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Workshop on Rossby Waves, Heat Waves, and Compound Extreme Events What: A workshop on Rossby waves, heat waves, and compound extreme events was co-organized by the Institute for Atmospheric Sciences and Climate (ISAC) of the National Research Council of Italy (CNR) and the University of Trento, Italy. The workshop gathered experts from different fields, such as extreme events analysis, atmospheric dynamics, climate modeling, and numerical weather prediction, with the aim to discuss state-of-the-art research, open challenges, and stimulate networking across different communities. When: 28-30 November 2023 Where: CNR Research Area, Bologna, Italy KEYWORDS: Atmosphere; Extratropics; Rossby waves; Extreme events; Heat wave, 1. Introduction The low-frequency variability of the atmosphere has long been the subject of intense investigation in the dynamical meteorology community (Benzi et al. 1986; Ghil 1987; Mo and Ghil [...]

The ever-increasing complexity and data volumes of numerical weather prediction demand innovations in the analysis and synthesis of operational forecast data. Here we show how dynamical thinking can offer directly applicable forecast information, taking as a case study the extreme northern Italy flooding of May 2023. We compare this event with long-lasting historical northern Italy rainfall events in order to determine (a) why it was so extreme, (b) how well it was predicted, and (c) how we may improve our predictions of such extremes. Lagrangian analysis shows, in line with previous work, that 48-hourly extreme rainfall in Italy can be caused by moist air masses originating from the North Atlantic; North Africa; and, to a lesser extent, eastern Europe, with compounding moisture contributions from all three regions driving the May 2023 event. We identify the large-scale precursors of typical northern Italy rainfall extremes based on geopotential height and integrated vapour transport fields. We show in European Centre for Medium-Range Weather Forecasts (ECMWF) operational forecasts that a precursor perspective was able to identify the growing possibility of the Emilia-Romagna extreme event 8 d beforehand – 4 d earlier than the direct precipitation forecast. Such dynamical precursors prove to be well suited for identifying and interpreting predictability barriers and could help build forecasters' understanding of unfolding extreme scenarios in the medium range. We conclude by discussing the broader implications and operational potential of dynamically rooted metrics for understanding and predicting extreme events, both in retrospect and in real time. [ABSTRACT FROM AUTHOR]

Research in the last few decades has revealed that rapidly ascending airstreams in extratropical cyclones – so-called warm conveyor belts (WCBs) – play an important role in extratropical atmospheric dynamics. However on the subseasonal timescale, the modulation of their occurrence frequency, henceforth referred to as WCB activity, has so far received little attention. Also, it is not yet clear whether WCB activity may affect tropospheric teleconnection patterns, which constitute a source of predictability on this subseasonal timescale. Using reanalysis data, this study analyzes the modulation of WCB activity by the Madden–Julian Oscillation (MJO). A key finding is that WCB activity increases significantly over the western North Pacific when the convection of the MJO is located over the Indian Ocean. This increased WCB activity, which is stronger during La Niña conditions, is related to enhanced poleward moisture fluxes driven by the circulation of subtropical Rossby gyres associated with the MJO. In contrast, when the convection of the MJO is located over the western North Pacific, WCB activity increases significantly over the eastern North Pacific. This increase stems from a southward shift and eastward extension of the North Pacific jet stream. However, while these mean increases are significant, individual MJO events exhibit substantial variability, with some events even exhibiting anomalously low WCB activity. Individual events of the same MJO phase with anomalously low WCB activity over the North Pacific tend to be followed by the known canonical teleconnection patterns in the Atlantic–European region; i.e., the occurrence frequency of the positive phase of the North Atlantic Oscillation (NAO) is enhanced when convection of the MJO is located over the Indian Ocean and similarly for the negative phase of the NAO when MJO convection is over the western North Pacific. However, the canonical teleconnection patterns are modified when individual events of the same MJO phase are accompanied by anomalously high WCB activity over the North Pacific. In particular, the link between MJO and the negative phase of the NAO weakens considerably. Reanalysis data and experiments with an idealized general circulation model reveal that this is related to anomalous ridge building over western North America favored by enhanced WCB activity. Overall, our study highlights the potential role of WCBs in shaping tropical–extratropical teleconnection patterns and underlines the importance of representing them adequately in numerical weather prediction models in order to fully exploit the sources of predictability emerging from the tropics. [ABSTRACT FROM AUTHOR]

