Abstract
ABSTRACTEddy covariance (EC) measurements of turbulent fluxes of momentum, sensible heat and latent heat – in addition to net radiation measurements – were conducted for three consecutive years in an urban environment: Helsinki, Finland. The aims were to: (1) quantify the detection limit and random uncertainty of turbulent fluxes, (2) assess the systematic error caused by EC calculation-procedure choices on the energy balance residual and (3) report the energy balance of the world's northernmost urban flux station. The mean detection limits were about 10% of the observed flux, and the random uncertainty was 9–16%. Of all fluxes, the latent heat flux – as measured with a closed-path gas analyser – was most prone to systematic calculation errors due to water vapour interactions with tube walls: using a lag window that is too small can cause a 15% lack of data (due to the dependency of lag time on relative humidity) and omitting spectral corrections can cause on average a 26% underestimation of the flux. The systematic errors in EC calculation propagate into the energy balance residual and can be larger than the residual itself: for example, omitting spectral corrections overestimates the residual by 13% or 18% on average, depending on the analyser.
Highlights
75% of Europe’s population lives in urban environments and a quarter of EU’s land surface has been directly affected by urbanisation (EEA, 2006)
The aims of this study are (1) to quantify the random uncertainty and detection limit of turbulent urban heat fluxes, (2) to provide estimates for the systematic errors caused by combinations of Eddy covariance (EC) flux calculations, (3) to assess the propagation of the systematic error into the urban energy balance residual term and (4) report 3 years’ energy flux data for the world’s northernmost urban EC flux measurement site
We presented data from three consecutive years from the world’s northernmost urban flux measurement station
Summary
75% of Europe’s population lives in urban environments and a quarter of EU’s land surface has been directly affected by urbanisation (EEA, 2006). This type of land-use change affects urban climate through alteration in surfaceÁatmosphere interactions through energy fluxes. The right-hand side includes turbulent fluxes of sensible heat (H) and latent heat (LE) in addition to advection through the volume (QA), which is not readily measurable. Which is negative when there is an additional energy source fueling H and LE, i.e. the building-air volume loses heat (DQSB0), there is an anthropogenic heat release (QF !0), or both. The determination of Res suffers from the differences in energy flux source areas, which is a common and unavoidable problem in urban micrometeorological measurements (Oke, 2006)
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