31/10/2020 - How reliable are long-term weather forecasts following weak polar vortex events?
New insights show that month-ahead temperature forecasts for Europe following sudden stratospheric warmings have to be interpreted with caution.
With the advent of winter, media reports on meteorologists speculating about the possible role of the so-called polar vortex in the Northern Hemispheric stratosphere will become more frequent again. Based on this, they will make predictions whether the coming months, or even the whole winter, will be rather mild and dry in Central Europe or whether we might rather experience cold air outbreaks with large amounts of snow.
Although such predictions might seem highly speculative at first glance, it makes indeed sense to carefully analyze the forecasts for the evolution of the polar vortex (a huge band of westerlies at 60 degrees north stretching from the lower to middle stratosphere at 10 to 50 km height in winter). This is because since some years science largely agrees on the fact that the stratospheric polar vortex provides one of the largest potentials for predictability of European weather on sub-seasonal timescales (15 – 60 days) during winter. This is due to the fact that the polar vortex directly connects to the upper troposphere (up to 10 km height) and thus the jet stream, which dominates the evolution of weather at our latitudes. Particularly in late winter, the polar vortex can weaken abruptly, which is called a “sudden stratospheric warming”. This weakening leads to a stronger meandering of the jet stream, which in turn can cause long-lasting Arctic cold air outbreaks into Europe and North America. In contrast, a particularly strong polar vortex is associated with a strong and zonally oriented jet stream, which causes rather long-lasting mild weather in Europe.
To exploit this large potential for sub-seasonal predictability, it is important that the numerical weather models correctly depict the polar vortex and its coupling with the troposphere. While recent studies often investigated and emphasized the enhanced large-scale predictability following sudden stratospheric warmings, a new study of our group in collaboration with the Swiss energy company Axpo Solutions  has come to the conclusion that the sub-seasonal forecasts for weather in Europe in such situations might not be as good as previously assumed. Instead, forecasts have shown to be much more reliable during phases of a particularly strong polar vortex. More specifically, the study has investigated how well the sub-seasonal weather model of the European Centre for Medium-Range Weather Forecasts (ECMWF) predicts country-averaged temperatures in Europe one month ahead, depending on the state of the polar vortex at the beginning of the forecast. This has revealed that the model on average predicts too cold anomalies particularly over Central and Southern Europe after an anomalously weak polar vortex. As a result, the forecast skill is even reduced for some countries (e.g., Spain, France, or Romania) compared to normal stratospheric conditions (see dark blue filled boxes compared to dark blue narrow boxes in Fig. 1). In contrast, the model has shown to predict the warm anomalies over large parts of Europe after an anomalously strong polar vortex remarkably well and thus significantly increase forecast skill for many countries (e.g., Germany, Spain, and France) compared to normal stratospheric conditions (see dark red filled boxes compared to dark red narrow boxes in Fig. 1). These new insights are surprising and important, as they question the regularly assumed enhanced sub-seasonal predictability following sudden stratospheric warmings at least from a regional perspective. At the same time, they show that sub-seasonal regional temperature forecasts following an anomalously strong polar vortex are much more reliable, which has been neglected so far. This becomes evident for countries like France and Spain revealing substantially enhanced forecast skill up to 4 weeks ahead, which is impressive and fascinating.
The reduced forecast skill for some regions following a weak polar vortex strongly indicates potential problems in the model in predicting the large-scale weather conditions, so-called weather regimes, in such situations. Indeed, further new findings of our group show that sub-seasonal forecast skill for weather regimes in Europe is significantly lower following a weak polar vortex compared to a strong polar vortex. Whether this is linked to the fact that the model might struggle to capture one of the very different weather regimes, which climatologically occur after a weak polar vortex [2, 3], is subject of our current research.
Dominik Büeler, Large-scale dynamics and predictability group
 Büeler, D., Beerli, R., Wernli, H., and Grams, C. M., 2020: Stratospheric influence on ECMWF sub-seasonal forecast skill for energy-industry-relevant surface weather in European countries. Q. J. R. Meteorol. Soc., https://doi.org/10.1002/qj.3866
 Beerli, R., and Grams, C. M., 2019: Stratospheric modulation of the large-scale circulation in the Atlantic-European region and its implications for surface weather events. Q. J. R. Meteorol. Soc., 145, 3732–3750, https://doi.org/10.1002/qj.3653
 Domeisen, D. I. V., Grams, C. M., and Papritz, L., 2020: The role of North Atlantic-European weather regimes in the surface impact of sudden stratospheric warming events. Weather Clim. Dynam., 1, 373–388, https://doi.org/10.5194/wcd-1-373-2020
07/05/2020 - Welcome Annika Oertel
We welcome Dr. Annika Oertel as a new postdoc in the „Cloud Physics“ and „Large-Scale Dynamics and Predictability“ groups. As part of the Transregio „Waves to Weather“ she will work on strongly ascending airstreams in extratropical cyclones, so-called warm conveyor belts (WCBs), and investigate the effect of ice microphysical processes on the evolution and intensity of WCBs using ICON and statistical emulation. She will also analyse the interaction between the representation of microphysical processes in WCBs and the large-scale circulation. During her PhD in atmospheric dynamics at ETH Zurich she combined satellite retrievals and radar observations of WCBs with high-resolution convection-permitting simulations to analyse the role of embedded convection in WCBs and its effect on the mesoscale and larger-scale circulation. Annika will be based on Campus South (office 13-5) and Campus North (office 316a, phone 26537) and her email is annika.oertel∂kit.edu. We wish Annika a good start at IMK-TRO and lots of fun with her future work and colleagues.
27/04/2020 - Why does it rain in Australia during El Niño?
Every 3-8 years, a significant increase in water temperature in the equatorial Pacific Ocean causes global changes in temperature and rainfall - El Niño. Though Peru and western North America expect above-average rainfall during El Niño, the densely populated southeastern Australia fears drought conditions and a high risk of bushfires. For example, the last El Niño event in 2015/2016 was classified as the third strongest El Niño event since 1965 and was characterized by long lasting heat waves, a very early start of the bushfire season and a record-breaking drought. Still, El Niño is not necessarily associated with reduced rainfall in southeastern Australia shown by close to average rainfall during the strong El Niño event of 1997/1998. So, why does it rain in Australia during El Niño? Are large-scale atmospheric flow patterns and embedded weather systems responsible for this rainfall variability between similarly strong El Niño events?
