University of Birmingham
Developing a research strategy to better
understand, observe and simulate urban
atmospheric processes at kilometre to sub-
kilometre scales
Barlow, J. F.; Best, M; Bohnenstengel, S. I.; Clark, P; Grimmond, C. S B; Lean, H; Christen,
A; Emeis, S; Haeffelin, M; Harman, I; Lemonsu , A; Martilli, A; Pardyjak, E; Rotach, M;
Ballard, S; Boutle, I; Cai, Xiaoming; Carpentieri, M; Coceal, O; Di Sabatino, S
DOI:
10.1175/BAMS-D-17-0106.1
Document Version
Publisher's PDF, also known as Version of record
Citation for published version (Harvard):
Barlow, JF, Best, M, Bohnenstengel, SI, Clark, P, Grimmond, CSB, Lean, H, Christen, A, Emeis, S, Haeffelin, M,
Harman, I, Lemonsu , A, Martilli, A, Pardyjak, E, Rotach, M, Ballard, S, Boutle, I, Cai, X, Carpentieri, M, Coceal,
O, Di Sabatino, S, Dou, J, Drew, D, Edwards, JM, Fallmann, J, Fortuniak, K, Gornall, J, Gronemeier, T, Halios,
CH, Hertwig, D, Hirano, K, Holtslag, AAM, Luo, Z, Mills, G, Nakayoshi, M, Schlünzen, KH, Smith, S, Soulhac, L,
Steeneveld, G-J, Sun, T, Theeuwes, N, Thomson, D, Voogt, JA, Ward, H, Xie, Z-T & Zhong, J 2017, 'Developing
a research strategy to better understand, observe and simulate urban atmospheric processes at kilometre to
sub-kilometre scales', Bulletin of the American Meteorological Society. https://doi.org/10.1175/BAMS-D-17-
0106.1
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THE INTEGRATION OF URBAN ATMOSPHERIC
PROCESSES ACROSS SCALES WORKSHOP
What: A Met Office/Natural Environment Research
Council Joint Weather and Climate Research
Programme workshop brought together 50
key international scientists from the United
Kingdom and the international community to
formulate the key requirements for an urban
meteorological research strategy. The workshop
was jointly organized by the University of
Reading and the Met Office.
When: 16–18 November 2016
Where: University of Reading, Reading, United Kingdom
DEVELOPING A RESEARCH STRATEGY
TO BETTER UNDERSTAND, OBSERVE,
AND SIMULATE URBAN ATMOSPHERIC
PROCESSES AT KILOMETER TO
SUBKILOMETER SCALES
Janet BarloW, Martin Best, sylvia i. Bohnenstengel, Peter Clark, sue griMMond, huMPhrey lean, andreas
Christen, stefan eMeis, Martial haeffelin, ian n. harMan, aude leMonsu, alBerto Martilli, eriC PardyJak,
Mathias W rotaCh, susan Ballard, ian Boutle, andy BroWn, XiaoMing Cai, Matteo CarPentieri,
oMduth CoCeal, Ben CraWford, silvana di saBatino, JunXia dou, daniel r. dreW, John M. edWards,
JoaChiM fallMann, krzysztof fortuniak, JeMMa gornall, toBias groneMeier, Christos h. halios,
denise hertWig, kohin hirano, alBert a. M. holtslag, zhiWen luo, gerald Mills, Makoto nakayoshi,
kathy Pain, k. heinke sChlünzen, stefan sMith, lionel soulhaC, gert-Jan steeneveld, ting sun,
natalie e theeuWes, david thoMson, JaMes a. voogt, helen C. Ward, zheng-tong Xie, and Jian zhong
W
ith the majority of people experiencing weather
in urban areas, it is critical to understand
cities, weather, and climate impacts. Increasing
climate extremes (e.g., heat stress, air pollution, flash
flooding) combined with the density of people means
it is essential that city infrastructure and operations
can withstand high-impact weather. Thus, there is a
huge opportunity to mitigate climate change effects
and provide healthier environments through design
and planning to reduce the background climate and
urban effects. However, our understanding of the
underlying urban atmospheric processes are primar-
ily derived from studies of separate aspects, rather
than the complete, human–environment system.
Air quality modeling has not been widely integrated
with aerosol feedbacks on local climate, while few
city-greening scenarios have tested the impacts on
boundary layer pollutant dispersion or the carbon
cycle. Building design guidelines have been devel-
oped without incorporating the impact of waste heat
on local temperatures, which, in turn, determines
building performance. Integration of such feedbacks
is imperative as they define, rather than just modify,
urban climate.
