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From Atmospheric Dynamics to Insurance Losses: An Interdisciplinary Workshop on European Storms

June 2019
Bulletin of the American Meteorological Society 100(6):ES175–ES178

DOI:10.1175/BAMS-D-19-0026.1

Authors:

Joaquim G. Pinto

Karlsruhe Institute of Technology

Florian Pantillon

Laboratoire d'Aérologie

Patrick Ludwig

Karlsruhe Institute of Technology

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The 7th European Storm Workshop gathered scientists and insurance industry experts from 10 countries to facilitate an interdisciplinary exchange regarding novel scientific advances and developments in risk modeling, allowing specialists with different backgrounds working in European windstorm research to discuss the priorities for future research.

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Content uploaded by Joaquim G. Pinto

Content may be subject to copyright.

INTERDISCIPLINARY WORKSHOP ON EUROPEAN

STORMS

What: The seventh European Storm Workshop

gathered scientists and insurance industry

experts from 10 countries to facilitate an

interdisciplinary exchange regarding novel

scientific advances and developments in risk

modeling, allowing specialists with different

backgrounds working in European windstorm

research to discuss the priorities for future

research.

When: 10–12 October 2018

Where: Karlsruhe, Germany

FROM ATMOSPHERIC DYNAMICS

TO INSURANCE LOSSES

An Interdisciplinary Workshop on European Storms

Joaquim G. Pinto, Florian Pantillon, Patrick ludWiG, madeleine-SoPhie déroche,

Giovanni leoncini, chriStoPh c. raible, len c. ShaFFrey, and david b. StePhenSon

AFFILIATIONS: Pinto and lu dWiG— Institute of Meteorology

and Climate Research, Karlsruhe Institute of Technology,

Karlsruhe, Germany; Pantillon —Institute of Meteorology and

Climate Research, Karlsruhe Institute of Technology, Karlsruhe,

Germany, and Laboratoire d’Aérologie, Centre National de la

Recherche Scientifique and Université de Toulouse, Toulouse,

France; déroch e—Group P&C Risk Management, AXA , Paris,

France; leoncini —Zurich Insurance Company Ltd, Zurich,

Switzerland; raib le—Climate and Environmental Physics, and

Oeschger Centre for Climate Change Research, University

of Bern, Bern, Switzerland; ShaFFre y—National Centre for

Atmospheric Science, and Department of Meteorology, University

of Reading, Reading, United Kingdom; StePhe nSon —Department

of Mathematics, University of Exeter, Exeter, United Kingdom

CORRESPONDING AUTHOR: Joaquim G. Pinto,

joaquim.pinto@kit.edu

DOI:10.1175/BAMS -D-19-0026.1

In final form 8 March 2019

For information regarding reuse of this content and general copyright

information, consult the AMS Copyright Policy.

W indstorms are extreme midlatitude cyclones

and one of the major natural hazards that

cause damage and losses in Europe. However,

the processes involved in their intensification and

generation of disastrous impacts, such as widespread

wind damage and f looding, are not fully understood.

Initiated in 2011, the European Storm Workshop

series (www.stormworkshops.org) brings together

the academic community, weather services, and risk

model developers from insurance and engineering

consulting companies. The goals are to stimulate

interdisciplinary research on midlatitude storms and

to bridge the gap between fundamental research and

practical implementations.

The seventh European Storm Workshop took

place in October 2018 at the Karlsruhe Institute of

Technology (KIT) in Germany. Over 60 partici-

pants from 10 countries discussed the latest results

and developments in windstorm research and its

industry applications, including 26 nonacadem-

ics. The workshop featured a total of 30 oral and

poster presentations split into three sessions, allowing

plenty of opportunities for exchange and discussion.

