UP1.1

Precipitation can have large dual societal impacts in regions with a dry climate. On the one hand, extreme precipitation can induce catastrophic floods, and on the other hand, replenish scarce freshwater resources. In contrast to wet (extra)tropical regions, the atmospheric processes that govern the precipitation formation in dry subtropical and extratropical regions are often overlooked by the scientific community. In this study we investigate the role of Rossby wave breaking for precipitation in (semi)arid regions at the global scale. To this end, we quantify the contribution of Rossby wave breaking to extreme precipitation days and annual precipitation amounts in regions with different degrees of aridity. Rossby wave breaking is represented by potential vorticity (PV) streamers and cutoffs on isentropic surfaces using ECMWF reanalysis data, while for precipitation we also use an observational product. We show that the relevance of Rossby wave breaking for precipitation increases from humid to hyper-arid regions. Equatorward breaking Rossby waves contribute to a large fraction of precipitation extremes and annual amounts in regions on the poleward-westward flanks of the world’s most arid regions where most precipitation occurs in the cool season. In contrast, precipitation in the equatorward-eastward parts of these arid regions has a negative association with Rossby wave breaking, implying that the tropical forcing governs the precipitation formation which occurs in these regions predominantly in the warm season. Overall, this study shows that Rossby wave breaking is of key importance for precipitation in (semi)arid regions that are exposed to both flood hazards and drying under global warming.

How to cite: De Vries, A. J., Armon, M., Klingmüller, K., Portmann, R., and Domeisen, D. I. V.: The role of Rossby wave breaking for extreme and annual mean precipitation in (semi)arid regions, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-316, https://doi.org/10.5194/ems2022-316, 2022.

The occurrence of cold spells over North America is statistically connected with an increased likelihood of extreme wind and precipitation events over Europe. This connection involves the state of the North Atlantic storm track in a nontrivial way. Cold spells over eastern Canada, for instance, are associated with windstorms over the United Kingdom in a configuration of positive North Atlantic Oscillation (NAO). However, cold spells over eastern Canada already tend to occur during the positive phase of the NAO, suggesting a chain of processes in which the internal dynamics of the storm track is responsible of both the cold spell and its European impact. Analogous considerations, although in the context of negative NAO, hold for cold spells hitting central United States and windstorms over the Iberian Peninsula.

Another key player in the dynamics of North American cold spells are Rossby wave trains propagating from the North Pacific towards North America and then the Atlantic Ocean. These low-frequency wave trains can impact the North Atlantic storm track directly, by reconfiguring the large-scale features defining locally the extratropical waveguide, or indirectly, by promoting anomalies in the genesis or propagation of extratropical cyclones.

We try to disentangle this complex interplay by analyzing systematically the state of the North Atlantic storm track before and after cold spells over several North American regions. Anomalies of wave activity flux, in the formulation by Takaya and Nakamura (2001), are computed to assess the relevance of Rossby wave trains for cold spells in each region. The main conclusion of this work is a reconsideration of the usual chain of processes connecting cold spells and wet/windy extremes over Europe: in some cases, the systematic connection between the former and the latter is likely mediated by the pre-existing state of the storm track and not simply due to the cold spells alone.

How to cite: Riboldi, J., Leeding, R., Segalini, A., and Messori, G.: A storm-track connection between North American cold extremes and European wet/windy extremes, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-136, https://doi.org/10.5194/ems2022-136, 2022.

From the 12^th to the 15^th of July 2021, Western Europe was confronted with an abnormal amount of precipitation leading to extreme floods and enormous damage in western Germany, Belgium, Luxembourg, and the Netherlands. Locally, almost thrice as much as the monthly precipitation amount was observed, up to 175 mm in two days. In this study, the atmospheric conditions resulting in this extreme precipitation are investigated, with a focus on understanding the enhanced moisture supply leading to the extreme precipitation amounts. Prior to the event, both the Baltic Sea and the Mediterranean Sea were exceptionally warm, and it was hypothesized that high evaporation rates over these regions could be transported towards western Europe to result in these enormous amount of rain.

To test this hypothesis we apply two state-of-the-art moisture tracking approaches. First, we use the Lagrangian moisture attribution diagnostic WaterSip, based on trajectories computed from ERA5 with the Lagranto trajectory model. Second, we use the Eulerian moisture tracking tool WAM-2layers, applied to ERA5 re-analysis data. Comparison of ERA5 with Eobs shows that the precipitation during the event was captured rather well. The moisture sources are mostly located over the continent, and to a lesser extent over the North and Baltic Seas. There is a height dependency of the atmospheric transport of air and moisture. For the surface layer, the moisture that led to precipitation mainly originated from the region of the Baltic Sea, North Sea, Denmark and Scandinavia. Higher up in the atmosphere, this was changing towards the European continent and parts of the Mediterranean Sea.

In addition, the impact on precipitation by adapting the sea surface temperature (SST) of both the Baltic and the Mediterranean was studied using WRF. This analysis showed that SST changes in the Mediterranean had the largest impact on precipitation in western Europe. Furthermore, our results suggest that the Mediterranean Sea, which had a positive SST anomaly of 2˚C, was the main moisture source preceding the precipitation event.

How to cite: Weijenborg, C., Benedict, I., Polak, F., Vermeulen, T., Kalverla, P., and Sodemann, H.: Moisture origin of the extreme precipitation event in Western Europe in July 2021, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-407, https://doi.org/10.5194/ems2022-407, 2022.

The summer monsoon provides most of the rain in India. Any variability in rainfall, particularly on the daily-to-weekly scale, strongly impacts the environmental systems and socioeconomic activity. This study focuses on cross-equatorial dry intrusions (DIs) in which dry air masses descend slantwise from the southern midlatitudes upper troposphere towards lower levels in the Arabian Sea during monsoon season. The presence of dry air masses in the Arabian Sea, a hot spot for such DIs, has great potential to modify the moisture availability and rainfall over India.

Using the Lagrangian analysis tool, LAGRANTO, forced by ERA-interim reanalysis data for 40 monsoon seasons (1979-2018), we identify 130 DI events. We show that 95% of DI air masses reach below 800 hPa in the central Indian Ocean within two days of their initiation in the southern hemisphere. Though rare (<1% frequency in time), their presence in the marine boundary layer triggers intense ocean evaporation due to enhanced moisture gradient in the lower levels. These DIs enhance low-level flow that facilitates more moisture transport from the central Indian ocean towards India. Depending on the wind pattern, increased moisture transport changes rain over land: 60% of the DI events are associated with more than 25% enhancement, whereas 40% with about 10% decrease in the rain compared to climatology. In summary, cross-equatorial dry air masses reaching the marine boundary layer over the tropical Indian Ocean during the summer monsoon season are responsible for developing anomalously moist air and heavy precipitation over the Indian region downstream and thus crucial for summer monsoon rainfall predictability, especially its extremes.

