Similarities and differences of advective fluxes of carbon dioxide and sensible heat

A main issue of the micrometeorological community is to give reliable estimates of the net ecosystem exchange (NEE) of CO2 for different ecosystems. This is an important input for estimating future global CO2 concentrations that will affect prospective climate change. During the last decade, it was tried to include advective fluxes of CO2 in the NEE budget assuming that these non-turbulent fluxes are important during night and hence change the NEE budget systematically. However, it is a complex task to measure advective fluxes of CO2 experimentally (e.g. measurements of horizontal CO2 gradients). Measurements of temperature gradients are possible with much less effort. Therefore, we test in this study whether advective fluxes of sensible heat could be used as a proxy for the corresponding flux of carbon dioxide despite somewhat differing sinks and sources. Data of the advection initiative ADVEX within CarboEurope-IP were used. Within this initiative, three experimental campaigns of same geometry and instrumentation were conducted at three European coniferous sites in 2005 and 2006. Advective fluxes of both scalars (CO2 and potential air temperature) were determined by exactly the same method. A surprisingly consistent pattern was found. Advective fluxes of sensible heat were of opposite sign in relation to the advective flux of carbon dioxide on average, especially during nighttime. Therefore, respective gradients must be of opposite sign. From a theoretical point of view this is quite evident for gradients in the vertical direction but less evident for gradients in the horizontal direction. A scheme was developed that shows the correlation of the respective gradients and a simple model was established to derive advective fluxes of carbon dioxide from advective fluxes of sensible heat. The results showed a satisfying agreement of mean diurnal variations between modelled and observed data of advective fluxes of carbon dioxide. Thus, it is possible to use advective fluxes of sensible heat as a proxy or an indicator for advection of CO2.