GEWEX is a Core Project of WCRP on Global Energy and Water Exchanges

Vol. 27 No. 2, May 2017

Workshop Reviews Aerosols-Clouds-Precipitation-and-Climate(ACPC) Initiative Progress in Case Studies

Initial results from Dipu Sudhakar, University of Leipzig show similarities to the satellite-derived relationships in some models, but not in all. Statistical relationship between cloud liquid water path (LWP) and cloud droplet number concentration (CDNC) expressed as conditioned probability(sum of LWP probability for each class of CDNC is normalized to unity as in Gryspeerdt et al. 2016) for the Variability of the American Monsoon
System (VAMOS) Ocean-Cloud-Atmosphere-Land Study (VOCALS) region and period, from (a) MODIS satellite retrievals, (b) a regional climate
model and (c) a global climate model. The dashed blue line shows the linear regression between the two quantities. See article by Johannes Quaas
et al. on page 7.

Contents
Commentary: Global Energy and Water Exchanges
in Times of Change
New SSG Member
Changes in Leadership in the GASS Panel
Director of WRMC Retires
News from the Young Earth System
Scientists (YESS) Community
Process Evaluation Study on Upper Tropospheric
Clouds and Convection

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2
3
3
3
4

First Results from ACPC Case Studies on
Aerosol Effects on Shallow and Deep Clouds
Meeting/Workshop Reports- 3rd PannEx Workshop- Workshop on Addressing the Challenge ofCompound Events
GEWEX/WCRP Calendar

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9
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Commentary
Global Energy and Water Exchanges inTimes of Change
Sonia I. Seneviratne
Co-Chair, GEWEX Scientific Steering Group (SSG), Professor for Land-Climate Dynamics, ETH Zürich, Switzerland

This issue of GEWEX News illustrates the ever-evolving
nature and dynamism of the research performed within
the GEWEX community. First, we are happy to welcome
Prof. Paul Dirmeyer as a new GEWEX SSG member, andDr. Daniel Klocke and Prof. Xubin Zeng as new co-chairs of
the Global Atmospheric System Studies (GASS) Panel. Paul
Dirmeyer has been a long-standing and pioneering memberof the GEWEX community and it is thus a great pleasurethat he can join the SSG and help steer new developments
within our core project. The new GASS co-chairs will leadand redesign the GASS Panel, which will include adding
stronger ties to the World Weather Research Programme.
As highlighted in the recent World Climate Research Programme (WCRP) workshop, “Climate Science: Thinking
Out of the Box,” developing research at the interface between weather and climate science constitutes one of thepromising new frontiers for climate research (Marotzke etal., 2017).
The activities and workshop reports in this newsletter exemplify the diversity and innovative nature of current GEWEX
initiatives, which range from the evaluation of upper tropospheric clouds and convection (page 4) to aerosol effects onshallow and deep clouds (page 7), to the analysis of multivariate extremes and compound events (page 10). The latter
topic is a new focus area of the WCRP Grand Challenge
on Weather and Climate Extremes, of which GEWEX is aco-leader.
It is also a great pleasure to see the successful development of
the Pannonian Basin Experiment (PannEx), which is working towards becoming a new Regional Hydroclimate Project of the GEWEX Hydroclimatology Panel (page 9). The
Pannonian Basin is a key agricultural and climate region inCentral and Eastern Europe, located in a hot spot of climatechange. For this reason, the developed research and monitoring program is of crucial relevance, in terms of both fundamental climate science and climate applications.
While embracing these new areas of development, we should
also applaud long-standing successful GEWEX activities,
such as the Baseline Surface Radiation Network (page 3).
We are thankful to the visionary initiators of this network,
including Atsumu Ohmura, Professor Emeritus of ETH
Zürich, and to the Alfred Wegener Institute for its current
invaluable support.
2

These and a wealth of other ongoing GEWEX activities were
presented at the annual Joint Scientific Committee (JSC)
meeting of WCRP held in early April. Potential joint initiatives within WCRP programs were discussed, and one of particular interest was the proposal for assessments of the global
energy and water balance as a measure of ongoing climatechange by the Climate and Ocean Variability, Predictability
and Change (CLIVAR) Project and GEWEX.
Looking at the many GEWEX projects bringing researchers
together across the world, we should be mindful that cooperation moves science forward. Being diligent and innovative is essential in these times of changing political views,
and we should be watchful that science is recognized forwhat it is: the search for truthful understanding, and not amatter of belief. We should be vigilant in ensuring that themerit of international research collaboration is recognized at
its true value.
Reference
Marotzke, J., C. Jakob, S. Bony, P. A. Dirmeyer, P. A. O’Gorman, E.
Hawkins, S. Perkins-Kirkpatrick, C. Le Quéré, S. Nowicki, K. Paulavets,
S. I. Seneviratne, B. Stevens and M. Tuma, 2017. Climate research must
sharpen its view. Nature Climate Change, 7, 89–91.

Paul A. Dirmeyer—
New GEWEX Scientific SteeringGroup Member
We welcome Paul A. Dirmeyer as a
new member of the GEWEX SSG.
He is a Professor and Senior Research
Scientist at the Center for OceanLand-Atmosphere Studies at George
Mason University in Fairfax, Virginia,
USA. Dr. Dirmeyer was a co-founderand former chair of the Global Land/
Atmosphere System Study (GLASS)
Panel and leader of the Global Soil
Wetness Project (GSWP). His scientific interests lie in the role of the land surface in the climate system. This includes the development and applicationof land-surface models, studies of the impact of land surface
variability on the predictability of climate, interactions between the terrestrial and atmospheric branches of the waterand energy cycles and the impacts of land use change on regional and global climate.
For a listing of all the GEWEX SSG members, see: http://
www.gewex.org/about/organization/.
May 2017

Changes in Leadership in GASS Panel
The new co-chairs for the Global Atmospheric System Studies (GASS) Panel are Drs. Daniel Klocke and Xubin Zeng.
GASS coordinates studies related to atmospheric and radiative processes.
Dr. Daniel Klocke is the researcharea director at the Hans Ertel Center for Weather Research at the German Weather Service in Offenbach,
Germany. Dr. Klocke is a meteorologist
with research interests in atmosphericmodeling for weather and climate acrossspatial scales with a focus on convection.
Previously he worked at the European
Centre for Medium-Range Weather
Forecasts and the Max Planck Institutefor Meteorology.
Dr. Xubin Zeng is the Agnese N. Haury
Chair for Environment, Professor of Atmospheric Science, and Founding Director of the Climate Dynamics and Hydrometeorology Center at the Universityof Arizona in Tucson, Arizona, USA. His
research interests include land-atmosphere-ocean interface processes, climate
modeling, hydrometeorology, remotesensing and nonlinear dynamics.