How to cite: Rai, D. and Raveh-Rubin, S.: Unraveling cross-equatorial dry intrusion influence on Indian summer monsoon rainfall, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-652, https://doi.org/10.5194/ems2022-652, 2022.

Equatorial waves (EWs) are synoptic- to planetary-scale propagating disturbances at low latitudes with periods from a few days to several weeks. Here this term includes Kelvin waves, equatorial Rossby waves, mixed-Rossby gravity waves and inertio-gravity waves, which are well described by linear wave theory, but also other tropical disturbances such as easterly waves and the intraseasonal Madden-Julian Oscillation with more complex dynamics. EWs can couple with deep convection, leading to a substantial modulation of clouds and rainfall. EWs are amongst the dynamic features of the troposphere with the longest intrinsic predictability and models are beginning to forecast them with an exploitable level of skill. Most of the methods developed to identify and objectively isolate EWs in observations and model fields rely on (or at least refer to) the adiabatic, frictionless linearized primitive equations on the sphere or the shallow water system on the equatorial β-plane. Common ingredients to these methods are zonal wavenumber-frequency filtering (Fourier or wavelet) and/or projections onto predefined empirical or theoretical dynamical patterns. This paper gives an overview of six different methods to isolate EWs and their structures, discusses the underlying assumptions, evaluates the applicability to different problems and provides a systematic comparison based on a case study (20 February-20 May 2009) and a climatological analysis (2001-2018). In addition, the influence of different input fields (e.g. winds, geopotential, outgoing longwave radiation, rainfall) is investigated. Based on the results, we generally recommend employing a combination of wavenumber-frequency filtering and spatial-projection methods (and of different input fields) to check for robustness of the identified signal. In cases of disagreement, one needs to carefully investigate which assumptions made for the individual methods are most probably not fulfilled. This will help choose an approach optimally suited to a given problem at hand and avoid misinterpretation of the results. 

How to cite: Knippertz, P., Gehne, M., Kiladis, G. N., Kikuchi, K., Rasheeda Satheesh, A., Roundy, P. E., Yang, G.-Y., Žagar, N., Dias, J., Fink, A. H., Methven, J., Schlueter, A., Sielmann, F., and Wheeler, M. C.: The Intricacies of Identifying Equatorial Waves, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-95, https://doi.org/10.5194/ems2022-95, 2022.

Recent studies have emphasized the relevance of synoptic-scale atmospheric processes in driving extreme events in the Arctic. In particular, warm extremes and episodes of anomalous sea ice melting have been attributed – in part – to intrusions of warm and moist air from lower latitudes associated with mid-latitude weather systems such as cyclones and blocks. At the same time, also local processes such as subsidence in anticyclones and diabatic heating have been found to play an important role in some cases. Accordingly, the relative importance of remote vs. local processes remains a matter of debate.

Here, we focus on the atmospheric components of the surface energy budget (SEB) over Arctic sea ice. Specifically, we present a novel methodology to identify and track extreme (> 95^th percentile) anomalies of the SEB in space and time in ERA5 reanalysis data (extended winters 1979 – 2020). The resulting 142 so-called SEB life-cycle events are predominantly caused by enhanced downward longwave radiation and turbulent heat fluxes. A synoptic analysis reveals that most of these events occur in narrow gateways in the Atlantic and Pacific sectors and are associated with a poleward deflection of the storm tracks and with blocks over northern Eurasia and Alaska.

Kinematic backward trajectories from the tropospheric column collocated with the events show that mid- and upper tropospheric air has predominantly been transported poleward from mid-latitudes, thereby ascending along slanted isentropes. Lower tropospheric air, in contrast, is largely of Arctic origin and subject to substantial (diabatic) air mass transformations (warming and moistening) during the days preceding the events. Despite the differences in origin of the air at different altitudes, the entire tropospheric column shows a pronounced warm anomaly (~ 10 K) building up over 2-4 days prior to the events. A decomposition of the temperature anomalies emphasizes the relevance of horizontal advection and latent heating for the generation of the mid- and upper tropospheric temperature anomalies, whereas subsidence and heating by surface heat fluxes over ocean are important for the transformation of the originally cold Arctic air at lower altitudes.

Thus, this work emphasizes that a rich chain of processes is required for the generation of the most extreme Arctic SEB anomalies, resulting from a combination of air mass injections from (remote) lower latitudes and local air mass transformations in the Arctic.

How to cite: Papritz, L., Murto, S., Caballero, R., Messori, G., Röthlisberger, M., Svensson, G., and Wernli, H.: Local versus remote origin of wintertime extreme surface energy budget anomalies in the Arctic, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-73, https://doi.org/10.5194/ems2022-73, 2022.

The Arctic region has undergone a substantial climatic changes in recent years with dramatic sea-ice loss, evident particularly in late summer and early autumn. According to many findings the near-surface temperature in high latitudes of the Northern Hemisphere has been rising at rates double that of the lower latitudes and was amplified by rapid loss of sea ice in the Arctic through the snow and ice albedo feedback. Open, free of ice water has a smaller albedo than ice, absorbs more energy incoming from the Sun, resulting in much faster increase in temperature than other areas. The abrupt acceleration in Arctic warming well seen since the middle 1990s coincides with changes in climate in the mid-latitudes. The aim of this paper is to analyse the changes in atmospheric circulation in the mid to high latitudes in the response to low extent of ice cover  in Arctic in the preceding months.

To analyze the variability of sea ice cover in Arctic region we used daily sea-ice fraction data which are defined as the fraction of a grid box which is covered by sea ice. These data were multiplied by field of each grid box. After summing up all recalculated grid points, a new parameter named – area ice cover (AIC) was defined. We analyzed the AIC variability in the whole Artcic and in three sectors Atlantic - from 75°W to 60°E, Asian - from 60.25°E to 189.75°W and American - from 170°W to 75.25°W.

Then AIC index was correlated to selected northern hemisphere teleconnection: North Atlantic Oscillation (NAO), the Scandinavian pattern (SCAND), East Atlantic (EA), Polar/Eurasia index (POL). The Monthly NAO, SCAND, EA and POL values were provided by U.S. National Centers for Environmental Prediction (NCEP). The AIC preceded circulation indices by 1 to 12 months.

In regions and periods of significant correlation, composite maps of temperature anomalies, sea level pressure and geopotential altitude of 100 and 200 hPa in the years with low icing were created and analysed.

The analysis showed that the greatest changes in AIC are observed in Asian sector of Arctic and they have the strongest impact on the atmospheric circulation. AIC changes modulate circulation in both winter and summer period. However the nature of the impact is clearly different. A sensitive regions turns to be Barents and Kara Seas and Novaya Zemlya.

How to cite: Jędruszkiewicz, J., Piotrowski, P., and Wibig, J.: Impact of low ice area in Arctic on atmospheric circulation and climate extremes in mid to high latitudes of the Northern Hemisphere, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-339, https://doi.org/10.5194/ems2022-339, 2022.

