Displays

Ray paths of stationary Rossby waves emanating from a local mid-latitude source are usually refracted equatorward. However, this general tendency for equatorward propagation is mitigated by the presence of a midlatitude jet which acts as a zonal waveguide. This opens the possibility for circum-global teleconnections and quasi-resonance, which suggests that the ability of a jet to guide a wave in the zonal direction is an important property.

This paper investigates waveguidability of idealized midlatitude jets in a barotropic model on the sphere. A forced-dissipative model configuration with a local source for Rossby waves is used in order to quantify waveguidability by diagnosing the latitudinal distribution of waviness in a longitudinal sector far downstream of the forcing. Systematic sensitivity experiments show that waveguidability increases smoothly with increasing jet amplitude and with decreasing jet width. This result is contrasted with the predictions from two idealized theoretical concepts based (1) on ray tracing as derived from WKB theory and (2) on a sharp jet with a zonally oriented front of potential vorticity. The existence of two so-called turning latitudes, which is the key diagnostic for a zonal waveguide according to ray tracing theory, turns out to be a poor predictor for the dependence of waveguidability on jet amplitude and jet width obtained in the numerical simulations. By contrast, the meridional gradient of potential vorticity correlates fairly well with the diagnosed waveguidability. The poor prediction from ray tracing is not surprising, because the underlying WKB assumptions are not satisfied in the current context.

How to cite: Wirth, V.: Waveguidability of idealized midlatitude jets and the limitations of ray tracing theory , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2585, https://doi.org/10.5194/egusphere-egu2020-2585, 2020.

Rossby wave packets (RWPs) are a fundamental ingredient of midlatitude dynamics and organize the formation, propagation and decay of midlatitude weather systems. They may also constitute precursors to high-impact weather events. It is often expected that RWPs, as large-scale flow features obeying balanced dynamics, exhibit a large degree of predictability. Recent work, however, has shown that there is increased forecast uncertainty, in particular associated with the impact of moist processes, which may compromise medium-range predictability in the downstream region.

As a contribution to an improved understanding of these inherent uncertainties, we employ a quantitative potential vorticity (PV) – potential temperature framework to quantify different processes governing the evolution of troughs and ridges. This PV framework allows to fully separate the dynamics into four processes, namely the group propagation of Rossby waves, baroclinic growth, the impact of upper-tropospheric divergent flow, and direct diabatic PV modification.

The dynamical evolution of the amplitude of troughs and ridges within RWPs is examined from a composite perspective. The composite is based on the new ERA5 dataset and comprises 7164 RWPs. The direct diabatic contribution is estimated by the physical tendencies of the ‚Year of tropical convection‘ (YOTC) data. Additional to baroclinic downstream development, the composite analysis reveals a first-order impact of upper-level divergent flow for the amplification of ridges and the decay of troughs. We interpret divergent outflow as an indirect diabatic process associated with latent heat release below. Based on these results, we suggest extending the prevailing paradigm of downstream baroclinic development to include the systematic impact of moist processes. In the end potential implications for the predictability of RWPs are shown.

How to cite: Teubler, F. and Riemer, M.: Dynamical Evolution of Troughs and Ridges within Rossby Wave Packets: A Composite Study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7332, https://doi.org/10.5194/egusphere-egu2020-7332, 2020.

Tropical convection drives extratropical variability on subseasonal to interannual time-scales by exciting Rossby wave trains in the upper troposphere. Traditionally the relevant Rossby wave source is considered to be the sum of vortex stretching and vorticity advection by the divergent horizontal flow ( - ∇·u_χ (ζ+f) - u_χ·∇ (ζ+f)). Since absolute vorticity is very small at the equator, the equatorward flanks of the upper tropospheric jets have been regarded the source region of Rossby wave trains. In these considerations vertical momentum advection is neglected, although, it is an important source for westerly momentum at the equator. The curl of vertical momentum advection is the sum of vertical vorticity advection and vortex tilting ( -  ω ζ_p - ω_x v_p + ω_y u_p). These contributions are smaller than the traditional Rossby wave source in midlatidues by about one order of magnitude but they are of similar size in the tropics.

How to cite: Wicker, W. and Greatbatch, R.: A more complete Rossby wave source, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10398, https://doi.org/10.5194/egusphere-egu2020-10398, 2020.

