An evaluation of aircraft flux measurements of CO2, water vapor and sensible heat

Abstract

Ground-based flux measurements of carbon dioxide and water vapor integrate physiological processes taking place on a field scale. Aircraft flux measurements have recently been undertaken to attempt to widen the scope of applicability of such measurements. However, because of the intermittency of turbulent transfer, flux measurements must be averaged over long periods of time or long distances to give reproducible results. This requirement makes it difficult to relate aircraft flux measurements to local surface processes. Flux measurements of COz, latent and sensible heat obtained from repeated passes in four directions and at three elevations over a homogeneous wheat-growing area are compared with ground-based measurements. Averages based on four runs of 4 km in length gave results consistent with ground-based measurements. The largest percentage differences were in the sensible heat flux. Cospectral analyses showed no significant high frequency losses for the data from flight levels of 25 and 50 m, but an underestimation of approximately 10% resulted at 10 m. Flight direction with respect to wind direction was relatively unimportant at 10 and 25 m but some effects were observed at 50 m. It was also shown that at 25 m, over a relatively smooth and homogeneous surface, the means of either three or four runs 4 km in length were similar to the means of 12-16 km runs. This confirms that at this altitude, most of the flux contribution is contained at wavelengths less than 4 km and that the mean of 3 to 4 passes accounts for most of the intermittency of turbulent transfer.

Winter observations of CO<sub>2</sub> exchange between sea ice and the atmosphere in a coastal fjord environment

“…Subsequently, surface flux estimates of CO 2 , sensible and latent heat were derived using ogive optimization (Sievers et al, 2015). The approach allows for separation of vertical turbulent flux and contributions from larger-scale motions by optimization of a model, ogive spectral distribution (Desjardins et al, 1989;Foken et al, 2006), to a density distribution of a large number of ogive spectral distributions, for which the data set length and detrending by running mean are varied simultaneously. Flux estimates are discarded only if an excessive number of gaps are present in the raw data set or if no theoretical model ogive distribution can be optimized sufficiently.…”

Energy balance closure for the LITFASS-2003 experiment

“…Special efforts were made by Foken et al (2006b) to analyse the effects of the long-wave part of the turbulence spectra by the use of the ogive function (Desjardins et al 1989;Friehe 1991;Oncley et al 1990). This function was proposed in order to test whether all low frequency parts are included in the turbulent flux calculated with the eddycovariance method (Foken et al 1995.…”

Technical note: Water vapour concentration and flux measurements with PTR-MS

“…the flux "ogives". The ogive Og wc is defined as the cumulative sum of the finite co-spectrum Co wc (Desjardins et al, 1989;Oncley et al, 1996). The co-spectrum is the real part of the discrete Fourier transform of the full covariance function cov wc (t) with 0≤t≤T a .…”