Modelling Sensitivity of Urban Thermal Comfort on Street-Level Adaptation Measures; Case Study of Prague-Holesovice, Czech Republic

The citizens of towns and cities are suffering from adverse climate effects. These effects are often related to human thermal comfort and air quality. The causes of these effects are numerous, among others it is urban heat island, heavy traffic, winter smog situations etc. It is a concern of governments and municipalities to mitigate these adverse effects via proper urban planning and development measures. In order to implement effective measures, reliable integrated modelling approach able to simulate complex relations of the processes inside the urban canopy is needed.

Atmospheric modelling of urban areas usually considers either only a limited number of relevant processes or it is performed with coarser resolution. The newly developed model PALM-4U enabled modelling in the very fine street-level resolution of 1-10 m and at the same time covering all relevant processes (e.g. turbulent flow, radiative energy exchange inside natural and urban canopy, influence of resolved and unresolved plant canopy and air pollution). This progress in modelling technique opens new opportunities, not only in biometeorology. We are able to simulate fine-scale spatio-temporal meteorological conditions in street-level resolution. For example, it is possible to analyze the influence of buildings, pavements, natural surfaces, and trees to radiative and heat energy transfers and the influence of these heat changes to turbulent flow and consequently to street-level air quality. We can also study the influence of these processes on pedestrians to advise on possible improvements on biometeorology through urbanistic measures such as the choice of pavement materials and color (concrete or asphalt, light or dark), planting trees (positioning within the street canyon, choice of tree species) etc.

In this study we provide evaluation of these effects in terms of sensitivity assessment of the PALM-4U modelling system. This includes assessing both the modelling tool as well as the proposed urban development scenarios and adaptation measures. Sensitivity assessment of model was performed via changing various input parameters. Sensitivity assessment by means of PALM-4U modelling enhances the current trends in atmospheric modelling of urban areas. The modelling in very fine resolution covering all relevant processes is desirable but difficult to achieve. In this manner the sensitivity assessment provides an important insight into the dependencies of these processes.

First results for a short summer heat-wave period in a selected synthetic domain based on a real urban quarter in Prague-Holesovice (Czech Republic) shows real complexity of the urban environment. Interesting example can show in the implementation of a recent adaptation strategy "One million planted trees for Prague". In accordance with this strategy, planting 128 new acer platanoides trees was simulated in the analyzed domain. Location of the new trees does not respect the positioning of underground utility lines and thus in a real-life situation, most of them could not be planted. Most important changes were found in land surface temperature (LST) and mean radiant temperature (MRT). On average LST was about 1.3 K lower, the surface was about 0.4 K cooler during the night and about 1.9 K during the day. Differences of LST are between 0.4-4.4 K. MRT was lower about 0.5 during nighttime and about 7.8 and 4.4 during daytime on average for the whole domain. Air temperature was lower only about 0.25 K, day-time air temperature was lower about 0.33 K and at night about 0.15 K. This generally means that trees are very useful for improving the thermal comfort; they provide shading and through this they directly affect MRT above pavements. Consequently, PET and UTCI are significantly lower than in streets without trees; PET decreases about 1.7 K (daily average, differences are between -0.4 and 4.1 K) and UTCI about 1.0 K (daily average; differences are between -0.6 and 2.3 K). Negative differences can be surprising, but the main reason is the limited night-time cooling as the tree crowns shadow the ground surfaces from the cool night sky. At the same time, trees in the streets can very negatively affect concentrations of pollutants by creating a ventilation barrier. The modelled concentrations of PM10 were about 5.5 µg/m³ higher during the selected summer period (daily average, differences are between 1-30 µg/m³).

These first outputs have an important limitation; they are valid only for the specific conditions and season. On the other hand, they provide a new and very useful information. Local authorities and their urban planners can benefit from assessing the sensitivity of proposed urban climate adaptation measures. The major impact of these studies follows from the possibility to implement only the measures effective for mitigating the adverse climate effects. It also allows to verify the impact of proposed measures intended for improvement of thermal comfort to air quality and vice versa.