How to bring urban and global climate studies together with urban planning and architecture?

Climate friendly urban planning plays a key role in climate change mitigation and adaptation and allows for sustainable development of living conditions for future generations. It has been long understood that measures such as urban greening, planted facades and roofs or highly reflecting building materials are able to dampen excess heat and help reducing energetic costs. Transferring scientific and often theoretical knowledge into actual urban planning however necessarily involves an interdisciplinary dialogue. This paper intends to provide a review of existing literature from a meteorological perspective in order to answer the question how results from urban climate studies can be linked to architectural design of future urban areas. Results from state of the art research are evaluated and critically addressed, hence providing a catalogue for urban planners and stakeholders which should serve as basis for a re-evaluation of the term ‘smart city’

Unsatisfying Transfer of Climate Research to Urban Planning: The Regulatory Trap in the Triple Helix

Making urban areas more sustainable by transferring scientific results into the building, shaping, and governance of cities is a complicated process which involves – amongst other dimensions – science, local governance, and regulatory processes. There are non-linear interactions within each of these three dimensions which are influenced and enhanced by interactions between the three dimensions. After a short analysis of different sustainability concepts, this conceptual paper considers each of the three dimensions and finally tries to find some suggestions as to how these dimensions could interact more smoothly also considering Triple Helix theory. One basic suggestion is that without updating laws, norms, and standards, urban administrations will often not be able to integrate new scientific findings into procedures for more sustainable cities. That is, all three dimensions need to be aligned in the process of building sustainable cities

How to bring urban climate studies to application – A meteorological view from five decades of urban climate research and results from a current study

Atmospheric sciences have dealt with the special features of urban climate for about 200 years, starting with the seminb nal book of Howard. It therefore has been long understood that urban areas govern the dynamics and air chemistry of the atmb mospheric boundary layer, they are key drivers for the development of local circb culation patterns and can modify local and regional weather and climate. On the other hand, local meteorological conditb tions and large-scale weather patterns drive the formation of the urban heat islb land, can modify microclimate conditions and affect air quality regionally and locb cally. As such, holistic models have to be developed in order to properly represent interactions along both time and spatial scales and preferably have to incorporate both dynamics and air chemistry. A large amount of studies exist already, which highlight the importance of properly reprb resenting urban areas within mesoscale models via urban canopy parametrizations. Current coordinated model activities try to assess these parb rametrizations to be included e.g. in regional climate models as being considered within Cordex FPS URBRCC

Mitigation of urban heat stress – a modelling case study for the area of Stuttgart

In 2050 the fraction of urban global population will increase to over 69 %, which means that around 6.3 billion people are expected to live in urban areas (UN 2011). Cities are the predominant habitation places for humans to live and are vulnerable to extreme weather events aggravating phenomena like heat stress. Finding mitigation strategies to sustain future development is of great importance, given expected influences on human health. In this study, the mesoscale numerical model WRF is used on a regional scale for the urban area of Stuttgart, to simulate the effect of urban planning strategies on dynamical processes affecting urban climate. After comparing two urban parameterisation schemes, a sensitivity study for different scenarios is performed; it shows that a change of the reflective properties of surfaces has the highest impact on near-surface temperatures compared to an increase of urban green areas or a decrease of building density. The Urban Heat Island (UHI) describes the temperature difference between urban and rural temperatures; it characterises regional urban climate and is responsible for urban-rural circulation patterns. Applying urban planning measures may decrease the intensity of the UHI in the study area by up to 2 °C by using heat-reflective roof paints or by 1 °C through replacing impervious surfaces by natural vegetation in the urban vicinity – compared to a value of 2.5 °C for the base case. Because of its topographical location in a valley and the overall high temperatures in this region, the area of Stuttgart suffers from heat stress to a comparatively large extent

Numerical simulations to assess the effect of urban heat island mitigation strategies on regional air quality