Director of WRMC Retires
Dr. Gert König-Langlo, the Director of the World Radiation
Monitoring Center (WRMC) from 2008–2017, retired in
April. WRMC hosts the Baseline Surface Radiation Network(BSRN) archive. We thank Dr. König-Langlo for his dedicated years of participation in BSRN and his excellent leadership in establishing and running the WMRC at the Alfred
Wegener Institute (AWI) for Polar and Marine Research at
Bremerhaven in Germany. Dr. Amelie Driemel, who has been
the data curator for the BSRN archive for several years, has
assumed the role of Director of the WRMC.

News from the Young Earth System
Scientists (YESS) Community
Carla Gulizia and the YESS Council
Centro de Investigaciones del Mar y la Atmósfera (CIMA/
CONICET-UBA), Buenos Aires, Argentina

YESS is a unified international multidisciplinary Early Career Researcher (ECR) network with more than 900 members
from over 80 countries. It works closely with international
programs for active involvement of ECRs in leading scientificprograms and participates and organizes special ECR events atinternational scientific meetings.
A YESS office was established at the Argentinian National
Weather Service in Buenos Aires in March 2017. Valentina
Rabanal, the YESS officer, will assist in coordinating andstrengthening the YESS community. This represents a big
step forward to engage more ECRs and to build the YESS
community on international and interdisciplinary levels.
YESS annual elections took place in April 2017 and Regional
Representatives and the Executive Committee were elected by
the YESS Council members. Regional Representatives support
the YESS community by sharing experiences, special interests
or information from their region. They are responsible for thegrowth of YESS in their region and serve as the contact points
for specific regional questions, activities and tasks. The YESS
community currently has representatives for the following
regions: Africa, Central Asia, Europe, North America, SouthAmerica, South East Asia and the South West Pacific. The outcomes of the elections are available on the YESS website at:
http://www.yess-community.org.
YESS activities include organizing “scientific speed dating”
for ECRs at the 2017 European Geophysical Union General
Assembly. This activity is aimed at sharing scientific questions and establishing potential collaborations. Two YESS
Council members were invited to represent YESS membership in the ECR Network of Networks (ECR NoN) at the
Future Earth Science and Engagement Committee meetingin March 2017. In April 2017, the 38th Session of the Joint
Scientific Committee (JSC) of the World Climate Research
Programme was held in Paris, France, and YESS memberspresented their achievements, plans for the coming year andrecommendations for consideration by the JSC. YESS was
also invited to present at the “Women in Science–Breaking
the Cliché” event at the World Meteorological Organizationin Switzerland in May 2017. Want to know more about YESS
or join our community? Visit our website or follow us on
Facebook, Twitter and LinkedIn.
Recently publish a paper related to GEWEX research?

From left to right: Atsumu Ohmura (architect of BSRN archive and first
director of WRMC), Gert König-Langlo and Amelie Driemel.

May 2017

If your research qualifies, highlights from it may be published on the GEWEX website or featured in GEWEX News.
For consideration, please summit your research highlight
at: http://www.gewex.org/latest-news/research-highlights/.

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Process Evaluation Study on Upper
Tropospheric Clouds and Convection(UTCC PROES)
Claudia Stubenrauch1, Graeme Stephens2 and GEWEXUTCC PROES Working Group
1
Laboratoire de Météorologie Dynamique/Institut Pierre
Simon Laplace, Centre National de la Recherche Scientifique, University Pierre and Marie Curie, Paris, France; 2Jet
Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA

The goal of activities of the GEWEX Process Evaluation Study(PROES) is to provide observation-based metrics for a better understanding of climate-related physical processes and toadvance their representation in research, weather and climate
models. A process that affects many components of the Earthsystem is convection. This process is a fundamental mode of
mixing and transport, moving heat, moisture, momentum
and atmospheric constituents through the Earth’s atmosphere.
Convective storms are the sole source of precipitation in many
regions of our planet and are widely understood to play a vitalrole in the Earth’s weather and climate. Advancing our understanding of these storm systems is viewed as one of the major challenges for atmospheric sciences in the coming decade(Bony et al., 2015). Convection is also a topic that is central
to various national agency objectives seeking to “improve thecapability to predict weather and extreme weather events.”
The GEWEX Upper Tropospheric Clouds and Convection(UTCC) PROES initiative specifically aims to develop new
diagnostic methods using existing observations to examine
the processes that detrain Upper Tropospheric (UT) clouds
from convection and the interconnection between convection
and the radiative heating induced by outflowing cirrus anvil
clouds. This heating is affected by at least three factors:

and the radiative heating associated with the UT clouds produced by convection.
UTCC PROES Goals• Understand and quantify the relation between convection and outflowing cirrus anvils by developing observation-based metrics.
• Determine the radiative heating of the UTC systems andquantify related feedbacks between UT clouds and convection in order to evaluate these processes in models.
Important UTCC PROES Elements• Develop new observational diagnostics designed specifically to address UTCC goals.
• Apply these diagnostics to evaluate and improve parameterizations affecting the relationships between convection and UT clouds in both cloud-resolving and global
weather and climate models.
• Develop process study experiments with these models,
including transport models, to strengthen our understanding of the processes critical to representing the
response of UT clouds to changes in environmental
forcing.
A working group is being formed that links, at present,
about 50 scientists from different communities. The first
UTCC PROES workshop was held at the Pierre and Marie
Curie University (UPMC) in Paris, France in November
2015. Twenty participants presented and discussed feedback
hypotheses and resources related to cloud systems and atmospheric environment from observations, Lagrangian transport to determine cirrus origin and life cycle, process modeling and large-scale parameterizations and radiative transfer.
Further meetings during the International Radiation Sym-