The Arctic Oscillation (AO) describes a seesaw pattern of variations in atmospheric mass over the polar cap. It is by now well established that the AO pattern is in part determined by the state of the stratosphere. In particular, sudden stratospheric warmings (SSWs) are known to nudge the tropospheric circulation toward a more negative phase of the AO, which is associated with a more equatorward shifted jet and enhanced likelihood for blocking and cold air outbreaks in mid-latitudes. SSWs are also thought to contribute to the occurrence of extreme AO events. However, statistically robust results about such extremes are difficult to obtain from observations or meteorological (re-)analyses due to the limited sample size of SSW events in the observational record (roughly 6 SSWs per decade). Here we exploit a large set of extended-range ensemble forecasts within the subseasonal-to-seasonal (S2S) framework to obtain an improved characterization of the modulation of AO extremes due to stratosphere-troposphere coupling. Specifically, we greatly boost the sample size of stratospheric events by using potential SSWs (p-SSWs), i.e., SSWs that are predicted to occur in individual forecast ensemble members regardless of whether they actually occurred in the real atmosphere. For example, for the ECMWF S2S ensemble this gives us a total of 6101 p-SSW events for the period 1997-2021. 

A standard lag-composite analysis around these p-SSWs validates our approach, i.e., the associated composite evolution of stratosphere-troposphere coupling matches the known evolution based on reanalyses data around real SSW events. Our statistical analyses further reveal that following p-SSWs, relative to climatology: 1) persistently negative AO states (> 1 week duration) are 16% more likely, 2) the likelihood for extremely negative AO states (< −3σ) is enhanced by at least 35%, while that for extremely positive AO states (> +3σ) is reduced to almost zero, 3) approximately 50% of extremely negative AO states that follow SSWs may be attributed to the SSW, whereas about one quarter of all extremely negative AO states during winter may be attributed to SSWs. A corresponding analysis relative to strong stratospheric vortex events reveals similar insights into the stratospheric modulation of positive AO extremes.  

How to cite: Spaeth, J. and Birner, T.: Stratospheric Modulation of Arctic Oscillation Extremes as Represented by Extended-Range Ensemble Forecasts, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-107, https://doi.org/10.5194/ems2022-107, 2022.

So-called "attribution studies" can be used to estimate the extent to which human-induced climate change is responsible for the change of occurrence frequency and intensity of individual weather or climate extremes. For such statistical analyses, mainly climate simulations with specially selected boundary conditions are used, as observational time series are often not yet sufficiently long.

For the production of rapid attribution studies, which publish results shortly after the event, a protocol has been developed in recent years by the World Weather Attribution Initiative (WWA, https://www.worldweatherattribution.org) and a Copernicus project. Many of the processing steps are currently carried out in the KNMI Climate Explorer (https://climexp.knmi.nl), an online application developed by the Dutch weather service and made freely available. The German Weather Service (DWD) uses the present protocol for its contributions to international attribution studies of the World Weather Attribution and carries out its own analyses.

Within the framework of “ClimXtreme - Climate Change and Extreme Events” (https://climxtreme.net), a research network funded by the Federal Ministry of Education and Research, the DWD is developing a workflow that automates many steps of an attribution analysis for extreme events occurring in Germany, including but not limited to performing calculations and creating figures. This reduces the processing time as well as ensures the robustness of results and enables researchers to focus on aspects of the attribution study that require expert knowledge.

The presentation will provide an overview of the current state of research, show results of attribution studies, and give an outlook on developments in the coming years.  

How to cite: Lorenz, P., Tradowsky, J., and Kreienkamp, F.: Extreme event attribution at Deutscher Wetterdienst - status and plans, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-666, https://doi.org/10.5194/ems2022-666, 2022.

Atmospheric blocking is often responsible for high-impact surface weather conditions such as heatwaves, cold spells, and droughts. Very long blocking events are particularly problematic due to the persistence of hazardous surface conditions. Despite their many impacts, understanding the dynamical behaviour of blocking events is still incomplete despite recent progress. Here, the relationship between block persistence and anticyclonic eddies contributing towards these blocks is investigated. Anticyclonic eddies that contribute to blocking are defined as large positive 500 hPa geopotential height (Z500) anomalies, obtained by filtering in both space and time, that pass through a blocked region. These features are then followed using an objective feature tracking algorithm. It emerges that stationary blocking conditions are often the result of more than one transient anticyclonic eddy entering the block itself, which is a reflection of the selective absorption mechanism (SAM) for block maintenance, proposed by Yamazaki and Itoh (2013). A relationship is found between the number of anticyclonic eddies contributing to a block and the persistence of the block itself, with longer-lasting blocks typically absorbing more eddies than less persistent events; this behaviour is particularly noticeable in winter. The contribution of the smaller eddies to the blocks via the SAM is also observed as the anomalies speed up slightly just before entering the blocking region, before intensifying, becoming slow-moving, and sometimes reversing in direction inside the block itself. In addition to this climatological viewpoint, case studies have also been analysed to obtain a more detailed view of the process. From these, it is observed that some of the Z500 anomalies that contribute to blocking events originate from a long way upstream and travel along the wave guide until they are absorbed into a block, and this is again most evident in winter. The results from this work suggest there is an inherent link between repeated block maintenance and the persistence of block events, and also provides evidence that block maintenance processes may differ according to time of year.

How to cite: Suitters, C., Martinez-Alvarado, O., Hodges, K., Schiemann, R., and Ackerley, D.: The Importance of Anticyclonic Transient Eddies for Atmospheric Block Persistence, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-272, https://doi.org/10.5194/ems2022-272, 2022.

Atmospheric blocking events are high-pressure weather events which block the mid-latitude westerly flow and can last from days to weeks. Owing to their persistence they can lead to extreme surface weather in the affected regions. Until now the largest forecast busts are associated with the onset of atmospheric blocking due to their complex dynamical nature. 

In this work we investigate atmospheric blocking dynamics through the lens of four year-round blocked weather regimes in the Atlantic-European region. It is of our interest to compare the mid-latitude dynamics of these weather regimes, namely Atlantic Ridge, Greenland Blocking, Scandinavian Blocking and European Blocking by a PV framework and a local wave activity framework. The PV framework is applied on isentropes intersecting the tropopause. The PV dynamics quantify downstream dispersion of wave energy, PV eddy fluxes, baroclinic growth, and divergent outflow. The latter is assumed to communicate the impact of mid-tropospheric latent heat release to tropopause height. We complement the diagnostic by the finite-amplitude local wave activity budget.