Work in this thesis demonstrates a numerical modelling approach to analyse the effect of urban planning strategies on the urban heat island (UHI) intensity and further the feedback on the chemical composition of the urban atmosphere. The urban area of Stuttgart acts as test bed for the modelling. The mesoscale chemical transport model WRF-Chem is used to investigate the effect of these urban heat island mitigation strategies on the surface concentration of primary (CO, NO, PM10) and secondary pollutants (O3). Known mitigation strategies such as bright roofs and façades, urban greening and modification of the building density are in the focus. All these measures are able to reduce the urban temperature and thus mitigate urban heat island intensity. Model results reveal that the most efficient way to cool down urban areas is the increase in the surface reflectivity. Changing the building albedo in the model from 0.2 to 0.7, lead to a reduction of the urban heat island by about 2 °C. The effect of urban greening and decreased building density is less. The mitigation strategies which have been mentioned before promote changes in energetic and radiative properties of urban surfaces modifying the chemical nature of the urban atmosphere with regard to both primary and secondary compounds. A temperature reduction of 1 °C leads to an increase of NO and CO by 5-25 %, whereas the mean ozone concentration is projected to decrease by 5-8 %. Reduced temperature on the surface and in the urban canopy layer influences the dynamical structure of the atmosphere, which leads to a reduction in turbulent mixing. The depth of the mixing layer is decreased accordingly. As a result, an increase of the near surface concentration of primary compounds is projected. Additionally, temperature directly controls the reactivity of chemical reactions, which explains the reduction of ozone concentration. It has to be pointed out however, that different measures can generate secondary effects. The increased portion of short wave radiation due to a reflexion from white roofs for instance can promote photochemical reactions, leading to an increase of peak ozone levels although temperature has been reduced. The additional emission of biogenic compounds coming along with urban greening is not covered in this work. The main result of this work indicates the dominating role of atmospheric dynamics when analysing the impacts of urban heat island mitigation strategies on urban air quality. Whereas in earlier studies the main effort had been put on the positive effect of temperature dependent reduction of urban ozone concentration, this work analyses a complete air chemistry, being able to show negative effects on primary compounds like CO, NO and PM10 as well

High resolution climate projections to assess the future vulnerability of European urban areas to climatological extreme events

Results from high resolution 7-km WRF regional climate model (RCM) simulations are used to analyse changes in the occurrence frequencies of heat waves, of precipitation extremes and of the duration of the winter time freezing period for highly populated urban areas in Central Europe. The projected climate change impact is assessed for 11 urban areas based on climate indices for a future period (2021–2050) compared to a reference period (1971–2000) using the IPCC AR4 A1B Scenario as boundary conditions. These climate indices are calculated from daily maximum, minimum and mean temperatures as well as precipitation amounts. By this, the vulnerability of these areas to future climate conditions is to be investigated. The number of heat waves, as well as the number of single hot days, tropical nights and heavy precipitation events is projected to increase in the near future. In addition, the number of frost days is significantly decreased. Probability density functions of monthly mean summer time temperatures show an increase of the 95th percentile of about 1-3 °C for the future compared with the reference period. The projected increase of cooling and decrease of heating degree days indicate the possible impact on urban energy consumption under future climate conditions