1. areal coverage of UT clouds,
2. cloud emissitivity within the UTC systems, and
3. underlying structure.
Since we are also interested in the thinner part of anvil clouds,
we use cloud properties obtained from the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding
Interferometer (IASI), because their spectral resolution provides reliable cirrus identification, day and night.
There currently are no real constraints in modeling the processes that produce these UT clouds, which results in a large
model-to-model spread in UT cloud properties and theirrelated feedbacks in models. While many feedbacks built
around the UT cloud radiative effects have been hypothesized, UTCC PROES was founded on the hypothesis thatthe radiative heating of the atmosphere specifically associatedwith UT clouds is fundamental to the most important cloudconvection-climate feedbacks. Thus, an important outcomeof the UTCC activity is the relationship between convection
4

Figure 1. Main elements (ovals) and links in the cloud feedback problem, framed by the synergetic elements necessary to determine UT
cloud feedback mechanisms by advancing our understanding on thesecloud systems.

May 2017

posium in Auckland, New Zealand, and at UPMC in Paris,
France in April 2016 provided forums for discussing strategies and first results. The goal of the second workshop, which
was held at City College, University of New York (CUNY)
in New York on 28–29 March 2017, was to outline the next
steps of UTCC PROES, including bringing diagnostic observations to cloud resolving models (CRMs) and generalcirculation models (GCMs).
UTCC PROES Synergistic Data
In addition to existing databases, such as the Tropical Rainfall Measuring Mission (TRMM), Megha-Tropiques and theInternational Satellite Cloud Climatology Project (in particular, statistics on weather states), which have been extensively
used to study tropical mesoscale convective systems, UTCC
PROES will provide:• A new database of UT cloud systems created from AIRS
and IASI cloud top pressure (Protopapadaki et al.,2017) with horizontal extent, and more importantly,
emissivity distributions within these systems, will aid in
distinguishing between thin cirrus, cirrus anvil and deep
convective cores;• The vertical dimension of the A-Train satellite constellation lidar-radar track using CloudSat and the CloudAerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission data, including vertical cloud
extent and layering, height of ice phase and ice watercontent and effective ice crystal diameter (De) profiles,
as well as radiative heating rates;

Figure 2. Thirteen years of statistics from AIRS. The ratio of thin cirrus(emissivity < 0.5) to total cloud anvil area of convective systems increases as the temperature of the convective core (emissivity > 0.98) decreases. The latter is a measure of convective depth (from Protopapadaki
et al., 2017).

• Thermodynamic and dynamic information from meteorological reanalyses; and• Additional geostationary cloud data (e.g., data developed for Megha-Tropiques) placed in the context of thelife cycle of these systems (formation, maturity and dissipation).
UTCC PROES First Observational Metrics
Measures of convective intensity, strength and depth may be
given by vertical updraft, lightning flash rate, level of neutral
buoyancy, area of heavy rainfall, width of the convective core,
cold cloud top temperature and mass flux (see references inProtopapadaki et al., 2017). UTCC PROES is performing areview of the different merits of these properties and how theymight relate to precipitation (e.g., Figure 10 in Protopapadaki
et al., 2017) and other properties of convection.
The first UTCC PROES results are expected to provide new
diagnostic characterizations of the convective detrainment
process. Two mutually supporting examples of these are linking anvil properties to different proxies of convective depth(see Figures 2 and 3). Both of these analyses reveal the remarkable result that for mature tropical convective cloud systems,
the ratio of thin cirrus over total anvil area (thick and thincirrus) increases (AIRS data) as convective depth increases. For
the latter, two independent proxies are used—the minimum
May 2017

Figure 3. Three years of collocated AIRS-CloudSat convective cloudsystems statistics. The fraction of thick cirrus and thin cirrus within the
total anvil areas of convective systems decrease and increase, respectively, as the height of the maximum mass flux within the anvil increases.
(H. Takahashi et al., presented at UTCC PROES workshop).

5cloud top temperature within the convective core from AIRS
(Figure 2) and the height of the level of neutral buoyancy
maximum mass flux derived from CloudSat (Figure 3). These
different analyses suggest that the behavior of UT cloud properties with increasing convective strength is robust.
Next Steps for UTCC PROES
These first observational metrics can be used to evaluate detrainment processes in models. Thus, the second workshop had
a greater emphasis on modeling than earlier UTCC PROES
meetings (see program at: http://www.gewex.org/gewex-content/
uploads/2017/03/UTCC_PROES_Meeting.pdf). Steps towards
using CRM simulations were outlined, along with strategies
to evaluate GCM convective parameterizations, including entrainment and detrainment. Since the mechanism(s) responsible for the observed behavior of UT clouds revealed in Figures
2 and 3 are not yet understood, CRM experiments are now
being planned to simulate the observations and investigate
possible mechanisms.
An important part of the project has to do with the heating
rates of UT cloud systems. As a first step, these mechanisms
will be sorted per cloud type (with respect to cloud emissivity)
within UT cloud systems by propagating the lidar-radar nadir
track vertical structure information across AIRS-observed UT
cloud systems. A simulator for UT cloud systems was developed for the evaluation of the Laboratoire de Météorologie
Dynamique climate model and can be adapted for CRM studies and for the evaluation of other GCMs.
During the workshop, it was noted that an important observational constraint is the vertical velocity profile. An interesting

