FOCUS#1: LES modelling in Helsinki – by Antti Hellsten and Mona Kurppa (FMI)

Air quality in a city  boulevard affected by surrounding buildings

The Finnish Meteorological Institute and the University of Helsinki have used supercomputers to investigate what kinds of building block structures should be built along future city boulevards planned in Helsinki in order to limit the adverse effects of street traffic emissions on air quality as much as possible.

In this study we have demonstrated the utility of LES-modelling in computing urban concentrations at very high spatial resolutions, as required in URCLIM WP3. The demonstrated numerical methodology can be used to assess a wide range of future scenarios, including alterations to the urban boundary layer energy and momentum balance arising due to climate change or modifications to the urban plan.

The new Helsinki City master plan includes areas of new city development where the aim is to change the existing access routes inside the Ring Road I into urban boulevards lined with residential and commercial buildings. According to the study conducted by the Finnish Meteorological Institute and the University of Helsinki, the structure of the building blocks surrounding the boulevard streets has a significant impact on the air quality in the local streets. The study compared four different types of building blocks and one of the solutions proved to be clearly better than the others in this respect.

Block models studied: V1 – V4. In the top picture the colour indicates the height of the buildings and the ground. The darkest shade of green represents the maximum height. The picture below describes the cross sections of the building block models. The study indicated that block model V3 was best for overall ventilation and reducing street traffic induced air pollution both in the most commonly occurring weather conditions and most ventilation-inhibiting meteorological conditions.

 

“The best solution in terms of air quality are building blocks of different heights, which contribute to the transfer of air pollution from the street level. In this solution, the short façades of the blocks face the boulevard and the height of the blocks varies gradually from one block to the other in the direction of the street,” says Finnish Meteorological Institute researcher Antti Hellsten. More efficient ventilation reduces concentrations of harmful pollutants to which people are exposed at street level. According to the modelling this sort of construction method allows the air to flow providing good ventilation.

Supercomputers make exceptionally detailed dispersion modelling possible

Such detailed air pollution dispersion modelling in an urban environment has not been carried out anywhere else in the world. The area of study was focussed on four theoretical urban boulevard plans drawn up for the Hämeenlinnanväylä area. The simulations were made at the Finnish Meteorological Institute on a Cray XC30 supercomputer running four hundred computing processes in parallel. In the study the air flow and its entrained air pollutant concentrations were modelled using a self-nested large-eddy simulation model.

Increased resolution at the location of interest: A: Outer modelling domain comprising an area of 4 km x 2 km, with a resolution of 2 m. B: Inner modelling domain: area of 2 km x 1 km, described in the model at 1 m resolution

The study took into account not only the ground height and buildings but also the air braking effect of trees in the street and the surroundings, as well as the effect of the weather on the ventilation. Both the weather conditions most common in the Helsinki region and the weather conditions expected to be the most ventilation inhibiting were included in the study scenarios (characterized by averaged wind direction, temperature near the ground surface, and the stratification of the atmosphere). Analysis of the model run results focused on pollutant concentration, vertical flux and ventilation efficiency.

Ventilation in common meteorological conditions: average wind direction is from South-West, and ventilation is strongest at the South-West end of the studied boulevard in all building block configurations. Overall ventilation is the strongest with the V1 building block configuration and weakest in the V2 configuration. At the North end of the boulevard ventilation in the V1 configuration was weaker than in V4.The variation of ventilation of inner courts of the buildings was remarkably large.

Ventilation-inhibiting meteorological conditions: In this case clean air flows from the East and the side streets of the boulevard are efficiently ventilated, but the boulevard itself is not.

 

 

The work was done in collaboration with the University of Helsinki’s Research Division of Atmospheric Sciences, as well as the Helsinki Region Environmental Services Authority (HSY).  The study was commissioned by the Helsinki City Planning Department. The study used methods developed for the Academy of Finland funded CityClim research project, as well as other methods.

Links

Link to the final project report: http://www.hel.fi/hel2/ksv/julkaisut/yos_2016-5.pdf

Link to an animation describing the wind field:  https://youtu.be/hTi2HPikJvk

Link to an animation showing the migration of pollutant concentrations close to the street level: https://youtu.be/6HTeWcBy1Fw

Further information

Senior Research Scientist Antti Hellsten, Tel. 050 409 0477, antti.hellsten@fmi.fi

Researcher Mona Kurppa, Tel. 0400 991 944, mona.kurppa@helsinki.fi

Researcher Mikko Auvinen, Tel. 029 415 0714, mikko.auvinen@helsinki.fi

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