Presentations and the discussions they generated

focused on the dynamics of European windstorms

(extreme midlatitude cyclones), their predictability

and variability from weather to climate time scales,

risk assessments, and academic–insurance industry

ES175

JUNE 2019AMERICAN METEOROLOGICAL SOCIETY |

collaborations. The workshop included keynote

lectures given by speakers from both academia and

the insurance industry. Highlights of each session are

discussed below.

DYNAMICS OF EUROPEAN WIND-

STORMS. The aim of this session was to discuss

new progress in understanding European wind-

storm dynamics. A major discussion topic was the

combination of perils associated with windstorms

such as extreme precipitation, severe wind gusts and/

or storm surges (so-called compound events). The

complexity of these events in terms of impacts makes

them a crucial topic for both researchers and the in-

surance industry. For example, Margarida Liberato

(University of Trás-os-Montes and Alto Douro)

presented a consistent catalog of exceptional, high-

impact windstorms for Iberia, which lead to both

wind and rainfall extremes. Extreme events were also

the focus of the keynote by Helen Dacre (University

of Reading), who reported on advances in the under-

standing of the relationship between warm conveyor

belts and atmospheric rivers (Dacre et al. 2019). She

showed the importance of low-level cyclone airflow,

known as the feeder airstream, which originates

ahead of the cyclone and flows rearward toward the

cyclone center. Some of the moisture transported

by the feeder airstream is supplied to the base of

the warm conveyor belt where it ascends to form

precipitation, while the rest remains at low levels,

forming the leading edge of an atmospheric river. Lea

Eisenstein (KIT) presented a modeling study of the

first detected sting-jet windstorm over continental

Europe (“Egon” in January 2017). Devastating sting

jets are associated with strong wind gusts lasting for

a few hours over a distinct region located between

the cold and warm jet of Shapiro–Keyser cyclones

(Hewson and Neu 2015). Accurately modeling the

sting jet, which is essential to assess its loss poten-

tial, requires high spatial resolution. Convection-

permitting simulations show that the characteristics

of this storm were largely consistent with other

known cases over the North Atlantic and the British

Isles (Clark and Gray 2018), but the cyclone was

also clearly affected by topography over continental

Europe. Possible changes in the characteristics of

windstorms in a future climate were the subject of

Dominik Büeler’s (ETH Zurich; KIT) presentation.

Based on idealized studies, he reported that, while

the intensity of moderate cyclones may decrease in

a warmer world, an intensification is expected for

strong cyclones, which is partly associated w ith latent

heating effects. Such results are of great importance

for the insurance industry, as more windstorms have

the potential to cause higher losses.

PREDICTABILITY AND VARIABILITY

FROM WEATHER TO CLIMATE TIME

SCALES. This session discussed the current state

of knowledge of the predictability of cyclones on dif-

ferent time scales. Aiko Voigt (KIT) illustrated with

various examples the importance of cloud–radiative

interactions on the midlatitude atmospheric cir-

culation and cyclone activity. Understanding these

interactions is crucial for an adequate assessment

of climate change projections, as clouds are one

of the largest sources of uncertainty (Bony et al.

2015). Recent studies suggest that thermal radiation

effects can weaken idealized cyclones by modifying

potential vorticity (Schäfer and Voigt 2018). Given

that the cloud–radiative impact is important for

both weather and climate, a better understanding of

cloud–circulation coupling is needed to quantify the

response of cyclone activity to global warming. Len

Shaffrey (University of Reading) presented a critical

evaluation of the significant increase in Northern

Hemisphere storminess detected in ECMWF’s first

atmospheric reanalysis of the twentieth century

(ERA20C), which had not been reported in this form

by other studies. This century-long trend is appar-

ently related to a significant and unrealistic decrease

in surface pressure over the Arctic. This decreasing

trend in pressure is not seen in observational data and

leads to an increase of the meridional pressure gradi-

ent between the high and midlatitudes and therefore

of midlatitude storminess (Bloomfield et al. 2018).

