UP2.1

There is a scientific consensus on the need for spatially detailed information on urban landscapes at a global scale. This data can support a range of environmental services, as cities are acknowledged as places of intense resource consumption and waste generation and foci of population and infrastructure that are exposed to multiple hazards of natural and anthropogenic origin. In the face of climate change, urban data is also required to explore future urbanisation pathways and urban design strategies, in order to lock in long-term resilience and sustainability, protecting cities from future decisions that could undermine their adaptability.

To serve this purpose, we present a 100m resolution global map of Local Climate Zones (LCZs), an universal urban typology that can distinguish urban areas on a holistic basis, accounting for the typical combination of micro-scale land-covers and associated physical properties. The global LCZ map, composed of 10 built and 7 natural land cover types, is generated by feeding an unprecedented amount of labeled training areas and earth observation imagery into lightweight random forest models. Its quality is assessed using a bootstrap cross validation alongside a thematic benchmark for 150 selected functional urban areas using independent global and open-source data on surface cover, surface imperviousness, building height, and anthropogenic heat.

As each LCZ type is associated with generic numerical descriptions of key urban canopy parameters that regulate atmospheric responses to urbanisation, the availability of this globally consistent and climate-relevant urban description is an important prerequisite for supporting model development and creating evidence-based climate-sensitive urban planning policies.

How to cite: Demuzere, M., Kittner, J., Martilli, A., Mills, G., Moede, C., Stewart, I. D., van Vliet, J., and Bechtel, B.: A global map of Local Climate Zones to support earth system modelling and urban scale environmental science, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-83, https://doi.org/10.5194/ems2022-83, 2022.

Urban air temperature (T_air) is an essential variable for a variety of urban issues, and analyzing the spatial patterns of T_air is of great importance for urban planning and management. However, it is difficult to obtain T_air data with a high spatial resolution because of the absence of weather stations within a heterogeneous city. In this research, T_air predictive models were developed in Szeged by the help of an urban climate monitoring system which is a part of COST FAIRNESS project. Our investigation focused on the whole year instead of just the summer which has been the most studied season for urban heat issues. Two statistical mthods, multiple linear regression and random forest regression, were used for developing T_air estimation models. T_air data obtained from in situ meteorological stations from 2014 to 2017 were used as the dependent variable for models training. Land surface temperature data from numerous MODIS satellite images and 7 auxiliary variables were used as the independent variables. The auxiliary data are open source, including local climate zone (LCZ) lassification data containing a wide range of urban surface information and atmospheric parameters in the urban boundary layer associated with near-surface urban climate. The atmospheric parameters calculated from the ERA5 reanalysis data. We calculated RMSE based on 10-fold cross-validation to valadite the models. The results indicated that the random forest models performed better than multiple linear models with lower RMSE in four seasons. According to the importance analysis in random forest, both LCZ classification and atmospheric parameters are effective in reducing model errors. LCZ parameters affect the models significantly during the day, while atmospheric parameters affects the models significantly at night. Both effects of these two auxiliary variables showed their maximum in summer. In the end of this research, we used the final models to estimate T_air and mapped the seasonal average T_air patterns from 2018 to 2019. Our overall aim is to develop a generalized methodology using globally available land surface temperature and auxiliary data capable to estimate the urban air temperature.

How to cite: Guo, Y., Unger, J., and Gál, T.: Model development for the estimation of seasonal urban air temperature patterns using MODIS satellite data, a case study in Szeged, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-567, https://doi.org/10.5194/ems2022-567, 2022.

Understanding the physical processes that affect the turbulent structure of the nocturnal urban boundary layer is essential for improving forecasts of air quality and the air temperature in urban areas. Low-level jets have been shown to affect turbulence in the nocturnal urban boundary layer. We investigate the interaction of a mesoscale low-level jets with the urban boundary layer during a 60-h case study. We use observations from two Doppler lidars and results from two high-resolution numerical-weather-prediction models (Weather Research and Forecasting model, and the Met Office Unified Model for limited-area forecasts for the U.K.) to study differences in the occurrence frequency, height, wind speed, and fall-off of low-level jets between an urban (London, U.K.) and a rural (Chilbolton, U.K.) site. The low-level jets are elevated (≈ 70 m) over London, due to the deeper urban boundary layer, while the wind speed and fall-off are slightly reduced with respect to the rural low-level jets. Utilizing two idealized experiments in the WRF model, we find that topography strongly affects low-level jets characteristics, but there is still a substantial urban influence. Finally, we find that the increase in wind shear under the low-level jets enhances the shear production of turbulent kinetic energy and helps to maintain the vertical mixing in the nocturnal urban boundary layer.