The development of atmospheric blocks over the North Atlantic–European region can lead to extreme weather events like heat waves or cold air outbreaks. Despite their potential severe impact on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction (NWP) models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air (cold air outbreaks, CAOs) from the Arctic over the North Atlantic, and associated air–sea interactions over the Gulf Stream are key processes contributing to the onset, maintenance, and persistence of such flow regimes. To better understand the mechanism connecting air–sea interactions over the Gulf Stream with changes in the large-scale flow, we focus on an episode between 20 and 27 February 2019, when a quasi-stationary upper-level ridge was established over western Europe accompanied by an intensified storm track in the northwestern North Atlantic. During that time, a record-breaking winter warm spell occurred over western Europe bringing temperatures above 20 ∘ C to the United Kingdom, the Netherlands, and northern France. The event was preceded and accompanied by the development of several rapidly intensifying cyclones that originated in the Gulf Stream region and traversed the North Atlantic. To explore the mechanistic linkage between the formation of this block and air–sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using kinematic trajectories. This allows us to study the pathways and transformations of air masses that form the upper-level potential vorticity anomaly and interact with the ocean front. We establish that more than one-fifth of these air masses interact with the Gulf Stream in the lower troposphere, experiencing intense heating and moistening over the region due to the frequent occurrence of CAOs behind the cold front of the cyclones. Trajectories moistened by the advection of cold air over a warm ocean by one cyclone later ascend into the upper troposphere with the ascending airstream of a subsequent cyclone, fueled by the strong surface fluxes. These findings highlight the importance of CAOs in the Gulf Stream region, indicating that their intense coupling between the ocean and atmosphere plays a role in block development. Additionally, they provide a mechanistic pathway linking air–sea interactions in the lower troposphere and the upper-level flow. [ABSTRACT FROM AUTHOR]

Blocking over Greenland has substantial impacts on surface weather in particular over Europe and North America, and can increase melting of the Greenland Ice Sheet. Climate models notoriously underestimate the frequency of blocking over Greenland in historical periods, but the reasons for this are not entirely clear, as we are still lacking a full dynamical understanding of Greenland blocking from formation through maintenance to decay. This study investigates the dynamics of blocking life cycles over Greenland based on ERA5 reanalysis data from 1979–2021. A year-round weather regime definition allows us to identify Greenland blocking as consistent life cycles with an objective onset, maximum, and decay stage. By applying a new quasi-Lagrangian potential vorticity (PV) perspective, following the negative, upper-tropospheric PV anomalies (PVAs-) associated with the block, we examine and quantify the contribution from different physical processes, including dry and moist dynamics, to the evolution of the PVA- amplitude. We find that PVAs- linked to blocking do not form locally over Greenland but propagate into the region along two distinct pathways (termed 'upstream' and' retrogression') during the days before the onset. Remarkably, the development of PVAs- differs more between the pathways than between seasons. Moist processes play a key role in the amplification of PVAs- before the onset and are linked to midlatitude warm conveyor belts. Interestingly, we find moist processes supporting the westward propagation of retrograding PVAs- from Europe, too, previously thought to be a process dominated by dry-barotropic Rossby wave propagation. After onset, moist processes remain the main contribution to PVA- amplification and maintenance. However, moist processes weaken markedly after the maximum stage and dry processes, i.e. barotropic, non-linear wave dynamics, dominate the decay of the PVAs- accompanied by a general decrease in blocking area. Our results corroborate the importance of moist processes in the formation and maintenance of Greenland blocking, and suggest that a correct representation of moist processes might help reducing forecast errors linked to blocking in numerical weather prediction models and blocking biases in climate models. [ABSTRACT FROM AUTHOR]

Stochastic parametrisation techniques have been used by operational weather centres for decades to produce ensemble forecasts and to represent uncertainties of the forecast model. Their use has been demonstrated to be highly beneficial, as it increases the reliability of the forecasting system and reduces systematic biases. Despite the random nature of the perturbation techniques, the response of the model can be nonlinear and the mean state of the model can change. In this study, we attempt to provide a process-based understanding how stochastic model perturbations affect the model climate. Previous work has revealed sensitivities of the occurrence of diabatically driven, rapidly ascending air streams to the stochastically perturbed parametrisation tendencies (SPPT) scheme. Such strongly ascending air streams are linked to different weather phenomena, such as precipitation and upper-tropospheric ridge building in the midlatitudes, which raises the question whether these processes are also influenced by stochastic perturbations. First, we analyse if rapidly ascending air streams also show sensitivities to a different perturbation technique - the stochastically perturbed parametrisations (SPP) scheme, which directly represents parameter uncertainty in parametrisations and has recently been developed at the European Centre for Medium-Range Weather Forecasts (ECMWF). By running a set of sensitivity experiments with the Integrated Forecasting System (IFS) and by employing a Lagrangian detection of ascending air streams, we show that SPP results in a systematic increase of the occurrence of ascending air parcel trajectories compared to unperturbed simulations. This behaviour is very similar to that of SPPT, albeit some regional differences are apparent. We further show that the one-sided response to the stochastic forcing cannot be attributed to a single process (e.g. convection parametrisation), but rather that perturbations to different parametrisations have similar effects. Thereafter, we link the frequency changes of ascending air streams to closely related weather phenomena. Whereas the signal of increased ascending motion is directly transmitted to global precipitation sums for all analysed schemes, changes to the amplitude of the upper-level Rossby wave pattern are more subtle. In agreement with the trajectory analysis, both SPPT and SPP increase the waviness of the upper-level flow and thereby reduce a systematic bias of the model, even though the order of magnitude is small. Our study presents a coherent process chain that enables to understand how stochastic perturbations systematically affect the model climate. We argue that weather systems which are characterised by threshold behaviour on the one hand and that serve as a dynamical hinge between spatial scales on the other hand can convert zero-mean perturbations into an asymmetric response and project it onto larger scales. [ABSTRACT FROM AUTHOR]

