Lagrangian Dynamics of European heat waves

Hitzewellen sind meteorologische Extremereignisse mit gesundheitlichen und sozioökonomischen Auswirkungen. In einem sich verändernden Klima ist zu erwarten, dass diese Ereignisse zunehmen werden. Modernste numerische Wettervorhersagemodelle sind in der Lage, das Auftreten von Hitzewellen vorherzusagen. Beginn, Dauer, Ende und Ausmaß der Ereignisse stellen jedoch nach wie vor eine Herausforderung für die Vorhersagemodelle dar und das grundlegende Verständnis der Entstehung und Aufrechterhaltung von Hitzewellen ist noch immer nicht vollständig. Daher wird in dieser Arbeit untersucht, wie sich hohe oberflächennahe Temperaturen während Hitzewellen und die damit verbundenen obertroposphärischen Zirkulationsmuster entwickeln. Darüber hinaus wird die Vorhersagbarkeit ausgewählter Hitzewellen untersucht. Die erste Fallstudie analysiert die spätsommerliche Hitzewelle über Europa im Jahr 2016. Mittel-, West- und Südwesteuropa sind in erster Linie von den hohen Temperaturen betroffen. Sevilla (Spanien) erlebt am 5. September 2016 mit 44,8°C die höchste jemals gemessene Temperatur im September und in Trier (Deutschland) erreichen die Temperaturen am 13. September 2016 34,2°C. Die Hitzewelle ist durch drei deutliche Spitzenwerte gekennzeichnet, begleitet von Rekordwerten der geopotentiellen Höhe in 500 hPa und, in geringerem Maße, der Temperatur in 850 hPa. Diese Spitzenwerte stehen im Zusammenhang mit der Ankunft von hochamplitudigen Rossby-Wellenpaketen in Westeuropa. Letztere entstehen einige Tage vor dem Ereignis über dem Westen Nordamerikas. Während der drei Peaks der Hitzewelle ist nicht die lokale Temperaturadvektion, sondern das Absinken und die daraus resultierende adiabatische Kompression in der freien Atmosphäre in Kombination mit Grenzschichtprozessen für das Auftreten der extremen Temperaturepisoden verantwortlich. Operationelle Ensemblevorhersagen zeigen in Bezug auf die Modellklimatologie die höchsten Wahrscheinlichkeiten für extreme Temperaturen in Trier, gefolgt von Sevilla und Bordeaux. Die Entwicklung hoher oberflächennaher Temperaturen während Hitzewellen wird für den Zeitraum von 1979 bis 2016 für verschiedene Klimazonen in Europa analysiert. Hitzewellen werden mit Hilfe eines auf einem Perzentil basierenden Index definiert und die Hauptprozesse, die entlang der Trajektorien quantifiziert werden, sind die adiabatische Kompression durch Absinken sowie lokale und entfernte diabatische Prozesse in der oberen und unteren Troposphäre. Diese Lagrangesche Analyse wird durch eine Euler\u27sche Berechnung der horizontalen Temperaturadvektion ergänzt. Während typischer Sommer in Europa treten ein oder zwei Hitzewellen mit einer durchschnittlichen Dauer von fünf Tagen auf. Während hohe oberflächennahe Temperaturen über Skandinavien von omega-ähnlichen Verteilungen der geopotentiellen Höhe in 500 hPa begleitet werden, sind Hitzewellen über dem Mittelmeer mit vergleichsweise flachen Rücken verbunden. Wenn die Luftmassen von den Hitzewellen rückwärts verfolgt werden, können drei Trajektoriencluster mit kohärenten thermodynamischen Eigenschaften, vertikalen Bewegungen und geographischen Ursprüngen identifiziert werden. In allen Regionen ist die horizontale Temperaturadvektion eher vernachlässigbar. In zwei der drei Cluster ist das Absinken in der freien Atmosphäre sehr wichtig, um hohe Temperaturen nahe der Oberfläche zu erzeugen, während sich die Luftmassen im dritten Cluster hauptsächlich aufgrund der diabatischen Erwärmung nahe der Oberfläche erwärmen. Große interregionale Unterschiede treten zwischen den Britischen Inseln und Westrussland auf. In der letztgenannten Region scheinen oberflächennaher Transport und diabatische Erwärmung sehr wichtig für die Bestimmung der Intensität der Hitzewellen zu sein, während für die Britischen Inseln Absinken und adiabatische Erwärmung von Bedeutung sind. Obwohl das großräumige Muster während der Hitzewellentage quasi-stationär ist, werden während des