There is an urgent need to link processes that
people experience at street level (human scale) to
processes at neighborhood, city, and regional scales.
As these scales have traditionally been the focus for
specialists in different fields, few observation and
model systems cross these scales. However, under-
standing the interactions between these scales is
critical for the design of future parametrizations
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OCTOBER 2017AMERICAN METEOROLOGICAL SOCIETY
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and observation networks. Although models and
observational methods are emerging that permit
research into scale interactions [e.g., high-resolution
numerical weather prediction (NWP), large-domain
computational fluid dynamic (CFD) models, remote
sensing, extensive sensor networks, vertical remote
sensing], an integrated approach across methodolo-
gies is currently lacking.
To tackle these scale interactions requires diverse
skills from a wide range of research communities.
This is a daunting challenge. However, improved
understanding of urban atmospheric processes such
as clouds and precipitation, heat transfer, and convec-
tion would enable improvements in urban system
models to provide seamless hazard prediction at all
time scales. Hence, an initial focus on the meteoro-
logical aspects of the research challenge may be a
more manageable problem, even though the scope is
still large. As such, it was identified that within the
United Kingdom there is an urgent need to develop an
urban meteorological research strategy that integrates
interactions and feedbacks on all scales.
MAIN FINDINGS/SCIENCE BACKGROUND.
The workshop was structured around three questions:
1) What are the key scientific challenges in ob-
serving and modeling urban atmospheres from
minutes to decades and from building to regional
scales?
2) How can atmospheric observations, models,
and theory be better integrated to tackle these
challenges?
3) Which atmospheric feedbacks across which scales
are critical to include across urban system models
(including building design, engineering, plan-
ning, air quality, hydrology, etc.)?
Following short provocative keynote presentations
and intense discussion across the wide variety of is-
sues, two distinct science challenges were identified
that cut across the three workshop questions, namely,
heterogeneity and anthropogenic drivers.
1) Urban areas are heterogeneous within and across
a range of scales (obstacles at 1–10 m, neighbor-
hoods at 10
2
–10
3
m, city scale at 10
3
–10
5
m).
Heterogeneity impacts the mean flow and turbu-
lent structures generated by the obstacles across
these scales, which interact with the turbulent
characteristics of the boundary layer. Urban
meteorology has relied on traditional Monin–
Obukhov similarity theory (MOST) with assumed
horizontal homogeneity to parametrize the tur-
bulent flux terms in mesoscale models. However,
given the extensive size (and ever taller) rough-
ness elements, and the relatively narrow bound-
ary layer, the applicability of MOST is severely
limited. With surface characteristics changing
at many length scales MOST, and extensions
such as blending height theory and tiling, have
to be questioned. The current representation of
the turbulent exchange of momentum (drag),
heat, moisture, pollutants, and radiation at all
scales across the urban system all need to be
formally reconsidered. Treatment of clusters of
AFFILIATIONS: BarloW, Clark, griMMond, CraWford, dreW,
halios, hertWig, luo, Pain, sMith, sun, theeuWes, and Ward—
University of Reading, Reading, United Kingdom; Best, Bohnenstengel,
lean, Ballard, Boutle, BroWn, edWards, fallMann, gornall, and
thoMson—Met Office, Reading, United Kingdom; Christen—
University of British Columbia, Vancouver, British Columbia,
Canada; eMeis—Karlsruhe Institute of Technology, Karlsruhe,
Germany; haeffelin—Institut Pierre Simon Laplace, Paris, France;
harMan—CSIRO Oceans and Atmosphere, Yarralumla, Australian
Capital Territory, Australia; leMonsu—Météo-France, Paris, France;
Martilli—Centro de Investigaciones Energéticas, Medioambientales y
Tecnológicas (CIEMAT), Madrid, Spain; PardyJak—University of Utah,
Salt Lake City, Utah; rotaCh—University of Innsbruck, Innsbruck,
Austria; Cai and zhong—University of Birmingham, Birmingham,
United Kingdom; CarPentieri—University of Surrey, Guildford,
United Kingdom; CoCeal—National Centre for Atmospheric Science,
University of Reading, Reading, United Kingdom; di saBatino—
University of Bologna, Bologna, Italy; dou—Institute of Urban
Meteorology, China Meteorological Administration, Beijing, China;
fortuniak—University of Łódź, Łódź, Poland; groneMeier—Leibniz
Universität Hannover, Hannover, Germany; hirano—National
Research Institute for Earth Science and Disaster Resilience (NIED),
Tsukuba, Ibaraki, Japan; holtslag and steeneveld—Wageningen
University, Wageningen, Netherlands; Mills—University College
Dublin, Dublin, United Kingdom; nakayoshi—Tokyo University of
Science, Tokyo, Japan; sChlünzen—Universität Hamburg, Hamburg,
Germany; soulhaC—University of Lyon, Lyon, France; voogt—
Western University, London, Ontario, Canada; Xie—University of
Southampton, Southampton, United Kingdom
CORRESPONDING AUTHORS: Sylvia I. Bohnenstengel,
sylvia.bohnenstengel@metoffice.gov.uk;
C. S. Grimmond, c.s.grimmond@reading.ac.uk
The abstract for this article can be found in this issue, following the table of
contents.
