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23/04/2019 - Are mountain ranges important for extratropical transition?

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Fig. 1: Potential temperature (K, shaded) and geopotential height (up to 760 m, dashed cont.) at 925hPa at 12 UTC, 19 Sep. 2008, for the simulation with (left) and without (right) mountains. Gray cont.: coastlines, gray areas/triangles: orography.

Tropical cyclones (TCs), also called Hurricanes or Typhoons, are one of the most hazardous natural phenomena on Earth. When TCs move towards land, they often receive special attention from the media because of their destructive wind speeds, excessive rainfall and associated storm surge. Thus, they are a thread to human activity, especially in the densely populated coastal areas. Sometimes, a TC moves poleward out of the tropics and into the mid-latitudes. When a TC moves into the mid-latitudes, it experiences a decrease in sea surface temperature, an increase in vertical wind shear near the polar jet stream and it impinges on a horizontal temperature gradient. Due to these environmental changes, the characteristics of the cyclone shift from a typically tropical structure (e.g. axi-symmetric, dominated by deep convection) to a typically extratropical structure (e.g. asymmetric, frontal activity). This transformation is called extratropical transition (ET) [1] [2]. The local and downstream weather during ET is often not well forecasted and the combination of high impact weather and low predictability forms a risk for society. Therefore, ET is one of the topics studied at IMK-TRO.

Most of the ET events occur over the ocean but some TCs recurve and undergo ET along coastal regions like Japan, New Zealand or the United States East Coast. For example, Typhoon Sinlaku (2008) underwent ET along the southern coast of Japan. In order to investigate the influence of orography on the development of the cyclone, Typhoon Sinlaku was simulated with a numerical weather prediction model, both with and without mountains over Japan [3]. For the simulations the COSMO model is used with a high horizontal resolution (2.8 km). In general, orography behaves like a barrier and thus blocks the lower-level air flow. Air parcels either need to flow over or around the barrier. In the COSMO simulation with mountains, the low-level mid-latitude air north of Japan is forced to flow around the orography and does not interact with Sinlaku yet (Fig. 1, left). Once Sinlaku moves further eastward away from the orographic barrier, the cooler air flows southward and ET starts. However, without orography, cool air from the north can immediately interact with Sinlaku, starting the transition from a tropical into an extratropical cyclone (Fig. 1, right). Thus, ET is delayed in the presence of orography. This gives a new insight into the complex interaction when a TC undergoes ET near land and orography.

[1] Evans, C., and Coauthors, 2017: The extratropical transition of tropical cyclones. Part I: Cyclone evolution and direct impacts. Monthly weather review, 145, 4317–4344, https://doi.org/10.1175/MWR-D-17-0027.1 External Link.
[2] 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. Monthly weather review, 147, 1077-1106, https://doi.org/10.1175/MWR-D-17-0329.1 External Link.
[3] Lentink, H. S., Grams, C. M., Riemer, M., Jones, S. C, 2018: The effects of orography on the extratropical transition of tropical cyclones: a case study of Typhoon Sinlaku (2008). Monthly weather review, 146, 4231–4246. https://doi.org/10.1175/MWR-D-18-0150.1 External Link.


18/04/2019 - 3 Months Research Stay at Monash University in Melbourne, Australia

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AGGrams_Hauser_Melbourne

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

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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

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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

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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!


Improving weather forecasts on sub-seasonal time scales

IR-Sat+Jet
Fig. 1: Meteorological situation during the Central European summer heat wave in July 2015, IR satellite image, blocking anticyclone (blue hatching), deflection of the jet stream (green contour), warm conveyor belt (blue-to-red trajectories).

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 [1].

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 [2]. 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

[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).

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

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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

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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”

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picture: 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