We discuss the different dynamics for the four weather regimes by time-lagged composites from the last 40 years in the ERA5-reanalysis and -reforecast datasets. Our analysis reveals among others an intriguing variability in formation: Scandinavian blocking appears as part of an Rossby wave packet (RWP) propagating into northern Asia, whereas European blocking terminates the RWP.  We further find that moist-baroclinic growth has an important but mostly ‘’indirect’ effect on the formation of the blocked regimes: ”indirect” in the sense that the associated ridge amplification occurs upstream of the region over that the regime is subsequently established.

How to cite: Teubler, F., Polster, C., Wirth, V., Hauser, S., Grams, C., and Riemer, M.: Dynamics of blocked weather regimes in the Atlantic-European region: a combined PV and local wave activity approach, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-109, https://doi.org/10.5194/ems2022-109, 2022.

Atmospheric blocking describes a flow configuration that is associated with the meridional reversal of the large-scale atmospheric circulation in the midlatitudes. Due to the blocking of the mean westerly flow and the disruption of the eastward propagation of embedded synoptic-scale weather systems, such situations can lead to many kinds of extreme weather - within the block itself but also in adjacent regions. One blocking type of particular interest is the Greenland Blocking as it is more long-lived than blocking over other regions and strongly related to the negative phase of the North Atlantic Oscillation. Greenland Blocking has a strong surface impact, as the associated temperature anomalies can lead to ice sheet melting. It is therefore crucial to understand the underlying processes and dynamics of blocking in this region.

Here we look at Greenland Blocking from a year-round weather regime perspective based on ERA-5 reanalysis data from 1979-2021 focusing on the dynamical life cycle of the block. A diagnostic, quasi-Lagrangian potential vorticity (PV) framework has been developed and is applied to all Greenland Blocking weather regime life cycles. The framework allows the tracking of negative, upper-tropospheric PV anomalies in the Northern Hemisphere and assigns them to regime life cycles. With piecewise PV inversion, we partition the PV tendency equation into different contributions – separated into advective and diabatic PV tendencies – to disentangle different physical and dynamical processes that affect the amplitude evolution of the negative PV anomalies. This complements the quasi-Lagrangian framework and enables us here to distinguish the roles of dry and moist dynamics in the life cycle of the Greenland blocking regime.

Our results show that most negative, upper-tropospheric PV anomalies that are associated with Greenland Blocking regime life cycles are not generated locally over Greenland. We present different pathways by which the PV anomalies reach the area of the incipient block and analyze their seasonal dependence. A detailed investigation of the individual pathways and their associated negative PV anomalies reveals that the amplitude evolution of the PV anomalies is characterized by different proportions of dry and moist processes. Seasonal differences between the occurrence of these pathways emphasize the complexity of the underlying processes.

How to cite: Hauser, S., Knippertz, P., Quinting, J. F., Riemer, M., Teubler, F., and Grams, C. M.: A process-based understanding of Greenland Blocking regime life cycle dynamics in ERA-5 reanalysis from a potential vorticity perspective, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-120, https://doi.org/10.5194/ems2022-120, 2022.

How to cite: Lee, H.-I. and Nakamura, N.: Nonadiabatic and nonquasigeostrophic sources of upper tropospheric Rossby wave activity, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-250, https://doi.org/10.5194/ems2022-250, 2022.

An intense extratropical cyclone impacted western Europe on 9 February 2020 with wind gusts in excess of 35 m s^–1, heavy rainfall, and considerable inland and coastal flooding. This extratropical cyclone developed beneath the poleward-exit region of an anomalously strong North Atlantic jet streak that featured wind speeds in excess of 100 m s^–1 and enabled a British Airways 747 to break the record for a subsonic commercial flight between New York City and London at the time. Anomalously strong jet streaks, such as that which accompanied the aforementioned extratropical cyclone, also feature strong lower-tropospheric baroclinicity and strong horizontal and vertical wind shear favorable for the development of clear-air turbulence. Consequently, improved understanding of the processes that influence the development anomalously strong jet streaks is necessary not only because of their influence on the production of high-impact weather, but also their influence on aviation.

This presentation focuses on the kinematic processes that contribute to the development of anomalously strong jet streaks over the North Atlantic with wind speeds in excess of 100 m s^–1 during September-May 1979–2018, and how those processes vary across a large sample of cases identified within the NCEP Climate Forecast System Reanalysis. A composite analysis demonstrates that anomalously strong North Atlantic jets are most frequent during the winter compared with the fall and spring, and that their development is preceded by low-level warm-air advection, poleward moisture advection, and moist ascent within the warm conveyor belt of a surface cyclone beneath the equatorward jet-entrance region. A diagnosis of the irrotational and nondivergent components of the ageostrophic wind within the near-jet environment reveals that both wind components facilitate jet intensification via their nonnegligible contributions to negative potential vorticity (PV) advection and PV frontogenesis in the vicinity of the dynamic tropopause. The foregoing results indicate that a diagnosis of jet intensification during anomalously strong jet events is strongly dependent on an accurate representation of the cumulative effects of latent heating within the near-jet environment.

How to cite: Winters, A.: Kinematic processes contributing to the intensification of anomalously-strong North Atlantic jets, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-298, https://doi.org/10.5194/ems2022-298, 2022.

This study investigates the transition from current practical predictability of midlatitude weather to its intrinsic limit. For this purpose, estimates of the current initial condition uncertainty of 12 real cases taken from ECMWF's ensemble of data assimilations system are reduced in several steps from 100% to 0.1%. They are propagated in time with a global numerical weather prediction model (ICON at 40 km resolution) that is extended by a stochastic convection scheme to better represent error growth from unresolved motions. With the provision that the perfect model assumption is sufficiently valid, it is found that the potential forecast improvement that could be obtained by perfecting the initial conditions is 4-5 days. This improvement is essentially achieved with an initial condition uncertainty reduction by 90% relative to current conditions, at which point the dominant error growth mechanism changes: With respect to physical processes, a transition occurs from rotationally-driven initial error growth to error growth dominated by latent heat release in convection and due to the divergent component of the flow. With respect to spatial scales, a transition from large-scale up-amplitude error growth to a very rapid initial error growth on small scales is found. Reference experiments with a deterministic convection scheme show a 5-10% longer predictability, but only if the initial condition uncertainty is small. These results confirm that planetary-scale predictability is intrinsically limited by rapid error growth due to latent heat release in clouds through an upscale-interaction process, while this interaction process is unimportant on average for current levels of initial condition uncertainty.

How to cite: Selz, T., Riemer, M., and Craig, G.: The transition from practical to intrinsic predictability of midlatitude weather, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-398, https://doi.org/10.5194/ems2022-398, 2022.

Rossby Wave Packets (RWPs) are linked to extreme weather events and exert a strong influence on the predictability of weather systems in the midlatitudes. Considering the whole wave packet, in the sense of the packet envelope, RWPs can be viewed as entities that describe variability of the atmosphere beyond the synoptic scale.

