Cities and urban areas in the earth-atmosphere system

Cities and urban environments are a key aspect of the United Nations (UN) Agenda for Sustainable Development, as well as in recent the scientific and socio-economic perspectives. As urbanization processes continue across the world, its representation, impact, and understanding need to be further studied in order to fully comprehend the extent of their impact on weather and/or climate. These aspects are crucial both for the advancing of the current knowledge and the creation of effective sustainable solutions. Key challenges to this task are the level of complexity and multi-scale dimension of diverse urban environments.

Urban environments have a complex structure as they include different typologies, e.g. industrial, residential, and recreational/green areas, which have a different impact on the Earth system and can vary over time. These typologies have a diverse impact on the air and water quality and consumption issues, energy consumption/production. Further, urban environments have a low resilience to changes in climate change and extremes, which affects the overall population living conditions.

This session presents and explores aspects of cities and urban environments within the Earth system. We welcome modeling and observational studies that aim to investigate different aspects of urbanization (e.g. urban heat island, population vulnerability, urban/peri-urban agriculture) and their feedback on the climate system, with a particular focus on application for sustainable adaptation plans. Novel methods that aim to assess urban representation and/or to bridge the different scales of representation within numerical models are encouraged. The impact of cities on weather, climate and/or their extremes (e.g. drought, precipitation), as well as on climate change and on population and adaptation will also be discussed in this session.

Topics may include:

• new urban parameterizations, methods to derive urban parameters for numeric models
• implementation of climate mitigations, adaptation strategies and self-government policies in cities and urban context
• impact of the different urban parameterizations on the atmosphere dynamics and on the different scales
• the impact of the urbanization including estate industrial on weather and/or climate extremes
• field measurements of urban climate, e.g. urban heat island
• impact of different surfaces (green areas, impermeable outer surfaces etc.) on climate and/or its extremes in build-up areas
• population vulnerability to urban climate and climate change
• extreme events (e.g. drought, rainfall events) impacts on town agglomeration
• urban and peri-urban agriculture

Including Tromp Foundation Conference Award to Young Scientists winner
Conveners: Pavol Nejedlik, Arianna Valmassoi | Co-conveners: Silvana Di Sabatino, Jan Keller, Marina Neophytou
Lightning talks
| Fri, 10 Sep, 11:00–15:30 (CEST)

Lightning talks: Fri, 10 Sep

Chairpersons: Arianna Valmassoi, Silvana Di Sabatino, Marina Neophytou
Tromp Foundation Conference Award to Young Scientists winner
Ge Cheng, David Grawe, and K. Heinke Schlünzen

Nudging is a simple method that aims to dynamically adjust the model toward the observations by including an additional feedback term in the model governing equation. This method is widely applied in data assimilation due to its simple implementation and reasonable model results. The basic concept of nudging is similar to that of urban canopy parameterization, in which additional terms are usually added in the conservation equations of momentum and energy aiming to simulate the canopy effects. However, few studies have investigated the implementation of nudging methods in urban canopy parameterizations. In this study we developed a multi-layer urban canopy parameterization (UCP) by using a nudging approach to represent the impacts of vegetated urban canopies on temperatures and winds in mesoscale models.

The difficulty of developing UCP by using a nudging method lies in defining appropriate values for the nudging coefficients and the forcing fields (e.g. indoor temperature fields for temperature nudging). To determine nudging coefficients, we use three major urban canopy morphological parameters: building height, frontal area density and building density. The ranges of these parameters are taken from the values for the Local Climate Zones datasets, in our case for the city of Hamburg. The UCP is employed in the three -dimensional atmospheric mesoscale model METRAS. Results show that this UCP can well simulate wind-blocking effects induced from obstacles as buildings and trees and urban heat island phenomenon for cities. Thus, nudging is an efficient and effective method that can be used for urban canopy parameterizations. However, as well known for nudging, it is not conserving energy. Therefore, we investigated the energy loss by tracking the reduced kinetic energy and internal energy. The UCP and model results will be presented.

How to cite: Cheng, G., Grawe, D., and Schlünzen, K. H.: Parameterizing the impact of vegetated urban canopies on wind and temperature fields by using a simple nudging method, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-394, https://doi.org/10.5194/ems2021-394, 2021.

Giovanna Motisi and Björn Maronga

Vehicle-induced effects (VIE) and exhaust fumes interacting with turbulent flow has become known to be a critical factor when investigating the wind flow and the transport of pollutants in urban street canyons. Up to now, mainly the Reynolds-Averaged-Navier-Stokes (RANS) technique has been applied for CFD studies of the processes within urban street canyons; research studies using turbulence-resolving Large-Eddy Simulations (LES), however, were rather rare. As LES models explicitly resolve the dominant turbulent motions, whose knowledge is needed to fully understand the processes, the incorporation of moving objects into a turbulence-resolving model is essential for the accurate simulation of pollutant dispersion in urban environments. In this paper we outline our effort to account for VIE in the LES model PALM. For this purpose, an innovative and easy to implement method was realised to represent a common car shape within the environmental LES setup: the so-called air-block method. Its concept is based on an object (representing the vehicle) in which a fixed velocity is prescribed to the objects grid volumes that equals the driving speed of the vehicle. Control of its movement, however, is achieved via a Lagrangian particle located at its center of gravity. This approach is significantly different from conventional consideration of solid objects as obstacles, since the air-block representation assumes that frictional drag is much smaller (and can thus be neglected) than form drag. By the same token the implementation is much easier to achieve in a complex LES model such as PALM.

