Modeling of current and projected urban temperatures and related uncertainties in Finnish cities

Throughout the world, literally billions of people live their lives in cities, exposed to local weather conditions that are influenced by the presence of the built-up environment. In particular, the temperature within cities tends to be higher than in surrounding rural areas; a phenomenon known as the urban heat island (UHI). Even in the relatively moderate-sized cities of Finland, the strength of the UHI may often reach several degrees. As temperatures are generally rising due to both global climate change and the growth of cities, it is crucial to understand the behavior of the UHI. Conditions within cities vary significantly, and, for example, temperatures tend to be higher in heavily built-up areas compared to sparsely built-up ones abounding in vegetated surfaces. Thus, city planners have tools to mitigate the UHI and the effects of the global changes, for example, by adding more green areas within urban areas and favoring building materials that do not absorb as much heat, but the mitigating actions need to take into account that cities are unique and their temperature variation differs spatially and temporally. Although observations can be used to measure this variability, urban climate models play a crucial role in quantifying the differences between and within cities. This thesis quantified the temperature variability within Finnish cities by utilizing numerical weather prediction (NWP) modeling, observations, and climate projections. The NWP model HARMONIE-AROME was used to model local heatwave conditions in Turku at a horizontal grid spacing of 750 m. The model results were compared with measurements from an extensive observation network and with a linear regression model trained with the same observations. The accuracy of the NWP model’s land use description was assessed by comparing the dataset with the Corine 2018 dataset. The effect of land use on spatial temperature variability was assessed by studying the effect of a green intervention on the recent past and future climate. The model experiment, which simulated conditions in the capital region of Finland, was conducted with the surface-atmosphere interaction module SURFEX and atmospheric forcing modified with climate projections based on the RCP8.5 pathway. An idealized green intervention was applied to the model’s land use description representing a commercial area in Vantaa and involved decreasing the area’s building height and building fraction and increasing the vegetation fraction. The results show that HARMONIE-AROME can simulate urban temperatures accurately when the land use description corresponds well to the actual environment, although the model experiences a nighttime warm bias when modeling calm and cloudless conditions. In Vantaa, the cooling effect of the green intervention was found to be notable but not as strong as the warming effect of the projected climate change. The model results reveal how the temperature variability within the cities is dictated by land use during summer, and that during winter the distance to the sea is the most prominent factor. The new high-resolution NWP model simulation, together with the extensive network of temperature measurements, quantifies the UHI of a Finnish city in unprecedented detail and extent. The presented method for studying local temperature variability in different climates enables high-resolution modeling of projected future weather conditions without computationally heavy regional climate modeling and sheds light on the magnitudes of local temperature variability and climate change impact.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2026

Author dc:creator

• Saranko, Olli

• meteorologia

Statement dc:rights

• Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
• This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
• Publikationen är skyddad av upphovsrätten. Den får läsas och skrivas ut för personligt bruk. Användning i kommersiellt syfte är förbjuden.

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/631579

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F631579