We here examine the predictability of RWPs as such entities. As a skill metric we used the so-called Displacement and Amplitude Score (DAS) applied to the envelope field of the midlatitude flow. The DAS is based on a field deforming method and, as one of its major advantages, avoids the “double-penalty” verification problem without the need to identify single RWP-objects. Object-based methods tend to be highly sensitive to the choice of thresholds used to identify the objects and conceptual issues arise when assigning forecast to analysis objects. We assess RWP predictability using NOAA GEFSV12 ensemble reforecasts for RWPs that have been previously tracked in ERA5 data, due to the higher available temporal resolution. The ERA5 dataset was also used to quantify individual dynamical contributions to the amplitude evolution of single RWPs by partitioning the PV-tendency equation.

A prominent result is that RWP predictability depends on the stage of the RWP lifecycle: The propagation stage exhibits higher predictability than the decay or genesis stage. A small seasonal dependence is found, with summer being the least predicable season. No significant dependence is found on size and amplitude of RWPs or their geographical location. The presentation will further discuss differences in predictability that arise due to differences in the dynamical proccesses that contribute to the evolution of RWPs.

How to cite: Prestel-Kupferer, I., Schmidt, S., Riemer, M., and Teubler, F.: Predictability of midlatitude Rossby wave packets and their underlying dynamics, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-287, https://doi.org/10.5194/ems2022-287, 2022.

Warm Conveyor Belts (WCBs) are moist air streams that ascend from the warm sector of extratropical cyclones into the upper troposphere. Cloud-condensational processes along their ascent lead to the release of latent heat which further accelerates the ascending motion and results in cross-isentropic transport of lower-tropospheric air with low values of potential vorticity (PV) to the upper troposphere. This diabatically enhanced outflow intensifies the PV-gradient along the dynamical tropopause, accelerates the jet stream and ultimately modulates the upper-tropopsheric Rossby wave pattern through ridge amplification and downstream development. In numerical weather prediction (NWP) models, it is crucial to correctly represent this interaction of WCBs with the large-scale flow, as mis-representations thereof can lead to poor forecast skill in downstream regions. At the same time, this multi-scale interaction poses a major challenge for global NWP-models, as the governing moist-dynamical processes in WCBs are parameterized and introduce uncertainty into the forecast.

In this study, we investigate the role of WCBs for the error growth in medium-range forecasts over the North Atlantic in a systematic way. This is done by analyzing the relationship between commonly used error metrics of variables that characterize the large-scale flow and imprints of WCBs detected with trajectory analysis in three years of operational ECMWF ensemble forecasts.

Forecasts with high WCB activity are on average characterized by an amplified Rossby wave pattern and anticyclonic flow anomalies downstream. We find that the forecast skill is generally reduced when the WCB-activity is high, and that the WCB-activity is particularly increased around forecast times when the error growth is largest. To establish a relationship between the occurrence of WCBs and error growth, composites of normalized forecast error fields centered on WCB objects are computed. The composites reveal that anomalously high forecast errors associated with mis-representations of the Rossby wave pattern emerge along and downstream of WCB ascent and outflow. While there is considerable case-to-case variability in the mid-tropospheric error patterns, the structure of the errors associated with the upper-level jet stream is robust.

The WCB-relative errors are largest when the WCB-event occurs on forecast days 3-5, whereas WCBs at earlier leadtimes have a substantially reduced impact on the forecast errors, pointing towards an amplification of pre-existing errors by WCBs. Future work will attempt to further distinguish between situations in which WCBs amplify forecast errors, and in which WCBs are the source of forecast errors.

How to cite: Pickl, M., Grams, C. M., and Quinting, J. F.: The role of warm conveyor belts for medium-range forecast error growth, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-228, https://doi.org/10.5194/ems2022-228, 2022.

Subgrid-scale uncertainty for convective-scale forecasts and its sensitivity to synoptic conditions remain unclear. In this study, the relative importance of individual and combined perturbations representing various sources of uncertainties are addressed by simulating real case studies stratified into weak and strong synoptic convective-forcing situations using the quasi-operational setup of ICON-D2 model with the double moment microphysics scheme. We quantified and intercompared the impact given by three uncertainties, microphysics parameters (cloud condensation nuclei concentrations and shape parameters of cloud droplet size distribution), initial and boundary conditions (IBC), and physically-based stochastic perturbation scheme (PSP). The impact on two aspects of precipitation forecasts are assessed: daily accumulated precipitation amount averaged over Germany and temporal evolution of spatial variability of hourly precipitation. 
The relative impact on 24h precipitation is quantified against the mean of sub-ensembles, in which different parameters are perturbed. Microphysics perturbations and PSP show a minor impact compared to the impact of IBC perturbations, but combining IBC and microphysics perturbations extends the tails of the forecast distribution.  The responses to all the perturbations are relatively insensitive for strong-forcing situations.
Visual inspection of hourly precipitation fields reveals that microphysics perturbations slightly shift convective cells but vary precipitation intensities while IBC perturbations completely deform the spatial convective structure. We attempted to quantify the typical scales of modification using Fractions Skill Scores and found that microphysics perturbations have an explicit impact on the position of individual convective cells. On the other hand, PSP initiates convection earlier and affect the lifetime of cloud and convective precipitation, resulting in earlier and faster growth of spatial variability of hourly precipitation than microphysics. 
In the talk, additional comparisons against radar observations and benefits of practical forecast skills will be also presented. 

How to cite: Matsunobu, T., Keil, C., and Barthlott, C.: Impact of combined uncertainties on convective precipitation during different synoptic control, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-56, https://doi.org/10.5194/ems2022-56, 2022.

Ensemble forecasts have proven useful for diagnosing the source of forecast uncertainty in a wide variety of atmospheric systems. Ensemble Sensitivity Analysis (ESA) uses ensemble forecasts to evaluate the impact of changes in initial conditions on subsequent forecasts. Ensemble sensitivity is often used as a simple univariate regression as it approximates the analysis covariance matrix with diagonal elements on predicting the response to an initial perturbation. On the contrary, the multivariate ensemble sensitivity computes the ensemble sensitivities by incorporating the contribution from the full covariance matrix. In this work, the precipitation forecast responses of an extreme rainfall event from both univariate and multivariate ensemble sensitivity methods are analyzed. The ensemble forecasts and analyses are generated using an ensemble Kalman Filter (EnKF) coupled with the Advanced Research version of the Weather Research and Forecasting (WRF) model. Based on the results obtained the univariate ensemble sensitivity analysis shows broadly distributed sensitivity patterns, while the multivariate sensitivity analysis generally exhibits organized sensitivity patterns. The perturbation initial condition experiment applied to both methods proves that the ensemble sensitivity estimated by multivariate is more realistic compared to univariate ensemble sensitivity. In the presence of added model error, using the Stochastic Kinetic Energy Backscatter Scheme (SKEBS) it is found that the forecast response estimated by multivariate sensitivity compares better with the actual model response. Further, it is identified that if the localization used in multivariate is not sufficient its performance is contaminated by the occurrence of spurious correlations. The impact of lead times on both methods shows that multivariate provides better performance than univariate mainly at longer lead times when nonlinearity becomes important. The use of convection-permitting ensemble forecasts reveals that the multivariate ensemble sensitivity with localization ameliorates the sensitivity estimates in convective scales.