In collaboration with Prof. Michael Reeder and Dr. Shayne McGregor from Monash University in Melbourne, these questions were investigated in our research group "Large-scale dynamics and predictability" . By clustering monthly rainfall anomalies between June and November, a total of four monthly rainfall patterns could be found in southeastern Australia (Fig. 1): wet southeastern Australia (Cluster 1), dry southeastern Australia (Cluster 2), wet East Coast (Cluster 3) and wet South Coast (Cluster 4). Nearly 50% of the El Niño months considered are assigned to the dry pattern (Cluster 2), which reflects the general dry conditions expected in southeast Australia during El Niño.
In order to advance the meteorological understanding driving these patterns, objectively identified weather systems are used . The focus is on cut-off lows, extratropical cyclones, atmospheric blocking, moisture transport and warm conveyor belts (WCBs), which represent the main-ascending, precipitating part of extratropical cyclones. During dry El Niño conditions in southeastern Australia, a blocking high pressure system is centred over the southern part of the continent (Fig. 2b). This suppresses the occurrence frequency of rain-bringing WCBs, cut-off lows, and extratropical cyclones. In contrast, composites of the unusually wet Cluster 1 show a high frequency of blocking high pressure systems southeast of Australia and an increased frequency of cut-off lows along the South Coast and WCBs more inland. In addition, enhanced moisture transport from the Tropics across the continent supports rain-bringing WCB activity in southeastern Australia (Fig. 2a). It is the change in frequency of WCBS and cut-off lows that explains the wet conditions but not the change in rainfall intensity.
To return to the initial question, wet conditions during El Niño occur in southeastern Australia on monthly time-scales due to changes in the activity of weather systems. The results of this study highlight that these weather systems may override the effect of El Niño which is important when aiming for skilful seasonal forecast in this region.SH & JQ
09/01/2020 - Research stay at the University of Tsukuba
After the successful start of our collaboration with colleagues from the Center for Computational Sciences at the University of Tsukuba (Japan) in May 2019, we spent several weeks at Tsukuba University in November. The collaboration between our groups at the University of Tsukuba and KIT is funded by the DAAD joint research project "Weather regimes in Europe and Asia: sub-seasonal predictability (WEASP)".
Following our discussions in May, we continued to work on two main working packages: 1.) a year-round definition of East Asian weather regimes and 2.) the verification of these regimes in sub-seasonal numerical weather prediction models.
The definition of East-Asian weather regimes turned out to be a challenging task since the occurrence of some regimes is limited to summer or winter. Also, circulation patterns in this region of the world are more transient which requires a different approach than for weather regimes in the Atlantic-European sector. Despite these difficulties, we made huge progress in finding a year-round definition for East-Asian weather regimes. Thanks to in-depth discussions with our Japanese colleagues it was possible to come up with a year-around weather regime definition for East Asia including Siberia. Furthermore, we newly identified year-around weather patterns specifically focusing on weather and its extremes in Japan. These definitions lay the basis for future studies on dynamical processes that influence weather regimes and weather patterns in East Asia.
Concerning the second working package (the verification of East Asian weather regime forecast), we discussed on how to best calibrate forecasts in order to account for systematic model biases. When investigating the flow dependent forecast skill of numerical weather prediction models, typical bias corrections may misleadingly degrade the actual forecast skill. We therefore developed a new calibration techniques which will be useful to elucidate the overall forecast skill of weather regimes in East Asia but also in the Atlantic-European sector.
Besides the meetings with our colleagues at the University of Tsukuba, Christian gave a seminar at the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) in Tsukuba and visited colleagues at JAMSTEC (Japan Agency for Marine Earth Science and Technology). One of the highlights of our trip was a symposium on climate science at University of Tokyo with Professor Brian Hoskins, Mio Matsueda, Taroh Matsuno and other colleagues from Japan. After the symposium, we took the fast and frequently operating (every 5-10 minutes) Shinkansen train to spend a weekend in Kyoto (the former Japanese capital) where we visited many temples and shrines and enjoyed the Japanese culture. We would like to say arigato gozaimasu (ありがとうございます) (thank you very much) to our colleagues in Japan and look forward to their next visit in February./JW & JQ
18/12/2019 - White Christmas: A challenge for Subseasonal Prediction!?
Christmas 2019 is approaching and people are wondering early in December “What are the odds of a cold, snowy White Christmas?” or “Will it be a warm and windy holiday season as in previous years?” Questions alike are not only of public interest, they are also relevant to various socio-economic activities such as renewable energies or the transportation sector. Episodes of exceptionally warm or cold temperatures, strong or weak winds, or heavy precipitation typically last for several days and affect continent-size regions. Weather on these spatio-temporal scales is governed by weather regimes, which are large-scale circulation patterns in the lowest 10 km of the atmosphere. Further up – in the stratosphere – the circulation during winter is dominated by strong circumpolar westerly winds forming the stratospheric polar vortex (SPV). The intensity of the SPV usually changes gradually within a few weeks, but occasionally the westerly winds break down abruptly, which is called “sudden stratospheric warming” (SSW). Because of the dynamical coupling between the stratosphere and the troposphere below, extreme anomalous states of the SPV can induce persistent surface weather conditions. In particular, SSWs are believed to be followed by cold and calm conditions in Europe.
In a recent study , we found that not only the most extreme states of the SPV change the odds for specific weather regimes but more generally also above-normal, normal, or below-normal SPV conditions (Figure 1a). Nevertheless, large-scale weather extremes in different European sub-regions, happen during any SPV state. This can be explained by the fact that most large-scale weather extremes occur during more than one preferred weather regime (Figure 1b). Interestingly, a few weather regimes occur irrespective of the SPV state. These regimes thus provide pathways to weather extremes that might be unexpected with respect to the current SPV state. For example, the Atlantic Trough (AT) regime occurs during any SPV state and thus provides a pathway to high wind events in Central Europe also during weak SPV states, while other regimes causing such events (ZO, ScTr) are suppressed.