In this talk we will outline the newly-developed simplistic method to represent driving vehicles in an LES model and show its performance based on a validation study for the turbulent wake flow and dispersion of exhaust fumes. For this purpose we employ existing wind tunnel data and comparative PALM simulations using the conventional solid-obstacle approach (Carpentieri et al. 2012, Atmospheric Environment, 62:9-25, DOI:10.1016/j.atmosenv.2012.08.019; Kastner-Klein et al. 2001, J. Wind Eng Ind Aerodyn, 89:849-861, DOI:10.1016/S0167-6105(01)00074-5). 

How to cite: Motisi, G. and Maronga, B.: Introduction and validation of a simplistic method to represent vehicle-induced turbulence in high-resolution large-eddy simulations, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-6, https://doi.org/10.5194/ems2021-6, 2021.

Shuojun Mei, Chao Yuan, Wenhui He, and Tanya Talwar

Densely packed urban buildings trap outgoing long-wave radiation, leading to reduced surface cooling and increased building surface temperature. In calm conditions, poor natural ventilation causes both thermal comfort and air quality issue. The buoyancy flow generated by heated urban surfaces is the main driving of the urban flow and pollutant dispersion. A 3D numerical modelling is conducted to investigate the thermal plumes merging and buoyancy-driven airflow in urban areas. The performances of four different turbulence models, i.e., two URANS (Unsteady Reynolds-averaged Navier–Stokes equations) models and two LES (Large-Eddy Simulation) models are evaluated by comparing the velocity field with previous water tank measurements. Validation results show that all four turbulence models can capture the bending of thermal plumes toward the centre, and LES models provide a better prediction on the vertical velocity profiles, while both URANS models show underestimation. The plume merging mechanism is analysed with the high accuracy LES results. Both pressure difference and swaying motion caused by mean flow and turbulence are important for plume merging. The turbulence coherent structure of plume merging is analysed by a quadrant analysis, which shows ejection and sweep events could significantly change with the building density. A case study with complex urban geometry is conducted to show the impact of thermal plumes merging in the real high-density urban areas. The convergence airflow at the pedestrian level is estimated to 2 m/s under a surface-air temperature difference of 5 °C, which is comparable to wind-driven ventilation and beneficial to thermal comfort and air quality.

How to cite: Mei, S., Yuan, C., He, W., and Talwar, T.: Numerical simulation of urban thermal plume merging, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-5, https://doi.org/10.5194/ems2021-5, 2021.

Liqing Zhang and Chao Yuan


Urban heat island (UHI) is known as one of the severe environmental problems, and thus, research on UHI mitigation from the perspective of urban morphology is indispensable, especially in tropical regions like Singapore.


While studies were carried out to evaluate and relieve UHI effect in urban areas, research that conducted through integrated assessment of urban aerodynamic and long wave radiation is limited. This research aims to provide an integrated climate-sensitive planning framework to UHI mitigation by understanding urban morphology.


A district-scale case study in Paya Lebar Air Base (PLAB) was conducted to illustrate how the urban morphological study contributes to the initial planning by an integrated analysis of climate information. Two urban morphological indices, frontal area density (FAD) and sky view factor (SVF), were calculated to depict aerodynamic and long wave radiation, i.e., pedestrian-level wind speed and air temperature, respectively.


The SVF modelling results indicate that the UHI intensity at surrounding areas could be 2℃ to 3℃. With future development, there is a potential risk to create a spreading and more intensive UHI. Aiming at this problem, the FAD map indicates the importance of linking open spaces to create air paths, while the ΔT map implies the necessity of separating building clusters with intensive UHI. Integrated planning strategies are then developed based on the balance between link and separation, focusing on site layout and building geometry. For site layout, open spaces, e.g., major roads, building setbacks, low-rise built areas, and green corridor, should be linked to form the potential breezeways. At the same time, buffer zones like secondary forest should be arranged between site and surrounding areas to prevent new and existing UHI clusters from merging together. As for the building geometry, as the important design parameters, building height, footprint area, and building height to width ratio (H/W) should be carefully decided. Accordingly, a multi-step workflow is developed as an integrated climate-sensitive planning framework.


Urban morphology makes an important contribution to UHI effect. Integrated UHI mitigations can be developed by balancing the strategies for spatial link and separation in urban planning and design, based on climate information, e.g., aerodynamics and heat. The integrated climate-sensitive planning framework is generally applicable to tropical regions with cooling needs, as the key is to minimize temperature rise due to long wave radiation while introduce cool air to the site.

How to cite: Zhang, L. and Yuan, C.: Mitigating urban heat island effect through integrated climate-sensitive planning framework: a study based in Singapore, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-68, https://doi.org/10.5194/ems2021-68, 2021.