How to cite: George, B. and Kutty, G.: Multivariate Ensemble Sensitivity Analysis for an Extreme Weather Event Over Indian Subcontinent, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-382, https://doi.org/10.5194/ems2022-382, 2022.

For both the meso- and synoptic scale, reduced models exist that give insight into atmospheric dynamics. For the mesoscale, the weak temperature gradient approximation is one of several approaches, while for the synoptic scale,  the quasi-geostrophic theory is well established. However, how these two scales interact with each other, is usually not included in such reduced models, thereby limiting our current perception of flow dependent predictability and upscale error growth.

Here, we explicitly address the scale interactions by developing a two-scale asymptotic model for the meso- and synoptic scales with two, coupled sets of equations for the meso- and synoptic scale, respectively. The mesoscale equations follow a weak temperature gradient balance and the synoptic scale equations align with quasi-geostrophic theory. Importantly, the equation sets are coupled via scale interaction terms: eddy correlations of mesoscale variables impact the synoptic potential vorticity tendency and synoptic variables force the mesoscale vorticity (for instance due to tilting of synoptic scale wind shear). Furthermore, we impose different diabatic heating rates as proxies for the effect of latent heating on the different scales and distinguish between a weak, heating regime with O (4K/3.5h)  mesoscale heating rates and a strong heating regime with O (40K/3.5h) mesoscale heating rates. With weak heating, the upscale impact of the mesoscale on the synoptic scale is only of dynamical nature. With strong heating, the upscale impact also includes thermodynamical effects.
The scale interactions and the imposed diabatic heating can provide new insight into atmospheric dynamics, flow dependent error growth characteristics and predictability.

How to cite: Hirt, M. and Craig, G.: A two-scale model for the meso- and synoptic scales, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-104, https://doi.org/10.5194/ems2022-104, 2022.

In June 2021, the western North American continent experienced an intense heat wave with unprecedented temperatures and far-reaching socio-economic consequences. The magnitude of the heat wave was substantially underestimated by probabilistic weather forecasts for lead times beyond seven days. The record-breaking temperature anomaly coincided with a far northward extending upper-level ridge that was unambiguously linked to the intensity of the heat wave. During the 10 days preceeding the heat wave, the upper-level ridge was continuously fed by air masses originating to a substantial fraction from the lower troposphere that ascended in the West, Central, and East Pacific.
We analyze the role of strongly ascending airstreams, so called warm conveyor belts (WCBs), for the upper-level ridge amplitude, and illustrate how the anomalous WCB activity in the North Pacific limits the predictability horizon of this extreme event. We identify footprints of WCBs in operational ensemble forecasts from the European Centre for Medium-Range Weather Forecasts which is enabled through a machine-learning based diagnostic. The 51 member ensemble with lead times up to 15 days is stratified into a subset that best capture the upper-level ridge and potential vorticity anomaly ('good' members), and one with the largest discrepancy in the upper-level flow field ('bad' members). We find that the underestimation of the ridge amplitude over the North American continent in the bad forecasts is associated with a mis-representation of WCB activity across the West and East Pacific. While WCB outflow in the East Pacific maintains the ridge quasi-stationary and re-amplifies the pre-existing PV anomaly, WCB outflow in the West Pacific lifts the tropopause to anomalous heights and strengthens the upper-level jet, which facilitates East Pacific WCB ascent through downstream development. The mis-representation of this chain of synoptic events in the bad members finally results in an erroneous position and amplitude of the upper-level ridge and associated temperature anomaly. We conclude that the chain of synoptic events across the Pacific and their model representation play an essential role for the upper-level ridge position and amplitude and limit the predictability of the magnitude of the heat wave.

How to cite: Oertel, A., Pickl, M., Quinting, J. F., Hauser, S., Wandel, J., Magnusson, L., Balmaseda, M., Vitart, F., and Grams, C. M.: How warm conveyor belt activity across the North Pacific influenced the predictability of the North American heat wave 2021, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-227, https://doi.org/10.5194/ems2022-227, 2022.

The variability of the large-scale atmospheric circulation over the North Atlantic is governed by persistent and recurrent flow patterns, so-called weather regimes. Weather regimes describe preferential quasi-stationary states of the atmosphere that are characterised by persistent configurations of the large-scale flow patterns and the occurrence of synoptic weather systems in specific parts of the North Atlantic and western Europe. Increasing evidence suggests that the key processes responsible for the onset and persistence of such weather regimes are 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. However, how air mass transformations over the ocean, and the Gulf Stream, in particular, affect the large-scale flow and their role in the development of specific weather regimes are not fully understood.

Here, we focus on an episode of European blocking - a weather regime associated with an anticyclone over the British Isles - in the period between 20 and 27 of February 2019, which was accompanied by a record-breaking warm spell bringing temperatures above 20°C to the United Kingdom, Netherlands, and France. This particular event has been chosen due to its strong connection with air-sea interactions over the Gulf Stream region. Specifically, the formation of the anticyclone was preceded by several, rapidly intensifying cyclones originating in the Gulf Stream region and traversing the North Atlantic. These cyclones contributed to the establishment of the anticyclone over Europe by injection of low potential vorticity air into the upper troposphere. In addition, they were accompanied by multiple CAOs over the area of the strong sea surface temperature (SST) gradient associated with the Gulf Stream, resulting in intense upward fluxes of sensible and latent heat.  In order to quantify the dynamical linkage between the formation of this block and air-sea interactions over the Gulf Stream SST front, we adopt a Lagrangian perspective, using backward and forward kinematic trajectories to study the pathways of air masses forming the upper-level potential vorticity anomaly and interacting with the ocean front. The detailed investigation of the evolution of potential temperature, moisture and other variables along the trajectories, as well as of surface fluxes, SST and SST gradient underneath the trajectory path is carried out to examine the nature of the processes involved in upper-tropospheric flow variability. Determining the exact geographical location of moisture uptakes as well as their environment allows us to link air-sea interaction processes and the dynamical evolution of the flow. Thereby, we address the hypothesis that air-sea interaction processes along the Gulf Stream front, in particular during CAOs, is of fundamental importance for the maintenance of favourable conditions for cyclone intensification and the formation of European blocking.

How to cite: Wenta, M., Grams, C. M., Papritz, L., and Federer, M.: Air-sea interactions and diabatic processes in the Gulf Stream region and their role in the life-cycle of a blocking anticyclone: a case study of European Blocking in Feb 2019., EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-252, https://doi.org/10.5194/ems2022-252, 2022.