Hence, the long-term storminess trends present in

ERA20C should be regarded with caution. Finally,

Florian Pantillon (Centre National de la Recherche

Scientifique; KIT) presented recent advances on

the prediction of wind gusts over central Europe

based on statistical postprocessing of an operational

convection-permitting weather forecast ensemble.

While ensemble model output statistics (EMOS)

substantially improve the average gust forecasts,

there are still a few cases which are poorly forecast

despite the use of EMOS (Pantillon et al. 2018). For

these cases, it is crucial to accurately represent frontal

convection, which is the source of some of the most

destructive gusts during windstorms over central

Europe (Ludwig et al. 2015).

WINDSTORM RISK AND INSURANCE

COLLABORATIONS. Applications of wi ndstorm

research were discussed focusing on the insurance

industry. One key aspect to improve the assessment

ES176 JUNE 2019

of windstorm risk is access to both observational and

model data. Alan Whitelaw (CGI IT U.K. Limited)

presented the operational windstorm service for

the insurance sector provided by the Copernicus

Climate Change Service. Expanding upon previous

efforts (Roberts et al. 2014), it provides an extended

database of windstorm tracks and high-resolution

wind footprints (https://wisc.climate.copernicus.eu

/wisc/#/). The new developments combine dynamical

and statistical downscaling to cover a larger number

of events. The use of numerical prediction models for

windstorm risk purposes has increased in recent years,

but they remain computationally very expensive and

are not easily implemented by private companies. As

an example of collaboration with the scientific com-

munity, Robin Locatelli (AXA) and Bernd Becker (Met

Office) presented a research partnership aiming at

providing hig h-resolution gust footprints for historical

events. These footprints were combined with claims

data to develop vulnerability curves for the European

market. Using statistical modeling, Dav id Stephenson

(University of Exeter) discussed various approaches to

quantify the dominant extremal dependence class for

realist ic windstorm footprints and found lit tle evidence

of asymptotic extremal dependency. When fitting the

data with statistical dist ributions, the Gaussian copula

appears to perform well, which allows the statistical

simulation of windstorm footprints (Dawkins and

Stephenson 2018). This approach opens the possibil-

ity of using geostatistical models for fast simulation

of windstorm hazard maps, which can complement

dynamical modeling approaches. Finally, one crucial

issue for the estimation of aggregated insurance losses

is the occurrence of multiple windstorms within a

season, a phenomenon known as storm clustering.

Based on high-resolution climate model simulations,

Matthew Priestley (University of Reading) showed

that serial clustering leads to an increase in annua l ag-

gregated losses of 10%–20% for return periods longer

than 3 years (Priestley et al. 2018). This was another

successful example of how basic research can have

important industrial applications.

CURRENT CHALLENGES AND FUTURE

PERSPECTIVES. Future directions and emerging

topics were debated in three breakout groups, which

covered a wide range of areas including new oppor-

tunities to further our understanding of European

windstorm risk. For example, the potential to reassess

historical windstorm risk through international proj-

ects on data discovery and the development of new

multidecadal reanalysis was discussed to overcome

the current limitations associated with the short his-

toric record. Similarly, new opportunities to under-

stand climate change impacts on future windstorm

risk will arise with the upcoming phase 6 of the

Coupled Model Intercomparison Project (CMIP6)

and the High Resolution Model Intercomparison

Project (HighResMIP) climate model projections

(e.g., Haarsma et al. 2016). A better understanding

of the importance of the different cyclone relative

air flows (“conveyor belts”; Hewson and Neu 2015)

for total property damage was also identified as

a key priority. Other emerging areas of scientific

interest include improved seasonal forecasts of the

North Atlantic Oscillation (e.g., Scaife et al. 2014),

which might help windstorm risk estimation for

the insurance industry. Additionally, validation and

calibration methods of extreme storms were debated.