DOI:10.1175/BAMS-D-17-0106.1
In final form 21 April 2017
©2017 American Meteorological Society
For information regarding reuse of this content and general copyright
information, consult the AMS Copyright Policy.
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OCTOBER 2017
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tall buildings, deep urban canopies, and vegeta-
tion effects all need to be addressed.
The key problem is how we describe subgrid-
scale patchiness and its impact on momentum,
scalar exchange, and radiative forcing. This includes
challenging examples such as isolated groups of
tall buildings and spatially extended deep urban
canopies requiring vertically distributed processes
to be included in urban parametrizations. We lack
the observational knowledge to describe the scale
interaction between variations in surface-induced
turbulence and the stochastic nature of turbulence
in the planetary boundary layer. Observations are
fundamental to the development of both a theoretical
understanding and the models used. To capture the
scale interaction in the urban boundary layer, with
tall but sparse roughness elements (e.g., buildings
do not close the canopy as a forest may in leaf-on
state), will require new measurement technologies
and deployments. The shedding of heat, moisture,
and momentum from preferentially radiated volumes
with roof characteristics (e.g., heights, shapes) and
packing densities that modify the interaction with air
aloft are going to require new measurement technolo-
gies to be developed.
With NWP moving toward grid lengths of O(100)
m, we approach terra incognita (Wyngaard 2004)
and the building gray zone (where we need to resolve
large building blocks). We face the challenge of pa-
rametrizing turbulence at very different scales gen-
erated by a very nonuniform surface. This includes
dealing with stochastic transitions between filtered
and explicitly represented scales. One challenge is
the diurnal evolution of the boundary layer, where
the turbulence scales may no longer be resolved at
night. Models with grid lengths of O(100) m may
resolve the energy-containing eddies of a convective
boundary layer when they are forced by a uniform
rough surface, but the characteristics of these eddies
may change substantially with a more irregular urban
surface that creates localized peaks in scalar fluxes.
The workshop discussions highlighted the need
to agree on very specific research questions in order
to develop a robust theoretical framework beyond
MOST. To tackle some of the research questions, we
need high-quality long-term datasets, horizontally
and vertically distributed through the boundary layer,
over well-characterized urban areas. It is essential
that we design appropriate observational campaigns,
as well as measurement and evaluation techniques,
in collaboration with a community that includes
modelers.
2) While “dead” (unpopulated) cities pose many
physical problems associated with the grand
challenge above, anthropogenic drivers dramati-
cally change the properties of urban areas. This
includes, for example, urban energy, heat, water,
CO
2
, and spatial and temporal variability. The
dynamic changes of a city at subdaily, weekly, sea-
sonal, and longer time scales must be accounted
for (e.g., travel patterns, heating/cooling to retro-
fitting buildings, changing urban morphology,
and land cover).
It is critical that the fundamental data required to
capture these anthropogenic processes be properly
employed. This requires developing close collabora-
tion between those stakeholders with this expertise
(and also the likely end users of integrated weather,
climate, environment, and water services from
improved predictive capability), for example, the
energy sector, transport, water management, building
materials, building management, planning, and the
urban meteorological and atmospheric chemistry
communities, to ensure these data are available and
realistic. As a city evolves with technological, weather,
climate, and environmental changes, the services pro-
vided need to be dynamic in response to the people
living in the city. The inclusion of human behavior
is critical to providing realistic two-way interac-
tions with the urban–human environment system.
However, the complex nature of these feedbacks
requires the human system to be incorporated into
the physical system, requiring an integrated research
community with, for example, the socioeconomic,
political, psychological, and health disciplines
working together with climatologists, meteorologists,
atmospheric chemists, and others.
RECOMMENDATIONS. To expand upon these
two grand challenges, breakout groups considered
how research could tackle each challenge in turn.