Reference:

Tsiringakis, A., Theeuwes, N.E., Barlow, J.F., GJ. Steeneveld, 2022: Interactions Between the Nocturnal Low-Level Jets and the Urban Boundary Layer: A Case Study over London. Boundary-Layer Meteorol 183, 249–272. https://doi.org/10.1007/s10546-021-00681-7

How to cite: Tsiringakis, A., Theeuwes, N., Barlow, J. F., and Steeneveld, G.-J.: Interactions Between the Nocturnal Low-Level Jets and the Urban Boundary Layer: A Case Study over London, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-134, https://doi.org/10.5194/ems2022-134, 2022.

Changing land surfaces can have a profound impact on local and regional climates. Cities in particular are associated with a comprehensive transformation of the local surface structure, which, in combination with anthropogenic emissions of radiative active gases and particles, leads to a significant modification of the energy balance of the urban boundary layer. One of the most well-known urban climate phenomena is the urban heat island effect. In addition to the thermodynamic effect, a clear urban influence on the wind field and the modification of precipitation above and around cities is documented by many studies. Observational and modeling studies provide convincing evidence that precipitation patterns over and/or around urban areas are altered, and convective precipitation and flash flood events may be enhanced or even triggered. The urban influence often is result of a combination of thermal effects in connection with the heat island, obstacle effects an aerosol influences on microphysical processes and on the heating profile over the city. Precipitation can not only be increased by cities, it can also be weakened or even averted, especially in connection with particle emissions. Inprinciple urban effects on precipitation are fairly well known, but the many published studies on them show a wide range in terms of the magnitude of precipitation changes and their location relative to the urban area. Simple statements concerning urban precipitation modification cannot currently be made, if this is at all possible given the complexity resulting from the overlaying influencing factors. In many cases, the lack of standardized reporting of study results makes it considerably more difficult to compile generalized statements. As part of the Cluster of Excellence "Climate, Climatic Change and Society" (CliCCS), a project is focussing on sustainable adaptation scenarios of cities with regard to hydrological pressures in connection with climate change. Within this project, we conducted a systematic literature review on precipitation modification by cities with a focus on the last 10 years. The reason for this was that the last comprehensive review on the subject was several years ago and the number of publications is constantly increasing. Here we report the first results of our systematic review broken down by spatial scale, dominant processes and possible relevance to climate change. In doing so, we highlight the influence of cities on heavy precipitation and point out the development of the research landscape and alleged research gaps. Selected results from recent publications are shown for illustrative purposes.

How to cite: Quante, M., Hanf, F. S., Bär, F., Boettcher, M., Burgemeister, F., Grawe, D., Hoffmann, P., and Schlünzen, H.: Urban induced modification of precipitation - Findings from a systematic literature review with focus on studies from the last decade, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-121, https://doi.org/10.5194/ems2022-121, 2022.

It is well known that the CO2 is one of the main greenhouse gases. The traffic of non-electric vehicles is the main contribution to the emissions of this gas in the cities, enhancing the urban heat island effect and increasing the dangers of global warming. The changes in the amount of CO2 across the city occur on a small urban scale, easily measured by a person moving around the city, whether it be walking or cycling. A handheld CO2 sensor capable of recording this gas, temperature, atmospheric pressure, and relative humidity has been mounted on a bicycle to analyse local variations of this gas in a couple of cycling route transects in Barcelona city and some of municipality in its metropolitan area. The results presented here show a local CO2 variability, with some changes of this gas in 90 ppm over distances of few hundreds of meters (less than 1 km). The influence of parks, urban forests, and gardens in absorbing the gas has also been identified. The measurements obtained with this technique demonstrate that using a bicycle as a scientific vehicle is a viable method to analyse the C02, temperature, humidity and pressure distribution in urban and periurban areas, which could be used as a scientific citizen project helping to monitoring the amount of CO2 in urban areas. Many cities and municipalities worldwide are making an effort to adapt their infrastructures by including bike lanes, with the purpose of promoting bicycles as a common mobility vehicle. Bicycles are spreading out throughout cities as a useful and efficient vehicle. Thousands of cyclists moving through urban areas is also an opportunity to use the bicycle as a scientific vehicle to measure and helping monitoring some atmospheric variables (such as temperature, relative humidity, or CO2 values), thus inciting citizens to collaborate with scientists in quantifying the evolution of this gas in urban areas.