Lebenszyklus einer Hitzewelle ständig neue Luftmassen in die untere Troposphäre transportiert. Insgesamt bieten die Ergebnisse dieser Analyse einen Leitfaden, auf welche Prozesse und Diagnoseverfahren sich Wetter- und Klimastudien konzentrieren sollten, um die Schwere von Hitzewellen zu verstehen. Auf die klimatologische Analyse hoher Oberflächentemperaturen während Hitzewellen folgt eine Lagrangesche Analyse von obertroposphärischen Antizyklonen, die in verschiedenen europäischen Regionen im Zeitraum von 1979 bis 2016 mit bodennahen Hitzewellen in Verbindung stehen. Um die Bildung dieser Antizyklonen und die Rolle diabatischer Prozesse zu klären, werden Luftpakete rückwärts von den obertroposphärischen Antizyklonen verfolgt und das diabatische Heizen in diesen Luftpaketen quantifiziert. Etwa 25-45 % der Luftpakete werden in den letzten drei Tagen vor ihrer Ankunft in den obertroposphärischen Antizyklonen diabatisch geheizt, und dieser Anteil steigt in den letzten sieben Tagen auf 35-50 %. Der Einfluss des diabatischen Heizens ist bei hitzewellenbedingten Antizyklonen in Nordeuropa und Westrussland größer und in Südeuropa kleiner. Interessanterweise findet das diabatische Heizen in zwei geographisch getrennten Luftströmen statt. Drei Tage vor der Ankunft befindet sich ein diabatisch geheizter Luftstrom (entfernter Luftstrom) über dem westlichen Nordatlantik und der andere diabatisch geheizte Luftstrom (nahe gelegener Luftstrom) über Nordwestafrika/Europa südwestlich der obertroposphärischen Zielantizyklone. Das diabatische Heizen im entfernten Luftstrom steht im Zusammenhang mit warm conveyor belts in nordatlantischen Zyklonen stromaufwärts des sich entwickelnden obertroposphärischen Rückens. Im Gegensatz dazu wird der nahegelegene Luftstrom durch Konvektion diabatisch geheizt, was durch eine erhöhte konvektiv verfügbare potenzielle Energie entlang der Westseite der stärker ausgeprägten obertroposphärische Antizyklone deutlich wird. Die meisten europäischen Regionen werden von beiden Luftströmen beeinflusst, während Westrussland überwiegend vom nahe gelegenen Luftstrom betroffen ist. Der entfernte Luftstrom beeinflusst vorwiegend die Bildung der obertroposphärischen Antizyklone und damit der Hitzewelle, während der nahe Luftstrom während der Aufrechterhaltung der Antizyklone aktiver ist. Bei lang anhaltenden Hitzewellen regeneriert sich der entfernte Luftstrom wieder. Die Ergebnisse dieser Studie zeigen, dass die dynamischen Prozesse, die zu Hitzewellen führen, möglicherweise empfindlich auf kleinskalige mikrophysikalische und konvektive Prozesse reagieren, deren genaue Darstellung in Modellen daher für die Vorhersage von Hitzewellen auf Wetter- und Klimazeitskalen entscheidend sein dürfte. Die Arbeit schließt mit der Vorhersagbarkeit einer lang anhaltenden Hitzewelle, die vom 24. Juli bis zum 9. August 2018 weite Teile Mitteleuropas erfasste. Sowohl 3- als auch 7-tägige operationelle Vorhersagen unterschätzen oft die über das Gebiet der Hitzewelle gemittelten 2-m-Temperaturen. Fehler auf der 7-Tage-Zeitskala hängen mit der Dynamik in der oberen Troposphäre zusammen, wie eine konsistente Unterschätzung der geopotentiellen Höhe von 500 hPa zeigt. Allerdings sind die Fehler von 500 hPa geopotentieller Höhe bei 3-Tages-Vorhersagen erheblich reduziert, und für diese Vorlaufzeit hängen die Vorhersagefehler mit physikalischen Prozessen entlang der Trajektorien zusammen. In einem neuen und einzigartigen Ansatz, der auf einer Kombination von Trajektorien aus Reanalysen und Vorhersagen basiert, wird festgestellt, dass 2-m-Temperaturfehler von 3-Tages-Vorhersagen hauptsächlich auf diabatische Prozesse in der planetaren Grenzschicht zurückzuführen sind. Bei Vorhersagen, die die 2-m-Temperatur unterschätzen, wird die diabatische Erwärmung entlang von Trajektorien zwischen 12 und 18 UTC erheblich unterschätzt. Der Temperaturfehler am Ort der Hitzewelle ist auf die planetare Grenzschicht beschränkt und in der freien Atmosphäre deutlich reduziert