Hypothetical proposals were developed. From these,
it was evident how a research program could begin
to make significant contributions toward solving
some of the challenges facing the urban community.
The proposals demonstrate the key need of taking
a coordinated and integrated approach between
different groups and methods. For example, new
frameworks designed to treat heterogeneous surface
exchange at scales ranging from O(100) m to O(1)
km can be developed using large-eddy modeling and
wind tunnel modeling, but multiscale measurements
of real canopy and boundary layer flows are essen-
tial to understanding these processes. The need to
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test fundamental instrument applicability and the
probable need to develop suitable urban-specific
measurement technologies is likely. Similarly, while
specific questions (e.g., concerning urban moisture
transport) may be addressable through modeling,
ultimately anthropogenic drivers in real cities will
need to be studied. A combination of modeling and
observational studies is essential to advance our
knowledge, possibly focused on a single city to start
with, so as to build up a comprehensive dataset and
conceptual understanding of the process interac-
tions between the building scale, the city scale, and
the mesoscale.
Consensus from the workshop suggests benefits
from the following initiatives:
• An integrated approach across all aspects of
urban areas and not isolated individual studies is
required.
• An urban “laboratory” at a fixed site is needed
to bring together different communities and
measurements/modeling efforts. This would
enable short-term intensive observation periods
(IOPs) to be embedded into well-understood long-
term datasets. Historically, the difficulties associ-
ated with long-term funding to facilitate such an
initiative have meant many missed opportunities
of well-bounded IOP studies. To address ques-
tions of change (e.g., technology, understanding,
behavior, land cover, climate), ensuring that quali-
ty-controlled datasets, with extensive data storage
(i.e., raw, processed datasets) and with extensive
urban metadata (biophysical, behavioral, etc.), are
available allows for numerous and repeated solu-
tions to be considered.
• Four-dimensional observations of multiple vari-
ables are needed. Theoretical understanding and
frameworks designed to address MOST at neigh-
borhood scales and heterogeneity at short scales
are critical. We need to understand the transfers
of heat, mass, and momentum from the urban
canopy layer (UCL), roughness sublayer (RSL),
inertial sublayer (ISL), and beyond to develop new
model parametrizations.
• Development and deployment of appropri-
ate measurement and modeling techniques/
parametrizations requires coverage of scales
ranging from within the UCL through the RSL
and the ISL, to the city scale, the boundary layer
scale, and mesoscale.
• Turbulence schemes and urban surface exchange
parametrizations for Wyngaard’s terra incognita
need to be developed.
• Cross-cutting research collaboration between
social sciences and a range of atmospheric/
environmental sciences will be fundamental to
the development and deployment of an urban
environmental system model.
• Longer-term funding is essential for long-term
facilities, whereas development work requires
funding for short-term-focused blue skies
exploration.
• Data assimilation techniques in heterogeneous
areas impacted by human activities need to be
developed.
• Satellite-derived data have increasing potential.
New deployments would permit many traditional
challenges between the pixel scale and land-cover
variability to be addressed.
• With extensive nontraditional data sources in
cities [e.g., mobile phones, social media, vehicle
usage characteristics (windscreen wipers, speed)],
there are opportunities through data mining to
significantly enrich urban environmental system
modeling [e.g., data assimilation (DA), assessment].
• Linking with end users, with particular applica-
tion needs or concerns, is critical to ensuring the
benefit of improved predictive capacity is taken
through to service provision.
Although, it is unclear how much this research
will enhance NWP at scales larger than the urban
area, it is likely to significantly improve weather and
climate services for the management of cities and
their inhabitants. This has the potential to improve
the prosperity, health, and safety of urban residents.
And with global sources of greenhouse gases being
disproportionately urban, there are likely many other
benefits to better cities, beyond their borders.
ACKNOWLEDGMENTS. The workshop was funded
by U.K. NERC via the Joint Weather and Climate Research
Programme (JWCRP). The organizers wish to thank
Khalid Mahmood, Pip Gilbert, and Debbie Turner for
administrative assistance. GJS acknowledges NWO Grant
864.14.007. KHS acknowledges the DFG-funded Cluster of
Excellence “CliSAP” (EXC177). The authors are grateful to
Dr. Phil Newton for verifying the accuracy of the contents
of this report.
REFERENCE
Wyngaard, J. C., 2004: Toward numerical modeling in
the “terra incognita.” J. Atmos. Sci., 61, 1816–1826,
doi:10.1175/1520-0469(2004)061<1816:TNMITT>2
.0.CO;2.
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