How to cite: Mazon, J.: The bicycle as a weather vehicle: transects of the urban CO2 and weather measurements on a bicycle-based atmospheric sensor, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-248, https://doi.org/10.5194/ems2022-248, 2022.

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. Additionally, 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. This is valid for coupled atmospheric chemistry as well, thus even air pollution modelling has to consider the urban environment. The inclusion of the individual urban processes affecting energy balance and transport (i.e. heat, humidity, momentum fluxes, emissions) via special urban land-surface interaction 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.). Cities are becoming one of the most vulnerable environments under climate change. In 2013, the CORDEX community identified cities to be a prime scientific challenge. 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 was started in May 2021. Main aims and planning of this activity will be presented together with a call for potential participation in ensemble experiment for selected city following adopted coordinated simulations protocol. Preliminary results from the analysis of already available data will be presented as well.

How to cite: Halenka, T. and Langendijk, G.: Urban Environments and Regional Climate Change - CORDEX Flagship Pilot Study URB-RCC, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-700, https://doi.org/10.5194/ems2022-700, 2022.

A new urban canopy scheme for the ICON atmospheric model is presented. Increasing the resolution of atmospheric models for numerical weather prediction (NWP) or climate simulations allows, among others, for a more realistic description of the processes at the land surface. Here, one field of growing interest are the processes in urban areas. Beside their relevance for the meteorological modelling, there is a general trend in most countries that the number of people living in towns is significantly increasing. During the recent years, an urban canopy parameterization was developed for the multi-layer land surface scheme TERRA of the Consortium for Small-scale Modeling (COSMO) mesoscale atmospheric model. This parameterisation, TERRA_URB, originally developed for the climate version of COSMO and then ported to the NWP version, was shown to be able to reproduce the key urban meteorological features for different European cities. In the framework of the transition of the COSMO Consortium to the ICON model, TERRA_URB needs to be implemented in ICON. Furthermore, an updated set of urban canopy parameters needs to be provided, for describing the urban characteristics down to a mesh size of 1 km, and below. For these purposes, the COSMO Consortium organises the dedicated Priority Project CITTA’. First results are presented for TERRA_URB in the ICON limited-area model ICON-LAM for different cities of interest of the CITTA’ partners. The preliminary results indicate already that urban features like the urban heat island effect are well represented. This is in agreement with the experiences with TERRA_URB in the COSMO model, both the climate as well as the NWP version.

How to cite: Schulz, J.-P., Mercogliano, P., Adinolfi, M., Apreda, C., Bassani, F., Bucchignani, E., Campanale, A., Cinquegrana, D., De Lucia, C., Dumitrache, R., Fedele, G., Garbero, V., Interewicz, W., Iriza-Burca, A., Jaczewski, A., Khain, P., Levi, Y., Maco, B., Mandal, A., and Milelli, M. and the COSMO PP CITTA' team: A new urban parameterisation for the ICON atmospheric model, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-501, https://doi.org/10.5194/ems2022-501, 2022.

A multi-layer urban canopy parameterization using a nudging approach has been developed and integrated into the mesoscale atmospheric model METRAS to improve the urban surface representation. Through modifying the existing nudging equation in METRAS, urban canopy effects on local climate such as reduction of mean wind speeds, enhanced aerodynamic turbulent fluxes as well as urban heat island phenomenon are represented. The nudging technique has the advantages of simple implementation and reasonable model results, moreover, it is widely used as a data assimilation approach in global-scale models. One of the objectives of such a canopy parameterization using nudging is thus to prepare for its direct usage in the global-scale models with a typical resolution of 1 km so as to improve meteorological variables calculation for heterogeneous urban canopies.

