The City of Linz is enthusiastic about CLARITY
Unbearable heat in the city... Linz is one of the examples where the population is suffering extremely from heat exposure. Due to the common urban structures and large-scale climate change impacts, this Austrian City is an ideal demo case for many other cities in Central Europe. It can serve as a base for development of operating procedures and climate services in other regional centres.
Wolfgang Loibl and Tanja Tötzer from AIT presented to Eva Schobesberger from the City of Linz the CLARITY heat scenario. The press conference took place in the Old Town Hall of Linz on 24 June. Topics were urban heat islands, climate change and adaptation measures for Linz. Eva Schobesberger got very enthusiastic about the ideas and wants one thing above all for Linz.
Natural air conditioners for the city
Wolfgang Loibl and Tanja Tötzer talked about heat islands and micro climate. In former measuring, the temperature difference between the whole city and its surroundings was considered. Today, there is a change in the meaning of the term heat island: the city must be seen as a conglomerate of many heat islands of varying intensity depending on the building structure, greening and sealing. The good thing about it: you can have an impact on this microclimate by taking the necessary measures.
Facing the problem: Higher, wider, more
The city of Linz - like many others - is characterised by dense development and sealing and little open space and vegetation. Higher, wider and more buildings are coming into town. Added to this are anthropogenic (heat) emissions from industry, transport and households. Water hardly evaporates, the streets are so narrow that they are no longer well ventilated. The large-scale climate provides the second framework condition for the microclimate in the city. These two framework conditions are changing with climate change: cities are expanding and becoming denser, the climate is becoming more extreme: we have more temperature extremes, precipitation extremes and storms. This also changes the microclimate. This is the point at which CLARITY comes into play.
Mean radiant temperature is much higher than average
The AIT has developed adaptation measures to cope with the changes and mitigate the future consequences of climate change. They have made an analysis of the effect of measures on different spatial scales using different climate models.
The company found out that greening and water help more than expected: There is a cooling effect from evapotranspiration, there is shading, the radiation temperature is reduced, surfaces absorb less sunlight, less need for cooling and heating in buildings, water retention and delayed water discharge relieves the sewer system, more ecology and biodiversity and, ultimately, more quality of life: less noise and better air quality for the inhabitants. Of course, in general everywhere is the same temperature, but the feeling is much nicer to have shady places and light airs in the streets. Besides, the radiant temperature gets up to more than 40 degrees while under big treetops you can enjoy up to 12 degrees less.
Solution: Planting only 9 trees could make the main plaza much more comfortable
In the case study, Linz was analysed once with and once without adaptation measures or urban densification. AIT tested different cases and found out that for example in the old town you would only need 9 trees to have a strong cooling effect already. The trees also protect the buildings which no longer store as much heat. The road pavement also stores heat over night. The cemented areas also cannot absorb the rain water - trees, on the other hand, relieve the sewer system.
Not only in the Old Town, but also at the old tobacco factory the densification effect was created with high buildings. The so called "Brandland" should be reconstructed to give more shadow.
Transferable to most other cities
The AIT has uncovered clear advantages not only for Linz, but also for other cities: wide roads are more sunny than narrow ones, so in wide roads tree plantations can provide more additional shade. Large intersections are particularly exposed to sun and wind. At the same time, they are more frequented by the users of the road space. Tree plantings are particularly useful here. North-south oriented roads are only used during the noon hours. East-west oriented streets, on the other hand, are sunniest for the longest time during the day, with the sun shining at its steepest angle during the hottest midday and early afternoon periods. Therefore, planting trees in these streets brings the maximum improvement of thermal comfort. Tree plantings in front of the south and west facades are particularly effective.
The presenters spoke about planning recommendations such as having large yards which have stronger microclimatic fluctuations than small ones: They have more solar radiation and warming during the day, stronger radiation and cooling at night and the tree plantations significantly reduce the warming here and also increase the humidity and thus the cooling effect. The smaller the yards, the greater the shading and the less effective (further) tree plantings are. Too dense and high crowns reduce the Skyview factor and thus reduce air exchange and night cooling. Open courtyards are better ventilated, but also receive more sunlight. Viewed over the day tree plantations significantly reduce the air and average radiation temperature and increase the humidity and thus the cooling effect. By reducing the wind speeds, however, the heating can be slightly accelerated. In closed courtyards, tree plantations can be also contribute to a continuous lowering of the air temperature. The already low air exchange rate is not further obstructed here by the trees.