Localised regions of negative potential vorticity (PV) are frequently seen on the equatorward flank of the upper-tropospheric jet streams in analysis and forecast products. Their positioning, on the anticyclonic side of the jet and often close to the jet core, suggest they are associated with an enhancement of jet stream maximum winds. Given that PV is generally positive in the northern hemisphere and is conserved under adiabatic conditions, the presence of negative PV is indicative of recent diabatic activity. However, little is understood on the mechanisms for its generation and subsequent lifecycle.

In this study, aircraft measurements from a recent field campaign are used to provide direct observational evidence for the presence of negative PV on the anticyclonic side of an upper-tropospheric jet. Theory is then developed to understand the process by which PV can turn negative. The key ingredient is diabatic heating in the presence of vertical wind shear, and the resulting PV anomalies are shown to always result from a flux of PV directed 'down the isentropic slope'. This explains why, for the typical situation of heating in a warm conveyor belt, negative PV values appear on the equatorward side of the upper-tropospheric jet stream close to the jet core. These ideas are illustrated with a semi-geostrophic model and the processes responsible for the observed negative PV are explored using an operational forecast model with online PV tracer diagnostics.

The diabatic influence on jet stream winds and shear is of interest because it is pertinent to the predictability of extreme jet stream events and associated flight-level turbulence, and is crucial to the propagation of Rossby waves at tropopause level, development of mid-latitude weather systems and their subsequent impacts at the surface.

How to cite: Harvey, B. and Methven, J.: Diabatic generation of negative potential vorticity and its impact on the jet stream, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19683, https://doi.org/10.5194/egusphere-egu2020-19683, 2020.

An ensemble of CESM atmosphere only experiments with varying soil moisture anomalies over Australia (+1 , 0 ,-1 STD) is analysed with respect to the atmospheric response. Locally an intensification of the surface heat low and an upper-level anticyclone is found for the negative anomaly. The local response to the low soil moisture content is driven by increase sensible heat fluxes and associated positive near-surface temperature anomalies.

A remote response of the upper-level flow consists of a downstream Rossby wave train extending along the jet waveguide and an upstream response projecting upon the main mode of variability the southern annular. The downstream response is driven by linear wave dynamics while the upstream response is modulated by non-linear wave dynamics and associated eddy fluxes. The sensitivity of the response to the background flow, i.e., different phases of ENSO is explored.

How to cite: Martius, O., Wehrli, K., and Seneviratne, S.: Local and remote Rossby wave responses to an anomalously dry or wet Australian continent , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9290, https://doi.org/10.5194/egusphere-egu2020-9290, 2020.

Eddy-driven jets are sustained through momentum transport by Rossby waves, which propagate along potential vorticity (PV) gradients. In the atmosphere, spatial variations in time-mean PV are mostly dominated by the variation of the Coriolis parameter with latitude. However, at high southern latitudes, a significant perturbation to the distribution and mixing of PV is provided by the Antarctic Plateau, which rises up to 4km above sea level. It is therefore possible that this orography affects Rossby wave propagation and hence affects the circulation in mid-latitudes.

We show through a set of semi-realistic and idealised experiments, that Antarctic topography plays a fundamental role in shaping the structure of the Southern Hemisphere extratropics. In particular, we perform runs with and without the Antarctic Plateau and demonstrate that the Plateau alters Rossby wave structure and propagation, thereby changing the momentum fluxes. Removal of the Plateau weakens the Indian Ocean jet and has a substantial effect on the flow downstream over the South Pacific. Here, the characteristic split jet pattern is destroyed and the flow at high latitudes stagnates. This also illustrates the prevalence of downstream development in the Southern Hemisphere and the strong connections between the flow over the South Pacific and Indian Oceans.   

How to cite: Patterson, M., Woollings, T., and Bracegirdle, T.: The influence of Antarctic topography on jet streams and Rossby waves in the Southern Hemisphere., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17811, https://doi.org/10.5194/egusphere-egu2020-17811, 2020.

How to cite: Di Carlo, E., Ruggieri, P., Davini, P., Tibaldi, S., and Corti, S.: Effects of mean state of climate models on the response to prescribed forcing: Sensitivity experiments with the SPEEDY general circulation model., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-341, https://doi.org/10.5194/egusphere-egu2020-341, 2020.

It is evident that persistent large-scale weather phenomena are an important factor in extreme seasonal climate; this has been especially true in boreal summers over the last two decades. Large, relatively slowly changing modes of variability on the mid-latitude jet are key to understanding high impact weather events. High monthly precipitation totals in the summer, for example, are linked to stationary Rossby wave patterns; stationary winter jet patterns can direct North Atlantic cyclones towards the UK and Europe. These wave patterns are often diagnosed but without a link to their phase speeds or dynamics.