Stadtforschung, der schwierige Weg von der Erkenntnis zur Umsetzung

Stadtforschung in den atmosphärischen Wissenschaften findet seit über 200 Jahren (Howard 1818-20) statt. Sie umfasst alle Themen und Einflüsse, die die Lebensqualität der Stadtbewohner beeinflussen. Hierzu zählen an prominenter Stelle die städtische Luftqualität, das Strahlungsklima, die Windverhältnisse in Städten und die städtische Wärmeinsel. Laut übereinstimmenden Analysen (z.B. Eliasson 2000; Mills et al. 2010; Parasee etal. 2019) haben die naturwissenschaftlichen Erkenntnisse aus der Stadtklimaforschung bisher nur begrenzt Eingang in die Stadtplanung gefunden. In vier Bereichen, die sich von Mikro- bis zur Meso-Skala erstrecken, lassen sich hier Fortschritte erreichen: (1) Baumaterialien und Gebäudegestaltung, (2) Grün und Blau in der Stadt, (3) Stadtplanung und (4) Einbindung der Städte in regionale und überregionale Infrastrukturen. Der Beitrag wird hier Beispiele für alle vier Bereiche benennen. Ein wesentliches Mittel um die Auswirkung von Maßnahmen in den vier zuvor genannten Bereichen zu beurteilen und auch unerwünschte Nebenwirkungen einzelner Maßnahmen auf andere Handlungsfelder im Vorfeld zu erkennen, sind holistische Simulationsmodelle wie PALM4U, die von Atmosphärenwissenschaftlern entwickelt wurden (Scherer et al. 2019), aber durch eine entsprechende Oberfläche auch Planern zur Verfügung gestellt werden sollen. Pilotstudien, PALM4U einsatzfähig zu machen laufen derzeit. Der Beitrag wird Fragen zur Simulation der Luftqualität und der Strömung in komplexem Gelände mit PALM4U adressieren. Die Schaffung nachhaltiger Städte mit gesunden Lebensbedingungen für die Bewohner liegt nicht in der Analyse und Bearbeitung von Einzelaspekten, sondern in einer umfassenden Zusammenarbeit von Naturwissenschaftlern, Stadtplanern und Architekten, um vorhandene Städte systematisch zu transformieren und neue städtische Gebiete von Anfang an nachhaltig zu planen. PALM4U ist auf dem Wege, ein wichtiges „Met-Tool“ für diesen Zweck zu werden. Eliasson, I. 2000. The use of climate knowledge in urban planning. Landscape and Urban Planning 48, 1-2, 31–44. Fallmann, J., S. Emeis, 2020. How to Bring Urban and Global Climate Studies together with Urban Planning and Architecture? Devel. Built Environ., 4, 100023. Fallmann, J., S. Emeis, 2021: How to bring urban climate studies to application – A meteorological view from five decades of urban climate research and results from a current study. IAUC Newsletter 80, 12-17. Howard, L. 1818-20. The climate of London. Deduced from meteorological observations, made at different places in the neighbourhood of the metropolis. London. Mills, G; H. Cleugh; R. Emmanuel; W. Endlicher; E. Erell; G. McGranahan; E. Ng; A. Nickson; J. Rosenthal; K. Steemer. 2010. Climate Information for Improved Planning and Management of Mega Cities (Needs Perspective). Procedia Environmental Sciences 1, 228–246. Parsaee, M; M. M. Joybari; P. A. Mirzaei; F. Haghighat. 2019. Urban heat island, urban climate maps and urban development policies and action plans. Environ. Technol. & Innov. 14, 100341. Scherer, D., Antretter, F., Bender, S., Cortekar, J., Emeis, S., Fehrenbach, U., Gross, G., Halbig, G., Hasse, J., Maronga, B., Raasch, S., Scherber, K., 2019: Urban Climate Under Change [UC]2 – A National Research Programme for Developing a Building-Resolving Atmospheric Model for Entire City Regions. Meteorol. Z. (Contr. Atm. Sci.), 28, 95-104

Impact of urban imperviousness on boundary layer meteorology and air chemistry on a regional scale

It has been long understood that land cover change from natural to impervious modifies the surface energy balance and hence the dynamical properties of the overlying atmosphere. The urban heat island is manifested in the formation of an urban boundary layer with distinct thermodynamic features that in turn govern transport processes of air pollutants. While many studies already demonstrated the benefits of urban canopy models (UCM) for atmospheric modelling, work on the impact on urban air chemistry is scarce. This study uses the state-of-the-art coupled chemistry-climate modelling system MECO(n) to assess the impact of the COSMO UCM TERRA_URB on the dynamics and gas phase chemistry in the boundary layer of the urban agglomeration Rhine-Main in Germany. Comparing the model results to ground observations and satellite and ground based remote sensing data, we found that the UCM experiment reduces the bias in temperature at the surface and throughout the boundary layer. This is true for ground level NO2 and ozone distribution as well. The application of MECO(n) for urban planning purposes is discussed by designing case studies representing two projected scenarios in future urban planning – densification of central urban areas and urban sprawl. Averaged over the core urban region and 10‑days during a heat wave period in July 2018, model results indicate a warming of 0.7 K in surface temperature and 0.2 K in air temperature per 10 % increase in impervious surface area fraction. Within this period, a 50 % total increase of imperviousness accounts for a 3 K and 1 K spatially averaged warming respectively. This change in thermodynamic features results in a decrease of surface NO2 concentration by 10–20 % through increased turbulent mixing in areas with highest impervious fraction and highest emissions. In the evening and nighttime however, increased densification in the urban centre results in amplified canyon blocking, which in turn results in average increase in near surface NO2 concentrations of about 10 %, compared to the status quo. This work intends to analyse regional scale features of surface-atmosphere interactions in an urban boundary layer and can be seen as preparatory work for higher resolution street scale models