Chao Yuan, Shuojun Mei, Wenhui He, Ayu Sukma Adelia, and Liqing Zhang

Anthropogenic heat is one of the key factors that causes intensive urban heat island due to its direct impact on ambient temperature in urban areas. Stagnated airflow due to closely packed tall buildings causes weak dilution and removal of anthropogenic heat. Consequently, research is critically needed to investigate the effect of urban morphology on anthropogenic heat dispersion and provide effective planning strategies to reduce UHI intensity, especially at the extreme scenario, such as with very low wind speed and high heat emission. This study provides scientific understanding and develops a GIS-based modelling tool to support decision-making in urban planning practice. We start from a computational parametric study at the neighbourhood scale to investigate the impact of urban morphology on heat dispersion. Site coverage ratio ( ), and frontal area density ( ) are two urban morphological parameters. Ten parametric cases with two heat emission scenarios are designed to study representative urban areas. Furthermore, based on the energy conservation within the urban canopy layer, we develop a semi-empirical model to estimate spatially-averaged in-canopy air temperature increment, in which the exchange velocity between the street canyon and overlying atmosphere is estimated by the Bentham and Britter model. The performance of the new model is validated by cross-comparing with CFD results from the parametric study. By applying this new model, the impact of anthropogenic heat on air temperature is mapped in residential areas of Singapore for both long-term annually averaged and short-term extreme low wind speed to improve urban climate sustainability and resilience.

How to cite: Yuan, C., Mei, S., He, W., Adelia, A. S., and Zhang, L.: Mitigating Intensity of Urban Heat Island by Better Understanding on Urban Morphology and Anthropogenic Heat Dispersion, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-1, https://doi.org/10.5194/ems2021-1, 2021.

Robert Goler, Maja Žuvela-Aloise, Sandro Oswald, Brigitta Hollósi, Claudia Hahn, and Astrid Kainz

As the majority of the population live in cities, it is important to understand the urban climate and how it can change in the future. Accordingly, the ACRP-funded project LUCRETIA investigates how land use and land cover determine local climate characteristics within cities in Austria. 

Historical land use data has been obtained for Graz and Vienna for a number of years and used as input into the microscale urban climate model MUKLIMO_3 to simulate both cities in conditions representing a typical summer day. In conjunction with the cuboid method, climate indices such as the average number of summer and hot days per year have been calculated to establish how the heat load changes from one year to another. Differences in the heat load have been related to changes in the land use focusing on (i) the change that occurs in situ and (ii) the change that occurs in the neighbourhood. 
It is shown that land use categories can be ordered according to their heat load, with categories containing larger amounts of greenery generally having lower heat loads. With the land use categories sorted in such a way, it enables a relatively quick assessment to be made of the effect of replacing one land use category with another, without having to employ expensive modelling tools. Furthermore, it is shown that land-use changes not only affect the heat load of the changed area in situ, but also the neighbourhood around where the change was made. This demonstrates that land-use changes may have a broader spatial impact than initially anticipated. The results from this study can serve as guidance for city planners regarding future land use and land cover changes.

How to cite: Goler, R., Žuvela-Aloise, M., Oswald, S., Hollósi, B., Hahn, C., and Kainz, A.: Investigating the effect of land-use change on the heat load within two Austrian cities, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-165, https://doi.org/10.5194/ems2021-165, 2021.

Jérémy Bernard, Fredrik Lindberg, and Sandro Oswald

Wind speed is one of the key parameter affecting human thermal comfort: high wind speed during winter and low wind speed during summer may exacerbate respectively cold and heat stress. In urban areas, where more than 50% of the world population is currently living, the wind field is strongly affected by the size and the organization of the obstacles (mainly buildings and trees). Simple and quick estimation of the wind speed and direction in an urban setting could then be an interesting information for urban planning purpose. To calculate a high resolution three-dimensional wind field in an urban setting, Computational Fluid Dynamic (CFD) models are mostly used. They usually solve advection and turbulence equations by an iterative process which is too long for most of the urban planning applications. To reduce this time, Röckle (1990) proposed:

  • to decrease the number of iteration by initializing the wind field around buildings: this is done by modeling empirically the wind speed and direction using results from wind tunnel observations,
  • to solve only the advection equation from this initial wind field since the turbulence is supposed roughly “solved” by the initialization.

At our knowledge, at least two models have been developed using this approach: QUIC-URB (Brown et al. 2018) and the second is part of the SkyHelios software (Fröhlich and Matzarakis, 2018). However: (i) none of these softwares are open-source (i.e. source code is not freely available), it is then rather complicated to propose scientific improvements and (ii) none of them are integrated in a commonly used GIS-based urban planning tool which would popularize their use by urban planners.

Our presentation will focus on the development of our tool called URock, an open-source application of the Röckle methodology. If the results produced by this tool are consistent with observation, it should be included in QGIS (a commonly used urban planning GIS) through the plug-in UMEP (Lindberg et al. 2017).


Brown, Michael John. Quick Urban and Industrial Complex (QUIC) CBR Plume Modeling System: Validation-Study Document. No. LA-UR-18-29993. Los Alamos National Lab.(LANL), Los Alamos, NM (United States), 2018.

Fröhlich, Dominik, and Andreas Matzarakis. "Spatial estimation of  thermal indices in Urban Areas—Basics of the SkyHelios Model." Atmosphere 9.6 (2018): 209.

Lindberg F, Grimmond CSB, Gabey A, Huang B, Kent CW, Sun T, Theeuwes N, Järvi L, Ward H, Capel-Timms I, Chang YY, Jonsson P, Krave N, Liu D, Meyer D, Olofson F, Tan JG, Wästberg D, Xue L, Zhang Z (2018) Urban Multi-scale Environmental Predictor (UMEP) - An integrated tool for city-based climate services. Environmen tal Modelling and Software.99, 70-87 https://doi.org/10.1016/j.envsoft.2017.09.020

Röckle, R., 1990: Bestimmung der Strömungsverhältnisse im Bereich komplexer Bebauungsstruk-turen. PhD thesis Fachbereich Mechanik der Technischen Hochschule Darmstadt Darmstadt.