Warm conveyor belts (WCBs) produce a major fraction of precipitation in extratropical cyclones and modulate the large-scale extratropical circulation. Diabatic processes, in particular associated with cloud formation, influence the cross-isentropic ascent of WCBs into the upper troposphere and additionally modify the potential vorticity (PV) distribution, which influences the larger-scale flow. In this study we investigate heating and PV rates from all diabatic processes, including microphysics, turbulence, convection, and radiation, in a case study that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) campaign using the Icosahedral Nonhydrostatic (ICON) modeling framework. In particular, we consider all individual microphysical process rates that are implemented in ICON's two-moment microphysics scheme, which sheds light on (i) which microphysical processes dominate the diabatic heating and PV structure in the WCB and (ii) which microphysical processes are the most active during the ascent and influence cloud formation and characteristics, providing a basis for detailed sensitivity experiments. For this purpose, diabatic heating and PV rates are integrated for the first time along online trajectories across nested grids with different horizontal resolutions. The convection-permitting simulation setup also takes the reduced aerosol concentrations over the North Atlantic into account. Our results confirm that microphysical processes are the dominant diabatic heating source during ascent. Near the cloud top longwave radiation cools WCB air parcels. Radiative heating and corresponding PV modification in the upper troposphere are non-negligible due to the longevity of the WCB cloud band. In the WCB ascent region, the process rates from turbulent heating and microphysics partially counteract each other. From all microphysical processes condensational growth of cloud droplets and vapor deposition on frozen hydrometeors most strongly influence diabatic heating and PV, while below-cloud evaporation strongly cools WCB air parcels prior to their ascent and increases their PV value. PV production is the strongest near the surface with substantial contributions from condensation, melting, evaporation, and vapor deposition. In the upper troposphere, PV is reduced by diabatic heating from vapor deposition, condensation, and radiation. Activation of cloud droplets as well as homogeneous and heterogeneous freezing processes have a negligible diabatic heating contribution, but their detailed representation is important for, e.g., hydrometeor size distributions. Generally, faster-ascending WCB trajectories are heated markedly more than more slowly ascending WCB trajectories, which is linked to larger initial specific humidity content providing a thermodynamic constraint on total microphysical heating. Yet, the total diabatic heating contribution of convectively ascending trajectories is relatively small due to their small fraction in this case study. Our detailed case study documents the effect of different microphysical processes implemented in ICON's two-moment scheme for heating and PV rates in a WCB from a joint Eulerian and Lagrangian perspective. It emphasizes the predominant role of microphysical processes and provides a framework for future experiments on cloud microphysical sensitivities in WCBs. [ABSTRACT FROM AUTHOR]

The development of atmospheric blocks over the North Atlantic European region can lead to extreme weather events like heatwaves or cold air outbreaks. Despite their potential severe impact on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction (NWP) models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air (cold air outbreaks, CAOs) from the Arctic over the North Atlantic, and associated air-sea interactions over the Gulf Stream are key processes responsible for the onset, maintenance, and persistence of such flow regimes. In order to establish how air mass transformations over the ocean, and in particular over the Gulf Stream, affect the large-scale flow, we focus on an episode between 20 and 27 of February 2019, when a quasi-stationary upper-level ridge established over western Europe accompanied by an intensified storm track in the Northwestern North Atlantic. During that time a record-breaking warm spell occurred over Western Europe bringing temperatures above 20 °C to the United Kingdom, the Netherlands, and Northern France. The event was preceded and accompanied by the development of several, rapidly intensifying cyclones originating in the Gulf Stream region and traversing the North Atlantic. To explore the mechanistic linkage between the formation of this block and air-sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using backward and forward kinematic trajectories. This allows us to study the pathways and transformations of air masses forming the upper-level potential vorticity anomaly and interacting with the ocean front. We establish that more than one-fifth of these air masses interact with the Gulf Stream in the lower troposphere, experiencing intense heating and moistening over the region, due to the frequent occurrence of CAOs behind the cold front of the cyclones. Trajectories moistened within the cold sector of one cyclone, later ascent into the upper troposphere with the ascending air stream of a consecutive cyclone, fueled by the strong surface fluxes. These findings highlight the importance of CAOs in the Gulf Stream region with their intense coupling between the ocean and atmosphere for blocking development, and provide a mechanistic pathway linking air-sea interactions in the lower troposphere and the upper-level flow. [ABSTRACT FROM AUTHOR]