In this work, the city Hamburg with heterogeneous surfaces was selected as the study area. Urban canopy information for Hamburg such as building height and building surface fraction (i.e., the ratio of the surface area occupied by buildings to the total plan area) were obtained from the 3D city model of Hamburg LoD1 (Level of Detail 1). We also used the Local Climate Zone map, which defines a range of values of the urban canopy parameters for each urban class. Sensitivity studies using these two types of canopy data are performed to gain a better understanding of how detailed should urban canopy data be to serve as input for an urban canopy parameterization. Model simulations are made for summer 2020 with particularly focusing on the maximum urban heat island phenomena. Simulation results for different meteorological situations are presented and the impacts of model resolution will be discussed.

How to cite: Cheng, G., Grawe, D., and Schlünzen, K. H.: Parameterizing the impact of heterogeneous urban canopies on local climate using nudging, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-537, https://doi.org/10.5194/ems2022-537, 2022.

Rapid urban sprawl has been evident in sub-Saharan Africa; however, little is known about the implication on urban energy fluxes. Thus, this study analyses the urban expansion processes of Lagos, Nigeria, a hot-humid and Africa’s most populous city between 2000 – 2020; and quantifies the effect of the associated urban form and function on the partitioning of  Surface Energy fluxes in the study period using the Surface Urban Energy and Water Balance Scheme (SUEWS). The urban canopy was parameterized for the years 2000 and 2020 using the Local Climate Zone Scheme via the LCZ generator (https://lcz-generator.rub.de/).  

Most of the LCZs are built types, corresponding to LCZ 1 and 2 in the microcenter, LCZ 3 in the macrocenter, and LCZ 6 and 9 in the city suburban area. Results showed a significant increase of  60%, 105% and 165% in LCZ 3, 6 and 9, respectively within the 20 years period at the detriment of 35%, 85%, and 55% loss in the vegetation classes, LCZ A, C, and F respectively. The impact of these landscape changes on the energy fluxes was accessed with SUEWS model forced with ERA5 surface data. Model validation exercise showed an acceptable comparison between modelled and observed meteorological outputs which align with previous findings for midlatitude regions, suggesting that the model could perform well in a humid tropical climate. The spatio-temporal distributions of SEB showed that the turbulent heat fluxes increased significantly particularly in the micro and macro urban LCZs whereas suburban LCZs have greater latent heat flux magnitudes due to the large conversion of vegetation to suburban LCZs. We also noted a significant increase in the Bowen ratio in micro-urban LCZs particularly at night which is marked by great variability across other LCZs.  Similarly, we observed a 96% increase in anthropogenic heat flux in all the urban LCZs with marked intra-urban variabilities.

The results demonstrate the capability of SUEWS model with readily available inputs in estimating changes in the surface energy balance because of urbanization in a tropical humid city.  

How to cite: Obe, B., Morakinyo, T. E., and Mills, G.: Modelling the Urbanization Effect on Urban Energy Fluxes over a Tropical Humid City, Lagos, Nigeria using SUEWS , EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-147, https://doi.org/10.5194/ems2022-147, 2022.

The population in cities is steadily increasing, requiring more sustainable and secure urban environments. This includes the adaptability and resilience towards natural disasters. Floods and storms can have a large negative impact on livelihoods and infrastructure. At the same time, we are increasing our ability to predict such events. Numerical weather prediction models nowadays can perform on large regions with convection resolving grid resolution. 

A current research question is to what extent forecasting improvements can be obtained by running locally nested large-eddy simulations in advance of convective storms. To investigate this, we use a setup with the atmospheric model ICON in the numerical weather prediction mode (ICON-NWP, 2.5 km resolution) as well as in the large eddy simulation mode (ICON-LEM). The study is embedded into the WMO-WWRP endorsed Paris Research Demonstration Project (RDP) focussing on the Olympic Games of Paris in 2024 with the objective to advance meteorological research for future weather forecasting systems in urban areas.