To examine these slow modes we define an atmospheric background state as a function of isentropic and materially conserved co-ordinates (potential temperature and PV), resulting in a slowly changing, zonally symmetric background state. We then extract patterns of variability from the set of perturbations by employing an alternative Empirical Orthogonal Function (EOF) technique which utilizes a conserved wave activity as a weighted covariance. This results in statistical (EOF) patterns which possess an intrinsic dynamical phase speed and frequency, which are predicted from the conservation properties pseudomomentum and pseudoenergy. These statistical modes are a recombination of the dynamical normal modes in a system with quasi-linear dynamics.

We examine long runs with relaxation to unstable background jets but without orography, diurnal or seasonal effects, where large amplitude wave activity emerges. These simplified situations are used to test whether or not the predicted phase speeds from theory (given the structures found) matches with the observed phase speeds deduced from the principal component time series of the ENMs. Our hypothesis is that slow wave motion is explained by the structure and conservation properties of the modes. We are able to explore the dependence on the structures by varying the background state.

How to cite: Jones, D., Methven, J., Frame, T., and Berrisford, P.: Finding Dynamical Modes of Atmospheric Variability Using Conservation Properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22454, https://doi.org/10.5194/egusphere-egu2020-22454, 2020.

Studies of the Southern Hemisphere (SH) extratropical circulation are dominated by investigations of the zonally symmetric component of the Southern Annular Modular (SAM). However, there are significant asymmetries embedded in the zonal flow. In particular, a zonal wave 3 (ZW3) pattern is one of the dominant features of the SH circulation on daily, seasonal and interannual timescales. While the ZW3 circulation has had significant impacts on meridional heat transport and Antarctic sea ice extent in recent years, the physical mechanisms responsible for its presence still remain elusive. In this study, we use the Community Earth System Model (CESM) to understand the mechanisms that give rise to and modulate the ZW3 pattern in the SH extratropics. We examine, among other things, the popular belief that the ZW3 pattern is present due to the existence of three separate land-masses in the SH, namely Australia, Africa and South America, and whether it is modulated by both the land-ocean contrast and tropical forcing.

How to cite: Goyal, R., Jucker, M., Sen Gupta, A., and England, M.: Why is there a Zonal Wave 3 pattern in the Southern Hemisphere extratropical circulation?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12114, https://doi.org/10.5194/egusphere-egu2020-12114, 2020.

Recent climatological studies based on trajectory calculations have pointed to an important role of latent heating during cloud formation for the dynamics of blocking anti-cyclones. However, the causal relationship between latent heating and blocking formation has not yet been fully elucidated. To explicitly study this causal relationship, we perform sensitivity simulations of selected blocking events with a global weather prediction model in which we artificially eliminate latent heating in clouds upstream of the blocking anti-cyclones. This elimination has substantial effects on the upper-tropospheric circulation in all case studies, but there is also significant case-to-case variability: some blocking systems do not develop at all without upstream latent heating, while for others the amplitude of the blocking anticyclones is merely reduced. This strong influence of latent heating on the upper-level circulation is due to a combination of two effects: the direct injection of air masses with low potential vorticity (PV) into the upper troposphere in strongly ascending “warm conveyor belt” airstreams, and the indirect effect owing to the interaction of the associated divergent outflow with the upper-level PV structure. The important influence of diabatic heating demonstrated with these experiments suggests that an accurate parameterization of microphysical processes in weather prediction and climate models is crucial for adequately representing blocking dynamics.

How to cite: Pfahl, S., Steinfeld, D., Boettcher, M., and Forbes, R.: The sensitivity of atmospheric blocking to changes in upstream latent heating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17897, https://doi.org/10.5194/egusphere-egu2020-17897, 2020.

The horizontal and temporal variation of static stability prior to blocking onset is characterized through composite analysis of blocking events in the Southern Hemisphere. It is found that a local minimum of static stability in the upper troposphere and on the tropopause is achieved over the block-onset region when blocking onset takes place. From the perspective of isentropic potential vorticity, blocking onset is accompanied by extratropical tropopause elevation and a local low isentropic potential vorticity anomaly that is formed right under the elevated tropopause. This low isentropic potential vorticity anomaly is coincident with a local minimum of static stability over the block-onset region. In addition, based on static stability budget analysis, it revealed that the decrease of static stability in the upper troposphere and on the tropopuase prior to blocking onset is attributable to horizontal advection of low static stability from subtropics to midlatitude as well as the stretching effect associated with upper-level convergence, with the horizontal advection forcing being the primary contributor. On the other hand, the vertical advection of static stability tends to oppose the decreasing static stability through advecting more stable air downward such that it stabilizes the local air over the block-onset region. Furthermore, the indirect and direct effect of latent heat to the local change of static stability over the block-onset region are also discussed, respectively.

