Measurement of horizontal and vertical advection of CO2 within a forest canopy

Abstract

Accurate observations of the carbon cycle are essential as inputs to global climate models. Observations made by the micrometeorological technique of eddy covariance, whist widespread, may be incorrect if air is advected away from below the sensor system. This is potentially a serious issue for FLUXNET, a global network of eddy flux sites. The approach in this thesis to investigate this problem was twofold: A full micrometeorological mass balance using an instrumented 50 m long by 50 m wide by 6 m high Cartesian control volume (CV) covering the understorey vegetation of a 40 m high Eucalyptus forest was carried out; situated adjacent to the Tumbaruma eddy covariance site in Australia. At night positive (into the atmosphere) advection fluxes caused by down-slope katabatic drainage within the forest trunk space, dominated the CO 2 flux budget of the CV, with both vertical and horizontal advection terms having predominantly positive values. The nighttime estimates of advection were subject to large systematic errors that were of the same order of magnitude as the advection signal. Nevertheless, the nocturnal respiration flux of the understorey vegetation was clearly resolved by the diurnal full mass balance flux curve that resulted from the experiment, having a typical value of 5 µmol m-2 s-1. A second experiment carried out at the Griffin forest in Scotland demonstrated the presence of sub-canopy katabatic/gravity flows at night that would be likely to cause scalar advection resulting in underestimation of the nocturnal respiration flux of CO 2. Finally, it is recommended that the micrometeorological mass balance technique should not be deployed across FLUXNET because of financial cost and issues of systematic error.

Contribution of advection to the carbon budget measured by eddy covariance at a steep mountain slope forest in Switzerland

“…Several studies have been published so far where advection was measured by multiple EC towers in a complex threedimensional multi-level set-up (e.g., Staebler and Fitzjarrald, 2004;Heinesch et al, 2007;Feigenwinter et al, 2008;Leuning et al, 2008;Yi et al, 2008). However, this is not a practicable option for most of the flux tower sites.…”

Direct CO2 advection measurements and the night flux problem

“…Several campaigns aiming at directly measuring advection have been designed recently (Aubinet et al, 2003;Feigenwinter et al, 2004;Fitzjarrald, 2004, 2005;Marcolla et al, 2005;Heinesch et al, 2007Heinesch et al, , 2008Tóta et al, 2008;Sun et al, 2007;Leuning et al, 2008;Yi et al, 2008). However, these different experiments are difficult to compare as the results are sensible to the design of the experimental setup and the applied methodology to derive the fluxes from measurements.…”

“…One important issue is the correction of the measured wind field according to the law of mass conservation, a topic which was brought up by Vickers and Mahrt (2006), Heinesch et al (2007) and by Leuning et al (2008), for the computation of the vertical wind velocity, and later extended to the three dimensions by Montagnani et al (2009). Montagnani et al (2010) used a correction factor cf to account for mass conservation of the wind field within the external surfaces of the control volume and compare their advection computations with the results of Feigenwinter et al (2008) for all three ADVEX sites.…”

Exchange of Carbon Between the Atmosphere and the Tropical Amazon Rainforest

“…The search for this missing CO 2 has spurred a great deal of research with the goal of explicitly estimating advective fluxes in field experiments during the last decade, in order to correct the NEE bias over single tower eddy covariance measurements (Aubinet et al, 2003(Aubinet et al, , 2005Staebler andFitzjarrald, 2004, 2005;Feigenwinter et al, 2004;Marcolla et al, 2005;Sun et al, 2007;Leuning et al, 2008;Tóta et al, 2008;Yi et al, 2008;Feigenwinter et al, 2009a, b).…”