Processes determining heat waves across different European climates

This study presents a comprehensive analysis of processes determining heat waves across different climates in Europe for the period 1979–2016. Heat waves are defined using a percentile‐based index and the main processes quantified along trajectories are adiabatic compression by subsidence and local and remote diabatic processes in the upper and lower troposphere. This Lagrangian analysis is complemented by an Eulerian calculation of horizontal temperature advection. During typical summers in Europe, one or two heat waves occur, with an average duration of five days. Whereas high near‐surface temperatures over Scandinavia are accompanied by omega‐like blocking structures at 500 hPa, heat waves over the Mediterranean are connected to comparably flat ridges. Tracing air masses backwards from the heat waves, we identify three trajectory clusters with coherent thermodynamic characteristics, vertical motions, and geographic origins. In all regions, horizontal temperature advection is almost negligible. In two of the three clusters, subsidence in the free atmosphere is very important in establishing high temperatures near the surface, while the air masses in the third cluster are warmed primarily due to diabatic heating near the surface. Large interregional differences occur between the British Isles and western Russia. Over the latter region, near‐surface transport and diabatic heating appear to be very important in determining the intensity of the heat waves, whereas subsidence and adiabatic warming are of first‐order importance for the British Isles. Although the large‐scale pattern is quasistationary during heat wave days, new air masses are entrained steadily into the lower troposphere during the life cycle of a heat wave. Overall, the results of the present study provide a guideline as to which processes and diagnostics weather and climate studies should focus on to understand the severity of heat waves

Application of an object-based verification method to ensemble forecasts of 10‑m wind gusts during winter storms