Given the limited sample size of the historic records,

there is a need to develop methods beyond the stan-

dard quantile mapping approaches in order to cor-

rect biases of extreme events. Overall, the breakout

discussions highlighted the strong synergy between

academia and the insurance industry in terms of open

research questions, providing compelling evidence of

the need for sustained collaboration and dialogue. A

general consensus was reached to keep organizing

future workshops. The program and presentations

are available on our website (www.stormworkshops

.org/workshop2018.html).

ACKNOWLEDGMENTS. The authors thank all

the workshop participants for their contributions and

discussions. The workshop was made possible thanks

to support from the AXA Research Fund and the

Transregional Collaborative Research Center SFB/TRR

165 “Waves to Weather” funded by the German Research

Foundation (DFG).

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Nov 2016
GMD

Robust projections and predictions of climate variability and change, particularly at regional scales, rely on the driving processes being represented with fidelity in model simulations. The role of enhanced horizontal resolution in improved process representation in all components of the climate system is of growing interest, particularly as some recent simulations suggest both the possibility of significant changes in large-scale aspects of circulation as well as improvements in small-scale processes and extremes. However, such high-resolution global simulations at climate timescales, with resolutions of at least 50 km in the atmosphere and 0.25° in the ocean, have been performed at relatively few research centres and generally without overall coordination, primarily due to their computational cost. Assessing the robustness of the response of simulated climate to model resolution requires a large multi-model ensemble using a coordinated set of experiments. The Coupled Model Intercomparison Project 6 (CMIP6) is the ideal framework within which to conduct such a study, due to the strong link to models being developed for the CMIP DECK experiments and other model intercomparison projects (MIPs). Increases in high-performance computing (HPC) resources, as well as the revised experimental design for CMIP6, now enable a detailed investigation of the impact of increased resolution up to synoptic weather scales on the simulated mean climate and its variability. The High Resolution Model Intercomparison Project (HighResMIP) presented in this paper applies, for the first time, a multi-model approach to the systematic investigation of the impact of horizontal resolution. A coordinated set of experiments has been designed to assess both a standard and an enhanced horizontal-resolution simulation in the atmosphere and ocean. The set of HighResMIP experiments is divided into three tiers consisting of atmosphere-only and coupled runs and spanning the period 1950–2050, with the possibility of extending to 2100, together with some additional targeted experiments. This paper describes the experimental set-up of HighResMIP, the analysis plan, the connection with the other CMIP6 endorsed MIPs, as well as the DECK and CMIP6 historical simulations. HighResMIP thereby focuses on one of the CMIP6 broad questions, “what are the origins and consequences of systematic model biases?”, but we also discuss how it addresses the World Climate Research Program (WCRP) grand challenges.

The XWS open access catalogue of extreme European windstorms from 1979 to 2012

Article

Full-text available

Sep 2014
NAT HAZARD EARTH SYS

The XWS (eXtreme WindStorms) catalogue consists of storm tracks and model-generated maximum 3 s wind-gust footprints for 50 of the most extreme winter windstorms to hit Europe in the period 1979–2012. The catalogue is intended to be a valuable resource for both academia and industries such as (re)insurance, for example allowing users to characterise extreme European storms, and validate climate and catastrophe models. Several storm severity indices were investigated to find which could best represent a list of known high-loss (severe) storms. The best-performing index was Sft, which is a combination of storm area calculated from the storm footprint and maximum 925 hPa wind speed from the storm track. All the listed severe storms are included in the catalogue, and the remaining ones were selected using Sft. A comparison of the model footprint to station observations revealed that storms were generally well represented, although for some storms the highest gusts were underestimated. Possible reasons for this underestimation include the model failing to simulate strong enough pressure gradients and not representing convective gusts. A new recalibration method was developed to estimate the true distribution of gusts at each grid point and correct for this underestimation. The recalibration model allows for storm-to-storm variation which is essential given that different storms have different degrees of model bias. The catalogue is available at http://www.europeanwindstorms.org .