strategy to study the evolution of tropical convection is compositing a measure of vertical velocity estimated from CloudSat
against the TRMM convection time series (Masunaga andLuo, 2016).
To find out how much of the heating can be traced to convectively generated cirrus, we need to separate cirrus systems(UT cloud systems not containing convection) originatingfrom convection and in situ freezing. Since the A-Train observations provide only snapshots, Lagrangian transport studies
will have to be performed. In addition, the evolution of cloudsystems could be investigated over specific regions (e.g., theWest Pacific Warm Pool) by adding high temporally-resolved
information from geostationary imagers.
The next UTTC PROES workshop is scheduled for autumn
2018. However, shorter meetings are foreseen during international conferences.
References
Bony, S., B. Stevens, D. M. W. Frierson, C. Jakob, M. Kageyama, R. Pincus,
T. G. Shepherd, S. C. Sherwood, A. P. Siebesma, A. H. Sobel, M. Watanabe
and M. J. Webb, 2015. Clouds, Circulation and Climate Sensitivity. Nat.
Geosci., 8, 261–268.
Masunaga, H., and Z. J. Luo, 2016. Convective and large-scale massflux profiles over tropical oceans determined from synergistic analysis of a suite of satellite observations. J. Geophys. Res. Atmos., 121,
doi:10.1002/2016JD024753.
Protopapadaki, E.-S., C. J. Stubenrauch and A. Feofilov, 2017. Upper Tropospheric Cloud Systems Derived from IR Sounders: Properties of Cirrus
Anvils in the Tropics. Atmos. Chem. Phys., 17, 3845–3859, doi:10.5194/
acp-17-3845-2017.

Participants of the 2nd UTCC PROES Workshop at City College, University of New York.

6

May 2017

First Results from ACPC Case Studies on
Aerosol Effects on
Shallow and Deep Clouds
Johannes Quaas1, Daniel Rosenfeld2, Meinrat Andreae3,
Graham Feingold4, Ann Fridlind5, Ralph Kahn6, Philip
Stier7, Kentaroh Suzuki8, Sue van den Heever9 andRobert Wood10
1
University of Leipzig, Germany; 2Hebrew University ofJerusalem, Israel; 3Max Planck Institute for Chemistry,Mainz, Germany; 4NOAA Earth System Research Laboratory, Boulder, USA; 5NASA Goddard Institute for Space Studies, New York, USA; 6NASA Goddard Space Flight Center,Greenbelt, USA; 7University of Oxford, United Kingdom;
8
University of Tokyo, Japan; 9Colorado State University, Fort
Collins, USA; 10University of Washington, Seattle, USA

The Aerosols-Clouds-Precipitation-and-Climate (ACPC) Initiative aims at a better understanding and quantification of the
impact of aerosol perturbations on clouds, radiation, precipitation, latent heating and atmospheric circulation. Following
roadmaps defined and iterated at earlier meetings (see: http://
research.uni-leipzig.de/acpc/meeting2017.html and summarized
in workshop reports, Quaas et al., 2015; 2016), and a comprehensive review paper summarizing earlier work within ACPC
(Rosenfeld et al., 2014), the initiative focuses on two cloud
regimes—shallow marine clouds and deep convective clouds.
Ongoing work and recent results were discussed at a workshop
held at the Physikzentrum Bad Honnef in Germany from 2–6
April 2017.
Research on deep convective clouds is currently focused on
isolated convective cells over Houston, Texas, USA. This research is guided by the substantial perturbation in cloud condensation nuclei (CCN) concentrations caused by pollution
from Houston in onshore flow that is in contrast to much lesspolluted conditions in the vicinity as observed by new satellite
products (Rosenfeld et al., 2016). The aerosol perturbation
under onshore flow conditions offers the possibility of observing the evolution of convective cells under relatively uniform
thermodynamic conditions.
On the observations side, emphasis is placed on the analysisof radar measurements. Groups at the National Aeronautics
and Space Administration (NASA) Goddard Institute forSpace Studies (GISS), Texas A&M University, the National
Atmospheric and Oceanic Administration and the National
Severe Storms Laboratory identified convective cells in polarimetric radar data from the Next-Generation Weather Radar(NEXRAD) network and tracked them over their lifetime.
Preliminary analysis of the NEXRAD data and collocatedLightning Mapping Array observations indicate that characteristics of isolated cell evolution differ between situations subject to relatively high versus low CCN conditions. One study
by the Texas A&M University and the Hebrew University ofJerusalem proposed from a statistical analysis of observations
May 2017

that these clouds had greater vertical development, larger hydrometeors and enhanced lightning, hypothesizing that thismight be due to invigoration (Andreae et al., 2004; Rosenfeld
et al., 2008). Through interaction with experts on radar observations, the group proposes new radar observation strategies that may allow for improved tracking of rapidly evolving cell development, in particular using mobile polarimetricradars that can offer rapid scan capabilities. A group at Stony
Brook University and NASA GISS used forward simulationfrom preliminary simulations to demonstrate potential minimum requirements (distances from target, number of radars)
for radars to adequately observe isolated cells and potentiallyretrieve vertical wind speed.
On the modeling side, a common case study protocol for
simulations of deep convective clouds has been defined (details are available at: www.acpcinitiative.org) and first simulations were conducted with two cloud-resolving models. Thesesimulations from Colorado State University and the University of Oxford showed distinct differences between the highand low-CCN simulations in vertical wind and specific ice
content, albeit with little signal in surface precipitation. Thislatter finding was also evident in an ensemble of simulations
conducted by the University of Leeds for a different convective case, where it was very clear that even very large aerosol
perturbations do not produce signals in precipitation distinguishable from the uncertainty range as represented by theensemble spread. However, other properties differed notably
but not entirely consistently across the models tested; suchdifferences between models are expected based on substantial
uncertainties in microphysics schemes (White et al., 2016), inpart motivating a strong parallel effort on the observation side.
Based on a new analysis of data from the air quality focusedfield study, Deriving Information on Surface Conditions from
Column and Vertically Resolved Observations Relevant to Air
Quality (DISCOVER-AQ), by the Max Planck Institute for
Chemistry, the prescribed CCN profiles of the case study protocol will be revised. A next step is to invite the wider modeling community to contribute more simulations and to forward-simulate polarimetric radar signals from the simulations
for comparison to the hypotheses on aerosol signals proposedon the basis of the observations analysis.
Based on these results, ACPC is working towards a first field
campaign where at least one mobile radar from potential U.S.
sources can be deployed and the methods for rapid scanningand statistical assessment of the observations can be tested onsite in the Houston area. The model simulations in the deep
convective cloud case study will help to develop and test observational strategies for the field campaign; examples include
testing the potential capability to measure wind convergenceprofiles using high-resolution dropsonde arrays over 100-kmdiameter columns (based on preliminary results by the Laboratoire de Météorologie Dyanmique and Max Planck Institutefor Meteorology) and assessing the observational strategy toachieve a box closure experiment (Rosenfeld et al., 2014).
7

