Estimation of advective fluxes from CO2 flux profile observations at the Cabauw Tower

The exchange of carbon dioxide (CO2) between the atmosphere and the terrestrial biosphere is an important link in climate related research. In this field of research, there is a need to quantify regional scale vertical fluxes of CO2 to bridge the gap between ecosystem scale and the global scale. These regional scale fluxes may be obtained by accurate observations higher up in the atmosphere. In doing so, complicating factors arise such as storage of CO2 beneath the level of observation and advection. The question of advective contribution to the net ecosystem exchange (NEE) of CO2 is investigated at the 213 m tall-tower near Cabauw, The Netherlands. In addition, vertical and temporal variability of CO2 at Cabauw is studied. This site is particularly interesting because advection is assumed to be small under convective conditions, because of the relatively homogeneous and flat surrounding landscape. Moreover, the Cabauw tower is equipped with CO2 flux and concentration instrumentation at several levels, which enables detailed advection estimations based on a scalar budget methodology. To judge advection under convective conditions a summer half year was studied. During monthly averaged diurnal cycles the advective contribution to NEE was found to be in the order of 20%, but also showed a significantly noisy signal in general. The advection is typically alternately CO2 enriching and depleting. However, with a fairly constant easterly wind direction over the course of a month the advective contribution was observed to be persistently and significantly enriching during daytime. This is probably related to the presence of a village in the footprint of the flux observations. This study shows that in assessments of NEE for Cabauw and similar landscapes advection should be considered.