How to cite: Dong, L. and Colucci, S.: Static Stability Associated with Southern Hemisphere Blocking Onsets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3991, https://doi.org/10.5194/egusphere-egu2020-3991, 2020.

Petoukhov et al.’s (2013, PNAS) hypothesis of quasi-resonant Rossby waves as a mechanism for destructive weather extremes—both heat- and rain-related, observed and projected—has received a great deal of attention in recent years.  Most notably, it has been used for diagnostic studies of reanalysis products and full-physics atmospheric or coupled general circulation models. However, studies of this sort essentially assume (rather than test) the validity of the underlying theory.

Since the quasi-resonance theoretical arguments do not explicitly involve the full complexity of atmospheric physics, it ought to be possible to test them within the much simpler framework of an idealized general circulation model. By carefully constructing the forcing fields for such a model, we will achieve control of its zonal mean state and thus the waveguide properties of the zonal jet. We will explore the properties of the quasi-stationary Rossby waves in such simulations to test whether they have the properties predicted by Petoukhov et al. By testing this dynamical mechanism in a simplified model, we can better understand its applicability and limitations for investigations of future climate.

How to cite: Mooring, T. and Linz, M.: Investigating quasi-resonant Rossby waves with an idealized general circulation model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12992, https://doi.org/10.5194/egusphere-egu2020-12992, 2020.

Several studies on extreme precipitation events (EPEs) in the alpine area reported, as the main triggering factor, a meridionally elongated upper-level trough (i.e., a breaking Rossby wave) as part of an incoming Rossby wave packet (RWP). In this work, we investigate a vast number of EPEs occurring between 1979 and 2015 in northern-central Italy. The EPEs are subdivided into three categories (Cat1, Cat2, Cat3) according to thermodynamic conditions over the affected region. The three categories do not only differ locally but also in the evolution of precursor RWPs. These differences cannot be solely explained by the apparent seasonality of the flow; therefore, the relevant physical processes in the RWP propagation of each case are further investigated. In particular, we show that RWPs associated with the strongest EPEs, namely the ones falling in Cat2, undergo a substantial amplification over the western N. Atlantic due to anomalous ridge-building two days before the event; arguably due to diabatic heating sources. This type of development induces a downstream trough which is highly effective in focusing water vapour transport towards the main orographic barriers of the Apennines and the Alps. Finally, we identify an increasing trend of water vapour transport over the western N. Atlantic which is likely associated with the observed increase in Cat2 and Cat3 events

How to cite: Grazzini, F., Fragkoulidis, G., Teubler, F., Wirth, V., and Craig, G.: Rossby wave packets associated with extreme precipitation events over Northern-Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13233, https://doi.org/10.5194/egusphere-egu2020-13233, 2020.

Synoptic-scale Rossby wave-packets have a recurrent pattern during several episodes of persistent surface weather which is termed as 'recurrent Rossby wave-packets' (RRWP). They result in a statistically significant increase in winter cold and summer hot spells over large areas of the Northern Hemisphere mid-latitudes.

We present a global climatology of the RRWPs to study its spatial and seasonal variation. We also investigate the link of RRWPs to persistent surface extremes in the Southern Hemisphere (SH).  We find that RRWPs result in a statistically significant increase in winter cold and summer hot spells over broad areas in Australia and South America. Furthermore, we discuss the effects of climatological oscillations (Madden Julian Oscillation, ENSO, etc) on influencing the RRWPs.

How to cite: Ali, S. M., Martius, O., and Röthlisberger, M.: Are Recurrent Rossby wave packets linked to persistent extreme weather events in the Southern Hemisphere?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5168, https://doi.org/10.5194/egusphere-egu2020-5168, 2020.

The Red Sea Trough (RST) is an inverted trough of low-pressure system at lower tropospheric levels over the northeast Africa and the Red Sea. The previous research conducted on the RST suggests that when this system is activated, heavy rainfall occurs in large parts of the eastern Mediterranean and southwest Asia. The main aim of this article is to investigate the way Rossby wave activity at the upper level troposphere and its interaction with the lower tropospheric circulation activate the RST.

