Rossby Wave Packets and their Role in Temperature Extremes

Eastward propagating Rossby wave packets (RWPs) are a dominant feature of the mid-latitude circulation. They are reflected in the upper-tropospheric meridional wind field as a longitudinally-confined group of northerlies and southerlies. Their documented relevance to weather extremes necessitates the development of diagnostic methods that can identify and investigate their properties locally in space and time. The overarching goal of this work is to develop such diagnostics, investigate the local properties of RWPs and quantify their role in temperature extremes. The diagnostic methods are based on the analytic signals of the filtered upper-tropospheric meridional wind and its envelope function along latitude circles. RWP properties like the local amplitude, wavelength, phase velocity, and group velocity collectively provide the main characteristics of the upper-tropospheric flow in the mid-latitudes at any instance in time, but also reveal its distinct climatological patterns. Based on them, the role of RWPs in the occurrence and duration of temperature extremes is investigated. In particular, regression analyses show that the probability for a temperature extreme in many parts of the Northern Hemisphere increases significantly with RWP amplitude; a linkage that cannot be revealed so clearly when employing non-local metrics like the Fourier amplitudes of meridional wind. The role of RWPs as large-scale upstream precursors is further emphasized in an investigation of Southeastern European hot and cold extremes. Identifying and following the spatiotemporal evolution of RWPs also proves beneficial in exploring the lifetime of the 2003 and 2010 heat waves in Western Europe and Russia respectively. In doing so, it is shown that a single one or several successive non-circumglobal RWPs can create the large-scale environment where temperature anomalies can amplify and --- in combination with physical processes of smaller scale --- lead to extreme events. During such cases of persistent temperature extremes the role of below-normal RWP phase velocity is found to be critical. The combined effect of RWP amplitude and phase velocity in the occurrence and duration of temperature extremes is quantified using a sufficiently large sample of short-lived and persistent events in 40 years of reanalysis data (1979--2018). Global climatologies of local RWP amplitude, phase velocity and group velocity are produced for the first time and reveal the major differences in the upper-tropospheric circulation of the Northern and Southern Hemispheres. Moreover, important aspects of the seasonal variability in particular regions are discussed. Finally, an investigation of medium-range forecast biases of the Northern Hemisphere RWP properties using 6 years of ECMWF operational forecasts (2013--2018) suggests possible implications for the practical predictability of temperature extremes occurrence and duration. Overall, this work contributes to the overarching goal of improving our understanding of RWPs and temperature extremes. The aforementioned findings and the novel diagnostics will be beneficial for future research on the processes that affect the RWP evolution and its implications to the occurrence and predictability of extremes at both the weather and climate time scales

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

Global climatologies of Eulerian and Lagrangian flow features based on ERA-Interim

This paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zürich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first application, the very unusual Northern Hemispheric winter of 2009/10 is identified as the season when most of the considered features show maximum deviations from climatology. The second application considers dry winters in the western United States and reveals fairly localized anomalies in the eastern North Pacific of enhanced blocking and surface anticyclones and reduced cyclones

The extremely hot and dry 2018 summer in central and northern Europe from a multi-faceted weather and climate perspective

The summer of 2018 was an extraordinary season in climatological terms for northern and central Europe, bringing simultaneous, widespread, and concurrent heat and drought extremes in large parts of the continent with extensive impacts on agriculture, forests, water supply, and the socio-economic sector. Here, we present a comprehensive, multi-faceted analysis of the 2018 extreme summer in terms of heat and drought in central and northern Europe, with a particular focus on Germany. The heatwave first affected Scandinavia in mid-July and shifted towards central Europe in late July, while Iberia was primarily affected in early August. The atmospheric circulation was characterized by strongly positive blocking anomalies over Europe, in combination with a positive summer North Atlantic Oscillation and a double jet stream configuration before the initiation of the heatwave. In terms of possible precursors common to previous European heatwaves, the Eurasian double-jet structure and a tripolar sea surface temperature anomaly over the North Atlantic were already identified in spring. While in the early stages over Scandinavia the air masses at mid and upper levels were often of a remote, maritime origin, at later stages over Iberia the air masses primarily had a local-to-regional origin. The drought affected Germany the most, starting with warmer than average conditions in spring, associated with enhanced latent heat release that initiated a severe depletion of soil moisture. During summer, a continued precipitation deficit exacerbated the problem, leading to hydrological and agricultural drought. A probabilistic attribution assessment of the heatwave in Germany showed that such events of prolonged heat have become more likely due to anthropogenic global warming. Regarding future projections, an extreme summer such as that of 2018 is expected to occur every 2 out of 3 years in Europe in a +1.5 ∘C warmer world and virtually every single year in a +2 ∘C warmer world. With such large-scale and impactful extreme events becoming more frequent and intense under anthropogenic climate change, comprehensive and multi-faceted studies like the one presented here quantify the multitude of their effects and provide valuable information as a basis for adaptation and mitigation strategies