The ICON-LEM is set-up and run for Paris and its urban surroundings using a circular domain applying different horizontal resolutions (100 - 600m). We use a two-moment cloud microphysics parameterization scheme and the Smagorinsky scheme for the subgrid scale turbulence. For the evaluation on the observational side, we use the super site SIRTA. There, for instance, we can use the ground-based remote sensing profilers (e.g. microwave radiometer, Doppler lidar) as well as radiosondes and surface measurements. We ran the model for single cases with convective storms and high temperatures.

We analyzed the boundary layer growth in the simulations using ICON-NWP during the day, which showed good agreement between model and observations. The vertical profiles of the radiosondes showed that the height of the inversions was not accurately met and there is less humidity in the model. We plan to further extend our analysis to evaluate the precipitation and establish if and what refinements to certain parameters such as the roughness length can improve the simulations.

How to cite: Bruder, L., Kiszler, T., Schemann, V., and Löhnert, U.: High resolution simulations using ICON-LEM to study convective storms in an urban environment around Paris, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-517, https://doi.org/10.5194/ems2022-517, 2022.

Climate conditions in urban areas considerably differs from the climate of their rural surroundings manifesting as urban heat island (UHI). In addition to impacts of regional factors (such as latitude, topography, distance to large water bodies etc.), the urban climate is largely determined by effects of urbanization (the structure of the city, like height and spacing of buildings, characteristics of constructing materials etc.). However, interaction between urban climate and large-scale climate variability (e.g. heat-waves, North Atlantic Oscillation, El-Niño Southern Oscillation) may amplify UHI and therefore increase the heat stress leading to more unpleasant urban environment for its inhabitants with potential negative economic, health and social consequences. Here, we investigate the role of North Atlantic Oscillation (NAO) in modifying urban heat island of Zagreb, Croatia. Based on analyses of ground measurements and MUKLIMO_3 modelled data, it is demonstrated that both winter (wNAO) and summer (sNAO) NAO affect the summertime urban heat load. The strongest increase in the heat load is found for those summers when positive wNAO is followed by negative sNAO, while the opposite situation leads to the strongest decrease. Situations associated with wNAO and sNAO of the same polarity resulted in much weaker response due to their cancellation effect. Although the NAO is a large-scale phenomenon, its impact is not spatially uniform over the urban domain. Results indicate soil-moisture and related processes as modifying factors. Additional targeted modelling experiments based on standardized precipitation evapotranspiration index (SPEI) uphold the modifying effect of drought conditions on spatial characteristics of urban heat load. Results also imply that efficiency of green infrastructure considerably depends on soil moisture availability indicating the need of their irrigation in dry conditions together with increased water demands for irrigation in future.

How to cite: Herceg-Bulic, I., Nimac, I., and Zuvela-Aloise, M.: North Atlantic Oscillation as a modifcator of urban climate, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-270, https://doi.org/10.5194/ems2022-270, 2022.

The co-benefit of urban trees for  heat mitigation and air quality improvement is not fully understood, and conflicting recommendations on whether trees should be integrated into (core) urban areas exist in literature. For climate-sensitive urban design,  trees help mitigate urban overheating. However, in terms of air quality-related effect, there is conflicting recommendation depending on the perspective of the study: dispersion (aerodynamic effect) or deposition (removal of pollutants). For the former, trees minimize pollutant dispersion and increase concentration levels, thus not recommended for urban integration. On the other hand, trees have been found to remove particulate pollutants through their filtration and deposition mechanisms, thus, improving air quality – recommended. This study leveraged the Particle(gas) Dispersion and Deposition Module (PDDM) and atmospheric modules in the ENVI-met modelling system to investigate the relationship between 3D tree form, urban morphology, urban microclimate and air quality. Parametric models were developed with variable urban morphology (SVF:0 to 1) and different 3D tree form based on tree height, trunk height, crown diameter and foliage density; with fixed traffic and meteorological information as boundary conditions. Thereafter, the micro-climate and air-quality related output were analysed using a proposed integrated climate-air quality index that accounts for the impact of trees on pollutant dispersion, deposition, and heat mitigation to estimate the co-benefits of different trees forms in different urban morphology. Results show  Findings are useful in practice as it helps recognize a set of tree form (species) with similar and simultaneous heat mitigation and air quality improvement capacities thereby aiding decision making in selecting the right set of species for the right place.