This study was carried out in three stages: first, the climatological behavior of RST in winter (December to February) was studied and then, cyclones were identified and tracked in the northeast Africa and the Red Sea using a cyclone tracking scheme. In the second stage, the Rossby wave activity flux at the 300 hPa level was considered in the region. Finally, the interaction between the wave activity flux and the RST was investigated. Two critical phases for the wave flux entering the region were considered. The critical positive (negative) phase corresponds to the month when on average the highest (lowest) values of the wave activity flux enter the northeast Africa and Red Sea regions. The results show that, during the critical positive phase, the RST strengthens and extends to the northeast of the Mediterranean Sea and cyclogenesis is increased in the northeast of Africa and especially in the northeast of the Red Sea.

With regard to the divergence of wave activity flux with an associated southward flux, the source of activity needed for cyclogenesis and reinforcement of the RST is provided by the North Atlantic storm track and the divergence core over the Mediterranean Sea. The results of the wave activity time series show that part of the activity from the northeast is integrated with the convergence core of the Mediterranean storm track, leading to enhancement of the cyclones in the northeast of the Red Sea and the extension of the RST to the northeast. But most of the activity joins the flux divergence core of the Mediterranean storm track in the west of the region and results in amplification of Sudan’s cyclones and activation of the RST along both the meridional and zonal directions; the important point to consider is that the wave activity flux entering the region is greater in the zonal direction. In addition to the southward propagation of the wave activity, the packets of flux convergence and divergence in the central North Atlantic are tilted in the southwest–northeast direction, indicating the dominance of anticyclonic Rossby wave breaking. Associated with the upper-level wave activity fluxes entering the region, there is jet enhancement and low-level cold advection from higher latitudes to the tropical and subtropical regions. The difference of RST between the critical positive and negative phases is turned out to be statistically significant with confidence levels of greater than 90%.

Keywords: Red Sea Trough, Northeast Africa and Red Sea cyclones, wave activity flux, critical positive and negative phases, Mediterranean storm track, North Atlantic storm track

How to cite: Alizadeh, Z., Mohebalhojeh, A., Ahmadi-Givi, F., Mirzaei, M., and Khansalari, S.: The impact of southward propagation of the upper-tropospheric Rossby wave activity on the Red Sea trough, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-762, https://doi.org/10.5194/egusphere-egu2020-762, 2020.

Floods or wet spells have increased over Pakistan in recent years, however a long-term classification of large-scale and synoptic-scale configuration for these events has been lacking. In this study, a total of 53 wet spells during the period of 1951-2015 over the core monsoon domain of Pakistan are identified. Based on daily geopotential height fields from NCEP/NCAR re-analysis, the dominant synoptic-scale systems, displaying distinct low-level circulation and moisture transport, are found during these wet spells over Pakistan. They are categorized as trough with low pressure system (LPS, 30 cases), trough without LPS (19 cases), and LPS only (4 cases) wet spells. Without the accompanying LPS over India, the trough tends to be deep and intrudes to south Pakistan with moisture transport mainly from Arabian Sea. In contrast, the trough is relatively shallow and interacts with presence of the LPS to steer moisture from the Bay of Bengal towards Pakistan. We found that subtropical trough associated with the blocking ridge over west Asia is an essential ingredient of wet spells over Pakistan. The patterns observed from wet spells over Pakistan are different from wet spells over the core monsoon domain of India, which is mainly dominated by LPS. The ridge development and blocking over Siberia is a precursor to wet spells over Pakistan and provides guidance for prediction.

How to cite: Sadaf, N., Lin, Y., and Dong, W.: Atmospheric blocking modulates the odds of heavy precipitation over Pakistan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4364, https://doi.org/10.5194/egusphere-egu2020-4364, 2020.

Transient Rossby wave packets (RWPs) are a prominent feature of the synoptic to planetary upper-tropospheric flow at the mid-latitudes. This prompts the development of diagnostic methods to identify and investigate the spatiotemporal evolution of key RWP properties. Such properties include the RWP phase speed and group velocity, the diagnosis of which has so far remained non-local in space and/or time. To this end, a novel diagnostic approach is presented here, which is based on the analytic signal of upper-tropospheric meridional wind velocity and thus allows the evaluation of RWP properties locally in space and time. The detailed insight into these properties can be utilized toward a better understanding of the upper-tropospheric circulation, its interplay with local weather features, and its model representation. In particular, climatologies of RWP amplitude, wavenumber, phase speed, and group velocity are investigated using reanalysis data for the time period 1979 – 2018. Pronounced features of seasonal and interregional variability are highlighted. Moreover, the role of RWP amplitude and phase speed in the occurrence and duration of temperature extremes in Europe is explored. Finally, indications of systematic biases in medium-range forecasts of these fields suggest that a correct representation of the RWP evolution is crucial for the predictability of temperature extreme events.

