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 (F_c) 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. F_c 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].