How to cite: Bernard, J., Lindberg, F., and Oswald, S.: Urban wind field calculation through the Röckle based method: the basics for a GIS implementation, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-27, https://doi.org/10.5194/ems2021-27, 2021.

Arjan Willemse, Alberto Martilli, Bert Heusinkveld, Oscar Hartogensis, and Gert-Jan Steeneveld

With increasing urbanization and ongoing climate change there is a need to develop and evaluate modelling infrastructure for urban weather and climate. In this study we evaluate three urban canopy models for a heat-wave case study in Amsterdam (The Netherlands), notably the single-layer urban canopy model (SLUCM) and the building environment parameterization (BEP) and the BEP+BEM (BEP+Building Energy model) urban canopy models within the WRF infrastructure. Model results are evaluated against a network of near surface observations of air temperature, turbulent surface fluxes, SODAR wind profiles, and radio soundings of temperature and humidity taken in the city center of Amsterdam.

We find that the BEP+BEM model outperforms the other schemes for the near surface air temperature, with a bias of -0.66 K for BEP+BEM, -1.51 K for BEP and-1.56 K for SLUCM. However, WRF produces an elevated inversion level that, at the same time, is substantially (~ 2-8 K) weaker than observed in the radiosoundings.

To estimate the future increase in energy demand by air conditioning, we project this heatwave case study to the future. To do so, we force the WRF model with increased temperatures in initial and boundary conditions following the four KNMI climate scenarios. With the climate scenario with the largest warming (WH-scenario) we find a 2-m temperature increase of ~3 K during daytime compared to the current climate. Finally we find that for this scenario the energy consumption by air-conditioning increases between 25% and 40% in the city center compared to the current climate (with constant number of airco’s installed).

How to cite: Willemse, A., Martilli, A., Heusinkveld, B., Hartogensis, O., and Steeneveld, G.-J.: Evaluating three urban canopy models against in-situ observations for a heat-wave case in Amsterdam, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-86, https://doi.org/10.5194/ems2021-86, 2021.

The urban fingerprint in the sea-breeze hodograph reveled by high resolution WRF simulations.
David Avisar, Ran Pelta, Alexandra Chudnovsky, and Dorita Rostkier-Edelstein
Valeria Garbero, Massimo Milelli, Francesca Bassani, Edoardo Bucchignani, Paola Mercogliano, Mikhail Varentsov, Inna Rozinkina, Gdaliy Rivin, Denis Blinov, Hendrik Wouters, Jan-Peter Schulz, Ulrich Schaettler, Matthias Demuzere, and Francesco Repola

Nowadays, cities are the preferred location for more than half of the human population and the places where major human-perceived climate change impacts occur. In an increasingly urbanized world, it is essential to represent such areas adequately in Numerical Weather Prediction (NWP) models, not only to correctly forecast air temperature, but also the human heat stress and the micro-climate phenomena induced by the cities. Among them, the best known is the Urban Heat Island (UHI) effect, which refers to the significantly higher temperatures experienced by a metropolitan area than its rural surroundings. Currently, the COSMO model employs a zero-order urban description, which is unable to correctly reproduce the UHI effect: cities are simply represented as natural lands with increased surface roughness length and reduced vegetation cover. However, the reproduction of the urban climate features in NWP and regional climate models is possible with the use of the so-called urban canopy models, that are able to parameterize the interaction between the urbanized surface and the overlying atmosphere. In this context, a new bulk parameterization scheme, TERRA_URB (TU), has been developed within the COSMO Consortium. TU offers an intrinsic representation of urban physics: the effect of buildings, streets and other man-made layers on the surface-atmosphere interaction is described by parameterizing the impervious water balance, translating the 3D urban-canopy parameters into bulk parameters with the Semi-empirical Urban canopy parameterization (SURY) and using the externally calculated anthropogenic heat flux as additional heat source. In this work, we present high-resolution simulations with the TU scheme, for different European cities, Turin, Naples and Moscow. An in-depth evaluation and verification of the performances of the recent COSMO version with TU scheme and new implemented physical parameterizations, such the ICON-like surface-layer turbulence scheme and the new formulation of the surface temperature, have been carried out. The validation concerned the 2-meter temperature and was performed for 1- or 2-week selected periods over the 3 European cities characterized by different environment and climate, namely the Moscow megacity in Russia and Turin and Naples in Italy. Even if the three domains are morphologically different, the results follow a common behavior. In particular, the activation of TERRA_URB provides a substantial improvement in capturing the UHI intensity and improving air temperature forecasts in urban areas. Potential benefits in the model performance also arise from a new turbulence scheme and the representation of skin-layer temperature (for vegetation). Our model framework provides promising perspectives for enhancing urban climate modelling, although further investigations are needed.

How to cite: Garbero, V., Milelli, M., Bassani, F., Bucchignani, E., Mercogliano, P., Varentsov, M., Rozinkina, I., Rivin, G., Blinov, D., Wouters, H., Schulz, J.-P., Schaettler, U., Demuzere, M., and Repola, F.: Evaluating the Urban Canopy Scheme TERRA_URB in the COSMO Model for Selected European Cities, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-303, https://doi.org/10.5194/ems2021-303, 2021.

