Climate change is increasing the frequency and intensity of urban heat islands and stormwater flooding. In order to mitigate these threats cities are turning toward green infrastructure to restore the hydrologic cycle in a way that increases the ecosystem services provided by trees. Strategically designed green infrastructure can mitigate runoff volume by rainfall interception through tree canopies and redirect impervious runoff into bioswales that promote infiltration. In addition, urban greens mitigate extreme heat via evapotranspiration and shading. Here we applied the i-Tree HydroPlus model to the German city of Karlsruhe and its twenty-seven districts with varying initial conditions of tree cover to analyze the potential for both runoff and heat mitigation during dry and wet periods throughout a 5-year period. After analyzing initial tree cover and drainage conditions, we used the model to simulate a green infrastructure scenario for each district with restored hydrology and tree cover at 30%. Regarding trade-offs between runoff and heat mitigation, the results confirm that dry soils before storm events lead to greater runoff reduction by 10%, and wet soils prior to heatwaves resulted in a greater evaporative cooling. Compared to current conditions, the green infrastructure scenarios resulted in decreasing the number of extreme heat hours (Heat Index > 31 °C) per year on average by 64.5%, and to reduce runoff in average by 58% across all city districts. Thus, our simulation results show that investing into a greener infrastructure, has positive impacts on microclimate and hydrology. Finally, we discuss synergies and trade-offs of the investigated management options as well as the transferability of results to other cities.

Traditional heat health warning systems focus on severe and extreme heat events at the district or regional level, often overlooking localized risk and protective factors such as healthcare access and urban green spaces. This approach considers less the varying impacts of heat within cities, including the phenomenon of Urban Heat Islands (UHIs) and the diverse needs of different populations. To address these shortcomings, a need for the development of an Urban Heat Health Warning and Information System (UHHWIS) that operates within the framework of Heat Health Action Plans is needed. Such a system integrates national acute heat health warnings with city-specific assessments of UHI effects and other relevant factors. The technical implementation of the UHHWIS involves the calculation and preprocessing of basic factors such as the Normalised Difference Vegetation Index (NDVI), imperviousness, and UHI intensity. Additionally, further factors are assessed, spatially processed, and provided in accordance with Open Geospatial Consortium (OGC) standards. An iso-area analysis is conducted to evaluate the accessibility of protective factors, such as urban green spaces, drinking wells, hospitals, physicians, and pharmacies, based on the city’s road topology. One crucial factor considered in the system is the casting of shadows, which is influenced by both time and location and facilitated through deck.gl. The developed template encompasses all these components into a unified system aimed at protecting vulnerable and risk groups, such as the elderly, through resilient, climate-adapted urban planning. The system provides warnings and information tailored to the urban morphology and prevailing conditions, complemented by a catalogue of potential short- to long-term measures focused on behavioral changes and climate-resilient urban planning strategies. The template can be adapted for use in various European cities, offering valuable insights to decision-makers in city administration for mitigating thermal stress and enhancing resilience against urban heat nowadays and in future.

Citizens in urban areas are affected by the urban heat island (UHI) effect, resulting in increased thermal heat compared to rural areas. This threat is exacerbated by global climate change. Therefore, it is necessary to assess human thermal comfort and risk for decision making. This is important for planners (climate resilience), the health sector (information for vulnerable people), tourism, urban designers (aesthetics), and building architects. Urban structures modify local meteorological parameters and thus human thermal comfort at the microscale. Knowledge of the pattern of a city’s UHI is typically limited. Based on previous research, generalized additive models (GAMs) were built to predict the spatial pattern of the UHI in the city of Karlsruhe. The models were trained with administrative, remotely sensed, and land use and land cover geodata, and validated with measurements in Freiburg. This identified the hot and cold spots and the need for further urban planning in the city. The model had some limitations regarding water bodies and anthropogenic heat production, but it was well suited for applications in mid-latitude cities which are not topographically characterized. The model can potentially be used for other cities (e.g., in heat health action plans) as the training data are freely available.