These findings suggest that numerical weather prediction models require not only a correct stratosphere-troposphere coupling but also a correct representation of weather regimes to fully exploit the potential predictability from the stratosphere. Indeed, the subseasonal forecast issued by ECMWF correctly indicated a dominance of mild and windy regimes (ZO, ScTr, AT) in the first half of December 2019 more than four weeks ahead (Figure 2). These regimes will dominate until Christmas. So at least for Central Europe the chances for White Christmas are low. However, models consistently predict a change of the prevailing regime over the holidays, giving a little hope for belated snow at least for some of us.
Ongoing research in the LSDP group now investigates systematically how the forecast skill for surface weather depends on the SPV state and the correct representation of weather regimes  and what determines the specific regime response and associated surface weather following SSW events ./ DB & CG & JQ
 Beerli, R., and C. M. Grams, 2019: Stratospheric modulation of the large-scale circulation in the Atlantic–European region and its implications for surface weather events. Q.J.R. Meteorol. Soc., 145, 3732–3750, doi:10.1002/qj.3653.
 Büeler, D., R. Beerli, H. Wernli, and C. M. Grams, 2020: Stratospheric influence on ECMWF sub-seasonal forecast skill for European energy-industry-relevant surface weather. In preparation for Q.J.R. Meteorol. Soc.
 Domeisen, D. I. V., C. M. Grams, and L. Papritz, 2019: The role of North Atlantic-European weather regimes in the surface impact of sudden stratospheric warming events. Weather and Climate Dynamics, submitted.
06/10/2019 - 19th Cyclone Workshop
For the 19th time, scientists from all over the world attended the “Cyclone Workshop”. The workshop is traditionally held every two years in the United States or Canada. Due to the 100th anniversary of the famous Bergen School of Meteorology paper, this year’s workshop took place at the beautiful scenery of the Monastery Seeon from 29 September to 04 October. The intense and exciting program included presentations on the dynamics of weather systems at all spatio-temporal scales, from supercells and tornadoes to cyclone families, atmospheric blocks and large-scale weather regimes. Christian, Dominik, Moritz, Julian and Seraphine attended the workshop and presented their research highlights in posters and talks. In addition to scientific talks and poster presentations, recreational breaks as well as interactive evening sessions helped to establish new contacts in the community. As a follow up of the workshop, Maria Madsen (University of Wisconsin) and Ben Moore (NOAA) will visit the SPREADOUT group to work on future collaborations.
01/08/2019 - Welcome Seraphine Hauser
We welcome back Seraphine Hauser in the group “Large-scale Dynamics and Predictability”. Seraphine studied Meteorology at KIT and always had a strong interest in weather systems. Consequently already in her Bachelor’s thesis she performed a synoptic analysis of a dust storm in spring 2015 over the Arabian Peninsula. For her Master thesis she joined our team and discovered an important impact of weather systems in ENSO related precipitation anomalies for Southeastern Australia. This included an intense collaboration with colleagues in Australia and a three-month research stay at Monash University. Seraphine now starts her PhD research in which she will investigate blocked weather regime life-cycles from a potential vorticity dynamics perspective making extensive use of ERA5-reanalysis data. This research is embedded within the Transregional Collaborative Research Center “Waves2Weather” and in close collaboration with colleagues at University of Mainz and the Young Investigator group “SPREADOUT” at IMK-TRO. Seraphine is based at Campus North temporarily in the Master’s room of IMK-TRO (building 435, office 204, phone: 23174). We wish Seraphine a happy start, a successful PhD time, and a lot of fun working in W2W and at IMK. Welcome! /CG
26/06/2019 - European heat also develops locally
This text is adapted from an official KIT expert’s statement published on 26.6.2019. The original text (in german) written by Timo Schreck is available at http://www.sek.kit.edu/kit_express.php.
Central Europe suffers from the current heat wave. The danger of forest fires and health impacts accompanies such extreme weather. Today, we expect the hottest day of the week with maximum temperatures of up to 40 degree Celsius. Often air masses from the Sahara are said to cause these hot temperatures. Researchers at IMK-TRO now show that this is only half of the story – the near-surface air causing the blazing heat in Central Europe is also produced locally.
„Heat waves are periods of at least three consecutive days with maximum temperature in excess of the top 10% of the hottest days in a season”, explains Andreas Fink from IMK-TRO at KIT. To define these hottest days, mostly a reference period of the last 30 years is considered. However, the current hot near-surface air does not originate from the Sahara but is produced locally, as recent research by Philipp Zschenderlein’s project “Predictability of heat waves in Europe” within the DFG-funded collaborative research center “Waves to Weather” (https://www.wavestoweather.de/) shows: „On the way to southern Germany, the air mass in the lower-levels (up to 2km) descend and warm by so-called adiabatic compression. This process can be imagined like the warming in a bicycle tyre inflator. This time the warming due to descent is unusually high compared to other heat waves.“
Julian Quinting and Christian Grams of the Young Investigator Group “Large-scale Dynamics and Predictiability” at IMK-TRO computed the origin of the current near-surface air mass which reveals that it originates from the Baltic Sea region (Fig. 1). On Saturday, its air temperature was only about 10 degree Celsius, but at a higher altitude. Air from the Sahara still plays a role as the two researchers add: „It conditions the large-scale environment to allow local warming over Europe. In mid-levels of 3-5 kilometres air from North Africa are advected towards Europe and enhance the high pressure system called ‘Ulla’. Numerical weather forecasts indicated the start of this large-scale weather regime characterised by high pressure over Europe and low pressure over the East Atlantic already 10 days ahead of the peak of the heat wave which allowed a very reliable forecast.“
The mid-level air from the Sahara transports unusually high amount of dust, as revealed by novel dust forecasts with the numerical modelling system ICON-ART, run by Heike Vogel in collaboration with the Deutscher Wetterdienst (DWD) (Fig. 1). This also affects the forecast of maximum temperatures, as her colleague and leader of the working group “Aerosols, Trace Gases and Climate Processes”, Bernhard Vogel, adds: „The high amount of dust cause uncertainty in the forecast of maximum temperature. The dust reduces the direct insolation but could also influence the formation of mid- and high-level clouds and thus reduces insolation significantly.” This is an important topic of current research activities at IMK-TRO.