Chairpersons: Pavol Nejedlik, Jan Keller, Arianna Valmassoi
Nils Eingrüber, Wolfgang Korres, and Karl Schneider

In the context of climate change, more frequent and intensive exposure to heat stress is observed and predicted for many cities worldwide. Urban climatological studies in recent decades have shown significant positive trends in the number of hot days. As heat stress poses a considerable health risk, adaptation measures need to be developed. Against this background, the research study aims to measure and model the urban microclimate of a 15 ha study area in Cologne. A network of IButtons and Netatmo weather stations with ultrasonic anemometers is used to measure temperature, humidity and wind speed/direction for assessing the climate character of the study area. The low cost sensors are calibrated against built up research grade meteorological stations. Utilizing low cost sensors also provides opportunities to activate citizens in microclimate research and to foster participation in mitigating climate change effects. The measurement network is set up as transects along street corridors and is used to a) identify the local climatic impacts of different surface types, vegetation areas and building structures, and b) to later calibrate and validate the ENVI-met model. Processes affecting the urban energy balance and microclimate are identified focussing particularly on source areas of excessive heat. Effects of urban green infrastructures are analysed with regard to their mitigation potential for heat stress, water demand for evapotranspiration, and their potential to modify the partitioning of the radiation balance into sensible heat and latent heat flux. We will use the validated ENVI-met model to simulate various adaptation scenarios and climate change scenarios. Adaptation measures will comprise changes in surface (e.g. urban water bodies and vegetation areas), facade/roof greenings or cooling materials. Climate projections until 2099 will be used with ENVI-met by downscaling meteorological data using the Statistical DownScaling Model (SDSM) and assuming the HadCM3 future emission scenarios.

How to cite: Eingrüber, N., Korres, W., and Schneider, K.: Pathways for climate change adaptation in urban areas - first results from field measurements and ENVI-met modeling, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-374, https://doi.org/10.5194/ems2021-374, 2021.

Achim Drebs, Tim Sinsel, and Kirsti Jylhä

Due to geographical location extended heat-waves occurs in coastal high-latitude areas or cities rather seldom.

In our research we describe influences of roof- and wall-greening on micro-meteorological conditions around (and in?) a moderate insulated stand-alone six-storey concrete building in Helsinki, Finland. The block of flats serves as an old people's home and being built in the late 1970’s, it represents the prevailing construction type of that era. The building is located on a slightly southwards declining slope, and the neighbouring buildings are 30 meters away.

We applied the holistic ENVI-met simulation soft-ware and used real meteorological forcing-data as input for the simulations.  The study focused on a 24-day heat-wave event in summer 2018. During the period from July, 13th until August, 5th, the daily maximum air temperature reached almost every day 25 °C and more, sometimes even more than 30 °C. All one-hour air temperature, wind, humidity, and solar radiation (cloudiness) measurements were conducted at a near-by synoptic weather station.

 The ENVI-met soft-ware has standard set-ups for green roof and green wall properties for simulations of their impacts on the thermal condition. We simulated different levels of insulation (poor, moderate, and good) and used the roof- and wall-greening separately or together in order to find the optimal combination of different greening options. Furthermore, we analysed the physical environment around the old people’s home from the aspect of human comfort, especially the influences of the simulated green infrastructures in front of the building.

The research is part of the HEATCLIM-project financed by the Academy of Finland Science Program CLIHE (2020-2023).

How to cite: Drebs, A., Sinsel, T., and Jylhä, K.: About simulated influence of roof- and wall-greening on an old people’s home in Helsinki, Finland, during the 2018 heatwave event - Drebs, Achim, …, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-221, https://doi.org/10.5194/ems2021-221, 2021.

K. Heinke Schluenzen, Sue Grimmond, and Alexander Baklanov

Today, every second person lives in a city, and urbanization is continuously increasing. For 2050, it is to be expected that 2 out of 3 people will live in a city and thus the vast majority of the world's population will be affected not only by global climate change but also by locally induced climatic changes. The canopy layer urban heat island (CL-UHI) is one of the most well-known meteorological characteristics of urban areas found in cities small and large around the world. Its characteristics differ between cities, across a city and with time. The climate change induced warming cities experience is additionally impacted by the CL-UHI.

Despite the city-scale importance of CL-UHI, the WMO has not had any specific guidance on this. In response to the request of the 18th World Meteorological Congress (Resolutions 32 and 61) experts from WMO GAW (Global Atmosphere Watch) Urban Research Meteorology and Environment (GURME) initiated in 2020 preparation of a guidance on measuring, modelling and monitoring the CL-UHI. The guidance is a community-based development with 30 contributors providing expertise in all different aspects of CL-UHI. This includes a clear definition of what a CL-UHI is and clarifications of what it is not, how it develops (e.g. meteorological and morphological influences), methods to assess CL-UHI intensities (measurements,  modelling approaches) as well as when its assessment  (applications) is needed and how it can be reduced (or when it is beneficial).

The presentation will specifically focus on the key questions addressed in the guidance: what a CL-UHI is and what it is not, where CL-UHI values are relevant for and the many challenges that exist in simulating the CL-UHI with different models.

How to cite: Schluenzen, K. H., Grimmond, S., and Baklanov, A.: Guidance to Measuring, Modelling and Monitoring the Canopy Layer Urban Heat Island, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-301, https://doi.org/10.5194/ems2021-301, 2021.