With regard to shallow clouds, research has focused on thesoutheastern Pacific stratocumulus region, where the Variabilityof the American Monsoon System (VAMOS) Ocean-CloudAtmosphere-Land Study (VOCALS) field campaign madecomprehensive measurements in October–November 2008.
Statistical relationships between aerosol optical depth and cloud
droplet number concentration, Nd, and between Nd and cloudliquid water path (LWP, Michibata et al., 2016) are being assessed from available simulations and satellite retrievals and put
into the context of anthropogenic perturbation (cf. Gryspeerdt
et al., 2017).
First results from the University of Leipzig show similarities tothe satellite-derived relationships in some models, but not in all(see figure on page 1). From a set of simulations by the University of Leeds, new results suggest the possibility of identifying
clear signals of anthropogenic effects on clouds even in top-ofatmosphere radiation (Grosvenor et al., 2017). A new effort isnow directed at running large eddy simulations (LES) along
Lagrangian trajectories derived from a coarse grid Weather Research and Forecasting (WRF) model. This amounts to a downscaling exercise in which a LES provides a more detailed view ofaerosol and cloud processes along the stratocumulus to cumulustransition. Trajectories were initiated near the Chilean coast in
closed cell stratocumulus decks under polluted conditions andfollowed towards the open ocean where the regime changed to
broken cumulus cloudiness, with a corresponding reduction inaerosol loading. Sensitivity to WRF forcing has been exploredbut much more work needs to be done.
The Shallow Cloud Working Group, along with continuing research along these lines, will broaden the focus to also consider
stratocumulus and their transition to cumulus in the Southeast
Atlantic under the influence of biomass burning smoke [theObservations of Aerosols above Clouds and their interactions(ORACLES)/CLoud-Aerosol-Radiation Interactions and Forcing (CLARIFY)/Layered Atlantic Smoke Interactions with
Clouds (LASIC) campaigns]; also trade-wind cumulus clouds
will be observed along with a comprehensive characterizationof the large-scale weather conditions in the Elucidating the Roleof Cloud-Circulation Coupling in Climate (EUREC4A) campaign (http://eurec4a.eu/). Finally, because the Shallow CloudGroup to a large extent assesses satellite data, one ongoing effort
within ACPC is to characterize capabilities and uncertainties ofcurrent cloud droplet number concentration calculations basedon satellite-based cloud optical depth and drop effective radiusretrievals, and to assess new and upcoming approaches.
A follow-up workshop is planned for 4–6 April 2018 in
Colorado, USA. The ACPC group welcomes interested persons or groups to join the activities.
References
Andreae, M. O., D. Rosenfeld, P. Artaxo, A. A. Costa, G. P. Frank, K. M.
Longo and M. A. F. Silva-Dias, 2004. Smoking rain clouds over the Amazon. Science, 303, 1337–1342, doi:10.1126/science.1092779.

8

Grosvenor, D. P., P. R. Field, A. A. Hill and B. J. Shipway, 2017. The relative importance of macrophysical and cloud albedo changes for aerosol
induced radiative effects in stratocumulus: insight from the modeling ofa case study. Atmos. Chem. Phys., 17, 5155–5183, doi:10.5194/acp-175155-2017.
Gryspeerdt, E., J. Quaas and N. Bellouin, 2016. Constraining the aerosol influence on cloud fraction. J. Geophys. Res., 121, 3566–3583,
doi:10.1002/2015JD023744.
Gryspeerdt, E., J. Quaas, S. Ferrachat, A. Gettelman, S. Ghan, U. Lohmann, H. Morrison, D. Neubauer, D. G. Partridge, P. Stier, T. Takemura,Hailong Wang, M. Wang and K. Zhang, 2017. How accurately can theforcing from aerosol-cloud interactions be constrained? Proc. Nat. Acad. Sci.
USA, in press.
Michibata, T., K. Suzuki, Y. Sato and T. Takemura, 2016. The source of
discrepancies in aerosol–cloud–precipitation interactions between GCM
and A-Train retrievals, Atmos. Chem. Phys., 16, 15413–15424, doi:10.5194/
acp-16-15413-2016.
Quaas, J., D. Rosenfeld, A. Fridlind and R. Wood, 2015. Workshop on
the Aerosols-Clouds-Precipitation and Climate (ACPC) Initiative, GEWEX
News, 25, No. 2, 11–12.
Quaas, J., D. Rosenfeld, A. Fridlind, R. Wood, G. Feingold, S. van den
Heever and P. Stier, 2016. ACPC Initiative to identify signatures of aerosolcloud interactions in high-resolution modeling and observations. GEWEX
News, 26, No. 3, 5–7.
Rosenfeld, D., Y. T. Zheng, E. Hashimshoni, M. L. Pöhlker, A. Jefferson,
C. Pöhlker, X. Yu, Y. N. Zhu, G. H. Liu, Z. G. Yue, B. Fischman, Z. Q.
Li, T. Giguzin, T. Goren, P. Artaxo, H. M. J. Barbosa, U. Pöschl and M. O.
Andreae, 2016. Satellite retrieval of cloud condensation nuclei concentrations by using clouds as CCN chambers, Proc. Nat. Acad. Sci. USA, 114(21), 5828–5834.
Rosenfeld, D., M. O. Andreae, A. Asmi, M. Chin, G. Leeuw, D. P.
Donovan, R. Kahn, S. Kinne, N. Kivekäs, M. Kulmala, W. Lau, S. Schmidt,
T. Suni, T. Wagner, M. Wild and J. Quaas, 2014. Global observations ofaerosol-cloud-precipitation-climate interactions. Rev. Geophys., 52, 750–
808, doi:10.1002/ 2013RG000441.
Rosenfeld, D., U. Lohmann, G. B. Raga, C. D. O’Dowd, M. Kulmala,S. Fuzzi, A. Reissell and M. O. Andreae, 2008. Flood or drought: How
do aerosols affect precipitation?, Science, 321, 1309–1313, doi:10.1126/science.1160606.
White, B., E. Gryspeerdt, P. Stier, H. Morrison, and G. Thompson, 2016.
Can models robustly represent aerosol–convection interactions if theircloud microphysics is uncertain? Atmos. Chem. Phys. Discuss. doi: 10.5194/
acp-2016-760.