Research on heat waves at KIT is one of the key competences of IMK-TRO. Next to a Young Investigator Group, KIT is also involved in several research projects: The DFG-funded transregional collaborative research centre „Waves to Weather“ (W2W) aims – amongst others – to advance predictability of heat waves. Within the sub-project „The role of multi-scale Dynamical Processes in shaping recent and future extreme Heat waves over Germany (DynProHeat)“ of the BMBF initiative „Climate Change and Extremes“ (ClimXtreme) Andreas Fink and Joaquim Pinto will investigate how climate change will affect heat waves. The project „Solar power reduction due to Saharan dust“ (PerduS) in collaboration with the Deutscher Wetterdienst (DWD) aims to improve forecasts of solar power output during Saharan dust events.
WG Atmospheric Dynamics https://www.imk-tro.kit.edu/english/5874.php
WG Large-scale Dynamics and Predictability http://www.imk-tro.kit.edu/english/7425.php
WG Aerosols, Trace Gases and Climate Processes http://www.imk-tro.kit.edu/english/3487.php
Bieli, M., S. Pfahl, and H. Wernli, 2015: A Lagrangian investigation of hot and cold temperature extremes in Europe. Q.J.R. Meteorol. Soc., 141, 98–108, doi:10.1002/qj.2339 .
Quinting, J. F., and M. J. Reeder, 2017: Southeastern Australian Heat Waves from a Trajectory Viewpoint. Mon. Wea. Rev., 145, 4109–4125, doi:10.1175/MWR-D-17-0165.1 .
Rieger, D., and Coauthors, 2015: ICON–ART 1.0 – a new online-coupled model system from the global to regional scale. Geosci. Model Dev., 8, 1659–1676, doi:https://doi.org/10.5194/gmd-8-1659-2015 .
Schaller, N., J. Sillmann, J. Anstey, E. M. Fischer, C. M. Grams, and S. Russo, 2018: Influence of blocking on Northern European and Western Russian heatwaves in large climate model ensembles. Environ. Res. Lett., 13, 054015, doi:10.1088/1748-9326/aaba55 .
Sprenger, M., and H. Wernli, 2015: The LAGRANTO Lagrangian analysis tool – version 2.0. Geosci. Model Dev., 8, 2569–2586, doi:10.5194/gmd-8-2569-2015 .
Zschenderlein, P., G. Fragkoulidis, A. H. Fink, and V. Wirth, 2018: Large-scale Rossby wave and synoptic-scale dynamic analyses of the unusually late 2016 heatwave over Europe. Weather, 73, 275–283, doi:10.1002/wea.3278 .
14/05/2019 - Successful first research stay of our partners from University of Tsukuba at KIT
During the first week of May, we had the opportunity to welcome our research colleagues from University of Tsukuba (Japan), Mio Matsueda (Assistant Professor), Akio Yamagami (PostDoc), and Takumi Matsunobu (MSc Student), at KIT. Their stay was the kick-off visit in the framework of our two-year DAAD joint research project “Weather regimes in Europe and Asia: subseasonal predictability (WEASP)”, which aims to foster collaboration between our groups at University of Tsukuba and KIT.
The visit entailed numerous fruitful meetings, in which we discussed first results of our common research, exchanged expertise on open challenges, and ultimately defined specific goals and tasks we want to achieve during the upcoming year. More specifically, we advanced the two following main work packages: first, we will come up with a year-round definition of East Asian weather regimes, investigate their dynamical characteristics with a focus on diabatic outflow, and verify their predictability in subseasonal numerical weather models, similarly to what we already do for the Euro-Atlantic weather regimes. Considering the strongly varying and thus fascinating climate of Japan, which ranges from heavy snow falls triggered by the mixing of cold Siberian air masses with humid maritime air masses from the Subtropics to devastating Typhoons that regularly hit the island, this research may ultimately help to improve operational subseasonal forecasting in Japan. To this end, the knowledge of our Japanese colleagues about East Asian climate perfectly complements the dynamical, weather system-oriented expertise about weather regime life cycles at SPREADOUT. In a second work package, we will draw upon the strong technical and statistical expertise of our colleagues to verify the subseasonal predictability of year-round Euro-Atlantic weather regimes, which will be the foundation for many research goals of SPREADOUT. Beside these main topics, we exchanged further ideas related to ongoing work on extreme events in Japan, tropical influences on Japanese climate, and dynamical links between Euro-Atlantic and East Asian weather regimes. The diverse research topics of our guests from Tsukuba was also of interest in a number of side meetings with colleagues at IMK-TRO. As a final scientific highlight, the delegation from Japan presented highlights from their past and ongoing work in the “Karlsruhe Meteorologisches Kolloquium” (the IMK-wide external seminar): Mio presented some of his extensive work on the verification of medium-range predictability of Euro-Atlantic winter weather regimes and introduced the publicly available TIGGE and S2S museums. Akio continued with an overview of his research on the dynamics of extraordinary Arctic Cyclones and Takumi finished with his work on the predictability of a past heavy precipitation event over Japan.
Since this was the first longer visit of Europe for some of our guests, we of course did not want to miss out on introducing them to some of the natural and culinary beauties of Southern Germany. We thus went for a hike in the Black Forest on the sunny 1st of May, visited Heidelberg on a cold Sunday, and ate delicious local food in various restaurants in Karlsruhe, which included tasty asparagus, good beer, and a ride on the legendary slide in the “Badisch Brauhaus”. In return, Mio, Akio and Takumi told us a lot about their certainly different but very interesting culture, about their traditions, and about how science works in their country. After an intense week, we all agreed on the success of their stay and the potential of our collaboration. Therefore, we are excited to continue with our joint research project WEASP and hopefully exchange many new results when we from SPREADOUT will visit them in fall 2019.
For more information about the research of our colleagues, visit the webpage of Mio Matsueda’s group at University of Tsukuba. / DB
29/04/2019 - Spreadout at the S2S/TIGGE workshop at ECMWF in Reading, UK
We recently had the opportunity to attend the inspiring and perfectly organized "Workshop on predictability, dynamics and applications using the TIGGE and S2S ensembles" at the European Centre for Medium-Range Weather Forecast (ECMWF) in Reading (UK). The workshop gathered a diverse group of people from research, industry, and national weather services from all over the world who work with the two popular model intercomparison datasets for medium-range (TIGGE) and subseasonal (S2S) weather prediction.