Tomas Halenka and Gaby Langendijk

Cities play a fundamental role on climate at local to regional scales through modification of heat and moisture fluxes, as well as affecting local atmospheric chemistry and composition, alongside air-pollution dispersion. Vice versa, regional climate change impacts urban areas and is expected to increasingly affect cities and their citizens in the upcoming decades. Simultaneously, the share of the population living in urban areas is growing, and is projected to reach about 70% of the world population up to 2050. This is especially critical in connection to extreme events, for instance heat waves with extremely high temperatures exacerbated by the urban heat island effect, in particular during night-time, with significant consequences for human health.

Cities are becoming one of the most vulnerable environments under climate change. In 2013, the CORDEX community identified cities to be one of the prime scientific challenges. Therefore, we proposed this topic to become an activity at CORDEX platform, within the framework of so called flagship pilot studies, which was accepted and the FPS URB-RCC activity has been started in May this year.

Indeed, from the perspective of recent regional climate model developments with increasing resolution down to the city scale, proper parameterization of urban processes is starting to play an important role to understand local/regional climate change. The inclusion of the individual urban processes affecting energy balance and transport (i.e. heat, humidity, momentum fluxes) via special urban land-use parameterization of distinct local processes becomes vital to simulate the urban effects properly. This will enable improved assessment of climate change impacts in the cities and inform adaptation and/or mitigation options by urban decision-makers, as well as adequately prepare for climate related risks (e.g. heat waves, smog conditions etc.).

The main goal of this FPS is to understand the effect of urban areas on the regional climate, as well as the impact of regional climate change on cities, with the help of coordinated experiments with urbanized RCMs. While the urban climate with all the complex processes has been studied for decades, there is a significant gap to incorporate this knowledge into RCMs. This FPS aims to bridge this gap, leading the way to include urban parameterization schemes as a standard component in RCM simulations, especially at  high resolutions.

How to cite: Halenka, T. and Langendijk, G.: CORDEX FPS on Urbanization - URBan environments and Regional Climate Change (URB-RCC), EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-477, https://doi.org/10.5194/ems2021-477, 2021.

Claudia Hahn, Sandro Oswald, Brigitta Hollosi, Robert Goler, Astrid Kainz, and Maja Zuvela-Aloise

Climate change impacts are amplified in cities due to the urban heat island effect and the high population density. Information about the intra-urban temperature patterns is therefore crucial to support resilient city planning. Within the ACRP funded project LUCRETIA, the intra-urban temperature patterns in Vienna, Austria, are investigated using urban climate models (MUKLIMO_3, PALM-4U) and data from citizen weather stations.

While the density of conventional weather station networks is usually too low to capture the temperature patterns in cities and to assess urban climate model results, citizen weather stations provide a dense monitoring network, especially in cities. In Vienna, more than 1000 citizen weather stations from the company Netatmo are available for our study period in August 2018, after the quality control. First investigations showed, that air temperature measurements from citizen weather stations are in good agreement with measurements from conventional stations. The observed differences are attributed to the different locations of the stations and micro-scale effects. A preliminary comparison of citizen weather station data with urban climate model results from MUKLIMO_3 for Vienna revealed for some of the stations similar patterns as the comparison between conventional stations and model results: a reasonably good agreement during the day, after model initialization, and a temperature overestimation at night. Within LUCRETIA we are assessing in more detail the model results (MUKLIMO_3, PALM-4U) for a three day period in August 2018, thereby looking at the effect of the different land-use classes within the city. In addition, we will investigate whether similar spatial temperature patterns are identified when using urban climate models and data from citizen weather stations.

How to cite: Hahn, C., Oswald, S., Hollosi, B., Goler, R., Kainz, A., and Zuvela-Aloise, M.: Assessing urban climate model results with crowd-sourced data, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-152, https://doi.org/10.5194/ems2021-152, 2021.

Vivien Voss, David Grawe, and K. Heinke Schlünzen

Numerical modeling makes it possible to represent complex processes in small-scale and complex areas like cities. For resolving obstacles, grid sizes in the order of meters are needed. Due to small grid sizes and numerical restrictions, such high-resolution investigations require a great deal of resources. Therefore, a re-use of the results by others would enhance the value of any of these model results. However, the subsequent use of model results is still poorly developed. Comparisons of model data, dissemination of results, or reproduction of simulations are hampered by inconsistent data structures, non-standardized variable names, and lack of information on model setup. In general, to ensure the reusability and accessibility of model data, data standards should be used. The most common data standard for atmospheric model output data are the CF conventions, a data standard for netCDF files, but this standard is currently not extended to cover the model output of obstacle resolving models (ORM).

The AtMoDat (Atmospheric Model Data) project developed a model data standard (ATMODAT standard) which ensure FAIR (Findability, Accessibility, Interoperability, and Reuse) and well documented data. We involved the micro-scale modelling community in this process with a web based survey (http://uhh.de/orm-survey) to find out which micro-scale ORMs are currently in use, their model specifics (e.g. used grid, coordinate system), and the handling of the model result data. Furthermore, the survey provides the opportunity to include suggestions and ideas, what we should consider in the development of the standard. We already identified typical variables used by ORMs (i.e. building structures, wall temperatures) and will propose them to be included in the CF convention.  The application of this standard is tested on the model output of the ORM MITRAS. The standard and experiences with its application will be presented.

How to cite: Voss, V., Grawe, D., and Schlünzen, K. H.: How to develop and apply a model data standard on microscale model data., EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-420, https://doi.org/10.5194/ems2021-420, 2021.