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May 2017

Meeting/Workshop Reports
3rd PannEx Workshop
Cluj-Napoca, Romania
20–22 March 2017
Ivan Güttler1 and Mónika Lakatos2
1
Meteorological and Hydrological Service (DHMZ), Zagreb,
Croatia; 2Hungarian Meteorological Service (OMSZ), Budapest, Hungary

The Pannonian Basin Experiment (PannEx) is working towards becoming a new Regional Hydroclimate Project of the
GEWEX Hydroclimatology Panel (GHP). The 3rd PannEx
Workshop was held at the Facultatea de Geografie of the
Universitatea Babeş-Bolyai with more than 60 participantsfrom Romania, Hungary, Croatia, Serbia, Ukraine, Slovakia,
Italy, France, Spain and The Netherlands. Dănuţ Petrea,
Dean of the Faculty of Geography, and Daniel David, ViceRector for Research, Competitiveness-Excellence, and Scientific Publications of Babeş-Bolyai University, opened themeeting. Mónika Lakatos, Chair of the PannEx International Planning Committee (IPC), Jan Polcher of Laboratoirede Météorologie Dynamique (CNRS), and Joan Cuxart of
the University of the Balearic Islands, the latter representing
GEWEX, also provided opening remarks.
The goal of PannEx is to enhance Earth science in the
Pannonian Basin and contribute to the overall objective of
the scientific community to improve the understanding of theclimate system, its predictability and its effect on human activities. The workshop had five sessions with targeted scientificdiscussions led by the authors of the White Book, which lays
out the structure and plans for PannEx.
Agricultural Response to Climate Change and Weather
Extremes
Links between climate change and climate variability and crop
production in Europe, and the impacts of crops and activities on crop health and yield, were examined using data fromfield experiments held in Hungary and Croatia. The extensionof existing field campaigns and the setup of new field agrometeorological observations were recommended, as they areneeded to increase our understanding of climate extremes oncrop growth, yield amount and quality.
Extreme Weather and Climate Events as a Risk to Sustainable
Development
Julia Keller of the World Weather Research Programme(WWRP) gave an overview of the program’s research activities and discussed synoptic conditions for cold waves andcold spells in Romania, and the impacts of climate variability
on forest growth. A suggestion was given to consider various
emerging subseasonal and seasonal forecast products available
for the countries in the Pannonian Basin.
May 2017

Water for a Secure Society in Changing Climate Conditions
A review of water balance studies confirms that there is no
uniform method in the modeling of the water balance to study
the water cycle, which makes intercomparisons between models difficult. Formulas recently used for estimation of the components of the water balance sometimes contain coefficientsthat were developed under different climate conditions. Oneof the important issues raised in this session concerned themany diverse approaches in estimating evaporation in different countries and in different research fields. A common harmonization effort is therefore necessary.
Monitoring and Estimation of Climate Change for Extreme
Weather Events
The results of the European Commission 7th Framework Programme (FP7) eartH2Observe Project were presented. Activities on drought monitoring in Slovakia; regional climate
modeling in Croatia and Hungary; and emerging rapid prototyping software products related to Earth observations in
Romania were discussed. Also, climate characteristics over the
Danube and Pannonian Basins and Ukraine were explored,
and results of the urban climate observational targeted activities in Budapest were presented to the participants.
Impact and Vulnerability Assessments of Climate Change
and Extreme Weather Events on Different Sectors
Hydrological processes over small watersheds in Romania andadvanced hydrological prognostic systems developed in Serbia
were discussed. Other sectors tackled in this session included
extensive boundary layer observations in Hungary, a projectin Croatia that links boundary layer observations with the observed effects of climate change on wine production, evaluation of the low-level jets over the Pannonian Basin as simulatedby the European Centre for Medium-range Weather Forecasts(ECMWF) prognostic system, and common meteorologicaland carbon dioxide observations in the town of Cluj-Napoca.
Future Actions
The first draft of the PannEx White Book was presented at
the GHP meeting held in Gif sur Yvette, France in October
2016. The PannEx IPC have incorporated comments received
from the GHP and will send the revised version to the PannEx
mailing list for comments i=n May. After the White Book is
finalized, the PannEx Science and Implementation Plan will
be distributed for comments.
IPC members Mónika Lakatos and Tamás Weidinger will explore the possibility of organizing the next PannEx workshop
in parallel with the European Meteorological Society Conference in Budapest in September 2018. In the meantime, thenext IPC meeting on the Science and Implementation Plan
will be coordinated by Vladimir Djurdjevic and held in September 2017 at the University of Belgrade.
Discussions concerning funding options, a logo for the initiative, and community growth resulted in concrete suggestions
that will be explored in the following months. Workshop presentations and posters are available at: https://sites.google.com/
site/projectpannex/workshops/pannex2017presentations.
9Workshop on Addressing the Challengeof Compound EventsETH Zurich, Switzerland
19–21 April 2017
Jakob Zscheischler1, Seth Westra2, Bart van den Hurk3,
Philip Ward4, Andy Pitman5 and Sonia I. Seneviratne1
1
Institute for Atmospheric and Climate Science, ETH Zurich,
Zurich, Switzerland; 2University of Adelaide, Adelaide,
Australia; 3KNMI, De Bilt, The Netherlands; 4Vrije Universiteit Amsterdam, The Netherlands; 5University of New South
Wales, Australia