Reading's busy morning traffic does not make it easy for the buses to reach the "Weather Centre", as the friendly Britains like to call it, on time. But with some patience and strengthened by a classic English breakfast, we succeeded and were ready to start the workshop. Jan and Christian both presented our operational weather regime forecast products. They showed their usefulness not only for operational weather forecasting but also for understanding how synoptic-scale processes, such as cloud-condensational heating in midlatitude cyclones, affect the subseasonal predictability of weather regime life cycles. Both contributions triggered lots of discussions with researchers working on large-scale dynamics, colleagues working on numerical model development, and forecasters seeking for better operational tools. Dominik presented an applied research project in cooperation with Remo Beerli (Axpo Solutions AG), which quantifies the skill of the ECMWF subseasonal model in predicting month-ahead, country-averaged surface weather over Europe after particularly strong and weak states of the stratospheric polar vortex during winter. Due to the sudden stratospheric warming at the beginning of this year and the failure of weather models (and thus the energy industry) to correctly predict its impact on surface weather, this contribution gained particular attention from energy meteorologists participating in the workshop, but also from model developers trying to better understand model biases associated with such dynamically complex events.
Numerous interesting contributions highlighted the great potential in using ensemble data for better predictions on subseasonal time scales. However, many studies also documented the lack of forecast skill for the midlatitudes beyond two weeks. It became obvious that models still struggle to fully exploit potential sources of extended range predictability (such as the stratosphere or the MJO). Working group discussions elaborated recommendations to the WMO WWRP/WCRP programs on how to make even better use of the TIGGE and S2S datasets and on the research needed to further improve numerical models. It clearly emerged that there is a need for a better understanding of the processes involved in teleconnections on subseasonal time scales and that the focus for the midlatitudes should be on large-scale regimes. Furthermore, the workshop showed us once more that understanding predictability and improving forecast skill on subseasonal timescales will be one of the key focuses for the coming decade, not just at ECMWF but all around the world. With our process-oriented approach in SPREADOUT to better understand European weather regimes, we are keen to help tackling this challenge!
We ended our week in Reading with an in-depth personal discussion with our collaborators at ECMWF. This was fruitful and very motivating because it once more showed their interest in our research and allowed us to further deepen our plans and specify the next steps in our collaboration.
23/04/2019 - Highlights in research on extratropical transition
Tropical cyclones (TCs) that move into the midlatitudes undergo a chain of processes that is called “extratropical transition” (ET). Key ideas of SPREADOUT research emerged from our roots in ET research and we are still engaged in this community: In autumn, Julian was rapporteur to the Ninth World Meteorological Organization (WMO) International Workshop on TCs Sub-Topic 4.3 “Extratratropical Transition” (webpage here, report here).
In this blog post we want to highlight recent research highlights on ET in which we were involved. Most importantly, Julian and Christian substantially contributed to a two-part review paper on ET published in Monthly Weather Review (Evans et al. 2017, Keller et al. 2019). Part 2 summarises our understanding on how diabatic outflow modifies the large-scale circulation and causes forecast uncertainty during the extreme case of a TC interacting with the midlatitude jet stream (Keller et al. 2019). Our colleagues at ETH Zurich now documented systematically when and how often ET actually triggers Rossby waves (Riboldi et al. 2018a). They further studied factors that favour an amplification of the wave guide and the eventual evolution of a remote blocking anticyclone (Riboldi et al. 2018b). Such an amplified wave guide is thought to cause high-impact weather in downstream regions. However, until recently it was difficult to quantify the effect of North Atlantic TCs on European high-impact weather. Now a climatological study lead by our colleagues at University of Bern revealed for the first time the conditions under which ET can double the likelihood of extreme precipitation in Europe (Pohorsky et al. 2019 and Uni Bern press release here). Focusing more on the actual transition of the former TC, IMK-research revealed that the interaction with orography can importantly delay ET or even hinder ET (Lentik et al. 2018). More details on the latter study are in this IMK-News highlight (link here). /CG
Evans, C., and Coauthors, 2017: The Extratropical Transition of Tropical Cyclones. Part I: Cyclone Evolution and Direct Impacts. Mon. Wea. Rev., 145, 4317–4344, doi:10.1175/MWR-D-17-0027.1.
Keller, J. H., and Coauthors, 2018: The Extratropical Transition of Tropical Cyclones. Part II: Interaction with the Midlatitude Flow, Downstream Impacts, and Implications for Predictability. Mon. Wea. Rev., 147, 1077–1106, doi:10.1175/MWR-D-17-0329.1.
Lentink, H. S., C. M. Grams, M. Riemer, and S. C. Jones, 2018: The Effects of Orography on the Extratropical Transition of Tropical Cyclones: A Case Study of Typhoon Sinlaku (2008). Mon. Wea. Rev., 146, 4231–4246, doi:10.1175/MWR-D-18-0150.1.
Pohorsky, R., M. Röthlisberger, C. M. Grams, J. Riboldi, and O. Martius, 2019: The Climatological Impact of Recurving North Atlantic Tropical Cyclones on Downstream Extreme Precipitation Events. Mon. Wea. Rev., 147, 1513–1532, doi:10.1175/MWR-D-18-0195.1.
Riboldi, J., M. Röthlisberger, and C. M. Grams, 2018a: Rossby Wave Initiation by Recurving Tropical Cyclones in the Western North Pacific. Mon. Wea. Rev., 146, 1283–1301, doi:10.1175/MWR-D-17-0219.1.
Riboldi, J., C. M. Grams, M. Riemer, and H. M. Archambault, 2018b: A Phase Locking Perspective on Rossby Wave Amplification and Atmospheric Blocking Downstream of Recurving Western North Pacific Tropical Cyclones. Mon. Wea. Rev., 147, 567–589, doi:10.1175/MWR-D-18-0271.1.
18/04/2019 - 3 Months Research Stay at Monash University in Melbourne, Australia
Seraphine Hauser, Master’s student in the Large-scale dynamics and predictability group, spent the last 3 months in Australia to work on her thesis about ‘A weather system perspective on cool-season rainfall variability in southeastern Australia during El Niño’ in the School of Earth, Atmosphere and Environment in Melbourne. Together with her supervisors abroad, Michael Reeder and Shayne McGregor, she discussed her latest results and got inspiration for further investigations. Seraphine’s analysis reveals the importance of midlatitude weather systems for the month-to-month rainfall variability that is observed during El Niño. In particular, the interplay of blocking anticyclones, cut-off systems and warm conveyor belts determines whether southeastern Australia experiences anomalously dry or wet conditions – an important information for the agricultural sector. During her stay in Melbourne, which was supported by the ARC Center of Excellence for Climate System Science, Seraphine also got the chance to visit the Bureau of Meteorology to further expand her expertise on the characteristics of weather and climate in Down Under.