Current and emerging developments in subseasonal to decadal prediction

Weather and climate variations of subseasonal to decadal timescales can have enormous social, economic and environmental impacts, making skillful predictions on these timescales a valuable tool for decision makers. As such, there is a growing interest in the scientific, operational and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) timescales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) timescales, while the focus remains broadly similar (e.g., on precipitation, surface and upper ocean temperatures and their effects on the probabilities of high-impact meteorological events), understanding the roles of internal and externally-forced variability such as anthropogenic warming in forecasts also becomes important. The S2S and S2D communities share common scientific and technical challenges. These include forecast initialization and ensemble generation; initialization shock and drift; understanding the onset of model systematic errors; bias correct, calibration and forecast quality assessment; model resolution; atmosphere-ocean coupling; sources and expectations for predictability; and linking research, operational forecasting, and end user needs. In September 2018 a coordinated pair of international conferences, framed by the above challenges, was organized jointly by the World Climate Research Programme (WCRP) and the World Weather Research Prograame (WWRP). These conferences surveyed the state of S2S and S2D prediction, ongoing research, and future needs, providing an ideal basis for synthesizing current and emerging developments in these areas that promise to enhance future operational services. This article provides such a synthesis

Κυματοπακέτα Rossby και ο ρόλος τους στα επεισόδια ακραίων θερμοκρασιών

Eastward propagating Rossby wave packets (RWPs) are a dominant feature of the midlatitude circulation. They are reflected in the upper-tropospheric meridional wind field as a longitudinally-confined group of northerlies and southerlies. Their documented relevance to weather extremes necessitates the development of diagnostic methods that can identify and investigate their properties locally in space and time. The overarching goal of this work is to develop such diagnostics, investigate the local properties of RWPs and quantify their role in temperature extremes.The diagnostic methods are based on the analytic signals of the filtered upper-tropospheric meridional wind and its envelope function along latitude circles. RWP properties like the local amplitude, wavelength, phase velocity, and group velocity collectively provide the main characteristics of the upper-tropospheric flow in the midlatitudes at any instance in time, but also reveal its distinct climatological patterns. Based on them, the role of RWPs in the occurrence and duration of temperature extremes is investigated. In particular, regression analyses show that the probability for a temperature extreme in many parts of the Northern Hemisphere increases significantly with RWP amplitude; a linkage that cannot be revealed so clearly when employing non-local metrics like the Fourier amplitudes of meridional wind. The role of RWPs as large-scale upstream precursors is further emphasized in an investigation of Southeastern European hot and cold extremes.Identifying and following the spatiotemporal evolution of RWPs also proves beneficial in exploring the lifetime of the 2003 and 2010 heat waves in Western Europe and Russia respectively. In doing so, it is shown that a single one or several successive non-circumglobal RWPs can create the large-scale environment where temperature anomalies can amplify and — in combination with physical processes of smaller scale — lead to extreme events. During such cases of persistent temperature extremes the role of below-normal RWP phase velocity is found to be critical. The combined effect of RWP amplitude and phase velocity in the occurrence and duration of temperature extremes is quantified using a sufficiently large sample of short-lived and persistent events in 40 years of reanalysis data (1979–2018).Global climatologies of local RWP amplitude, phase velocity and group velocity are produced for the first time and reveal the major differences in the upper-tropospheric circulation of the Northern and Southern Hemispheres. Moreover, important aspects of the seasonal variability in particular regions are discussed. Finally, an investigation of medium-range forecast biases of the Northern Hemisphere RWP properties using 6 years of ECMWF operational forecasts (2013–2018) suggests possible implications for the practical predictability of temperature extremes occurrence and duration.Overall, this work contributes to the overarching goal of improving our understanding of RWPs and temperature extremes. The aforementioned findings and the novel diagnostics will be beneficial for future research on the processes that affect the RWP evolution and its implications to the occurrence and predictability of extremes at both the weather and climate time scales.