Quality assurance aspects for laser based eddy covariance measurements of atmospheric trace gases
1 Institute of Meteorology and Climate Research IMK-IFU, Research Center Karlsruhe
O 2.5 in Turbulence structure in and above forests
06.10.2009, 11:40-12:05, Kutschenhaus
In performing and using measurements of atmospheric turbulence to determine ensemble parameters and second-order moments we must assume that the ensemble possesses certain properties in order to be able to statistically estimate this moment. Since frozen turbulence is assumed, spatial properties are translated into temporal properties. Almost all measurements are performed by averaging over time and we must assume that the atmosphere is stationary and ergodic over some limited time and vertical dimension. Today ecosystem researchers increasingly use laser based gas monitors to measure fluxes of greenhouse gases in and above forests. As these complex measurement devices under field conditions cannot be considered as stable, drift characterisation is an important issue to distinguish between atmospheric data and sensor drift. Therefore, in this contribution a concept based on the two sample variance is discussed to characterize the stationarity of spectroscopic and micrometeorological data in the time domain and will be applied to assess the optimum high-pass filter time constant for detrending of time series data. The method to be described provides information similar to existing characterizations as the ogive analysis, the normalized error variance of the second order moment as well as information about the spectral characteristics of turbulence in the inertial subrange. The approach is straight forward, easy to implement and, therefore, well suited to assist as a useful tool for a routine data quality check for both, new practitioners and experts using today´s laser gas analyzers to measure trace gas fluxes in the field.
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