Urban areas are characterized by the urban heat island effect. This phenomenon is caused by a change of the surface (or different surface properties like emissivity, albedo or heat capacity) and can be measured by air temperature as well as surface temperature. The heat island effect leads to a high thermal load and is additionally intensified by global climate change. In 2003, an additional 15,000 people in France and 9,600 people in Germany died as a result of extreme heat events.
A person's perception of heat and cold is individual and is influenced by air temperature, wind speed, solar radiation, and humidity in their environment. Urban green infrastructure can influence these meteorological parameters in the local context (urban climate) and thus human thermal comfort at the small-scale microscale. Trees can reduce sidewalk and street surface temperatures through shading and cool air temperatures through evaporation of water on the leaf surface. In addition, tree canopies have a slowing effect on wind and promote air quality in the streetscape by absorbing carbon and releasing oxygen.
To what extent do trees change the small-scale climate in the city? What role does the location and neighborhood of a tree play here? Which tree species can positively influence the climate? Are urban trees suitable for adaptation to climate change? In order to answer these and other questions, three task parts were distinguished.
First, the heat island effect in Karlsruhe was determined along the urban-rural gradients of Rheinstetten and Karlsruhe by means of mobile measurements. Afterwards, a neighborhood analysis was carried out in which the collected data were combined with additional data (e.g. land use, building development, tree species density and spatial tree density) about the immediate vicinity of the measurement route. Finally, a heat map of Karlsruhe was modeled which will serve as a basis for a special heat warning system in the future.
The mobile measurements were carried out in 2019 and 2020 during the heat period in June with a measurement vehicle of the DWD (see figure on the left). The meteorological measurement system was installed at the front of the measurement vehicle and included measurements of air temperature, relative humidity, surface temperature, and global radiation (on the roof). The meteorological parameters were recorded every second while the measurement vehicle traveled on two routes through the project area. In total, the northern and southern routes were each driven 26 times. All measurement trips were made at a time interval of three hours and the average trip duration was about 20 minutes trips, depending on the traffic situation.
After the extensive processing of the measurement runs, first observations on the urban heat island in Karlsruhe could be made. The relative air temperature at 2.0 m above ground is clearly dependent on the time of day and shows the typified expression of the urban heat archipelago with regard to a typical heat wave in Karlsruhe. This means that the time intervals during the day show a lower relative air temperature than during the night. The scale of the relative air temperature refers to the northern and southern route, so that the cooling effect of the open spaces in Rheinstetten and the Hardtwald in Karlsruhe can also be compared.
In the nighttime situation (see figure), the urban heat island effect for Karlsruhe and Rheinstetten is shown in its clearest form. The difference between the relative air temperature in urban and rural areas is clearly pronounced and the adjacent Hardtwald in the north is clearly the coolest. The influence of the Zoological City Garden is also noticeable, but much less pronounced. In addition, a decreasing air temperature gradient is pronounced between Karlsruhe city center and Karlsruhe Durchlach in the east.
The morning situation shows the weakening of the urban heat island effect due to morning solar radiation with the beginning of atmospheric mixing. The cooling effect of urban vegetation by transpiration and shading starts with increase of photosynthetically active radiation (PAR). This effect can be seen in the northern Hardtwald, but especially in the Zoologischer Stadtgarten. At the same time, small-scale warming is visible in open, unsealed squares and streets.
The midday situation shows the highest cooling effect due to the urban parks. The cooling effect of the northern Karlsruhe Hardtwald is heterogeneous at this time of day and in some cases even has a slight warming effect. Especially the Zoological City Garden in the south now unfolds its cooling potential. Environments with flat roofs, on the other hand, heat up particularly. The warmest area along the measured section is located on the busy main road (Reinhold-Frank-Straße).
The evening situation shows a similar structure as in the morning and the spatial development, or intensity of the urban heat island increases again. However, the expression is still lower, so that the urban heat island is not yet fully developed. In particular, the spatial variability of the relative air temperature is very low at night.
In the neighborhood analysis, the processed meteorological data were considered with different distances from the measurement route from 10 m to 500 m, and the immediate neighborhood was analyzed for built-up area, vegetation, tree species diversity, land use, and sealing.
Analysis of the degree of sealing
Here, a high degree of sealing in urban areas resulted in high values of relative normalized air temperature at 2.0 m, especially during the nighttime intervals 0 - 6PM and 6 - 24PM. During the morning hours, areas with low levels of sealing were cooler than areas with high levels of sealing, and unsealed areas were always the coolest. This increase in relative air temperature from a low to a high degree of sealing was observed mainly in the evening hours and during the day, on the other hand, was not clear. It is possible that the effect is not due to the sealing alone, but rather to the three-dimensional volume of the building mass. This is because relative air temperature is similarly affected by building height.
Analysis of tree diversity
Here, it was found that relative air temperature decreased with increasing vegetation cover or species diversity. However, this was not equally true for all genera. Within a time interval, however, large differences in relative air temperature were evident with an increase in the respective tree species density. A higher beech density resulted in a lower relative air temperature than sites with a low beech density.
In the end, the results of GrüneLunge 1.0 will be used to implement a specific heat warning system based on the DWD's National Warning System. The implementation of a specific heat warning system can be done, based on the predicted relative air temperature, for the complete Karlsruhe city area. This heat warning system should take into account the risk factors of the urban infrastructure (hospitals, retirement homes, nursing homes, schools and kindergartens).
If successfully implemented, the proposed method also offers the potential for implementation in other German/European cities with comparable northern latitudes.