Monika J. Hajto and Anita Bokwa

The urban heat island (UHI) effect is primarily related to the atmosphere, but may also refer to the surfaces. The atmospheric UHI (AUHI), determined using air temperature (Tair), and the surface UHI (SUHI), assessed using land surface temperature (LST), are distinguished. There is undoubtedly a relationship between SUHI and AUHI due to the modulation of Tair by LST. On hot days in the summer months, the SUHI/AUHI effect may increase the heat load, which is dangerous to the health and comfort of people staying in the city. Detailed characteristics of the spatial distribution of Tair and LST in urban areas are required to identify the parts of the city with the highest heat load. Spatially continuous Tair data, enabling better characterizing AUHI, can be obtained by modelling. Satellite thermal data (LST) can be used as input to the Tair spatial distribution model. Satellite data with 1 km spatial resolution, due to availability several times a day, are most useful in characterizing SUHI diurnal variability and the relationship of LST with Tair. The detailed knowledge of LST and Tair correlation should be helpful in the development of the Tair estimation algorithm based on the LST values. Better recognition of the relationship between LST and Tair, and thus improving the quality of modelling the spatial distribution of Tair in urban area, can possibly be achieved through downscaling of LST data to higher spatial resolution. In the study the method of LST downscaling from 1 km to 100 m was developed, using LST derived from AVHRR, Landsat, ASTER and ECOSTRESS data. The LST-Tair correlation in the diurnal course was examined and the influence of LST downscaling on the correlation was assessed. A Tair regression model was developed based on LST, depending on local climate zone (LCZ). LST and Tair maps for Kraków and its vicinities were prepared, and on the basis of them the intensities of AUHI and SUHI in the multi-year period (2010-2019) in the summer months (June, July, August) were determined, separately for day and night.

How to cite: Hajto, M. J. and Bokwa, A.: Multi-annual variability of summertime atmospheric and surface urban heat island in Kraków, Poland, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-64, https://doi.org/10.5194/ems2021-64, 2021.

Aleksandra Renc, Ewa Łupikasza, and Małgorzata Błaszczyk

Urbanization results in the increase of impervious surfaces build of materials with high heat capacity and low albedo. Therefore, urban areas heat up more than the surroundings, leading to the development of the urban heat island (UHI). The UHI intensifies the effect of global warming by increased intensity and frequency of heatwaves in the city. In summer, the UHI is considered a phenomenon hazardous for the life and health of city inhabitants. The problem of UHI has not been studied in the Górnośląska-Zagłębiowska Metropolis (GZM), which is the most populated area in Poland. This study aims to determine the spatial structure and intensity of the surface urban heat island (SUHI) in GZM and identify the areas exposed to the most intense heat island. The relationship between the type of land use and the SUHI occurrence was also studied. Four LANDSAT 8 images were converted to the form of land surface temperature (LST). Based on the mean and standard deviation of the LST the extent of the overall SUHI was determined without considering the type of land use. Based on the same method, three classes of SUHI intensity (standard, strong and extreme) were defined and distinguished within anthropogenic areas determined based on the Corine Land Cover 2018 classification of land use. The intensity of SUHI was defined as a difference between urban and non-urban areas. The relationships between various types of land use and LST were also examined. In GZM, SUHI has a structure of 'archipelago' rather than an 'island'. In each image, the highest LSTs were identified for industrial areas represented by Katowice Smeltery. The standard SUHI, defined as average LST +1 standard deviation (only within an anthropogenic area) accounted for 12.7% to 14.4% of the GZM area in individual years, which indicates a small temporal variability of its extent. The extreme SUHI was identified mainly for shopping and logistic centers, industrial facilities, or coal dumps. The intensity of SUHI ranged from 5 to 9 °C depending on the image and method of urban and non-urban areas delineating. The highest average LST was characteristic of the discontinuous urban fabric and industrial or commercial units. Water and forest areas had the lowest average LST. The discontinuous urban fabric and industrial or commercial units constituted more than 70% of the overall SUHI area.

How to cite: Renc, A., Łupikasza, E., and Błaszczyk, M.: Spatial structure of the Surface Urban Heat Island in summer based on Landsat 8 imagery in the Górnośląsko – Zagłębiowska Metropolis, Southern Poland , EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-100, https://doi.org/10.5194/ems2021-100, 2021.

Marco Possega, Leonardo Aragão, Paolo Ruggieri, Marco Antonio Santo, and Silvana Di Sabatino