The object-based method SAL (Structure, Amplitude and Location) was adapted for investigating the errors of forecasts of extreme 10‑m wind gusts associated with winter storms in Germany. It has been applied to a statistically downscaled version of the 51 member ECMWF (European Centre for Medium Range Weather Forecasts) operational ensemble forecast. The horizontal resolution of both downscaled data and of the German weather service's operational analysis data used for verification is 7 km. Forecast errors are subdivided in terms of storm intensity, location and extent. After identifying a set of storm events, objects of moderate and intense 10‑m wind gusts were identified with a local percentile-based threshold (90th percentile for moderate and 98th percentile for intense gust objects). Depending on the intensity of the storm, the gust objects differ in terms of size, shape and intensity. The characteristics of the ensemble forecasts of 10‑m wind gusts can basically be assessed in two different ways. Individual forecast members can be evaluated with respect to the location, intensity and extent of the gust field, and then address the ensemble characteristics by the score distributions. Alternatively, the gust fields' location, intensity and extent can be evaluated by directly using the ensemble mean forecast instead of the individual members. The results of the identified set of storms clearly indicate a high case-to-case variability in the predictability of 10‑m wind gusts objects, particularly when focusing on the structure of intense wind gust objects. It is found, that the gust fields' location and overall intensity can be better estimated from the ensemble mean forecast, compared to the individual forecast members. From a forecaster's perspective this means, that a storms' location and intensity can be well estimated by considering the ensemble mean wind forecasts. Considering the structure of the gust objects, results are different. While for longer lead times, there also seems to be a benefit from applying ensemble averaging, at short lead times the ensemble mean forecast performs equally or worse than most of the individual forecast members. The amplitude error is often the smallest component of the three error types. The findings are particularly relevant when deriving warning information, by giving guidance to forecasters when interpreting ensemble forecasts for severe storms

Large-scale Rossby wave and synoptic-scale dynamic analyses of the unusually late 2016 heatwave over Europe

This paper analyses the late summer heatwave over Europe in 2016. Central, western and southwestern Europe were primarily affected by the high temperatures. Seville, Spain, for example, experienced the highest September temperature on record on 5 September 2016, reaching a maximum of 44.8°C, and temperatures in Trier, Germany reached 34.2°C on 13 September 2016. The heatwave was marked by three distinct peaks, accompanied by record‐breaking values for 500hPa geopotential heights and, to a lesser extent, 850hPa temperatures. These peaks were associated with the arrival of high‐amplitude Rossby wave packets in western Europe. The latter originated several days before the event over western North America. During the three peaks of the heatwave, subsidence and the ensuing adiabatic compression in the free atmosphere in combination with boundary layer processes, rather than local temperature advection, were instrumental in the occurrence of the extreme temperature episodes

A Lagrangian analysis of upper-tropospheric anticyclones associated with heat waves in Europe

This study presents a Lagrangian analysis of upper-tropospheric anticyclones that are connected to surface heat waves in different European regions for the period 1979 to 2016. In order to elucidate the formation of these anticyclones and the role of diabatic processes, we trace air parcels backwards from the upper-tropospheric anticyclones and quantify the diabatic heating in these air parcels. Around 25 %–45 % of the air parcels are diabatically heated during the last 3 d prior to their arrival in the upper-tropospheric anticyclones, and this amount increases to 35 %–50 % for the last 7 d. The influence of diabatic heating is larger for heat-wave-related anticyclones in northern Europe and western Russia and smaller in southern Europe. Interestingly, the diabatic heating occurs in two geographically separated air streams; 3 d prior to arrival, one heating branch (remote branch) is located above the western North Atlantic, and the other heating branch (nearby branch) is located over northwestern Africa and Europe to the southwest of the target upper-tropospheric anticyclone. The diabatic heating in the remote branch is related to warm conveyor belts in North Atlantic cyclones upstream of the evolving upper-level ridge. In contrast, the nearby branch is diabatically heated by convection, as indicated by elevated mixed-layer convective available potential energy along the western side of the matured upper-level ridge. Most European regions are influenced by both branches, whereas western Russia is predominantly affected by the nearby branch. The remote branch predominantly affects the formation of the upper-tropospheric anticyclone, and therefore of the heat wave, whereas the nearby branch is more active during its maintenance. For long-lasting heat waves, the remote branch regenerates. The results from this study show that the dynamical processes leading to heat waves may be sensitive to small-scale microphysical and convective processes, whose accurate representation in models is thus supposed to be crucial for heat wave predictions on weather and climate timescales