Hazards such as floods, wildfires, heat waves and droughts often result from a combination of interacting physical processes
that take place across a wide range of spatial and temporalscales. The combination of physical processes leading to an
impact is referred to as a “compound event.” In a recent workshop at ETH Zurich, more than 30 scientists from three continents and 11 different countries, including 11 early career
scientists, discussed the current understanding of compound
events, and how to move forward. Compound events in thecontext of climate change have been introduced to the broader
climate science community by the Special Report on Climate
Extremes (SREX) of the Intergovernmental Panel on Climate
Change (IPCC) as: “(1) two or more extreme events occurring
simultaneously or successively; (2) combinations of extreme
events with underlying conditions that amplify the impact of
the events; or (3) combinations of events that are not themselves extremes but lead to an extreme event or impact whencombined. The contributing events can be of similar (clustered multiple events) or different type(s)” (Seneviratne et al.,2012). More recently, Leonard et al. (2014) suggested a more
confined definition focusing on impacts and statistical dependence between the drivers: “A compound event is an extreme
impact that depends on multiple statistically dependent variables or events.” Intense discussions about how to define com-pound events were a common theme across all sessions of theworkshop. This question is more than a matter of definition: it
delineates very much the way to approach, study and predictcompound events.
The Challenge
Compound events have been identified as an important challenge by the World Climate Research Programme (WCRP)
Grand Challenge on Weather and Climate Extremes (https://www.wcrp-climate.org/grand-challenges/gc-extreme-events),
which highlights their relevance to climate science. A better
understanding of the nature of compound events should lead
to better understanding and improved predictions of extreme
impacts with direct benefits to society. Because of their complex nature, compound events require climate scientists, impact modelers and social scientists to work closely together
to understand them. The insight that impacts rarely depend
upon only one driver may lead to novel ways of examining
climate model output if the goal is to simulate those extreme
events that are likely to have the largest impacts. In this context, working together should enable the community to provide valuable insights on problems such as identifying those(combinations of ) climatic drivers that lead to the largest impacts. In addition, the robust estimation of probabilities ofco-occurring extremes, such as storm surge and precipitation(Wahl et al., 2015) or droughts and heat waves (Mazdiyasni
and AghaKouchak, 2015), requires well-calibrated models or
the usage of novel statistical tools from the active area of multivariate extreme value theory.
The workshop was organized in sessions corresponding to thefour themes of the WCRP Grand Challenge on Extremes:
Document, Understand, Simulate and Attribute. Each session
was opened by a keynote talk, which provided an overview
of the current state-of-the-art knowledge and presented major questions related to compound events and the respective
theme. To keep the workshop highly interactive, participantspresented recent results and research questions in short pitches.
Furthermore, Mentimeter (http://www.mentimeter.com) wasused for polls throughout the workshop
to obtain an instant picture about theparticipants’ thoughts. For instance, thequestion “What is a compound event
to you?” was asked in the opening session (see figure on the left) and again at
the end of the workshop, demonstrating
how the importance shifted from theword “extreme” to “multivariate” overthe course of the workshop.

Word cloud created from the responses of participants to the question, “What is a compound event
to you?” This was done at the beginning of the workshop.

10

Document
In this session, the participants discussed
questions related to data availability
such as: (i) which climate variables need
to be assessed jointly in order to characterize the relevant class of a compound
event? (ii) How much is currently
known about the dependence between
these variables? and (iii) Are the obserMay 2017

vations sufficient to underpin the assessment of their dependencies in the historical climate and potential trends in theiroccurrence and/or dependence? The multivariate nature ofcompound events demands much longer time series to reachequally robust statistics as compared to the univariate case.
Furthermore, to identify the most relevant climatic drivers,
compound events require information on some type of impact
such as flood depth, economic damage or lives lost. Impact
data are, however, often highly heterogeneous and rarely centrally recorded. Hence, even identifying which data exist canbe a challenge (data discoverability). To overcome these difficulties, the idea of a meta-database of available data sets fromthe different communities was discussed.
Understand
Understanding compound events is the first step towards a better simulation and prediction of potentially extreme impacts. In
this session, the participants discussed questions such as: What
drives compound events? How do drivers interact? How dodrivers generate impacts? And what is the dependence between
drivers? Important points raised in the discussion included theidea that in order to rank events (e.g., for computing return
periods or other relevant statistics), there is a need to map multivariate drivers onto a univariate measure. This will generally
be some type of potential or realized impact. To detect dependences in the drivers, multivariate extreme value theory might
be necessary because the dependence in the tails may be different from the dependence for the remainder of the distribution,
leading to particularly devastating impacts (e.g., a combinationof extreme storm surge and extreme precipitation, Wahl et al.,2015). It was also emphasized that human decision can play a
key role in alleviating or aggravating the impacts of compound
events, and methods are required to capture the behavior ofdecision-makers, such as agent-based modeling approaches.
The session also discussed the idea that very rare events cannot
easily be characterized by their return time, as this is a quantitythat is hard to quantify. Storyline approaches and “black swan”

