Revised eddy covariance flux calculation methodologies-effect on urban energy balance

Abstract

Eddy 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. © 2012 A. Nordbo et al.