Comparing CO2 Storage and Advection Conditions at Night at Different Carboeuroflux Sites

Abstract

Abstract. Anemometer and CO 2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While storage was observed to be significant only during periods of both low turbulence and low advection, the advective fluxes strongly influence the nocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. On the basis of the main factors determining the onset of advective fluxes, the 'advection velocity', which takes net radiation and local topography into account, was introduced as a criterion to characterise the conditions of storage enrichment/depletion. Comparative analyses of the six sites showed several common features of the advective fluxes but also some substantial differences. In particular, all sites where advection occurs show the onset of a boundary layer characterised by a downslope flow, negative vertical velocities and negative vertical CO 2 concentration gradients during nighttime. As a consequence, vertical advection was observed to be positive at all sites, which corresponds to a removal of CO 2 from the ecosystem. The main differences between sites are the distance from the ridge, which influences the boundarylayer depth, and the sign of the mean horizontal CO 2 concentration gradients, which is probably determined by the source/sink distribution. As a consequence, both positive and negative horizontal advective fluxes (corresponding respectively to CO 2 removal from the ecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO 2 exchange rates.

On the Anomalous Behaviour of Scalar Flux–Variance Similarity Functions Within the Canopy Sub-layer of a Dense Alpine Forest

“…It is difficult to explore these processes via single tower measurements, though some theoretical and experimental attempts are presented in Assouline et al (2008) for heat and water vapour above small reservoirs. Recent studies (Feigenwinter et al 2004;Aubinet et al 2005;Sun et al 2007) found that advection may be significant even during daytime especially with low intensity turbulence. However, the main concern refers to nocturnal conditions when an underestimation of CO 2 fluxes using the eddy-covariance technique frequently occurs because non-turbulent transport processes, not taken into account by the eddy-covariance system, become significant.…”

“…It is possible that departures from MOST scaling for FV may provide additional clues as to when CO 2 advection is important. However, Feigenwinter et al 2004, Aubinet et al 2005, Sun et al 2007 do not discuss the role of advection on FV similarity functions (only on CO 2 fluxes via the mean scalar continuity).…”

Continuous CO<sub>2</sub>/CH<sub>4</sub>/CO measurements (2012–2014) at Beromünster tall tower station in Switzerland

“…Similar to previous studies (Haszpra et al, 2005;Winderlich et al, 2014) the advective flux component was not included in the calculations as this information was not available. Advective fluxes were shown to depend largely on topography and site characteristics (Aubinet et al, 2005;Feigenwinter et al, 2008) with an estimated contribution of about 10 % of the overall flux for a very tall tower (Yi et al, 2000), but may occasionally be the dominating component. However, these fluxes are difficult to measure and vertical advection is likely to be compensated for by horizontal advection (Feigenwin-ter et al, 2004).…”

Application of a mass consistent flow model to study the CO2 mass balance of forests

“…Half hourly means of the measured variables were used further for the calculations, following both Feigenwinter et al (2008), where the same data were analysed, and other studies for FLUXNET sites (e.g., Feigenwinter et al, 2004;Aubinet et al, 2005;Heinesch et al, 2007;Turnipseed et al, 2004, Staebler andFitzjarrald, 2004;Yi et al, 2008).…”

“…Goulden et al, 1996;Suyker et al, 2005;Moureaux et al, 2006;Zhao et al, 2006); and (ii) the "correction approach", based on a direct estimation of all the CO 2 balance terms, including the advection terms (e.g. Aubinet et al, 2003Aubinet et al, , 2005Feigenwinter et al, 2004;Staebler and Fitzjarrald, 2004;Marcolla et al, 2005;Sun et al, 2007;Yi et al, 2008;Montagnani et al, 2009). …”