Future changes in North Atlantic winter cyclones in CESM-LE – Part 1: Cyclone intensity, potential vorticity anomalies, and horizontal wind speed

Strong low-level winds associated with extratropical cyclones can have substantial impacts on society. The wind intensity and the spatial distribution of wind maxima may change in a warming climate; however, the involved changes in cyclone structure and dynamics are not entirely clear. Here, such structural changes of strong North Atlantic cyclones in a warmer climate close to the end of the current century are investigated with storm-relative composites based on Community Earth System Model Large Ensemble (CESM-LE) simulations. Furthermore, a piecewise potential vorticity inversion is applied to associate such changes in low-level winds to changes in potential vorticity (PV) anomalies at different levels. Projected changes in cyclone intensity are generally rather small. However, using cyclone-relative composites, we identify an extended wind footprint southeast of the center of strong cyclones, where the wind speed tends to intensify in a warmer climate. Both an amplified low-level PV anomaly driven by enhanced diabatic heating and a dipole change in upper-level PV anomalies contribute to this wind intensification. On the contrary, wind changes associated with lower- and upper-level PV anomalies mostly compensate for each other upstream of the cyclone center. Wind changes at upper levels are dominated by changes in upper-level PV anomalies and the background flow. Altogether, our results indicate that a complex interaction of enhanced diabatic heating and altered non-linear upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones

Similarity and variability of blocked weather-regime dynamics in the Atlantic-European region

Weather regimes govern an important part of the sub-seasonal variability of the mid-latitude circulation. Due to their role in weather extremes and atmospheric predictability, regimes that feature a blocking anticyclone are of particular interest. This study investigates the dynamics of these ''blocked'' regimes in the North Atlantic-European region from a year-round perspective. For a comprehensive diagnostic, we combine wave activity concepts and a piecewise potential-vorticity (PV) tendency framework. The latter essentially quantifies the well-established PV perspective of mid-latitude dynamics. All blocked regimes during the 1979&ndash;2021 period of ERA5 reanalysis are considered. Wave activity characteristics exhibit distinct differences between blocked regimes. After regime onset, one regime (Greenland Blocking) is associated with a suppression of wave activity flux, whereas two other regimes (Atlantic Ridge and European Blocking) are associated with a northward deflection of the flux without a clear net change. During onset, the envelope of Rossby wave activity retracts upstream for Greenland Blocking, whereas the envelope extends downstream for Atlantic Ridge and European Blocking. The fourth regime (Scandinavian Blocking) exhibits intermediate wave activity characteristics. From the perspective of piecewise PV tendencies projected onto the respective regime pattern, the dynamics that govern regime onset exhibit a large degree of similarity: Linear Rossby wave dynamics and nonlinear eddy PV fluxes dominate and are of approximately equal relative importance, whereas baroclinic coupling and divergent amplification make minor contributions. Most strikingly, all blocked regimes exhibit very similar (intra-regime) variability: a retrograde and an upstream pathway to regime onset. The retrograde pathway is dominated by nonlinear PV eddy fluxes, whereas the upstream pathway is dominated by linear Rossby wave dynamics. Importantly, there is a large degree of cancellation between the two pathways for some of the mechanisms before regime onset. The physical meaning of a regime-mean perspective before onset can thus be severely limited. Implications of our results for understanding predictability of blocked regimes are discussed. We further discuss the limitations of projected tendencies in capturing the importance of moist processes, which tend to occur at the fringes or outside of the regime pattern. Finally, we stress that this study investigate the variability of the governing dynamics without prior empirical stratification of data by season or by type of regime transition. We demonstrate, however, that our dynamics-centered approach does not map predominantly on variability that is associated with these factors. The main modes of dynamical variability revealed herein, and the large similarity of the blocked regimes in exhibiting this variability are thus significant results.</p