Heat waves (HWs) are extreme weather conditions characterized by persistent high temperatures with considerable impacts on society in terms of
mortality, thermal stress and energy demand of the population. One of the most interesting aspects of HWs concerns the interaction with the phenomenon
of urban heat island (UHI). The UHI is the tendency of urbanized areas to have warmer temperatures than the surrounding rural areas, mainly due to the thermal
properties of materials forming urban environment and the heat produced by human activities. Some studies analyzed the behavior of UHI during periods of
extreme heat, showing an amplification of the gradient of temperature between urban and rural areas in HW conditions, but results are often limited to case
studies with a single HW and/or a specific city. Other papers dealt with the same topic by examining events on various cities using outputs of global models,
but with resolution insufficient to include in detail urban-scale processes and therefore to take into account specific properties of the cities investigated. The
approach of this work consisted in providing observational evidence and extending the aforementioned results, studying the effect of HWs on UHI in 41 European cities
with different characteristics (geography, topography, urban planning) through the analysis of daily maximum / minimum temperatures data measured by
meteorological stations for the summers of period 2000-2019. In particular, the intensity of UHI was assessed through the computation of a Composite UHI Index
(UHII), defined as the difference between averaged urban and non-urban values. The different behavior of UHII during HWs compared to "normal" summer days
(NO) in selected European cities was investigated, detecting an intensification of index values regarding periods of extreme heat for the majority of examined
locations. More specifically, the analysis of temporal evolution of UHII was conducted, revealing an average increase of this index during the occurrence of
HW events due to higher urban than rural temperatures. This work provides an indication of how European urban areas respond to severe hot periods and could
be useful to validate numerical model simulations for more detailed analysis, for example regarding mitigation strategies. Finally, the emergence of some outliers,
namely cities whose UHI manifested a different reaction to HWs, may deserve dedicated studies in the future.


How to cite: Possega, M., Aragão, L., Ruggieri, P., Santo, M. A., and Di Sabatino, S.: Observational evidence of urban heat island intensification during heatwaves in European cities, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-160, https://doi.org/10.5194/ems2021-160, 2021.

Irena Nimac, Ivana Herceg-Bulić, Maja Žuvela-Aloise, and Matej Žgela

Dry periods and heat waves are becoming more frequent and intense in changing climate. Effect and impact of such specific events on local climate and ecosystems may vary spatially. One of the critical spots regarding extreme warm events are urban areas due to increased heat load. Here, we examine impact of drought conditions on characteristics of summertime urban heat island (UHI) for Zagreb, Croatia. For these purposes daily air temperature, precipitation, relative humidity, wind speed and wind direction data from the station Zagreb-Maksimir in a period 1928–2019 are used in the analysis. To define dry and wet conditions, standardized precipitation evapotranspiration index (SPEI) is used. The effect of drought conditions on summer UHI is analyzed from the perspective of preceding (i.e. wintertime and springtime) as well as concurrent (i.e. summertime) drought conditions. To estimate urban heat load in the city, urban climate model MUKLIMO_3 combined with cuboid method is used. Urban heat load is here represented as a number of summer days, i.e. days with maximum air temperature above 25°C. Landsat-8 satellite data were employed to analyze land surface temperature for specific situations. Results indicated substantial increase in heat load for situations when dry summer was preceded by dry late winter-spring period. However, when late winter-spring period was wet and followed with dry summer, much weaker increase in heat load is obtained. On the other hand, decrease in heat load is found for wet summer preceded by wet late winter-spring season. It is also showed that intensity of UHI is affected with drought conditions.


How to cite: Nimac, I., Herceg-Bulić, I., Žuvela-Aloise, M., and Žgela, M.: Impact of drought conditions on urban climate of Zagreb (Croatia), EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-315, https://doi.org/10.5194/ems2021-315, 2021.

Tim van der Schriek, Konstantinos V. Varotsos, Dimitra Founda, and Christos Giannakopoulos

Historical changes, spanning 1971–2016, in the Athens Urban Heat Island (UHI) over summer were assessed by contrasting two air temperature records from established meteorological stations in urban and rural settings. When contrasting two 20-year historical periods (1976–1995 and 1996–2015), there is a significant difference in summer UHI regimes. The stronger UHI-intensity of the second period (1996–2015) is likely linked to increased pollution and heat input. Observations suggest that the Athens summer UHI characteristics even fluctuate on multi-annual basis. Specifically, the reduction in air pollution during the Greek Economic Recession (2008-2016) probable subtly changed the UHI regime, through lowering the frequencies of extremely hot days (Tmax > 37 °C) and nights (Tmin > 26 °C).

Subsequently, we examined the future temporal trends of two different UHIs in Athens (Greece) under three climate change scenarios. A five-member regional climate model (RCM) sub-ensemble from EURO-CORDEX with a horizontal resolution of 0.11° (~12 × 12 km) simulated air temperature data, spanning the period 1976–2100, for the two station sites. Three future emissions scenarios (RCP2.6, RCP4.5 and RCP8.5) were implanted in the simulations after 2005. The observed daily maximum and minimum air temperature data (Tmax and Tmin) from two historical UHI regimes (1976–1995 and 1996–2015, respectively) were used, separately, to bias-adjust the model simulations thus creating two sets of results.

This novel approach allowed us to assess future temperature developments in Athens under two different UHI intensity regimes. We found that the future frequency of days with Tmax > 37 °C in Athens was only different from rural background values under the intense UHI regime. There is a large increase in the future frequency of nights with Tmin > 26 °C in Athens under all UHI regimes and climate scenarios; these events remain comparatively rare at the rural site.

This study shows a large urban amplification of the frequency of extremely hot days and nights which is likely forced by increasing air pollution and heat input. Consequently, local mitigation policies aimed at decreasing urban atmospheric pollution are expected to be also effective in reducing urban temperatures during extreme heat events in Athens under all future climate change scenarios. Such policies therefore have multiple benefits, including: reducing electricity (energy) needs, improving living quality and decreasing heat- and pollution related illnesses/deaths.


How to cite: van der Schriek, T., Varotsos, K. V., Founda, D., and Giannakopoulos, C.: Historical and future temporal trends in the summer Urban Heat Island of Athens (Greece) , EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-318, https://doi.org/10.5194/ems2021-318, 2021.


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