How to cite: Morakinyo, T. E.: A simulation study of the co-benefit of in-canyon trees for heat mitigation and air quality, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-319, https://doi.org/10.5194/ems2022-319, 2022.

Cities are not homogeneous, but areas with similar microclimatic characteristics can be singled out based on land surface features, type of material, human activity, etc. These areas are called local climate zones (LCZ) and they are extremely important in the field of urban climatology. In this study, for the first time, we present LCZs for nine Croatian cities and their wider surroundings: Zagreb, Split, Rijeka, Osijek, Pula, Varaždin, Slavonski Brod, Zadar and Dubrovnik. LCZs were classified using the Local Climate Zone Generator web application. To analyse the thermal characteristics of LCZs in selected cities, we calculated the average land surface temperature (LST) and normalized difference vegetation index (NDVI) for the summer seasons 2017 - 2021 using Google Earth Engine, a cloud-based geospatial analysis platform. An approach for the heat risk estimation based on LCZ, LST and NDVI data is proposed and calculated for the locations of the facilities used by the vulnerable population. The heat risk classes for nursing homes and kindergartens were calculated for two largest Croatian cities, Zagreb and Split.

The results show high classification accuracy among nine cities, with an average of 83 %. Continental cities have higher accuracy as they historically expanded more evenly. On the other hand, coastal cities show slightly lower classification accuracy, which can be attributed to the confined expansion of the cities in the narrow coastal zone and to karst topography. Statistical analysis shows significant differences of LST among LCZs. In all cities, the highest LST values ​are associated with dense built-up parts (i.e. compact LCZs) but also with industrial zones. Out of natural LCZs, the highest LST is found in the karst hinterlands of the coastal cities, i.e. for LCZ E (Bare rock). Except for LCZ G (Water), the lowest values ​​of LST are obtained for forested areas outside the city and in city parks with dense trees, where LST is significantly lower compared to compact LCZs.

Estimation of the heat risk of the vulnerable population of Zagreb and Split shows that facilities with high heat risk are located in most built-up parts of the cities connected with high LST values, but a small share of vegetation. This analysis highlights the importance of the position of such facilities in the city from the aspect of reducing the heat load and improving the standard of life of the vulnerable population.

How to cite: Žgela, M., Herceg Bulić, I., Lozuk, J., and Jureša, P.: Comparison of land surface temperature of local climate zones in Croatia and estimation of the vulnerable population heat risk, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-299, https://doi.org/10.5194/ems2022-299, 2022.

Cities worldwide will be affected by anthropogenic climate change and additionally cope with additional heat due to greater heat storage capacity of artificial surfaces, less ventilation, and a higher risk of extreme floods due to sealed surfaces. Urban heat island mitigation strategies (such as rooftop greening, increasing surface albedo of the city and irrigation of green surfaces, which leads to significant evaporative cooling) are well known, but the magnitude in air temperature reduction is still not fully understood and significantly differs between cities.

A small area (“Triangel area”) was desealed and planted with 18 young trees at a 1 ha area in the center of the Swiss City Basel in March 2021. This urban heat island mitigation strategy was validated with a high dense low-cost IoT measurement network, which was installed in Basel in 2020 to detect urban heat islands. To validate the mitigation strategy properly, 3 air temperature measurements were installed in the Triangel area in 2020 and compared with more than 20 air temperature measurements in the reference area outside the mitigated area.

The measurements showed that the Triangel area is in general around 0.2 K cooler than the reference area. Furthermore, the differences in air temperatures between Triangel and reference area were calculated before and after the mitigation action to test the effectiveness of the method. An air temperature reduction of the Triangel area of 0.4 K after the mitigation action (in comparison with the reference area) was observed by the measurements.

The results from the measurement campaign were compared with model results using the surface energy budget model SUEWS. The SUEWS model results confirm an air temperature decrease of 0.4 K for the chosen urban heat island mitigation strategy and suggest other mitigation strategies (such as rooftop greening, watering and more). This approach allows to estimate the best possible mitigation strategy for the Triangel area with the largest reduction of surface and air temperatures. In summary, the approach helps city councils taking the right decision by choosing the optimal cost-value ratio in urban heat island mitigation strategies and prevents costly (and non-climate effective) strategies.