Regarding the last 3 months, Seraphine is very happy about the experiences and exchange with researchers at Monash University and grateful that the final thesis will feature many of Michael’s and Shayne’s suggestions. /SH
20/02/2019 - Welcome Moritz Pickl
We welcome Moritz Pickl in the group “Large-scale Dynamics and Predictability”. Moritz received a Bachelor’s degree in Freiburg and then moved to the University of Berne for a Master’s degree in Climate Sciences with specialization in Atmospheric Sciences. In his thesis he studied the variability of North Atlantic teleconnections and ocean-atmosphere interaction during the last millennium. He continued with a one-year internship at MeteoSwiss where he contributed to the preparation of the Swiss Climate Change Scenarios CH2018.
Moritz now starts a PhD in which he will investigate the sensitivity of diabatic outflow and its impact on the large-scale circulation with numerical experiments in ICON and with data from the IFS ensemble. This will be an important component of the project “SPREADOUT” and inform us if the correct representation of diabatic outflow is critical during weather regime life cycle stages. Moritz is based at Campus North (435, office 316a). We wish Moritz a great start, success and a lot of fun with his work and colleagues at IMK. Welcome! / CG
13/01/2019 - Representation of synoptic‐scale Rossby Wave Packets and Blocking in the S2S Prediction Project Database
Equatorward and poleward wind perturbations propagating eastward along the fast flowing air currents in midlatitudes are commonly referred to as Rossby wave packets (RWPs). Typically, the waves form in the entrance region of the midlatitude storm tracks, that is, over the western North Pacific and the western North Atlantic. Regions of RWP decay are the exit regions of the storm tracks over North America and the East Atlantic/European region.
The occurrence of RWPs has been linked to extreme weather events such as intense winter storms, heat waves, and heavy precipitation. Hence, an adequate representation of RWPs in state‐of‐the‐art numerical weather prediction models is desirable to better predict these weather extremes.
In a recent study , we now verify for the first time the representation of RWPs in a set of 11 numerical weather prediction models on time‐scales of up to 28 days. It is shown that fundamental properties such as their climatological frequency of occurrence, their life time, and their mean propagation distance are represented reasonably well. However, models ‐ especially those with a rather coarse horizontal grid spacing ‐ struggle to adequately represent the frequency of decay of these waves in the exit region of the storm tracks over the Atlantic/European sector. Instead of decaying over the eastern North Atlantic, RWPs propagate into far eastern Europe likely due to an underestimation of the occurrence frequency of long‐lasting and stationary high pressure systems – commonly referred to as blocking highs. The observed systematic errors in the frequency of blocking highs and in the RWP decay is most pronounced but not unique to models with coarse resolution. That the observed errors are not purely resolution dependent points to the effect of the different representation of key physical processes for RWP dynamics in models of the S2S database. To pinpoint these processes with process-oriented diagnostics is one goal of the Large-scale dynamics and predictability group./JQ
 Quinting, J.F., and F. Vitart, Representation of synoptic‐scale Rossby Wave Packets and Blocking in the S2S Prediction Project Database, Geophys. Res. Lett., 46. https://doi.org/10.1029/2018GL081381 (2019).
17/10/2018 - Process-oriented Understanding of weather forecast error
Despite huge progress made in numerical weather prediction, occasionally severe forecast errors occur affecting large regions. In Europe, such “forecast busts” are related to a misforecast of the large-scale circulation over the Atlantic-European region. An example on how this affects 2m temperature forecast over Europe is shown in Figure 1. In this six-day forecast issued on 07 March 2016, the model predicts too mild conditions for wide parts of western and Central Europe whereas it predicts too cold conditions in Italy and the Balkans (Figure 1; note that data from more weather stations is available in central Europe and thus the density of available surface observations is much higher there).
The March 2016 forecast bust was related to the onset of a stationary high pressure system over the North Sea region – a so-called European blocking regime. Such weather regimes typically last for several days to a few weeks and affect entire Europe. Thus, it is important to understand why numerical models struggle in correctly predicting their life cycles.
In a recently published study , we now reveal that condensational processes associated with the warm conveyer belt (WCB) of an extratropical cyclone effectively amplify a small error early in a weather forecast and projects it on the large-scale circulation resulting in the severe forecast bust for entire Europe for the later forecast hours.
The group now investigates if this is a singular case or if WCBs and other processes acting on weather time scales generally dilute forecast skill for the large-scale weather regimes on medium-range to subseasonal time scales (10-30 days). Therefore, we investigate dynamical processes driving weather regime life cycles using reanalysis and historical weather forecast data. This includes the investigation of how slower climate modes such as the stratosphere, the ocean state, or the Madden-Julian-Oscillation affect predictability of weather regimes  and how weather regimes modulate surface weather on subseasonal time scales [2, 3].
The group is funded by the Helmholtz Association with a Helmholtz Young Investigator Group Grant for the project “Subseasonal Predictability: Understanding the Role of Diabatic Outflow” (SPREADOUT). /CG.
 C. M. Grams, L. Magnusson, and E. Madonna, An atmospheric dynamics‘ perspective on the amplification and propagation of forecast error in numerical weather prediction models: a case study. Quarterly Journal of the Royal Meteorological Society, in press, doi:10.1002/qj.3353 (2018).
 C. M. Grams, R. Beerli, S. Pfenninger, I. Staffell, H. Wernli, Balancing Europe’s wind-power output through spatial deployment informed by weather regimes. Nature Climate Change. 7, 557–562, doi:10.1038/NCLIMATE3338 (2017).
 L. Papritz, C. M. Grams, Linking Low‐Frequency Large‐Scale Circulation Patterns to Cold Air Outbreak Formation in the Northeastern North Atlantic. Geophysical Research Letters. 45, 2542–2553, doi:10.1002/2017GL076921 (2018).