Global climate change and its thermal implications on cities makes it necessary to react with long- and short-term climate-adapted urban planning and action. This should be organised and implemented by municipalities as part of heat action plans, to minimise future risks of overheated city districts on the city dwellers and especially on vulnerable groups. The evaluation of the thermal impact, based on thermal indices (depicting human thermoregulation) is most important in order to allow for a safe and risk-minimised but also human-adapted urban planning. Out of more than 200 thermal indices, the three most important ones have emerged in the literature (Physiologically Equivalent Temperature, Universal Thermal Climate Index and Perceived Temperature). These indices contain the complete energy balance equation of a human body under prevailing meteorological conditions. This contribution demonstrates the thermal vulnerabilities, strengths and similarities of the indices. Min-Max-Normalisation was applied to relate and spatially compare the indices, independently of the physical unit and range. Subsequent regression analysis revealed the relationship between each index in turn. In this context, the indices were calculated for the urban district Rieselfeld in Freiburg, Germany, using the numerical, urban microscale model SkyHelios. The model is suitable to predict the meteorological outdoor conditions of future building- and local climate- scenarios. Our investigation showed that the different thermal indices are not so different and differ mainly in the areas where the modification of radiation and wind is most prominent. These are also precisely the zones in which automated clothing becomes a key driving factor and differs among the indices. We want to emphasise, that in future it will be necessary to compare not only the thermal indices with each other, but also the underlying implementations of the indices and the higher-level urban microscale models. This will increase confidence of these models, providing additional information for future action in heat action plans.

Global climate change and its thermal implications on cities makes it necessary to react with long-term climate-adaptive urban planning. This should be part of heat action plans to be implemented by municipalities, to minimize future risks of overheated city districts on the city dwellers and especially on risk and vulnerable groups. The evaluation of the thermal impact, based on thermal indices (depicting human thermoregulation) is most important in order to allow for a safe and risk-minimized but also human-adapted urban planning. The assessment of thermal impacts can be achieved using numerical urban microscale models, which are suitable to analyse the human thermal outdoor conditions of future building- and local climate- scenarios. This chapter aims to demonstrate the applicability of the urban microscale model SkyHelios and thermal indices to an urban district in Freiburg, Germany. The findings demonstrate the thermal vulnerabilities, strengths and similarities of the indices and provide additional information for future action.

Global warming is a growing trend that challenges cities to adapt to future thermal conditions. Heat stress is one of the consequences that must be tackled through urban planning and architectural design by linking urban geometry and human biometeorology. The present research quantifies the effects of urban street configuration on human thermal comfort to formulate urban guidelines for city growth and protection of human life in the era of climate change. Meteorological data from a weather station (2010-2019) was used to assess thermal indices such as Physiologically Equivalent Temperature (PET) in open spaces and in various urban canyon configurations. A parametric study was conducted using the RayMan model in two urban development schemes: in-filling and sprawl. Simulations applied to the urban canyon addressed aspect ratio and orientation using temporal analyses. Findings reveal a similar pattern of thermal conditions in both schemes, except for sprawl with H/W = 2.5. In the summer months, the east-west axis displays the highest PET discomfort value, while the north-south axis offers the highest PET comfort value. The pattern showed that the lower the aspect ratio, the greater the heat stress in January, but performance was stable in July with aspect ratio >= 1.5.

Urban areas are affected by the urban heat island effect, resulting in increased heat strain compared to rural areas. In this article, we aimed to study how urban morphology (e.g., building structure, sealing, and availability of green space) influenced meteorological variables and human thermal comfort on the microscale. Mobile measurements were performed during autochthonal weather conditions in Karlsruhe. Post-processing includes the transfer from a temporal (1 observation per s) to a spatial resolution (1 observation per 5 m), the normalization of the air temperature to the data range [0,1], and the spatial aggregation to account for inaccuracies in the Global Navigation Satellite System. The processed data were linked to the neighborhood's characteristics for a spatial analysis, specified by remotely-sensed and administrative geodata. The analysis encompassed the relationship of land use, sealing factor, and normalized difference vegetation index to the normalized air temperature. It revealed the dependency of the cooling effect of urban vegetation on the size as well as on the greenness of green spaces, while sealing showed an inverse effect. Our results will help urban planners take action during heat waves in the future. Furthermore, normalization enables the comparability of urban climate studies of varying latitudes and climate regions.

