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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 7 | Copyright
Atmos. Meas. Tech., 11, 4567-4581, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 31 Jul 2018

Research article | 31 Jul 2018

New calibration procedures for airborne turbulence measurements and accuracy of the methane fluxes during the AirMeth campaigns

Jörg Hartmann1, Martin Gehrmann1, Katrin Kohnert2, Stefan Metzger3,4, and Torsten Sachs2 Jörg Hartmann et al.
  • 1Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 2GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
  • 3National Ecological Observatory Network, Battelle, 1685 38th Street, Boulder, CO 80301, USA
  • 4University of Wisconsin-Madison, Dept. of Atmospheric and Oceanic Sciences, 1225 West Dayton Street, Madison, WI 53706, USA

Abstract. Low-level flights over tundra wetlands in Alaska and Canada have been conducted during the Airborne Measurements of Methane Emissions (AirMeth) campaigns to measure turbulent methane fluxes in the atmosphere. In this paper we describe the instrumentation and new calibration procedures for the essential pressure parameters required for turbulence sensing by aircraft that exploit suitable regular measurement flight legs without the need for dedicated calibration patterns. We estimate the accuracy of the mean wind and the turbulence measurements. We show that airborne measurements of turbulent fluxes of methane and carbon dioxide using cavity ring-down spectroscopy trace gas analysers together with established turbulence equipment achieve a relative accuracy similar to that of measurements of sensible heat flux if applied during low-level flights over natural area sources. The inertial subrange of the trace gas fluctuations cannot be resolved due to insufficient high-frequency precision of the analyser, but, since this scatter is uncorrelated with the vertical wind velocity, the covariance and thus the flux are reproduced correctly. In the covariance spectra the −7∕3 drop-off in the inertial subrange can be reproduced if sufficient data are available for averaging. For convective conditions and flight legs of several tens of kilometres we estimate the flux detection limit to be about 4mgm−2d−1 for w′CH4, 1.4gm−2d−1 for w′CO2 and 4.2Wm−2 for the sensible heat flux.

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Short summary
We present new in-flight calibration procedures for airborne turbulence measurements that exploit suitable regular flight legs without the need for dedicated calibration patterns. Furthermore we estimate the accuracy of the airborne wind measurement and of the turbulent fluxes of the traces gases methane and carbon dioxide.
We present new in-flight calibration procedures for airborne turbulence measurements that...