12/10/2018 - Spreadout at the WWRP/WCRP S2S and S2D conference in Boulder, CO
The conference aimed to foster the exchange of information between the S2S and S2D communities, to identify challenges for transferring S2S and S2D research into operations, and to identify new collaborations, initiatives and urgent science issues (https://www.wcrp-climate.org/s2s-s2d-2018-home). Our contributions covered various of the conference themes: Dominik presented his applied research with Remo Beerli on the importance of the wintertime stratospheric polar vortex in serving as a predictor of month-ahead wind electricity generation in Europe. Julian’s and Christian’s contributions focused on current science issues. Christian pointed out that diabatic processes within rapidly ascending midlatitude airstreams (warm conveyor belts - WCBs) contribute to the formation and maintenance of blocked weather regimes. Julian then stressed in one of his contributions that subseasonal numerical weather prediction models generally underestimate the occurrence of stationary anticyclones (blocking) over the Atlantic-European region. This may be due to an inadequate representation of diabatic processes – a hypothesis which we now study in further detail. Our research combining process understanding and applications received quite positive feedback. Motivated by fruitful and very inspiring discussions with the S2S community, we now continue to identify processes acting on weather time scales that might dilute forecast skill on subseasonal time scales. /JQ.
18/07/2018 - Welcome Jan Wandel
We are very happy to welcome Jan Wandel as a student assistant in our group. Jan got a Bachelor’s and Master’s degree in Meteorology from KIT. During his studies he was engaged in IMK’s “early weather hazards warning” (Wettergefahrenfrühwarnung). In his Masterthesis at IMK-TRO Jan studied the synoptic environment triggering hailstorms in Europe.
Jan will now take care of operational weather regime forecast products in our group and investigate the linkage of weather regimes and hailstorms in collaboration with his former group. Jan will also develop operational forecast products for weather regimes on sub-seasonal time-scales. We wish Jan a great start, success and a lot of fun with his work and colleagues at IMK. Welcome!
Improving weather forecasts on sub-seasonal time scales
Advances in numerical weather prediction currently push the weather forecast horizon into sub-seasonal time scales of several days to a few weeks. On these time scales so-called weather regimes – quasi-stationary, recurrent, and persistent flow patterns – govern the variability of the large-scale circulation. They modulate the character of daily weather for continent-size regions and prolonged periods. Therefore, weather regimes have strong implications for socio-economic sectors such as agriculture, transport, or renewable energies .
The correct prediction of weather regime life cycles still is a key challenge for current sub-seasonal forecasting systems because weather regimes are concurrently modified by processes on very different spatial and temporal scales: From a weather perspective, the life cycles of these regimes are influenced by meso- to synoptic-scale weather systems such as extratropical cyclones or convective systems. From a climate perspective, modes of the climate system such as the Madden-Julian-Oscillation or the state of the stratosphere are potential sources of sub-seasonal predictability for such regimes.
The newly established group “Large-scale Dynamics and Predictability” aims to provide a comprehensive investigation of the physical and dynamical processes that control predictability and forecast skill on sub-seasonal time scales, with a focus on the life cycle of large-scale flow regimes in the Atlantic-European region. In addition, the group explores novel probabilistic forecast products on sub-seasonal time scales in collaboration with official weather services.
An example of such a novel forecast product is shown for the “early heat wave” in Central Europe that peaked from 19. – 22. April 2018 (Figure 2). The overview regime plot indicates how likely a specific weather regime occurs within the subsequent 15 days (Figure 2a). The detailed regime product shows how well the different regimes are established in a probabilistic forecast (Figure 2b). In this case these forecast products correctly indicated – more than one week in advance – the actual transition from a “Scandinavian blocking regime” (ScBL) into a “zonal regime” (ZO) at the end of the heat wave (Figure 2c compared to Figure 2b). However, particularly such transitions from one regime to another are often not well predicted by current sub-seasonal forecast systems.
The group now investigates in detail how well sub-seasonal forecasting systems represent weather regime life cycles and their underlying physical processes on shorter synoptic time-scales as well as their modulation by slower climate modes, e.g. the Madden-Julian-Oscillation or the state of the stratosphere . Working at the interface of these different spatial and temporal scales will not just improve the understanding of weather regimes but ultimately also contribute to the overarching goal of a seamless prediction of weather and climate.
The group is funded by the Helmholtz Association with a Helmholtz Young Investigator Group Grant for the project “Sub-seasonal Predictability: Understanding the Role of Diabatic Outflow” (SPREADOUT).
Link: Group „Large-scale Dynamics and Predictability“ http://www.imk-tro.kit.edu/english/7425.php
 C. M. Grams, R. Beerli, S. Pfenninger, I. Staffell, H. Wernli, Balancing Europe’s wind-power output through spatial deployment informed by weather regimes. Nature Climate Change. 7, 557–562, doi:10.1038/NCLIMATE3338 (2017).
 Papritz L., Grams C. M., Linking Low‐Frequency Large‐Scale Circulation Patterns to Cold Air Outbreak Formation in the Northeastern North Atlantic. Geophysical Research Letters. 45, 2542–2553, doi:10.1002/2017GL076921 (2018).
|Fig. 2: Example of novel weather regime forecast products. (a) Ensemble forecast initialised at 12 UTC 15 April 2018. Bars indicate relative number of members projecting in one of 7 weather regimes (colors) or no regime (grey). Bottom rows show the attribution of the ensemble mean, control, and high resolution (up to 240h) forecasts, respectively. (b) Ensemble distribution of projection in one of 7 weather regimes at each forecast step. Light shades indicate maximum and minimum projection, dark shades show the 75th and 25th percentile. Lines show the projection of the control forecast (bold solid), high resolution forecast (bold dashed, only up to 240h), and ensemble mean (thin dashed). (c) Actual verifying projection into 7 weather regimes in the 30day period starting on 7 April 2018.|
11/06/2018 - Visit of AXPO Trading
The SPREADOUT group visited AXPO Trading in Baden (Switzerland) on 11 June 2018. The main purpose of the visit was to inform energy traders and meteorologists at AXPO about current research activities and to discuss forecast tools useful to the energy sector. After an introductory presentation on SPREADOUT by Christian Grams and Dominik Büeler, Remo Beerli (energy meteorologist at AXPO) showed us the trading floor. There we gained interesting insights in the day-to-day business of energy meteorologists and learned about the forecast products that are needed to provide reliable information to the energy traders. Future collaborations and research avenues were elaborated in a lively discussion in the afternoon.