The mysterious long-range transport of giant mineral dust particles

Giant mineral dust particles (>75 µm diameter) found far from their source have puzzled scientists for a long time. These wind-blown particles impact the atmosphere’s radiation balance, clouds and the ocean carbon cycle but are generally ignored in models. Here we report new observations of individual giant Saharan dust particles of up to 450 µm in diameter sampled in air over the Atlantic Ocean at 2,400 and 3,500 km from the west African coast. Past research points to fast horizontal transport, turbulence, uplift in convective systems and electrical levitation of particles as possible explanations for this fascinating phenomenon. We present a critical assessment of these mechanisms using order-of-magnitude estimates and backward trajectories. Results show that established concepts are merely able to explain our new observations. Therefore we propose several lines of research we deem promising to further advance our understanding and modelling

How intense daily precipitation depends on temperature and the occurrence of specific weather systems – an investigation with ERA5 reanalyses in the extratropical Northern Hemisphere

Precipitation and surface temperature are two of the most important variables that describe our weather and climate. Several previous studies investigated aspects of their relationship, for instance the climatological dependence of daily precipitation on daily mean temperature, P(T). However, the role of specific weather systems in shaping this relationship has not been analysed yet. This study therefore identifies the weather systems (WSs) that are associated with intense precipitation days as a function of T, focusing on the question of how this relationship, symbolically expressed as P(T, WS), varies regionally across the Northern Hemisphere and between seasons. To this end, we first quantify if intense precipitation occurs on climatologically warmer or on colder days. In winter, over most continental and ocean regions, intense precipitation falls on warmer days apart from the Mediterranean area and regions in the lee of the Rocky Mountains, where intense precipitation is favoured on colder days. In summer, only at high latitudes is intense precipitation favoured on warmer days, whereas continental areas experience intense precipitation on colder days. For selected regions in Europe and North America, we then identify the weather systems that occur preferentially on days with intense precipitation (referred to as wet days). In winter, cyclones are slightly dominant on colder wet days, whereas warm conveyor belts and atmospheric rivers occur preferentially on warmer wet days. In summer, the overall influence of atmospheric rivers increases, and the occurrence of weather systems depends less on wet day temperature. Wet days in the lee of the Rocky Mountains are influenced by most likely convective systems in anticyclones. Finally, we investigate P(T, WS) during the wettest and driest season in central Europe and the central United States (US). In qualitative agreement with the results from the first part of this study, the wettest winter is warmer than normal in central Europe but colder in the central US, and the wettest summer is colder in both regions. The opposite holds for the driest winter and summer, respectively. During these anomalous seasons, both the frequency and the precipitation efficiency of weather systems change in central Europe, while the wettest and driest seasons in central US mainly arise from a modified precipitation efficiency. Our results show that the precipitation–temperature–weather system relationship strongly depends on the region and that (extreme) seasonal precipitation is influenced by the frequency and precipitation efficiency of the different weather systems. This regional variability is reflected in the relative importance of weather system frequency and efficiency anomalies for the formation of anomalously wet and dry seasons. Interestingly, in some regions and seasons, the precipitation efficiency of weather systems is increased during anomalously cold seasons.ISSN:2698-4016ISSN:2698-400

A Lagrangian analysis of upper-tropospheric anticyclones associated with heat waves in Europe