thinking need to enter the arena, giving room for experience
with real world events. Nevertheless, for assessing the risk fromcompound events, we need to examine compound events of all
sizes, not only the “black swans.”
Simulate
The Simulate session was dedicated to questions related to thesimulation of compound events such as: Are models able to reliably simulate the dependence between climate variables andhow they might change in a future climate? What methods
are available to evaluate the capacity of models to simulate an
appropriate level of dependence between variables? What is
the modeling capability to model the impacts of the variable
dependence adequately? Acknowledging the multivariate nature of many high-impact events will affect our views on how
to use and evaluate models. Models play a major role in theunderstanding of compound events, as data are often scarce.
However, if dependences are misrepresented in models, compound events will also likely be modeled unreliably, compromising risk assessments. Furthermore, new ideas are required
to extend bias correction methods to the multivariate case andto explore the space of potential high-impact events, as currently the extent to which bias correction methods maintainkey dependences in the climate system is largely unknown.
This exploration could be done, for instance, with storylines;that is, by shifting past events in space or time in the model
world and estimating potential impacts. In order to get to socioeconomic impacts, it is important to be able to simulate
the effects of compound events at the local scale. However, inmany parts of the world, state-of-the-art local impact models
are not available, and there is a need to nest local impact models within global physical models.
Trends and Attribute
The Trends and Attribute session dealt with the most uncertain parts of compound events research. Detecting trends
requires long time series. Detecting trends in dependences

Participants of the workshop.

May 2017

11

between variables requires even longer time series. The workshop discussed questions such as: to what extent are historicaland/or projected changes in the identified compound events
attributable to particular causes? Are the available modelingtools appropriate to enable attribution of compound eventsor classes of events? Do changes in external forcing change thephysical mechanisms involved in compound events, leading
to stronger or weaker connections? As an example, Wahl et al.
(2015) detected an increase in the co-occurrence of extreme
storm surge and precipitation events on U.S. coasts. Changes
in co-occurrence can also occur if only one variable changes,
such as increases in temperature, changes in water consumption and sea level rise.

GEWEX/WCRP Calendar
For the complete Calendar, see: http://www.gewex.org/events/
6–8 June 2017—Clouds, their Properties and their Climate Feedbacks–
What have we learned in the satellite era?—New York, New York, USA
22–23 June 2017—13th GRDC Steering Committee Meeting—Koblenz,
Germany
4–7 July 2017—10th HyMeX International Workshop—Barcelona, Spain
10–12 July 2017—Third Pole Science Summit–TPE-CSTP-HKT Joint
Conference—Kunming, China

Summary and Future Perspectives
Overall, the workshop demonstrated that advancing researchon compound events requires a collective effort by different
scientific communities, including not only climate modelingand impact modeling researchers, but also statisticians and disaster risk and resilience professionals. Progress will dependon the willingness to share knowledge between communities
across disciplinary boundaries. During the workshop, all participants were pushed out of their comfort zone and were required to interact with scientists from various backgrounds.
One main planned outcome will be a review paper reflecting
on the topics discussed during the workshop. We also considered applying for status as a European Commission Cooperation in Science and Technology (COST) Action to initiate
collaborations and to promote the dialog between disciplines
on this exciting topic. COST Actions are bottom-up scienceand technology networks that facilitate the creation of a newscientific community by providing funding for meetings overa period of four years.

10–14 July 2017—International WCRP/IOC Conference on Regional Sea
Level Changes and Coastal Impacts—New York, New York, USA

Acknowledgments
The workshop would not have been possible without generous
funding from WCRP, the Australian Research Council Centerof Excellence for Climate System Science (ARCCSS), ETH
Zürich, the Vrije Universiteit Amsterdam and The Netherlands
Organization for Scientific Research. The funding was primarily used to invite promising Early Career Scientists working on
compound events to the workshop.

9–13 October 2017—32th Session of WGNE—Exeter, United Kingdom

Leonard, M., S. Westra, A. Phatak, M. Lambert, Bart V. D. Hurk, K.
Mcinnes, J. Risbey, S. Schuster, D. Jakob and M. Stafford-Smith, 2014.
A compound event framework for understanding extreme impacts. WIREs
Clim. Change, 5, 113–128.
Mazdiyasni, O., and A. AghaKouchak, 2015. Substantial increase in concurrent droughts and heatwaves in the United States. Proc. Natl. Acad. Sci.
U.S.A., 112(37), 11484–11489.
Seneviratne, S. I., et al., 2012. Changes in climate extremes and their impacts on the natural physical environment. IPCC Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX)
report, pp. 109–230.
Wahl, T., S. Jain, J. Bender, S. D. Meyers and M. E. Luther, 2015. Increasing risk of compound flooding from storm surge and rainfall for major US
cities. Nat. Clim. Change 5(12), 1093–1097.

12

10–14 July 2017—Workshop on the Future of Cumulus Parameterization—Delft, The Netherlands
19–20 July 2017—1st International Surface Working Group Meeting—Monterey, California, USA
19–23 July 2017—5th WGNE Workshop on Systematic Errors in Weather
and Climate Models—Montreal, Canada
20–24 August 2017—7th International Workshop on Catchment Hydrological Modeling and Data Assimilation (CAHMDA VI)—Xi’an, Shanxi, China
11–14 September 2017—5th iLEAPS Science Conference—Oxford, United
Kingdom
18–20 September 2017—4th Satellite Soil Moisture Validation and Application Workshop and CCI Soil Moisture User Workshop—Vienna, Austria
18–22 September 2017—COSPAR 2017—Jeju, Republic of Korea
25–28 September 2017—CFMIP Meeting on Clouds, Precipitation, Circulation, and Climate Sensitivity—Tokyo, Japan

16–18 October 2017—25th SPARC Scientific Steering Group Meeting—Seoul, Republic of Korea
17–19 October 2017—GHP-TPE Workshop—Kathmandu, Nepal
25–26 October 2017—7th GVAP Workshop—Leicester, United Kingdom
13–17 November 2017—5th International Conference on Reanalysis(ICR5)—Rome, Italy

GEWEX NEWS
Published by the International GEWEX Project OfficePeter J. van Oevelen, Director
Dawn P. Erlich, Editor
Shannon F. Macken, Assistant Editor
International GEWEX Project Officec/o USRA
425 3rd Street SW, Suite 940
Washington, DC 20024 USA
Tel: 1-202-527-1827
E-mail: gewex@gewex.org
Website: http://www.gewex.org

May 2017