How to cite: Fragkoulidis, G. and Wirth, V.: Local diagnostics of Rossby wave packet properties – Seasonal variability and their role in temperature extremes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5411, https://doi.org/10.5194/egusphere-egu2020-5411, 2020.

Planetary waves are global scale waves in the atmosphere, which mainly dominate the atmospheric circulation in mid latitudes. It is discussed whether planetary wave activity increases due to the decrease of the meridional temperature gradient between the equator and the pole. As a result, large-scale weather patterns in mid latitudes should change, leading to a change in the occurrence of extreme weather events.

In order to analyze whether the occurrence of extreme temperature events has already changed, an algorithm was developed that identifies extreme temperature events in ERA5 temperature data from 1979 to 2019 in different height levels (1000hPa – 1hPa). We analyze the occurrence frequency of extreme temperature events in mid latitudes of the Northern Hemisphere as well as in Bavaria and in the Alpine region. To relate changes in the occurrence of extreme temperature events to possible changes of the planetary wave activity, we use the so-called dynamic activity index (DAI), which is operationally derived from ERA reanalysis temperature data at DLR.

In the troposphere, our analyses show that the occurrence frequency of heat events increases whereas the opposite holds for cold events. This is consistent with the expected effect of increasing average temperatures on the occurrence frequency of extreme temperature events. In the stratosphere, however, we observe an increase of cold events and a constant number of heat events. We conclude that tropospheric and stratospheric driving factors for the occurrence of extreme temperature events differ. The stratospheric development can be explained by increasing planetary wave activity as it is deduced from the DAI.

This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.

How to cite: Laux, D., Küchelbacher, L., Wüst, S., and Bittner, M.: First hints for the influence of planetary waves on extreme temperature events with a focus on Bavaria and the Alpine Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16147, https://doi.org/10.5194/egusphere-egu2020-16147, 2020.

Atmospheric blocking is a phenomenon in which a large, quasi-stationary anticyclone develops in the mid-latitudes and persists for several days or longer, blocking the ambient westerly winds and weather systems. Extremes on both ends of the temperature distribution are especially closely connected to atmospheric blocking (Brunner et al. 2017).

In this study the link between atmospheric blocking and Ukrainian cold and warm spells is investigated during winter and summertime in the period of 1991-2019 in order to provide better insight into the shifting role of blocking for extremes. Extreme temperatures are termed cold or warm spells if temperature stays outside the 10th to 90th percentile range at least six consecutive days. The detection of temperature extremes is based on daily minimum and maximum temperatures obtained for 12 meteorological stations that evenly cover territory of Ukraine. In the database obtained only the high-impact extreme temperature episodes are selected to be investigated in the further study.

The atmospheric blocking is detected on the basis of the daily 500 hPa geopotential height fields from the NCEP/NCAR reanalysis and potential temperature fields on the dynamical tropopause (PV = 2 PVU) obtained from ERA-Interim. In order to objectively diagnose atmospheric blocking two standard detection techniques are used. The first method utilizing the reversal of mid-latitude 500hPa geopotential height gradients was elaborated by Tibaldi and Molteni (1990) and detailed in Trigo et al. (2004), and the other one using reversal of potential temperature gradients was developed in Pelly and Hoskins (2003). These blocking detection algorithms identify fairly well the breaking of upper-level Rossby waves on 500 hPa height and on the dynamic tropopause, associated with onset of mid-latitude atmospheric blocking.

Up to 80% of winter cold and summer hot temperatures in Ukraine are associated with a collocated blocking. Large positive anomalies of 500 hPa geopotential height play a key role in maintaining prolonged extreme temperature spells and atmospheric blocking, though spell and blocking periods are much shorter than periods of positive anomalies. Spatio-temporal distribution of both indices are uneven, which meant that the wave-breaking process is not steady either at the 500 hPa surface or on the dynamical tropopause. Thus, during each episode the prolonged existence of ridges are maintained due not only to breaking of Rossby waves, but other mechanisms It should be mentioned that atmospheric blocking is more frequently revealed with the Tibaldi-Molteni indices than the Pelly-Hoskins ones, meaning that breaking of Rossby waves occurs more frequently at the 500 hPa geopotential height than on the tropopause.