Sting jets in extratropical cyclones: a review

Article

Mar 2020

Linking Atmospheric Rivers and Warm Conveyor Belt Airflows

Article

Apr 2019

Extreme precipitation associated with extratropical cyclones can lead to flooding if cyclones track over land. However, the dynamical mechanisms by which moist air is transported into cyclones is poorly understood. In this paper we analyze airflows within a climatology of cyclones in order to understand how cyclones redistribute moisture stored in the atmosphere. This analysis shows that within a cyclone’s warm sector the cyclone-relative airflow is rearwards relative to the cyclone propagation direction. This low-level airflow (termed the feeder airstream) slows down when it reaches the cold front, resulting in moisture flux convergence and the formation of a band of high moisture content. One branch of the feeder airstream turns toward the cyclone center, supplying moisture to the base of the warm conveyor belt where it ascends and precipitation forms. The other branch turns away from the cyclone center exporting moisture from the cyclone. As the cyclone travels, this export results in a filament of high moisture content marking the track of the cyclone (often used to identify atmospheric rivers). We find that both cyclone precipitation and water vapor transport increase when moisture in the feeder airstream increases, thus explaining the link between atmospheric rivers and the precipitation associated with warm conveyor belt ascent. Atmospheric moisture budgets calculated as cyclones pass over fixed domains relative to the cyclone tracks show that continuous evaporation of moisture in the precyclone environment moistens the feeder airstream. Evaporation behind the cold front acts to moisten the atmosphere in the wake of the cyclone passage, potentially preconditioning the environment for subsequent cyclone development.

Sting Jets in extratropical cyclones: a review: Sting Jets: a review

Article

Mar 2018

This paper reviews the current state of knowledge of sting jets in extratropical cyclones. Sting jets were formally identified in 2004 by the pioneering work of Keith Browning. Reviewing this and subsequent studies, we define the sting jet as a coherent air flow that descends from mid‐levels inside the cloud head into the frontal‐fracture region of a Shapiro‐Keyser cyclone over a period of a few hours leading to a distinct region of near‐surface stronger winds. It therefore lies above the cold conveyor belt during some stage its life, but, at least in some cases, descends to reach the top of boundary layer ahead of the cold conveyor belt. It is not attributed to a specific mechanism in this definition. We conclude that it is likely that a continuum of sting jet descent and speed‐up mechanisms exists. At one extreme is balanced descent partly associated with frontolysis in the frontal‐fracture region. More horizontally small‐scale and stronger frontolytic descent may occur associated with weak stability to slantwise convective downdraughts. Instability to slantwise convective downdraughts may occur in some systems, leading to multiple slantwise convective downdraughts associated with the release of conditional symmetric instability and even, possibly, symmetric instability. The global climatology of sting jets and the interaction between the atmospheric boundary layer and sting jets are revealed as specific areas where more research is needed. Finally, we describe eight myths and misunderstandings that exist in the current literature with the aim of guiding future research into the sting jet phenomenon.

Radiation Weakens Idealized Midlatitude Cyclones

Article

Feb 2018

Mid-latitude cyclones are strongly affected by diabatic processes. While the importance of latent heating is well established, the role of radiation has received little attention. Here, we address this question for idealized cyclones by performing baroclinic life cycle simulations in the global atmosphere model ICON with and without radiation, and with transparent clouds. Radiation substantially weakens the simulated cyclone: peak eddy kinetic energy reduces by 50%, and minimum storm central pressure increases by 17hPa. An analysis of the Lorenz energy cycle shows that the radiative weakening is not due to changes in the large-scale environment alone, but involves radiative processes within the cyclone. In fact, radiation warms the lower-tropospheric part of the cyclone's warm conveyor belt, and cools the upper-tropospheric part. We hypothesize that radiation weakens the cyclone by destroying mid-tropospheric potential vorticity in the warm conveyor belt.

From Atmospheric Dynamics to Insurance Losses: An Interdisciplinary Workshop on European Storms

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