Dynamik von Rossbywellenpaketen aus einer quantitativen PV-Perspektive

Rossbywellenpakete sind ein wichtiger Bestandteil der Dynamik in den mittleren Breiten und können insbesondere Vorläufer von Schwerwetterereignissen sein. Es wird davon ausgegangen, dass sich Rossbywellenpakete positiv auf die Vorhersagbarkeit auswirken, da sie in hohem Maße durch balancierte Dynamik beschrieben werden können. Neuere Studien haben jedoch gezeigt, dass diabatische Prozesse, insbesondere Feuchtprozesse, die Amplitude und Ausbreitung von Wellenpaketen beeinflussen können und so Unsicherheiten in den Vorhersagemodellen entstehen, die sich stromabwärts ausbreiten. In dieser Arbeit wird eine Diagnostik entwickelt und angewandt, die einen Beitrag zu einem besseren Verständnis dieser immanenten Unsicherheiten leisten kann. Rossbywellenpakete werden in dieser Arbeit als Anomalien der potentiellen Vorticity (PV) auf isentropen Flächen, die die Tropopause schneiden, identifiziert. Diese Betrachtungsweise wurde gewählt, da die PV nicht nur eine intuitive Beschreibung der Dynamik ermöglicht, sondern gerade auch die Vorhersagefehler der PV entlang der Tropopause maximal werden. Mit Hilfe dieser Schlüsselgröße lässt sich die Dynamik der Atmosphäre vollständig in vier Prozesse unterteilen, die im Kontext der Rossbywellendynamik in dieser Arbeit erstmals quantifiziert werden. Die Gruppenausbreitung eines Rossbywellenpaketes wird durch einen quasi-barotropen Ausbreitungsmechanismus beschrieben. Findet außerdem eine Wechselwirkung mit der Dynamik in der unteren Atmosphäre statt, wird das Wellenpaket durch barokline Wechselwirkung modifiziert. Die Entwicklung der Anomalien durch quasi-barotrope Ausbreitung und barokline Wechselwirkung wird durch das bekannte Konzept der baroklinen Entwicklung stromabwärts beschrieben. Zusätzlich werden in dieser Arbeit divergentes Ausströmen in der Höhe und direkte diabatische Modifikation untersucht. Besonders ausgeprägtes divergentes Ausströmen ist in hohem Maße mit dem Freisetzen latenter Wärme unterhalb des Ausströmens verbunden und kann daher als indirekt diabatisch betrachtet werden. In dieser Arbeit wird eine Diagnostik entwickelt, die diese Prozesse unterteilt und ihren relativen Einfluss auf die Entwicklung von Wellenpaketen quantifiziert. Insbesondere die relative Wichtigkeit der diabatischen Prozesse kann durch die neu entwickelte Diagnostik schärfer erfasst werden als mit existierenden Methoden. Die entwickelte Diagnostik wird in dieser Arbeit auf drei Fallstudien von Wellenpaketen angewandt. Um die dabei gefundenen Ergebnisse mit Hilfe vieler Wellenpakete zu untermauern, wird abschließend eine Composite-Analyse relativ zur maximalen Amplitude durchgeführt. Barokline Entwicklung stromabwärts ist ein wichtiger Prozess für die Amplitudenentwicklung, wird jedoch durch starkes divergentes Ausströmen in der Höhe beeinflusst. Innerhalb der Composite-Analyse ist divergentes Ausströmen sogar der dominante Prozess für die Verstärkung der Rücken und Abschwächung der Tröge. Da der divergente Einfluss in hohem Maß durch Feuchtprozesse verstärkt wird, reicht balancierte Dynamik nicht aus, um die Entwicklung des Wellenpaketes zu beschreiben. Direkte diabatische Modifikation spielt hingegen eine untergeordnete Rolle für die Amplitudenentwicklung. Die relative Wichtigkeit der verschiedenen Prozesse zwischen den einzelnen Anomalien ist jedoch einer großen Variabilität unterworfen. Die Ergebnisse dieser Arbeit zeigen, wie wichtig es ist divergentes Ausströmen und damit das Freisetzen latenter Wärme in numerischen Vorhersagemodellen möglichst korrekt zu repräsentieren, um die Vorhersagbarkeit von Rossbywellenpaketen und den damit verbundenen Wetterereignissen zu verbessern.Rossby wave packets (RWPs) are a fundamental ingredient of midlatitude dynamics and can constitute precursors to high-impact weather events. It is often assumed that RWPs, as large-scale flow features obeying balanced dynamics, exhibit a large degree of predictability. Recent work, however, has shown that diabatic processes, in particular moist processes, can modify the amplitude and propagation of RWPs.This impact can lead to an increased forecast uncertainty, which may compromise medium-range predictability in the downstream region. As a contribution to an improved understanding of these inherent uncertainties, a framework is developed and employed in this work to quantify different processes governing RWP evolution. RWPs are identified as anomalies of potential vorticity (PV) on isentropic levels intersecting the mid-latitude tropopause. The utilization of PV allows an intuitive description of mid-latitude dynamics. Furthermore, PV errors are maximised along the mid-latitude tropopause. This PV framework allows to fully separate the dynamics into four processes which are quantified in the context of RWP dynamics for the first time. The group propagation of RWPs is described by a qusi-barotropic propagation mechanism. The RWP is also modified by baroclinic growth if there exists an interaction with low-level dynamics. The evolution of anomalies by quasi-barotropic propagation and baroclinic growth constitute the well-known concept of baroclinic downstream development. In this work diverent outflow and direct diabatic modification are investigated additionally. Arguably, prominent upper-tropospheric divergent flow is associated to a large extent with latent heat release below and can thus be considered as an indirect diabatic impact. Here a diagnostic is developed which separates the aforementioned processes and quantifies their relative impact on RWP evolution. The newly developed diagnostic is more apt to quantify the relative importance of diabatic processes as existing methods. Case studies of three RWPs and a composite-analysis based on the maximal strength of the individual anomalies are performed. In general, baroclinic downstream development is an important process for the amplitude evolution but it is strongly modified by divergent outflow. The composite analysis even reveals a first-order impact of upper-level divergent flow for the amplification of ridges and the decay of troughs. Since divergent outflow is invigorated to a large degree by moist processes, balanced dynamics are not capable to fully describe the evolution of RWPs. Direct diabatic PV modification makes a subordinate contribution to the evolution. The relative importance of the different processes exhibits considerable variability between individual troughs and ridges. The results of this work demonstrate the importance of a precise representation of divergent outflow and thus latent heat release in numerical weather prediction models for the predictability of RWPs and their associated smaller-scale weather features

