Evaluation of flux calculation methods

The aim of this study (Norman et al., 2012) was to compare different techniques for flux estimation and to improve the methods using high frequency measurement from the land based Östergarnsholm site. The methods are also applied on ship data from the Galathea 3 expedition (exernal) (Christiansen et al., 2007).

To estimate fluxes from high frequency measurements the most direct method is the eddy correlation method (EC). This method is a well known method for flux estimation over land. For measurements on a moving platform, e.g. a ship or a buoy, corrections for the movement of the platform must be done before the Eddy correlation method can be used. The movement can be estimated by accelerometers placed on the platform, and thus the movement can be corrected for. There are alternate methods for flux estimation where such a correction is not needed, e.g. the inertial dissipation method (ID) and the co-spectral peak method (CSP) (Sørensen and Larsen, 2010). The influence of the platform motion is usually seen in the frequency interval 0.1-1 Hz, and the inertial dissipation method and the co-spectral peak method both employ frequency intervals higher or lower than that.

The comparison between the three different methods is quite good both for the data from the Östergarnsholm site and the Galathea expedition. Figure 1 illustrates the comparisons between the different methods with the new modifications using measurements from the Östergarnsholm site. Figure 2 illustrates the same as figure 1 but for the Galathea data.

Figure 1. Fluxes of carbon dioxide (mol m-2 y-1) estimated with the three different methods using data from the Östergarnsholm site. (a) ECM plotted against IDM, (b) CSP plotted against IDM, (c) ECM plotted against CSP.

Figure 1. Fluxes of carbon dioxide (µmol m-2 s-1) estimated with the three different methods using data from the Östergarnsholm site. (a) EC plotted against ID, (b) CSP plotted against ID, (c) EC plotted against CSP.

Figure 2. Fluxes of carbon dioxide (µmol m-2 s-1) estimated with the three different methods using data from the Galathea expedition. (a) EC plotted against ID, (b) CSP plotted against ID, (c) EC plotted against CSP.

References:

Christiansen, M.B., Sørensen, L.L., Hasager, C.B. and Nissen, J., 2007. Air-sea fluxes of CO2 from Galathea 3. Ship and satellite measurements. IGBP Global Change Newsletter, 69(May): 6.

Norman, M., Rutgersson, A., Sørensen , L.L., Sahlée, E., 2012. Methods for Estimating Air-Sea Fluxes of CO2 Using High-Frequency Measurements, Boundary-Layer Meteorology, 144:379-400.

Sørensen, L.L. and Larsen, S.E., 2010. Atmosphere-Surface Fluxes of CO2 using Spectral Techniques, Boundary-Layer Meteorology, 136:59-81.