How to cite: Popova, L. and Khomenko, I.: Links of Atmospheric Blocking to Temperature Extremes over Ukraine , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-758, https://doi.org/10.5194/egusphere-egu2020-758, 2020.

Heat waves impose large impacts on various sectors. Meteorologically, these events are co-located to upper-tropospheric anticyclones. In order to elucidate the formation of these anticyclones and the role of diabatic processes, we trace air masses backwards from the upper-tropospheric anticyclones and quantify the diabatic heating in these air parcels. We analyse anticyclones that are connected to summer heat waves at the surface during the period 1979 – 2016 in different European regions. Around 25-45% of the air parcels are diabatically heated during the last three days prior to their arrival in the upper-tropospheric anticyclones and this amount increases to 35-50% for the last seven days. The influence of diabatic heating is larger for heat wave anticyclones in northern Europe and western Russia and smaller in southern Europe. Interestingly, the diabatic heating occurs in two geographically separated air streams. Three days prior to arrival, one heating branch (western branch) is located above the western North Atlantic and the other heating branch (eastern branch) is located to the southwest of the target upper-tropospheric anticyclone. The diabatic heating in the western branch is related to the warm conveyor belt of a North Atlantic cyclone upstream of the evolving upper-level ridge. In contrast, the eastern branch is diabatically heated by convection, as indicated by elevated mixed-layer convective available potential energy along the western side of the matured upper-level ridge. Central Europe is influenced by both branches, whereas western Russia is predominantly affected by the eastern branch. The formation of the upper-tropospheric anticyclone, and therefore of the heat wave, is highly depended on the western branch, whereas its maintenance is more affected by the eastern branch. For long-lasting heat waves, the western branch regenerates. The results from this study show that the dynamical processes leading to heat waves may be sensitive to small-scale microphysical and convective processes, whose accurate representation in models is thus supposed to be crucial for heat wave predictions on weather and climate time scales.

How to cite: Zschenderlein, P., Pfahl, S., Wernli, H., and Fink, A. H.: A Lagrangian analysis of upper-tropospheric anticyclones associated with heat waves in Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4574, https://doi.org/10.5194/egusphere-egu2020-4574, 2020.

Heat waves can have a devastating impact on human society and ecosystems, and thus improved understanding and predictability of such events would provide huge benefits. It has been shown previously that many extreme temperature events are associated with quasi-stationary, or recurrent, Rossby waves (hereafter QSWs). We show that these QSWs are often associated with atmospheric waveguides, providing some dynamical understanding of why such weather patterns persist. In the context of this framework, we study the subseasonal-to-seasonal (S2S) predictability of heatwaves, QSWs, and atmospheric waveguides. Operational seasonal forecasts can reproduce the observed climatological statistics of QSWs, and the observed connection between QSWs and extreme temperatures over Europe, although with some biases. To better understand the underlying dynamics of the seasonal forecast models, we explore whether such models are capable of reproducing the observed connection between QSWs and atmospheric waveguides,  linked to persistent, and thus high impact, extreme heat events. We examine the S2S predictability of atmospheric waveguides and high amplitude QSW events, to better understand the potential S2S predictability of heatwaves.

How to cite: White, R., Prodhomme, C., Fragkoulidis, G., Materia, S., and Ardilouze, C.: Heatwaves and Predictability - the Role of Rossby Waves and Atmospheric Waveguides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15989, https://doi.org/10.5194/egusphere-egu2020-15989, 2020.

Summer 2010 was characterized by two contemporaneous extreme events: the Russian heat wave and the Pakistan flood. Several studies have shown a link between the two events, and Quasi-Resonant Amplification (QRA) has been suggested as an atmosphere-dynamic mechanism leading to the anomalous wavy circulation pattern which connected both extremes. Here, we aim at reproducing the 2010 circulation conditions in the Northern Hemisphere by obtaining a large ensemble of simulations from the Weather@home project within climateprediction.net (CPDN). We identify those ensemble members exhibiting a specific latitudinal temperature profile characterised by amplified high-latitude land warming (QRA - fingerprint) and investigate their surface temperature and upper level circulation properties. We show that when the QRA - fingerprint is present, the mid-latitude circulation bears similar characteristics to those observed in the 2010 summer: hot temperatures over European Russia and a wavy pattern in the upper-tropospheric meridional winds. As temperature profiles are projected to become increasingly similar to the QRA-fingerprint under future emission scenarios, these results provide further evidence that high latitude warming might favour persistent surface weather in the mid-latitudes.