This study presents a Lagrangian analysis of upper-tropospheric anticyclones that are connected to surface heat waves in different European regions for the period 1979 to 2016. In order to elucidate the formation of these anticyclones and the role of diabatic processes, we trace air parcels backwards from the upper-tropospheric anticyclones and quantify the diabatic heating in these air parcels. Around 25 %–45 % of the air parcels are diabatically heated during the last 3 d prior to their arrival in the upper-tropospheric anticyclones, and this amount increases to 35 %–50 % for the last 7 d. The influence of diabatic heating is larger for heat-wave-related anticyclones in northern Europe and western Russia and smaller in southern Europe. Interestingly, the diabatic heating occurs in two geographically separated air streams; 3 d prior to arrival, one heating branch (remote branch) is located above the western North Atlantic, and the other heating branch (nearby branch) is located over northwestern Africa and Europe to the southwest of the target upper-tropospheric anticyclone. The diabatic heating in the remote branch is related to warm conveyor belts in North Atlantic cyclones upstream of the evolving upper-level ridge. In contrast, the nearby branch is diabatically heated by convection, as indicated by elevated mixed-layer convective available potential energy along the western side of the matured upper-level ridge. Most European regions are influenced by both branches, whereas western Russia is predominantly affected by the nearby branch. The remote branch predominantly affects the formation of the upper-tropospheric anticyclone, and therefore of the heat wave, whereas the nearby branch is more active during its maintenance. For long-lasting heat waves, the remote branch regenerates. The results from this study show that the dynamical processes leading to heat waves may be sensitive to small-scale microphysical and convective processes, whose accurate representation in models is thus supposed to be crucial for heat wave predictions on weather and climate timescales.ISSN:2698-4016ISSN:2698-400

Processes determining heat waves across different European climates

This study presents a comprehensive analysis of processes determining heat waves across different climates in Europe for the period 1979–2016. Heat waves are defined using a percentile‐based index and the main processes quantified along trajectories are adiabatic compression by subsidence and local and remote diabatic processes in the upper and lower troposphere. This Lagrangian analysis is complemented by an Eulerian calculation of horizontal temperature advection. During typical summers in Europe, one or two heat waves occur, with an average duration of five days. Whereas high near‐surface temperatures over Scandinavia are accompanied by omega‐like blocking structures at 500 hPa, heat waves over the Mediterranean are connected to comparably flat ridges. Tracing air masses backwards from the heat waves, we identify three trajectory clusters with coherent thermodynamic characteristics, vertical motions, and geographic origins. In all regions, horizontal temperature advection is almost negligible. In two of the three clusters, subsidence in the free atmosphere is very important in establishing high temperatures near the surface, while the air masses in the third cluster are warmed primarily due to diabatic heating near the surface. Large interregional differences occur between the British Isles and western Russia. Over the latter region, near‐surface transport and diabatic heating appear to be very important in determining the intensity of the heat waves, whereas subsidence and adiabatic warming are of first‐order importance for the British Isles. Although the large‐scale pattern is quasistationary during heat wave days, new air masses are entrained steadily into the lower troposphere during the life cycle of a heat wave. Overall, the results of the present study provide a guideline as to which processes and diagnostics weather and climate studies should focus on to understand the severity of heat waves.ISSN:0035-9009ISSN:1477-870

Dynamics of concurrent and sequential Central European and Scandinavian heatwaves

In both 2003 and 2018 a heatwave in Scandinavia in July was followed by a heatwave in Central Europe in August. Whereas the transition occurred abruptly in 2003, it was gradual in 2018 with a 12-day period of concurrent heatwaves in both regions. This study contrasts these two events in the context of a heatwave climatology to elucidate the dynamics of both concurrent and sequential heatwaves. Central European and, in particular, concurrent heatwaves are climatologically associated with weak pressure gradient (WPG) events over Central Europe, which indicate the absence of synoptic activity over this region. One synoptic pattern associated with such events is Scandinavian blocking. This pattern is at the same time conducive to heatwaves in Scandinavia, thereby providing a mechanism by which Scandinavian and Central European heatwaves can co-occur. Further, the association of WPG events with Scandinavian blocking constitutes a mechanism that allows heatwaves to grow beyond the perimeter of the synoptic system from which they emanated. A trajectory analysis of the source regions of the low-level air incorporated in the heatwaves indicates rapidly changing air mass sources throughout the heatwaves in both regions, but no recycling of heat from one heatwave to the other. This finding is line with a composite analysis indicating that transitions between Scandinavian and Central European heatwaves are merely a random coincidence of heatwave onset and decay