Atmospheric blocking describes a situation in which a stationary and persistent anticyclone blocks the eastward propagation of weather systems in the midlatitudes and can lead to extreme weather events. In the North Atlantic–European region, blocking contributes to life cycles of weather regimes which are recurrent, quasi-stationary, and persistent patterns of the large-scale circulation. Despite progress in blocking theory over the last decades, we are still lacking a comprehensive, process-based conceptual understanding of blocking dynamics. Here we combine three different perspectives on so-called "blocked" weather regimes, namely the commonly used Eulerian and Lagrangian perspectives, complemented by a novel quasi-Lagrangian perspective. Within the established framework of midlatitude potential vorticity (PV) thinking, the joint consideration of the three perspectives enables a comprehensive picture of the dynamics and quantifies the importance of dry and moist processes during a blocked weather regime life cycle. We apply the diagnostic framework to a European blocking weather regime life cycle in March 2016, which was associated with a severe forecast bust in the North Atlantic–European region. The three perspectives highlight the importance of moist processes during the onset or maintenance of the blocked weather regime. The Eulerian perspective, which identifies the processes contributing to the onset and decay of the regime, indicates that dry quasi-barotropic wave dynamics and especially the eastward advection of PV anomalies (PVAs) into the North Atlantic–European region dominate the onset of the regime pattern. By tracking the negative upper-tropospheric PVA associated with the "block", the quasi-Lagrangian view reveals, for the same period, abrupt amplification due to moist processes. This is in good agreement with the Lagrangian perspective indicating that a large fraction of air parcels that end up in the negative PVA experience diabatic heating. Overall, the study shows that important contributions to the development take place outside of the region in which the blocked weather regime eventually establishes, and that a joint consideration of different perspectives is important in order not to miss processes, in particular moist-baroclinic dynamics, contributing to a blocked regime life cycle. [ABSTRACT FROM AUTHOR]

The representation of warm conveyor belts (WCBs) in numerical weather prediction (NWP) models is important, as they are responsible for the major precipitation in extratropical cyclones and modulate the large-scale flow evolution. Their cross-isentropic ascent into the upper troposphere is influenced by latent heat release mostly, but not exclusively, from cloud formation whose representation in NWP models is associated with large uncertainties. The diabatic heating additionally modifies the potential vorticity (PV) distribution which influences the circulation. We analyse diabatic heating and associated PV rates from all physics processes, including microphysics, turbulence, convection, and radiation, in a case study of a WCB that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) campaign using the Icosahedral Nonhydrostatic (ICON) modelling framework. In particular, we consider all individual microphysical process rates that are implemented in ICON's two-moment microphysics scheme, which sheds light on (i) which microphysical processes dominate the diabatic heating and PV structure in the WCB, and thus, potentially influence the large-scale flow, and (ii) which microphysical processes are most active during the ascent and influence cloud formation and characteristics. For this purpose, diabatic heating and PV rates are integrated along online WCB trajectories. Our convection-permitting simulation setup also takes the reduced aerosol concentrations over the North Atlantic into account. Complementary Lagrangian and Eulerian perspectives on diabatic heating and PV modification confirm that microphysical processes are the dominant diabatic heating contribution during ascent. Near cloud top longwave radiation cools WCB air parcels. Due to the longevity of the WCB cloud band, the diabatic heating contributions from radiation, and corresponding PV modification in the upper troposphere, are non-negligible. The turbulence scheme is active in the WCB ascent region, despite large gradient Richardson numbers, and process rates from turbulence and microphysics partially counteract each other. From all microphysical processes condensational growth of cloud droplets and vapor deposition on frozen hydrometeors most strongly influence diabatic heating and PV, while below-cloud evaporation strongly cools WCB air parcels prior to their ascent and increases their PV value. PV production is strongest near surface, and extends up to 4 km height with substantial contributions from condensation, melting, evaporation, and vapor deposition. In the upper troposphere, PV is reduced by diabatic heating from vapor deposition, condensation, and radiation. Activation of cloud droplets as well as homogeneous and heterogeneous freezing processes have a negligible diabatic heating contribution due to small overall mass conversion, but their detailed representation is likely important as the hydrometeor size distributions influence other microphysical processes. Generally, faster ascending WCB trajectories are heated markedly more than more slowly ascending WCB trajectories, which is linked to larger initial specific humidity content of fast WCB trajectories providing a thermodynamic constraint on total microphysical heating. Yet, the total diabatic heating contribution of convectively ascending trajectories is relatively small due to their small fraction in this case study. [ABSTRACT FROM AUTHOR]