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The coupling between the atmosphere and underlying surface is controlled by the turbulent transport of momentum, energy and mass. These control the atmospheric flow, exchange or flux of air pollutants and mixing conditions of the lower atmosphere. These surface fluxes are different in urban areas when compared to natural surroundings. The reason lies in the significant changes in surface cover with reduced vegetation, and anthropogenic sources of heat and pollutants. Our group, led by university researcher Leena Järvi, studies the coupling between the urban ecosystem and the atmosphere using observations and modelling. The group is part of the Division of Atmospheric Sciences.

Observations

The observational part is based on measurements carried out in the SMEAR III station located in Helsinki. Currently we have two active sites, where the fluxes of CO2, H2O, heat and particles are measured using the eddy covariance (EC) technique (Figure 1). The two sites represent different land uses: The semi-urban Kumpula site (blue) has been running since 2004 (Vesala et al. 2008, Järvi et al. 2009) and the urban site, Hotel Torni (red), was started in 2009 in the city centre and it represents a highly built-up land cover (Nordbo et al. 2013, Kurppa et al. 2015). Online data can be found here. In addition to the ongoing measurements, we have conducted several EC measurement campaigns related to other compounds like VOCs and N2O. We examine how the different land uses affect on the exchange processes with particular emphasis in the role of vegetation.

Urban land surface model - SUEWS

We participate to the development of the Surface Urban Energy and Water Balance Scheme SUEWS (Järvi et al. 2011, 2014, Ward et al. 2016) in close collaboration with the University of Reading. The model simulates the energy and water balance components through the use of commonly measured meteorological variables and information on the surface cover in a neighborhood scale. The model is also part of the Urban Multi-Scale Environmental Prediction Tool (UMEP). Besides model development, we use SUEWS to examine both long- and short-term behavior of the surface balances in different urban areas. Current study interests are long-term hydrological changes in in urban areas using re-analysis data as forcing, and urban surface energy and water balances in cold climate regions.

Large eddy simulation model - PALM

We develop and use the Large Eddy Simulation model PALM in collaboration with Finnish Meteorological Institute and University of Hannover to examine the flow fields and air quality in realistic Helsinki city centre. The current work covers the calculation of footprints for our EC measurement sites and an application how future Helsinki city neighborhoods should be built so that the air quality would be optimal. Currently our work also covers the inclusion of aerosol particle model in PALM.

We collaborate with several national and international research groups. Main partners are

The observations are also part of the Helsinki UrBAN Urban Boundary-layer Atmosphere Network.

 

Figure 1. Location of the eddy covariance measurement stations in Helsinki (Kaupunkimittausosasto, Helsinki, 2011)

 

Figure 2. LES simulation of Helsinki city centre

 

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