The Rhine River valley of Germany has been facing recurrent and intense spells of drought and heatwaves threatening the health of trees in peri-urban forests. Crown damage intensified by climate change accelerates tree mortality, threatening its ecological, economic, and social benefits; however, the pattern of crown die-back in peri-urban forests remained unclear. We performed a field inventory to estimate the crown die-back of 2578 trees of 51 species from 68 randomly selected peri-urban forest plots in Karlsruhe region on the right bank of the Rhine, after the catastrophic summer heatwave and drought of 2018. We related crown die-back to species-specific drought tolerance, wood anatomical traits, tree size, canopy surface temperature, tree density, Shannon's diversity and Gini coefficient for tree height. Regression results indicate that small-size trees were found to be more susceptible to canopy damage than large trees, with a 1-meter increase in tree height associated with a 0.8 % reduction in crown die-back. This size-dependent process is also species-specific. Among the 12 species with significant (p < 0.05) linear relationship between height and die-back, 9 species demonstrated negative correlations and 3 species showed positive relationships. Species tolerant to drought or cavitation (e.g., trees with diffuse porous xylem, 21 species) had significantly lower crown dieback. For example, with a 1-point-scale increase in drought tolerance crown die-back declined 14.35 %. Trees that experienced high canopy surface temperature and grew with high tree density and species diversity (Shannon's diversity) had more crown die-back. However, high structural diversity (Gini coefficient) was related to lower crown die-back. Our results suggested that future research should focus more on tree species-specific hydraulic and thermal traits and tree density and structure management to improve tree health and species selection in peri-urban forests under future climate change.

Global climate change is increasingly threatening our life in cities due to intensified heat waves and is thereby stressing the resilience of our cities and our society. Multiple descriptions of action plans (e.g., Sustainable development goals, Sendai framework for Disaster Risk Reduction 2015 – 2030 and Heat-health action plans), reflect the public awareness of these threats with the demand for action. Local administrations must implement long-term but also short-term adaptation measures, to increase the thermal resilience of our cities to protect the urban population, especially the vulnerable group. This contribution will demonstrate how meteorological observations from traverse measurement campaigns, reflecting the urban heat island effect, can be utilized in the aforementioned action plans. The observations are comprehensively preprocessed to generalize from prevailing weather conditions to observe the typical, thermal response of the urban morphology for authochtonous weather conditions during heat waves. Absolute values of meteorological conditions are no longer considered, as they are mapped to a relative and normalized scale. A subsequently fitted, Generalized Additive Model, considering the local urban structures (gray and green infrastructure), provides this information spatially for the entire urban area. The spatial prediction can be combined with local risk and protection factors (location of vulnerable facilities (e.g., hospitals, nursing homes, kindergartens) and protective infrastructure (e.g., drinking fountains)) to create a valuable tool for short-term protection of vulnerable and exposed populations during a heat wave. The short-term adaptation (for days) mainly addresses the people's behavior with respect to heat and is often implemented by information and warning systems. The German Meteorological Service operates a national heat warning system at community level based on the forecasts of the ICON-EU model (~6.5km, < 72h). The official heat warnings are derived from perceived temperature for severe and extreme heat and are statistically related to heat mortality. Based on the previously, generalized results, a specific heat warning system on city level (e.g., city of Karlsruhe) is implemented upon the national heat health warning system. The specific warning system is coupled to the national system and provides small-scale warnings and information for short-term adaptation under consideration of spatially resolved vegetation and urban structures. These warnings provide valuable recommendations of action for vulnerable and exposed people on urban district level in a citizen-oriented manner via the municipal systems.

