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Impact of coordinate rotation on eddy covariance fluxes at complex sites

Rannik, Ullar and Vesala, Timo and Peltola, Olli and Novick, Kimberly A. and Aurela, Mika and Jarvi, Leena and Montagnani, Leonardo and Molder, Meelis and Peichl, Matthias and Pilegaard, Kim and Mammarella, Ivan (2020). Impact of coordinate rotation on eddy covariance fluxes at complex sites. Agricultural and Forest Meteorology. 287 , 107940 , 1-23
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Abstract

The choice of coordinate system to calculate eddy covariance fluxes becomes particularly relevant at complex measurement sites. The traditional way is to perform double rotation (DR) of the coordinate system i.e., to calculate turbulent fluxes in a coordinate system that is aligned with the flow streamlines within the flux averaging period (e.g., kaimal and Finnigan, 1994). The second approach, the so-called planar-fitted (PF) co-ordinate system, averages the flow over a longer period of time, in practice a month or more. The PF method allows to derive an intercept coefficient of the vertical wind speed which can be attributed to the offset of the sonic anemometer or the average vertical flow related to meteorological conditions. We evaluated the variants of the PF methods using data from a variety of sites ranging from complex urban and forest sites to nearly ideal forest and peatland sites. At complex sites, we found that the intercept of the vertical wind speed derived from the PF method is a function of wind direction, time of day and/or stability. The sector-wise PF (SPF) method frequently led to insignificant statistical relationships. We tested a continuous PF (CPF) method where the relationship establishing the coordinate frame was represented as the continuous function in the form of Fourier series. The method enabled to obtain the PF with lower uncertainty as compared to the SPF method, by selecting necessary number of harmonics for each site based on confidence intervals of estimated parameters. Therefore, we recommend to use the CPF method in cases when the number of observations in some wind direction interval is low or the obtained SPF is insignificant due to large variance in measurements. We also showed that significant systematic difference can exist in cumulative turbulent fluxes between the DR and PF methods over a longer period of time. Derived vertical advection of carbon dioxide exhibited large variability with wind direction due to topography at complex sites and therefore, without considering horizontal advection, cannot be used to improve the net ecosystem exchange estimation during nocturnal, low turbulence conditions.

Authors/Creators:Rannik, Ullar and Vesala, Timo and Peltola, Olli and Novick, Kimberly A. and Aurela, Mika and Jarvi, Leena and Montagnani, Leonardo and Molder, Meelis and Peichl, Matthias and Pilegaard, Kim and Mammarella, Ivan
Title:Impact of coordinate rotation on eddy covariance fluxes at complex sites
Year of publishing :2020
Volume:287
Article number:107940
Number of Pages:23
Publisher:Elsevier
ISSN:0168-1923
Language:English
Publication Type:Journal article
Article category:Scientific peer reviewed
Version:Published version
Copyright:Creative Commons: Attribution 4.0
Full Text Status:Public
Subjects:(A) Swedish standard research categories 2011 > 1 Natural sciences > 105 Earth and Related Environmental Sciences > Meteorology and Atmospheric Sciences
Keywords:Coordinate rotation, Planar fit, Eddy covariance, Stability, Complex sites
URN:NBN:urn:nbn:se:slu:epsilon-p-105687
Permanent URL:
http://urn.kb.se/resolve?urn=urn:nbn:se:slu:epsilon-p-105687
Additional ID:
Type of IDID
DOI10.1016/j.agrformet.2020.107940
Web of Science (WoS)000531095900013
ID Code:17094
Faculty:S - Faculty of Forest Sciences
Department:(S) > Dept. of Forest Ecology and Management
Deposited By: SLUpub Connector
Deposited On:11 Jun 2020 12:12
Metadata Last Modified:23 Jul 2020 10:06

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