Η διάδοση κυματοπακέτων Rossby (RWP) προς την ανατολή αποτελεί κυρίαρχο χαρακτηριστικό της ατμοσφαιρικής κυκλοφορίας στα μέσα γεωγραφικά πλάτη. Τα RWP αντικατοπτρίζονται στο πεδίο του μεσημβρινού ανέμου της ανώτερης τροπόσφαιρας ως μια περιορισμένου μήκους αλληλουχία βόρειων και νότιων ανέμων. Η παρατηρηθείσα σχέση των RWP με ακραία καιρικά φαινόμενα απαιτεί την ανάπτυξη διαγνωστικών μεθόδων εντοπισμού και διερεύνησης των ιδιοτήτων τους τοπικά στο χώρο και στο χρόνο. Ο πρωταρχικός στόχος αυτής της εργασίας είναι η ανάπτυξη τέτοιων μεθόδων, η διερεύνηση των τοπικών ιδιοτήτων των RWP και η ποσοτικοποίηση του ρόλου τους στα επεισόδια ακραίων θερμοκρασιών.Οι διαγνωστικές μέθοδοι βασίζονται στα αναλυτικά σήματα του φιλτραρισμένου μεσημβρινού ανέμου της ανώτερης τροπόσφαιρας και της περιβάλλουσας καμπύλης (πλάτους) του κατά μήκος κύκλων γεωγραφικού πλάτους. Οι τοπικές ιδιότητες των RWP όπως το πλάτος, το μήκος κύματος, η ταχύτητα φάσης και η ταχύτητα ομάδας αποτελούν συλλογικά τα κύρια χαρακτηριστικά της ανώτερης τροποσφαιρικής ροής στα μέσα γεωγραφικά πλάτη σε συγκεκριμένες χρονικές στιγμές, αλλά επίσης αποκαλύπτουν και τα διακριτά κλιματολογικά της χαρακτηριστικά. Με βάση αυτές, διερευνάται ο ρόλος των RWP στην εμφάνιση και τη διάρκεια επεισοδίων ακραίων θερμοκρασιών. Ειδικότερα, αναλύσεις παλινδρόμησης δείχνουν ότι η πιθανότητα εμφάνισης ακραίων θερμοκρασιών σε πολλά μέρη του βόρειου ημισφαιρίου αυξάνεται σημαντικά με το πλάτος των RWP· μια σύνδεση που δεν μπορεί να αποκαλυφθεί τόσο καθαρά όταν χρησιμοποιούνται μη τοπικές μέθοδοι όπως τα πλάτη Fourier του πεδίου του μεσημβρινού ανέμου. Ο ρόλος των RWP ως μεγάλης κλίμακας πρόδρομων ακραίων φαινομένων από τα δυτικά επαληθεύεται και μέσω μιας ανάλυσης των θερμών και ψυχρών ακραίων θερμοκρασιών της Νοτιοανατολικής Ευρώπης.Ο εντοπισμός και η παρακολούθηση της χωροχρονικής εξέλιξης των RWP αποδεικνύεται επωφελής και στη διερεύνηση των κυμάτων καύσωνα του 2003 και του 2010 στη Δυτική Ευρώπη και τη Ρωσία αντίστοιχα. Με αυτήν την ανάλυση αποδεικνύεται ότι ένα ή περισσότερα διαδοχικά RWP περιορισμένου μήκους μπορούν να θέσουν τις βάσεις για την ενίσχυση θερμοκρασιακών αποκλίσεων και - σε συνδυασμό με φυσικές διεργασίες μικρότερης κλίμακας — να οδηγήσουν σε ακραία φαινόμενα. Αποδείχτηκε ότι σχετικά μικρή ταχύτητα φάσης των RWP παίζει σημαντικό ρόλο σε τέτοιες περιπτώσεις που οι ακραίες θερμοκρασίες διαρκούν αρκετές μέρες. Στη συνέχεια, η συνδυαστική επίδραση του πλάτους και της ταχύτητας φάσης των RWP στην εμφάνιση και τη διάρκεια επεισοδίων ακραίων θερμοκρασιών ποσοτικοποιήθηκε στατιστικά χρησιμοποιώντας ένα αρκετά μεγάλο δείγμα επεισοδίων μικρής και μεγάλης διάρκειας και δεδομένα μετα-ανάλυσης 40 ετών (1979-2018).Παγκόσμιοι κλιματολογίκοι χάρτες του πλάτους, της ταχύτητας φάσης και της ομαδικής ταχύτητας των RWP τοπικά παράγονται για πρώτη φορά και αποκαλύπτουν τις μεγάλες διαφορές στην κυκλοφορία της ανώτερης τροπόσφαιρας μεταξύ Βόρειου και Νότιου Ημισφαίριου. Επιπλέον, υπογραμμίζονται και σχολιάζονται σημαντικές πτυχές της εποχιακής μεταβλητότητας αυτών των ιδιοτήτων σε συγκεκριμένες περιοχές. Τέλος, διερεύνηση των επιχειρησιακών προγνώσεων καιρού του ECMWF κατά την περίοδο 2013-2018 στο Βόρειο Ημισφαίριο υποδεικνύει οτί συστηματικά σφάλματα στις ιδιότητες των RWP πιθανώς να έχουν συνέπειες στην πρακτική προγνωσιμότητα της εμφάνισης και της διάρκειας των ακραίων θερμοκρασιών.Συνολικά, αυτή η εργασία συμβάλλει στον πρωταρχικό στόχο της βελτίωσης της κατανόησης των RWP και των επεισοδίων ακραίων θερμοκρασιών. Τα προαναφερθέντα ευρήματα και οι προτεινόμενες διαγνωστικές μέθοδοι θα ωφελήσουν μελλοντικές έρευνες σχετικά με τις διεργασίες που επηρεάζουν την εξέλιξη των RWP και τις επιπτώσεις τους στην εμφάνιση και την προγνωσιμότητα ακραίων φαινομένων τόσο σε καιρικές όσο και σε κλιματικές χρονικές κλίμακες