01/06/2018 - Welcome Nadine Schittko and Seraphine Hauser
SPREADOUT is growing further: We welcome Nadine Schittko and Seraphine Hauser who will join the group for the next year to complete their Master’s degree.
Nadine will analyse the representation of tropical cyclones in the global forecast model ICON. In the framework of the Master’s thesis, she will implement different tropical cyclone tracking algorithms to verify the intensity and motion of tropical cyclones against best track data. Some impact relevant North Atlantic Hurriances in 2016/17 will be studied in greater detail. The project is executed in close collaboration with DWD where the resulting tools may be used operationally in the future for verification purposes.
In her Master’s thesis, Seraphine is going to analyse “The effect of the El Nino Southern Oscillation on Australian climate variability from a weather system perspective”. Using a novel data set of objectively identified weather systems, the goal of the first part of the project is to develop a conceptual picture on how different states of the El Nino Southern Oscillation are related to the occurrence frequency of subtropical and midlatitude weather systems. The results will then be used to attribute the observed variability in temperature and precipitation to these weather systems.
We wish Nadine and Seraphine great success and a lot of fun with their work!
01/03/2018 - Welcome
We welcome Dominik Büeler as a new member of the group “Large-scale Dynamics and Predictability”. Dominik received his PhD from ETH Zurich for his thesis entitled "Potential vorticity diagnostics to quantify effects of latent heating in extratropical cyclones: methodology and application to idealized climate change simulations". Already before his PhD, Dominik gained experience both in climate and weather modelling: he analysed marine boundary layer clouds in ECHAM5-HAM and investigated the northern mid- and high-latitude climate in a climate change mitigation scenario in his Bachelor and Master thesis, respectively. During a one-year internship at MeteoSwiss, he worked on the potential of COSMO in predicting photovoltaic power, which offered him an insight into applied weather science. After a short PostDoc project at ETH Zurich last autumn on month-ahead predictability of European wind power, Dominik will continue research in the field of sub-seasonal predictability at KIT in the project “SPREADOUT”. He will study the representation of large-scale weather regimes in sub-seasonal numerical weather prediction models and physical processes governing weather regime life cycles. We wish Dominik a great start, success and a lot of fun with his work and colleagues at IMK.
15/02/2018 - Welcome
We welcome Julian Quinting as a senior scientist in the group “Large-scale Dynamics and Predictability”. Julian received his PhD from KIT for his thesis entitled “ The impact of tropical convection on the dynamics and predictability of midlatitude Rossby waves: a climatological study” His Diploma and PhD research was part of the PANDOWAE research unit. After his PostDoc time at ETH Zurich and Monash University, Melbourne, Julian is back at KIT and will work on Sub-seasonal predictability in the project “SPREADOUT”.
As a PostDoc in Zurich, Julian worked on upper-level frontogenesis, Rossby wave dynamics, and MJO teleconnections. In addition, he helped preparing the NAWDEX field campaign and in autumn 2016 contributed actively to flight planning and forecasting based in Iceland. His research focus during the last 2 years at Monash shifted to the understanding of physical processes driving extreme events in the Australian region (e.g. heat waves) and southern hemispheric Rossby wave dynamics. In SPREADOUT Julian will study the representation of physical processes in global NWP data sets, their modulation by global teleconnections (e.g. MJO, ENSO), and how they affect sub-seasonal predictability for Europe. We wish Julian great success and a lot of fun with his work and colleagues at IMK.
12/12/2017 - Statement regarding Nature Geoscience manuscript “Southward shift of the global wind energy resource under high carbon dioxide emissions”
Based on an ensemble of 10 global climate model simulations following the RCP4.5 and RCP8.5 scenarios, this study reports a strong decrease of potential wind electricity generation in the mid-latitudes during the XXI Century (https://www.nature.com/articles/s41561-017-0029-9). The authors use a simple methodology and data with low spatio-temporal resolution, and consider an exemplary wind energy turbine for the computations. Compared to other regions of the world (notably North America), the changes for Europe are comparatively small. These projections for Europe are partially in agreement with studies based on datasets with much higher spatial and temporal resolution (e.g., Tobin et al., 2015, Reyers et al., 2016, Moemken et al., 2018). These studies reveal rather small changes of wind energy potentials for Europe on the continental scale (+/- 5%). On the other hand, they point to increased variability of wind electricity generation in multiple time scales. The differences to the Nature Geoscience study are related with the different data resolution and methodology.
In particular, an increased occurrence of low wind speed (< 3m/s) events reported in Moemken et al. (2018) may cause challenges for the energy supply across Europe. However, this challenge can be overcome with suitable mitigation strategies and updated planning. For example, Grams et al. (2017) provide evidence that the concentration of wind parks in some areas (e.g. North Sea) is problematic to warrant a reliable wind electricity generation. A pan-European management strategy and a more de-central distribution of wind parks would permit to balance the weather and climate variability and thus contribute to a more reliable energy supply. Moreover, the joint management of different renewable sources (notably solar) would further contribute to mitigate the possible changes in wind energy production in future decades.
Grams, C. M., R. Beerli, S. Pfenninger, I. Staffell, and H. Wernli, 2017: Balancing Europe’s wind-power output through spatial deployment informed by weather regimes. Nature Climate Change, 7, 557–562, doi:10.1038/nclimate3338.
Moemken, J., M. Reyers, H. Feldmann, and J. G. Pinto, 2018: Wind speed and wind energy potentials in EURO-CORDEX ensemble simulations: evaluation and future changes, Journal of Geophysical Research: Atmospheres, in revision.
Reyers, M., J. Moemken, and J. G. Pinto, 2016: Future changes of wind energy potentials over Europe in a large CMIP5 multi-model ensemble. Int. J. Climatol., 36, 783–796, doi:10.1002/joc.4382.
Tobin, I., and Coauthors, 2015: Assessing climate change impacts on European wind energy from ENSEMBLES high-resolution climate projections. Climatic Change, 128, 99–112, doi:10.1007/s10584-014-1291-0.
Contact: Joaquim G. Pinto http://www.imk-tro.kit.edu/14_7131.php
Contact: Christian M. Grams http://www.imk-tro.kit.edu/14_7356.php