Heatwaves are prolonged periods of anomolously high temperatures. Also in combination with droughts, heatwaves can have devastating impacts on the environment, society and economy.  Dynamically, heatwaves are linked to large-scale, quasi-stationary atmospheric blocking patterns. Heatwaves have become more intense and more numerous over most land areas in the recent history. Due to the expected rise in global mean temperature, it is expected that this trend will continue. This general intensification in line with mean temperature may be modified by changes also in the underlying dynamical processes.

In order to study such potential changes in heatwave dynamics, we compare Lagrangian backward trajectories of air streams associated with historic (1991-2000) and future (2091-2100) heatwaves in six European regions. The heatwaves are identified based on a large ensemble of CESM climate simulations with  the help of the percentile-based Heat Wave Magnitude Index daily.

Compared to climatological values, air streams associated with heatwaves show a larger temperature increase along their trajectory, which is connected to stronger descent and stronger diabatic heating when the air parcels enter the boundary layer. For future heatwaves, the model projects a north-/northeastward shift of the origin of the air masses three days prior to the heat event in most study regions. Furthermore, we find larger diabatic temperature increases along the parcel trajectories in the future. This increasing importance of diabatic heating is more pronounced for heatwaves over continental regions. Boundary-layer diabatic temperature changes are driven by sensible heat fluxes, which are stronger over dry soils, associated with an increase of the Bowen ratio. The amplified diabatic heating associated with future heatwaves may thus be explained by an expected drying of the land surface.

How to cite: Schielicke, L., Allner, T., and Pfahl, S.: European heatwaves in present and future climate simulations: A Lagrangian analysis, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-602, https://doi.org/10.5194/ems2022-602, 2022.

Diagnostics of Lagrangian information about the atmospheric flow are usually obtained through the computation of trajectories. Trajectories provide detailed information along the pathways of individual parcels, but one has to consider a vast amount of them in order to obtain continuous-in-time, volume-filling information. As a consequence, analysing trajectories can be a painstaking task. To overcome this problem, we developed an alternative method allowing one to continuously diagnose Lagrangian information about the atmospheric flow on a Eulerian grid. The method is based on the advection of passive tracer fields and includes a relaxation term. Thus, it provides a field-based view on Lagrangian properties of the flow. The convenient output format allows one to analyse large data sets such as reanalysis data in a straightforward manner. Here, we make use of this method to quantify the processes of horizontal advection, downwelling, and diabatic heating associated with heat waves in Europe. More specifically, we compute the recent meridional and vertical parcel displacement and the recent parcel-based diabatic heating for each grid point at any time step to correlate these to the occurrence of temperature anomalies in Europe. In this way, we characterize European heat waves in terms of their Lagrangian bevahior. The analysis focuses on the recent decade, which experienced several severe heat waves in Europe, and encompasses case studies of recent major European heat waves as well as a more comprehensive statistical analysis. Our study furthers our knowledge on important mechanisms and drivers of European heat waves, which in turn will help to improve their forecasts.

How to cite: Mayer, A. and Wirth, V.: Lagrangian characterization of heat waves by Eulerian tracer advection with relaxation, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-46, https://doi.org/10.5194/ems2022-46, 2022.

Research on heat waves and extreme events in general is highly motivated by their impacts on human life and the economy. Therefore, the focus is on near surface variables and less research has been done on the state of the lower atmosphere as a whole. In a study of the mega heat waves of 2003 in France and 2010 in Russia, Miralles et al. (2014) investigate which factors have to come together to enable such extremes. One interesting finding is the gradual increase in planetary boundary layer height during those events. Also, their simulations display a correlation between mean potential temperature in the lower atmosphere and the boundary layer height. For these reasons, we believe that a systematic analysis of the planetary boundary layer during heat waves may provide valuable insights into their formation and persistence. We investigate whether these features are common traits of European heat waves in general. To this end, we apply a classification and regression scheme to vertical profiles taken from COSMO-REA6 data for the summers of 2014 to 2018. The reanalysis data is also used to identify heat waves, such that a comparison of boundary layers during normal and extreme conditions is possible. We analyse the distributions of planetary boundary layer heights for every grid point to check for regional differences. For validation, we make a comparison to radio soundings taken from the DWD Open Data service. The results of our work could possibly be used to improve the discriminability of different severity levels of heat waves or to formulate a heat wave measure that is not based solely on surface variables.

How to cite: Fohrmann, T., Hense, A., and Friederichs, P.: Investigation of the vertical structure of the lower atmosphere during heat wave conditions, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-524, https://doi.org/10.5194/ems2022-524, 2022.

Atmospheric hot extremes are amongst the deadliest climate hazards and a focal point of the public and scientific discourse on climate change. Yet, the relative importance of the involved physical processes, temperature advection, adiabatic warming due to subsidence, and diabatic heating, is still debated. Here, we resolve this controversy by quantifying the contributions of these processes to near surface temperature anomalies during the hottest days of the years 1979–2020 in the ERA5 data set (hereafter TX1day events) at a global scale. To this end, a novel temperature anomaly decomposition is developed which evaluates the Lagrangian temperature anomaly equation (derived from the thermodynamic energy equation) along kinematic backward trajectories. We first use this decomposition to show that the extreme near surface temperature anomalies (hereafter T') during the June 2021 heat wave in western North America were primarily produced by diabatic heating, and, to a smaller extend, by adiabatic warming, while advection did not contribute significantly. Then, we systematically decompose T' during TX1day events globally and find that their composition strongly varies geographically. Advection dominates over midlatitude oceans, adiabatic warming near mountain ranges, and diabatic heating over tropical and subtropical land masses. However, in many regions TX1day anomalies arise from a combination of these processes. The time and spatial scales of their formation are 60 hours and 1100 km in the global mean, respectively, with large variability. The formation of these extremes is, therefore, inherently non-local and features distinct formation pathways in different regions, which implies hitherto poorly explored mechanisms for changing magnitudes of hot extremes in a warming climate.

How to cite: Röthlisberger, M. and Papritz, L.: Quantifying the physical processes leading to atmospheric hot extremes at a global scale, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-76, https://doi.org/10.5194/ems2022-76, 2022.

Long-term projections of the future climate display a robust reduction of winter land-sea thermal contrast in the Northern Hemisphere (NH), caused by a faster warming of the cold continents compared to the warm oceans. The reduction is expected to be strong in the extratropics, a region where the thermal contrast is relevant for maintaining the strong baroclinicity near the western coasts of the continents and for shaping the NH jets and large-scale stationary waves.

