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Eddy covariance measurements in complex terrain with a new fast response, closed-path analyzer: spectral characteristics and cross-system comparisons
Citation:
Novick, K., Johnt Walker, S. Chan, C. Sobek, AND J. Vose. Eddy covariance measurements in complex terrain with a new fast response, closed-path analyzer: spectral characteristics and cross-system comparisons. AGRICULTURAL AND FOREST METEOROLOGY. Elsevier Science Ltd, New York, NY, 181:17-32, (2013).
Impact/Purpose:
The purpose of this study is to assess the performance characteristics of a new infrared gas analyzer for measurement of carbon dioxide and water vapor atmosphere-biosphere exchange in mountainous terrain.
Description:
In recent years, a new class of enclosed, closed-path gas analyzers suitable for eddy covariance applications has come to market, designed to combine the advantages of traditional closed-path systems (small density corrections, good performance in poor weather) and open-path systems (good spectral response, low power requirements), and to permit estimates of instantaneous gas mixing ratio. Here, we provide an independent assessment of the extent to which these advantages are realized in field deployment, and discuss the suitability of such an analyzer (the EC155, manufactured by Campbell Scientific) for long-term flux measurements in complex terrain. We show that the cospectra for CO2 fluxes measured with the EC155 are similar to those measured with a co-located open-path system. Attenuation of the EC155 water vapor fluxes at high frequencies is observed, though results from an ogive analysis suggest that eddies operating on these time scales contribute only a small fraction of the total turbulent flux (< 3%). Inertial sub-range decay for all mass and energy generally conform to a -7/3 power law rule during near-neutral atmospheric conditions, supporting the use of an analytical spectral correction approach to the raw measured fluxes. We show that EC155 fluxes computed directly from instantaneous mixing ratio agree well will those calculated from mass density concentration measurements, provided density corrections for temperature, water vapor, and pressure are applied. Biases were observed when the EC155 flux records were compared to those measured with the open-path system. We show that some of these differences are related to wind angle of attack and instrument self-heating, and that these biases are minimized after the application of a friction velocity filter. Finally, we show that the EC155 considerably outperforms open-path analyzers during adverse weather conditions characterized by fog events.
