Surface–atmosphere interactions over complex urban terrain in Helsinki, Finland

Abstract

Long-term measurements of fluxes of sensible heat (H), latent heat (LE) and carbon dioxide (Fc) were made from December 2005 to August 2006 over an urban landscape in Helsinki, Finland using the direct micrometeorological eddy covariance technique. Three distinguished sectors of land-use cover (vegetation, roads and buildings) allowed comparisons of fluxes over different urban surfaces. The normalized standard deviation of wind and scalars as a function of atmospheric stability were typical for rough surfaces, as were turbulence spectra and cospectra. Footprint analysis was performed by a boundary-layer one and half-order closure model allowing for discrimination of surface and canopy sinks/sources and complex topography. Fluxes were analysed as average diurnal courses over winter, spring and summer periods. H exceeded LE reaching 300 W m-2 over urban and road surfaces in the summer and it was close to 100 W m-2 in the winter. LE was highest in the summer over vegetation cover attaining 150 W m-2. The emission rate of CO2 was high over road sector [20 μmol (m2s)-1] [Correction added after online publication 16 Oct 2007: 30 μmol changed to 20 μmol] while in the vegetation sector it remained below 5 μmol (m2s)-1 and at summertime reached even −10 μmol(m2 s)-1 [Correction added after online publication 16 Oct 2007: wording of sentence altered]. Effluxes from soil measured by chambers were 1- 3 μmol (m2s)-1. Fc correlated with traffic density and a background non-vehicle flux was 1 μmol (m2s)-1 [Correction added after online publication 16 Oct 2007: 2 μmol changed to 1 μmol].