In this work idealised perpetual-winter experiments characterised by a reduced land-sea thermal contrast are compared to control simulations featuring a thermal contrast similar to that observed in present-day climate. We use an intermediate-complexity AGCM with prescribed sea-surface and land temperatures. Warm temperature anomalies in East Asia and/or North America set a reduced thermal contrast in the whole NH or in individual NH sectors. We find that the Pacific-sector land-sea thermal contrast is by far more important than the Atlantic one for the large-scale mid-latitude circulation, as it impacts strongly the jet streams and the stationary planetary waves. While the local effects are coherent with the changes in baroclinicity brought by the surface forcing, the remote effects seem to be mediated by the response of the thermal and orographic components of the stationary waves. Based on the idealised-modelling results it is possible to hypothesise how the projected change in winter land-sea thermal contrast influences climate scenarios for the end of the XXI century. This factor has been rarely considered as a possible source of dynamical changes for the mid-latitude winter season.

How to cite: Portal, A., Pasquero, C., D'Andrea, F., Davini, P., Hamouda, M., and Rivière, G.: How is the extratropical circulation affected by reduced Atlantic and Pacific land-sea thermal contrast?, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-19, https://doi.org/10.5194/ems2022-19, 2022.

Extreme weather in midlatitudes is strongly associated with extratropical cyclones, which propagate within preferred latitudinal bands known as the storm tracks. The position, orientation and intensity of storm tracks make a key contribution to regional climate variability and change. Therefore, understanding changes in storm track location and dynamics under the influence of anthropogenic climate change is crucial for quantifying the socioeconomic risks posed by their associated extreme events.

Deep tropospheric jets, which affect the entire depth of the troposphere are typically present over storm tracks and understanding their interactions and symbiotic life cycle is essential for quantifying trends and variability in regional climate in all regions affected by storm tracks. In this work, we analyse modifications in deep jet characteristics forced by anthropogenic climate change over the North Atlantic sector during the winter season (December-February) by using ERA5 reanalysis data and Community Earth System Model (CESM) simulations for historical and future periods.

Firstly, deep jets are separated into three regimes (southern, medium and northern) defined by different latitudinal positions and life cycle characteristics, such as Rossby wave breaking (RWB) type. Next, forced changes in regime frequency and life cycles are identified by applying machine learning techniques aimed at distinguishing anthropogenic forcing from natural variability. Further, the goal is to quantify the role of additional drivers, such as the Artic amplification, tropical amplification, polar stratospheric vortex or variations in El Niño Southern Oscillation for the observed change in jet regimes.

Preliminary analyses based on ERA5 indicate an increasing preference for cyclonic RWB and southern jet regimes, which favour the development of explosive cyclones. In contrast, anticyclonic RWBs, which are favoured in the northern regime and are linked to the presence of potential vorticity streamers and heavy precipitation over western Europe, tend to become less frequent.

How to cite: Hermoso, A. and Schemm, S.: Climate change-induced jet variability over the North Atlantic: Trends and drivers, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-40, https://doi.org/10.5194/ems2022-40, 2022.

Climate models aim at representing as closely as possible the observed state of the climate components such as the atmosphere or the ocean. This is a fundamental requirement to correctly project changes in their dynamics due to anthropogenic forcing. In order to evaluate how closely models match observations, we need algorithms capable of selecting, processing and evaluating relevant dynamical features of the climate components. This has to be reiterated efficiently for large datasets such as those issued from the Coupled Model Intercomparison Project 6 (CMIP6). In this work, we use Latent Dirichlet Allocation (LDA), a statistical learning method initially designed for natural language processing, to extract synoptic patterns from sea-level pressure data and evaluate how close the dynamics of CMIP6 climate models are to the state-of-the-art reanalyses datasets such as ERA5 or NCEPv2.

LDA allows for learning a basis of decomposition of maps into objects called "motifs". Applying it to sea-level pressure data, reanalysis or simulation, robustly yields motifs that are known relevant synoptic objects, i.e. cyclones or anticyclones. Furthermore, LDA provides their weight in each of the maps of the dataset, their most probable geographical position and their possible changes due to internal variability or external forcings. LDA decomposition is efficient because most of the information of a given sea-level pressure map is contained in about 5 motifs, making it possible to decompose any map in a limited number of easy-to-interpret synoptic objects. This allows for a variety of new angles for statistical analysis.

We look at the dominant motifs and their distributions either on entire datasets or conditionally to particular extreme events, such as cold or heat waves, and compare results between reanalysis data and historical simulations. This enables us to assess which models can or cannot reproduce statistical properties of the observations, and whether or not there are properties that no model yet demonstrates. We find that models can capture the statistical synoptic composition of sea-level pressure data in general, but that some drawbacks still exist in the modelling of extreme events. LDA can also be applied separately to each dataset, and the two resulting synoptic bases can be compared. We find the sets of motifs from reanalysis and historical simulations are very similar, even if different spatial resolutions are used.

How to cite: Malhomme, N., Faranda, D., Podvin, B., and Mathelin, L.: Latent Dirichlet Allocation: a new machine learning tool to evaluate CMIP6 climate models atmospheric circulation, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-560, https://doi.org/10.5194/ems2022-560, 2022.

Extra-tropical cyclones (ETCs) are the main cause of precipitation in the mid-latitudes. Some, but not all, ETCs can lead to heavy precipitation and flooding. Furthermore, not all extreme ETCs in terms of maximum vorticity – a commonly used measure of ETC intensity – lead to extreme precipitation. Therefore, this study quantifies the relationship between maximum vorticity and ETC related precipitation in the current and future climates, and determines how this relationship depends on the structure and characteristics of the ETC. These aims are addressed using three 10-year-long aqua-planet simulations performed with a state-of-the-art global model, OpenIFS. The simulations only differ in terms of the specified sea surface temperature (SST) distributions which are held fixed in time. The control simulation has the well-known “QObs” SST distribution, the uniform warming simulation has a 4K warming relative to the control simulation applied everywhere, and the polar amplification experiment has a 5K warming applied poleward of 45 degrees. The feature tracking software TRACK is applied to objectively identify and track all ETCs in each experiment and obtain the maximum 850-hPa vorticity of each ETC. k-means clustering is then applied to the precipitation field surrounding each ETC to group the ETCs into clusters with similar precipitation structures.

In all experiments, ETCs with stronger maximum vorticity were associated with more precipitation. This relationship was strongest in the uniform warming simulation and weakest in the control simulation. Considerable spread, particularly in the uniform warming experiment, was present in this correlation suggesting that not all ETCs have a strong correlation between maximum vorticity and precipitation. This is confirmed by the clustering results. The k-means clustering identifies 4 distinct types of ETCs which are present in all experiments. For two types of ETCs – those located at high latitudes with weak precipitation and those with the precipitation located mainly in the centre of the ETCs – there is only a very weak relationship between maximum vorticity and precipitation. The strongest relationship between ETC maximum vorticity and precipitation occurs for ETCs that have most precipitation associated with the warm front. These results will be presented along with dynamical interpretations.

How to cite: Sinclair, V. and Catto, J.: The relationship between precipitation and extra-tropical cyclone intensity in different idealised climates, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-573, https://doi.org/10.5194/ems2022-573, 2022.