The UKC3 regional coupled environmental prediction system

This paper describes an updated configuration of the regional coupled research system, termed UKC3, developed and evaluated under the UK Environmental Prediction collaboration. This represents a further step towards a vision of simulating the numerous interactions and feedbacks between different physical and biogeochemical components of the environment across sky, sea and land using more integrated regional coupled prediction systems at kilometre-scale resolution. The UKC3 coupled system incorporates models of the atmosphere (Met Office Unified Model), land surface with river routing (JULES), shelf-sea ocean (NEMO) and ocean surface waves (WAVEWATCH III®), coupled together using OASIS3-MCT libraries. The major update introduced since the UKC2 configuration is an explicit representation of wave–ocean feedbacks through introduction of wave-to-ocean coupling. Ocean model results demonstrate that wave coupling, in particular representing the wave-modified surface drag, has a small but positive improvement on the agreement between simulated sea surface temperatures and in situ observations, relative to simulations without wave feedbacks. Other incremental developments to the coupled modelling capability introduced since the UKC2 configuration are also detailed. Coupled regional prediction systems are of interest for applications across a range of timescales, from hours to decades ahead. The first results from four simulation experiments, each of the order of 1 month in duration, are analysed and discussed in the context of characterizing the potential benefits of coupled prediction on forecast skill. Results across atmosphere, ocean and wave components are shown to be stable over time periods of weeks. The coupled approach shows notable improvements in surface temperature, wave state (in near-coastal regions) and wind speed over the sea, whereas the prediction quality of other quantities shows no significant improvement or degradation relative to the equivalent uncoupled control simulations

The UKC2 regional coupled environmental prediction system

It is hypothesized that more accurate prediction and warning of natural hazards, such as of the impacts of severe weather mediated through various components of the environment, require a more integrated Earth System approach to forecasting. This hypothesis can be explored using regional coupled prediction systems, in which the known interactions and feedbacks between different physical and biogeochemical components of the environment across sky, sea and land can be simulated. Such systems are becoming increasingly common research tools. This paper describes the development of the UKC2 regional coupled research system, which has been delivered under the UK Environmental Prediction Prototype project. This provides the first implementation of an atmosphere–land–ocean–wave modelling system focussed on the United Kingdom and surrounding seas at km-scale resolution. The UKC2 coupled system incorporates models of the atmosphere (Met Office Unified Model), land surface with river routing (JULES), shelf-sea ocean (NEMO) and ocean waves (WAVEWATCH III). These components are coupled, via OASIS3-MCT libraries, at unprecedentedly high resolution across the UK within a north-western European regional domain. A research framework has been established to explore the representation of feedback processes in coupled and uncoupled modes, providing a new research tool for UK environmental science. This paper documents the technical design and implementation of UKC2, along with the associated evaluation framework. An analysis of new results comparing the output of the coupled UKC2 system with relevant forced control simulations for six contrasting case studies of 5-day duration is presented. Results demonstrate that performance can be achieved with the UKC2 system that is at least comparable to its component control simulations. For some cases, improvements in air temperature, sea surface temperature, wind speed, significant wave height and mean wave period highlight the potential benefits of coupling between environmental model components. Results also illustrate that the coupling itself is not sufficient to address all known model issues. Priorities for future development of the UK Environmental Prediction framework and component systems are discussed