How to cite: Di Capua, G., Kornhuber, K., Rousi, E., Sparrow, S., Wallom, D., and Coumou, D.: Wave-resonance fingerprint in the 2010 summer: a modelling experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16003, https://doi.org/10.5194/egusphere-egu2020-16003, 2020.

Rossby wave activity is intimately related to the day-to-day weather evolution over midlatitudes and to the occurrence of extreme events. Global warming trends may also affect their characteristics: for example, it has been hypothesized that Arctic warming with respect to midlatitudes, known as Arctic Amplification, may lead to a reduction in the speed of Rossby waves, to more frequent atmospheric blocking and to extreme temperature events over midlatitudes. Testing this hypothesis requires an estimate of the evolution and of the variability of phase speed in recent decades and in climate model simulations. However, measuring the phase speed of the global Rossby wave pattern is a complex task, as the midlatitude flow consists of a superposition of waves of different nature (e.g., planetary vs synoptic) across a broad range of wavenumbers and frequencies.

We propose here a framework, based on spectral analysis, to understand the variability of Rossby wave characteristics in reanalysis and their possible future changes. A novel, daily climatology of wave spectra based on gridded upper-level wind data is employed to study the evolution of Rossby wave phase speed over the Northern Hemisphere between March 1979 and November 2018. A global estimate of phase speed is obtained by doing a weighted average of the phase speed of each wave, with the associated spectral coefficients as weights.

Several insights about the drivers of phase speed variability at different time scales and their link with extreme temperature events can be gained from this diagnostic. 1) The occurrence of low phase speeds over Northern Hemisphere midlatitudes is related to a poleward displacement of blocking frequency maxima; conversely, the occurrence of high phase speed is related to blocking occurring at lower latitudes than usual. 2) Periods of low phase speed are associated with the occurrence of anomalous temperatures over Northern Hemisphere midlatitudes in winter, while this linkage is weaker during boreal summer. 3) No significant trend in phase speed has been observed during recent decades, despite the presence of Arctic Amplification. The absence of trend in phase speed is consistent with the evolution of the meridional geopotential gradient during recent decades. On the other hand, the high temporal resolution of the phase speed metric highlights the intraseasonal and interannual variability of Rossby wave propagation and points to 2009/10 as an extreme winter characterized by particularly low phase speed.

How to cite: Riboldi, J., Lott, F., D'Andrea, F., and RIvière, G.: A daily estimate of phase speed to explore the link between Arctic Amplification and Rossby waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9212, https://doi.org/10.5194/egusphere-egu2020-9212, 2020.

Rossby wave breaking on the dynamic tropopause (DT) occurs when synoptic-scale Rossby waves become highly amplified and undergo a breaking process.  This process can result in significant meridional transport of air masses resulting and intrusions of low latitude air poleward, high latitude air equatorward, or a combination of the two.  The ensuing modification of the troposphere and lower stratosphere in response to such events have been areas of considerable research due to their potential impacts on both high- and low-frequency mid- and high-latitude variability.  Furthermore, the processes and feedbacks associated with these events can result in notable changes to the jet structure and are frequently associated with atmospheric river events amongst other phenomena.  As such, the potential impacts of future changes in these events make them of considerable interest for identifying and studying in global climate model (GCM) simulations. 

Here, we apply a Rossby wave breaking identification scheme to three sets of 25-member Community Earth System Model simulations with prescribed sea surface temperature and sea ice conditions over the historical period (2010-2019), mid-Century (2050-2059) and late-Century (2090-2099).  This dataset represents a unique opportunity to study Rossby wave breaking processes in future climate simulations on a dynamically evolving surface rather than the more common pressure levels or isentropic levels as the DT is calculated for each of the CESM members.  Both anticyclonic and cyclonic Rossby wave breaking events are identified and tracked.  Events modeled in the historical period are compared to existing reanalysis data for the same period to explore the ability of the CESM model in this configuration to reproduce these events accurately.  Furthermore, the three periods of interest are examined to determine changes in the locations of Rossby wave breaking as well as the dynamic and thermodynamic characteristics of composited events. 

How to cite: Bowley, K. and Gervais, M.: Rossby wave breaking through the 21st century in a global climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19980, https://doi.org/10.5194/egusphere-egu2020-19980, 2020.