How to cite: Schlögl, S., Smalla, T., and Gutbrod, K.: Urban climate: Verfication of urban heat island mitigation strategies in the Swiss city Basel, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-336, https://doi.org/10.5194/ems2022-336, 2022.

Due to the ongoing rapid urbanization, built-up landcover has geometrically increased in many global cities in recent years. Specifically, many coastal cities are almost using up their landed area, thus, land reclamation is increasingly becoming popular as a solution for further urban and economic development to cater for the growing population. However, such landcover changes could significantly impact the existing environmental condition and should be investigated.  One such case is the ongoing massive development of an expanse of reclaimed land on the Atlantic coast of Lagos, Nigeria. Thus, this study investigates the potential micro-climatic impact of the conversion of the large ocean surface to a built-up area, locally in the new development and downwind in the existing neighbourhood. To achieve this, we applied the ENVI-met micro-climate model in which a parametric study were conducted. Four scenarios i.e. the pre-reclamation - “reference” case, and three post-reclamation - “highly densified”, “grey design” and “grey-green design” scenarios were evaluated for their micro-climate and thermal comfort situations. Analyses involve the comparison between the pre-and post-reclamation scenarios, and inter-scenario comparison to reveal the influence of urban structure and design decisions on insitu and downwind thermal conditions. The result revealed both the insitu and downwind areas became warmer both at day and night-time due to significant (about 50%) landcover change from water to paved built-up surface. Although the imposed high-rise development on the reclaimed area has an impact on the ventilation flow pattern and magnitude, the relatively warmer air generated in the newly built area is still advected into the existing neighbourhood areas downwind.  However, depending on the urban structure/layout and design features imposed on the reclaimed area, the intensity of the consequent daytime and night-time warmth and thermal discomfort varies across the study domain. The warming intensity is stronger during daytime than nighttime and highest with the high densified/grey-design and otherwise with the green-grey design indicating the role of greening as a key climate-resilient strategy. Similarly, the green-grey design scenario improved the daytime thermal comfort condition significantly compared to the highly densified and grey design counterparts. However, at night-time, the downward longwave radiation from the trees made the environment warmer but this effect is significantly outweighed by the positive effect observed during the daytime. Finally, this study gave insights to developers, urban planners, architects, and decision-makers on the need to mitigate the potential environmental impacts of such large-scale land cover change and development.

How to cite: Morakinyo, T. E., Simon, H., Sinsel, T., and Bruse, M.: The potential insitu and downwind thermal conditions of a new coastal city built on reclaimed land in Lagos, Nigeria, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-591, https://doi.org/10.5194/ems2022-591, 2022.

Budapest is the capital and the largest city of Hungary, with 1.7 million inhabitants, surrounded by a large agglomeration area. Using a 20-year surface temperature database measured by NASA's Terra and Aqua satellites, we analysed the spatial distribution, inter-annual variability and 20-year trends of the urban surface heat island in Budapest. As the presence of the urban heat island causes the most adverse effects in summer, we investigated the evolution of surface temperature and heat island intensity in details for the late spring and summer months, with a special focus on heat waves.

Our results show that the maximum surface heat island intensity usually occurs in June, during the daytime. It was found that during the studied months, the monthly average surface temperature in the urban area exceeds 30 °C in the early afternoon and approaches 40 °C in the summer months of the hottest years. During periods of intense heat waves, surface temperatures above 50 °C can be detected on some days in a large area of the city. Our studies show that in dry, heat-wave weather situations, when surface temperatures are extremely high, relatively low heat island intensities occur. In these cases, the lower heat island intensities are not caused by a decrease in urban surface temperatures, but by a more intense warming of the areas outside the city, which is a consequence of the lower latent heat content of the energy balance above natural surfaces in these hot, dry situations.

As the Intergovernmental Panel on Climate Change (IPCC) assessment report states, extreme weather events, including heat waves, will become more frequent, longer, and more intense as global warming increases, therefore, it is important to gain a better understanding of the urban heat island effect to help developing effective climate adaptation strategies on local/regional scale.

How to cite: Dezső, Z., Pongrácz, R., and Bartholy, J.: Surface urban heat island characteristics in Budapest (Hungary) during the summer period, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-515, https://doi.org/10.5194/ems2022-515, 2022.