” To answer this question, a model simulation of the event is used in three ways: 20-km grid-spacing output to produce a SPC-style convective outlook provided by the European Storm Forecast Experiment (ESTOFEX), 800-m grid-spacing output to analyze simulated reflectivity and surface winds in a nowcasting analog, and 800-m grid-spacing output to produce storm-total footprints of updraft helicity maxima to compare to observed tornado tracks. The model simulates a large supercell on 24 June and weaker embedded mesocyclones on 25 June forming along a stationary front, allowing the ESTOFEX outlooks to correctly identify the threat. Updraft helicity footprints indicate multiple mesocyclones on both days within 40–50 km and 3–4 h of observed tornado tracks, demonstrating the ability to hindcast a large European tornado outbreak.
Abstract
A synthesis of tornado observations across Europe between 1800 and 2014 is used to produce a pan-European climatology. Based on regional tornado-occurrence datasets and articles published in peer-reviewed journals, the evolution and the major contributions to tornado databases for 30 European countries were analyzed. Between 1800 and 2014, 9563 tornadoes were reported in Europe with an increase from 8 tornadoes per year between 1800 and 1850 to 242 tornadoes per year between 2000 and 2014. The majority of the reports came from northern, western, and southern Europe, and to a lesser extent from eastern Europe where tornado databases were developed after the 1990s. Tornadoes occur throughout the year with a maximum in June–August for most of Europe and in August–November for southern Europe. Tornadoes occur more frequently between 1300 and 1500 UTC over most of Europe and between 0900 and 1100 UTC over southern Europe. Where intensity was known, 74.7% of tornadoes were classified as F0 and F1, 24.5% as F2 and F3, and 0.8% as F4 and F5. Comparing this intensity distribution over Europe with the intensity distribution for tornadoes in the United States shows that tornadoes over western and eastern Europe are more likely to be supercellular tornadoes and those over northern and southern Europe are likely to also include nonsupercellular tornadoes.
Abstract
A synthesis of tornado observations across Europe between 1800 and 2014 is used to produce a pan-European climatology. Based on regional tornado-occurrence datasets and articles published in peer-reviewed journals, the evolution and the major contributions to tornado databases for 30 European countries were analyzed. Between 1800 and 2014, 9563 tornadoes were reported in Europe with an increase from 8 tornadoes per year between 1800 and 1850 to 242 tornadoes per year between 2000 and 2014. The majority of the reports came from northern, western, and southern Europe, and to a lesser extent from eastern Europe where tornado databases were developed after the 1990s. Tornadoes occur throughout the year with a maximum in June–August for most of Europe and in August–November for southern Europe. Tornadoes occur more frequently between 1300 and 1500 UTC over most of Europe and between 0900 and 1100 UTC over southern Europe. Where intensity was known, 74.7% of tornadoes were classified as F0 and F1, 24.5% as F2 and F3, and 0.8% as F4 and F5. Comparing this intensity distribution over Europe with the intensity distribution for tornadoes in the United States shows that tornadoes over western and eastern Europe are more likely to be supercellular tornadoes and those over northern and southern Europe are likely to also include nonsupercellular tornadoes.
Abstract
During 2–3 December 2012, the Black Sea and east coast of Romania were affected by a rapidly deepening Mediterranean cyclone. The cyclone developed a bent-back front along which short-lived (2–4 h) strong winds up to 38 m s−1 were recorded equatorward of the cyclone center. A mesoscale model simulation was used to analyze the evolution of the wind field, to investigate the physical processes that were responsible for the strong winds and their acceleration, and to investigate the relative importance of the stability of the boundary layer to those strong winds. The origin of the air in the wind maximum equatorward of the cyclone center was twofold. The first was associated with a sting jet, a descending airstream from the midlevels of the cloud head and the lower part of the cyclonic branch of the warm conveyor belt. The sting jet started to descend west of the cyclone center, ending at the frontolytic tip of the bent-back front. The second was a low-level airstream associated with the cold conveyor belt that originated northeast of the cyclone center and traveled below 900 hPa along the cold side of the bent-back front, ending behind the cold front. Both airstreams were accelerated by the along-flow pressure gradient force, with the largest accelerations acting on the sting-jet air before entering into the near-surface strong-wind area. The sensible heat fluxes destabilized the boundary layer to near-neutral conditions south of the cyclone center, facilitating downward mixing and allowing the descending air to reach the surface. Mesoscale instabilities appeared to be unimportant in the sting-jet formation.
Abstract
During 2–3 December 2012, the Black Sea and east coast of Romania were affected by a rapidly deepening Mediterranean cyclone. The cyclone developed a bent-back front along which short-lived (2–4 h) strong winds up to 38 m s−1 were recorded equatorward of the cyclone center. A mesoscale model simulation was used to analyze the evolution of the wind field, to investigate the physical processes that were responsible for the strong winds and their acceleration, and to investigate the relative importance of the stability of the boundary layer to those strong winds. The origin of the air in the wind maximum equatorward of the cyclone center was twofold. The first was associated with a sting jet, a descending airstream from the midlevels of the cloud head and the lower part of the cyclonic branch of the warm conveyor belt. The sting jet started to descend west of the cyclone center, ending at the frontolytic tip of the bent-back front. The second was a low-level airstream associated with the cold conveyor belt that originated northeast of the cyclone center and traveled below 900 hPa along the cold side of the bent-back front, ending behind the cold front. Both airstreams were accelerated by the along-flow pressure gradient force, with the largest accelerations acting on the sting-jet air before entering into the near-surface strong-wind area. The sensible heat fluxes destabilized the boundary layer to near-neutral conditions south of the cyclone center, facilitating downward mixing and allowing the descending air to reach the surface. Mesoscale instabilities appeared to be unimportant in the sting-jet formation.