To quantify the ecosystem services of trees in urban environments, it is necessary to assess received direct solar radiation of each tree. While the Sky View Factor (SVF) is suitable for assessing the total incoming short- and longwave radiation fluxes, its information is limited to specific points in space. For a spatial analysis, it is necessary to sample the area for SVF. A new geometrical method, Area View Factor (AVF), for the calculation of sunlit areas is proposed. AVF is the ratio of the unhidden, projected surface of an object to the whole projected surface of an object in a complex environment. Hereby, a virtual, orthographic camera is oriented in accordance to the sun's position in the 3D model domain. The method is implemented in the microscale model SkyHelios, utilizing efficient rendering techniques to assess AVF of all urban trees in parallel. The method was applied to Rieselfeld in Freiburg, Germany. The assessed sunlit area is compared to the SVF at the top of each tree and solar altitude angle, revealing a strong relationship between sunlit areas to solar altitude angles. This study shows that AVF is an efficient methodology to assess received direct radiation of urban trees. Based on AVF, it is possible to identify urban areas with shaded and sunlit trees, but it can also be applied to other objects in complex environments. Therefore, AVF is applicable for urban architecture or energetic research questions.

The issue of the quantification of thermal comfort or heat stress on humans is in vogue nowadays. This is evident for indices, which are trying to quantify these effects. Most known indices are PET, modified PET, SET*, PT and UTCI. All thermal indices require the same thermo-physiological and meteorological parameters. Air temperature, air humidity, wind speed, and short and long wave radiation fluxes in terms of mean radiant temperature are the required meteorological parameters. For human thermo-physiology, information about heat production and clothing are required. The meteorological parameters have to be available in appropriate spatial and temporal scales depending on the target and the specific issues demanded. The appropriate spatial and temporal resolution data cannot only be delivered by measurement stations. Meso and micro scale models, which compute meteorological parameter and thermal indices, can be helpful in the development of mitigation and adaptation strategies in the era of climate change.

Thermal indices are applied frequently for the estimation of thermal comfort and thermal stress of a human being exposed to the prevailing meteorological conditions. Among the most common thermal indices, there is the Perceived Temperature (PT), the Universal Thermal Climate Index (UTCI), and the Physiologically Equivalent Temperature (PET). They are calculated based on the same meteorological input parameters air temperature, vapour pressure, wind speed, and mean radiant temperature. PT, UTCI, and PET can be easily calculated e.g. using RayMan or the SkyHelios model. The meteorological input data for the calculation of thermal indices is to be selected carefully. Parameters with strong spatial fluctuation need to be representative for the desired location. Differences between the instrument height and the target location need to be considered as well. All indices show individual sensitivity to the individual input parameters. However, their responses do mostly agree in terms of tendency as found in a sensitivity analysis in the course of this study. A second part of this study is targeting the spatial applicability of the three thermal indices. While PT and PET are found to be quite suitable, there have been found limitations concerning the applicability of UTCI in spatially resolved analysis.

Wind speed is reduced above urban areas due to their high aerodynamic roughness. This not only holds for above the urban canopy. The local vertical wind profile is modified. Aerodynamic roughness (both roughness length and displacement height) therefore is relevant for many fields within human biometeorology, e.g. for the identification of ventilation paths, the concentration and dispersion of air pollutants at street level or to simulate wind speed and direction in urban environments and everything depending on them. Roughness, thus, also shows strong influence on human thermal comfort. Currently, roughness parameters are mostly estimated using classifications. However, such classifications only provide limited assessment of roughness in urban areas. In order to calculate spatially resolved roughness on the micro-scale, three different approaches were implemented in the SkyHelios model. For all of them, the urban area is divided into reference areas for each of the obstacles using a voronoi diagram. The three approaches are based on building and [+one of them also on] vegetation (trees and forests) data. They were compared for the city of Stuttgart, Germany. Results show that the approach after Bottema and Mestayer (J Wind Eng Ind Aerodyn 74–76:163–173 1998) on the spatial basis of a voronoi diagram provides the most plausible results.