A process-based anatomy of Mediterranean cyclones: from baroclinic lows to tropical-like systems

In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies online potential vorticity(PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual-surface pressure centre. Finally, we focus on 10 medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane-driving processes, we highlight the need for a physical definition of these systems

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

Enhanced tropospheric wave forcing of two anticyclones in the prephase of the January 2009 major stratospheric sudden warming event

Tropospheric forcing of planetary wavenumber 2 is examined in the prephase of the major stratospheric sudden warming event in January 2009 (MSSW 2009). Because of a huge increase in Eliassen–Palm fluxes induced mainly by wavenumber 2, easterly angular momentum is transported into the Arctic stratosphere, deposited, and then decelerates the polar night jet. In agreement with earlier studies, the results reveal that the strongest eddy heat fluxes, associated with wavenumber 2, occur at 100 hPa during the prephase of MSSW 2009 in ERA-Interim. In addition, moderate conditions of the cold phase of ENSO (La Niña) contribute to the eddy heat flux anomaly. It is shown that enhanced tropospheric wave forcing over Alaska and Scandinavia is caused by tropical processes in two ways. First, in a climatological sense, La Niña contributes to an enhanced anticyclonic flow over both regions. Second, the Madden–Julian oscillation (MJO) has an indirect influence on the Alaskan ridge by enhancing eddy activity over the North Pacific. This is manifested in an increase in cyclone frequency and associated warm conveyor belt outflow, which contribute to the maintenance and amplification of the Alaskan anticyclone. The Scandinavian ridge is maintained by wave trains emanating from the Alaskan ridge propagating eastward, including an enhanced transport of eddy kinetic energy. The MSSW 2009 is an extraordinary case of how a beneficial phasing of La Niña and MJO conditions together with multiscale interactions enhances tropospheric forcing for wavenumber 2–induced zonal mean eddy heat flux in the lower stratosphere