Abstract
Deep convection frequently occurs on the eastern side of upper-level troughs, or potential vorticity (PV) anomalies. This is consistent with uplift ahead of a cyclonic PV anomaly, and consequent reduction in static stability and increase of convective available potential energy (CAPE). Nevertheless, the causal link between upper-level PV and deep convection has not been proven, and given that lift, moisture, and instability must all be present for deep convection to occur it is not clear that upper-level forcing is sufficient. In this paper a convective rainband that intensified ahead of a cyclonic PV anomaly in an environment with little CAPE (~10 J kg−1) is examined to determine the factors responsible for its intensification. The key feature was a low-level convergence line, arising from the remnants of an occluded front embedded in the low-level cyclonic flow. The rainband’s intensity and morphology was influenced by the remnants of a tropopause fold that capped convection at midlevels in the southern part of the band, and by a reduction in upper-level static stability in the northern part of the band that allowed the convection to reach the tropopause. Ascent ahead of the trough appears to have played only a minor role in conditioning the atmosphere to convection: in most cases the ascending airstream had previously descended in the flow west of the trough axis. Thus, simple “PV thinking” is not capable of describing the development of the rainband, and it is concluded that preexisting low-level wind and humidity features played the dominant role.
Abstract
Deep convection frequently occurs on the eastern side of upper-level troughs, or potential vorticity (PV) anomalies. This is consistent with uplift ahead of a cyclonic PV anomaly, and consequent reduction in static stability and increase of convective available potential energy (CAPE). Nevertheless, the causal link between upper-level PV and deep convection has not been proven, and given that lift, moisture, and instability must all be present for deep convection to occur it is not clear that upper-level forcing is sufficient. In this paper a convective rainband that intensified ahead of a cyclonic PV anomaly in an environment with little CAPE (~10 J kg−1) is examined to determine the factors responsible for its intensification. The key feature was a low-level convergence line, arising from the remnants of an occluded front embedded in the low-level cyclonic flow. The rainband’s intensity and morphology was influenced by the remnants of a tropopause fold that capped convection at midlevels in the southern part of the band, and by a reduction in upper-level static stability in the northern part of the band that allowed the convection to reach the tropopause. Ascent ahead of the trough appears to have played only a minor role in conditioning the atmosphere to convection: in most cases the ascending airstream had previously descended in the flow west of the trough axis. Thus, simple “PV thinking” is not capable of describing the development of the rainband, and it is concluded that preexisting low-level wind and humidity features played the dominant role.
Abstract
Alfred Wegener (1880–1930) was a leading geophysicist, atmospheric scientist, and an Arctic explorer who is mainly remembered today for his contributions to the theory of continental drift. Less well known are his contributions to research on tornadoes in Europe. Published 100 years ago, book Wind- und Wasserhosen in Europa (Tornadoes and Waterspouts in Europe) is an impressive synthesis of knowledge on tornadoes and is considered the first modern pan-European tornado climatology, with 258 reports from 1456 to 1913. Unfortunately, Wegener’s book was overlooked after the 1950s amid declining interest in tornadoes by European researchers and meteorologists. The recent revival of tornado studies in Europe invites a reflection on Wegener’s book. Using a relatively small dataset, Wegener was able to describe characteristics of tornadoes (e.g., direction of movement, speed, rotation, formation mechanism), as well as their frequency of occurrence and climatology, comparable with the results from modern tornado climatologies. Wegener’s lasting scientific contributions to tornado research are presented in the context of European research on this topic. Specifically, his book showed the utility of reports from citizen scientists and inspired other researchers, namely, Johannes Letzmann, who continued to study European tornadoes after Wegener’s death.
Abstract
Alfred Wegener (1880–1930) was a leading geophysicist, atmospheric scientist, and an Arctic explorer who is mainly remembered today for his contributions to the theory of continental drift. Less well known are his contributions to research on tornadoes in Europe. Published 100 years ago, book Wind- und Wasserhosen in Europa (Tornadoes and Waterspouts in Europe) is an impressive synthesis of knowledge on tornadoes and is considered the first modern pan-European tornado climatology, with 258 reports from 1456 to 1913. Unfortunately, Wegener’s book was overlooked after the 1950s amid declining interest in tornadoes by European researchers and meteorologists. The recent revival of tornado studies in Europe invites a reflection on Wegener’s book. Using a relatively small dataset, Wegener was able to describe characteristics of tornadoes (e.g., direction of movement, speed, rotation, formation mechanism), as well as their frequency of occurrence and climatology, comparable with the results from modern tornado climatologies. Wegener’s lasting scientific contributions to tornado research are presented in the context of European research on this topic. Specifically, his book showed the utility of reports from citizen scientists and inspired other researchers, namely, Johannes Letzmann, who continued to study European tornadoes after Wegener’s death.