Extreme precipitation events over northern Italy. Part II: Dynamical precursors

The connection between weather extremes and Rossby wave packets (RWP) has been increasingly documented in recent years. RWP propagation and characteristics can modulate the midlatitude weather, setting the scene for temperature and precipitation extremes and controlling the geographical area affected. Several studies on extreme precipitation events (EPEs) in the Alpine area reported, as the main triggering factor, a meridionally elongated upper‐level trough as part of an incoming Rossby wave packet. In this work, we investigate a wide number of EPEs occurring between 1979 and 2015 in northern‐central Italy. The EPEs are subdivided into three categories (Cat1, Cat2, Cat3) according to thermodynamic conditions over the affected region. It is found that the three categories differ not only in terms of the local meteorological conditions, but also in terms of the evolution and properties of precursor RWPs. These differences cannot be solely explained by the apparent seasonality of the flow; therefore, the relevant physical processes in the RWP propagation of each case are further investigated. In particular, we show that RWPs associated with the strongest EPEs, namely the ones falling in Cat2, undergo a substantial amplification over the western North Atlantic due to anomalous ridge‐building 2 days before the event; arguably due to diabatic heating sources. This type of development induces a downstream trough which is highly effective in focusing water vapour transport toward the main orographic barriers of northern‐central Italy and favouring the occurrence of EPEs.The EPEs are subdivided into three categories (Cat1, Cat2, Cat3) according to thermodynamic conditions over the affected region. The three categories not only differ locally but also in the evolution of precursor RWPs as visible in the composite Hovmöller plots. RWPs associated with the strongest EPEs, the ones falling in Cat2, undergo a substantial amplification over the west North Atlantic due to anomalous ridge‐building 2 days before the event. This type of development induces a downstream trough which is highly effective in focusing water vapour transport toward the Apennines and the Alps.Ludwig‐Maximilians‐Universität München http://dx.doi.org/10.13039/501100005722German Research Foundation (DFG)Transregional Collaborative Research Centr