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13/01/2019 - Representation of synoptic‐scale Rossby Wave Packets and Blocking in the S2S Prediction Project Database

AGGrams_Quinting_Vitart_Fig2
Fig 1. Bias in RWP (top) initiation and (bottom) decay frequency for lead time of 21–27 days. Positive (negative) values indicate an overestimation (underestimation). Taken from Fig. 2 in [1].

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 [1], 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

[1] 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

Forecast error for 2m temperature
Fig. 1: Forecast error for 2m temperature in a six-day (+144h) forecast valid at 00 UTC, 13 March 2016. The ECMWF high resolution forecast is verified against surface observations. Taken from Fig. 1c in [1].

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 [1], 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 [2] 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.

[1] 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).

[2] 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).

[3] 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

AGGrams_S2S_collage
Impressions from the S2S conference in Boulder. Photo credits: University Corporation for Atmospheric Science

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

AGGrams_Groupphoto
The research group "Large-scale Dynamics and Predictability": Christian Grams, Jan Wandel, Dominik Büeler, Nadine Schittko, Seraphine Hauser, and Julian Quinting.

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!


Verbessern von Wettervorhersagen auf sub-saisonalen Zeitskalen

IR-Sat+Jet
Abb. 1: Meteorologische Situation während der Hitzewelle in Mitteleuropa im Juli 2015: Infrarot Satellitenbild, blockierendes Hochdruckgebiet (blaue Schattierung), Ablenkung des Strahlstroms (grüne Kontur), „warm conveyor belt“ (blau-rote Trajektorien).

Fortschritte in der numerischen Wettervorhersage ermöglichen immer langfristigere Vorhersagen auf sogenannten sub-saisonalen Zeitskalen von einigen Tagen bis Wochen im Voraus. Auf diesen Zeitskalen bestimmen beständige, quasi-ortsfeste, und wiederkehrende Wetterregime die Variabilität der großräumigen Strömung. Diese Wetterregime bestimmen den Charakter des täglichen Wetters für eine längere Zeitperiode und für Gebiete der Größe Europas. Daher beeinflussen Wetterregime zahlreiche gesellschaftliche und wirtschaftliche Aktivitäten, wie z.B. die Landwirtschaft, den Verkehr oder die erneuerbaren Energien [1].

Dennoch ist es weiter eine große Herausforderung den Lebenszyklus dieser Wetterregime auf sub-saisonalen Zeitskalen korrekt vorherzusagen. Grund dafür ist das gleichzeitige Wirken meteorologischer Prozesse auf sehr verschiedenen räumlichen und zeitlichen Skalen: Auf kürzeren Zeitskalen, werden Wetterregimelebenszyklen von Wettersystemen, wie etwa außertropischen Tiefdruckgebieten oder Gewittersystemen beeinflusst. Auf längeren Zeitskalen sind langsamere Elemente des Klimasystems, wie zum Beispiel die Stratosphäre oder tropische Konvektion (Madden-Julian-Oszillation), Einflussfaktoren für Wetterregime, die teilweise Vorhersagbarkeit für Wetterregime auf sub-saisonalen Zeitskalen geben.

Die neu am IMK-TRO eingerichtete Gruppe „Großräumige Dynamik und Vorhersagbarkeit“ erforscht die physikalischen und dynamischen Prozesse, die Vorhersagbarkeit und Vorhersagegüte auf sub-saisonalen Zeitskalen bestimmen. Dabei legt sie den Schwerpunkt auf den Lebenszyklus großskaliger Wetterregime im Atlantisch-Europäischen Raum. Darüber hinaus erforscht die Gruppe in Zusammenarbeit mit Wetterdiensten neue probabilistische Vorhersageprodukte für die sub-saisonale Zeitskala.

Im Folgenden wird ein erstes Vorhersageprodukt am Beispiel der frühen Hitzewelle in Mitteleuropa, die vom 19.-22. April ihren Höhepunkt erreichte, gezeigt (Abbildung 2). Die Übersichtsdarstellung zeigt zunächst wie wahrscheinlich ein spezifisches Wetterregime in den nächsten 15 Tagen auftritt (Abbildung 2a). Mehr Details liefert eine Darstellung, die anzeigt wie stark verschiedene Regime in einer probabilistischen Vorhersage ausgeprägt sind (Abbildung 2b). Im Fall der Hitzewelle im April 2018 zeigten diese Vorhersageprodukte korrekterweise den Übergang eines “Skandinavischen Hochs“ (ScBL) in ein „Zonales Regime“, und damit das Ende der Hitzewelle, bereits mehr als eine Woche im Voraus an (vgl. Abbildung 2c und 2b). Oft sind jedoch gerade solche Regimeübergänge mit heutigen Wettervorhersagesystemen überhaupt nicht gut vorhersagbar.

Die Gruppe untersucht nun im Detail wie gut sub-saisonale Wettervorhersagesysteme die Lebenszyklen von Wetterregimen darstellen. Insbesondere werden physikalische Prozesse auf kürzeren Zeitskalen,  sowie deren Veränderung durch langsamere Komponenten des Klimasystems wie der Madden-Julian-Oszillation oder der Stratosphäre [2] untersucht. Diese Grundlagenforschung an der Schnittstelle verschiedener räumlicher und zeitlicher Skalen, wird nicht nur das Verständnis von Wetterregimen verbessern, sondern auch zum übergeordneten Ziel von universellen Vorhersagesystemen für Wetter und Klima beitragen.

Die Helmholtz Gemeinschaft fördert diese Forschungstätigkeit als Helmholtz Nachwuchsgruppe “Sub-seasonal Predictability: Understanding the Role of Diabatic Outflow” (SPREADOUT).

Link: Gruppe „Großräumige Dynamik und Vorhersagbarkeit“ http://www.imk-tro.kit.edu/7425.php

[1] 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).

[2] 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).

Abb. 2: Beispiele für neuartige Wetterregime Vorhersageprodukte: (a) Ensemble Vorhersage von 12UTC 15. April 2018. Die Balken zeigen die relative Wahrscheinlichkeit eines von 7 verschiedenen Wetterregimen an. Die unteren Zeilen zeigen jeweils an, welches Regime das Ensemble-Mittel, die Kontroll- und die hochaufgelöste Vorhersage vorhersagen (letztere nur bis 10 Tage im Voraus). (b) Ensemble-Verteilung der jeweiligen Ausprägung eines der 7 Wetterregime. Blasse Farben zeigen die jeweils stärkste und schwächste Ausprägung an, dunklere Farben das 25. und 75. Perzentil. Linien zeigen die Ausprägung in der Kontrollvorhersage (dick durchgezogen), hochaufgelösten Vorhersage (dick strichliert, nur bis 10 Tage), und im Ensemble-Mittels (dünn strichliert) an. (c) Tatsächlich aufgetretene Ausprägung der 7 Wetterregime vom 7. April bis 7. Mai 2018.

11/06/2018 - Visit of AXPO Trading

AGGrams_Axpo
Julian Quinting, Dominik Büeler, energy meteorologist Remo Beerli, and Christian Grams in front of the AXPO main building in Baden (Switzerland).

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
Dominik Büeler, Nadine Schittko, Seraphine Hauser, Christian Grams, Julian Quinting

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”

Foto: Bernhard Mühr, www.wolkenatlas.de

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.

 

References:

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.

Link: http://www.sek.kit.edu/kit_express_3874.php

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