Gap Filling Comparison Workshop, September 18-20, 2006, Jena, Germany Corinna Rebmann Olaf Kolle Max-Planck-Institute for Biogeochemistry Jena, Germany Eddy covariance measurements and their shortcomings for the determination of the net ecosystem exchange of carbon dioxide Outline Introduction of measurement site and advection experiment Reasons for data gaps Special features of open path analyser Consequences for final flux data Summary Measurement Site: Wetzstein, Thuringia, Germany, flux measurements established end of 2001 main tower tower C tower B tower D tower A measuring heights: Main tower: 30.0m Tower C: 29.4m ADVEX06 (April 11 June 19, 2006) flux measurements for , H, E, CO 2 CO 2, wind and temperature profiles The ADVEX Experiment Advection experiment CarboEurope-IP: 4 towers around the main tower: A, B, C, D: profiles of [CO 2 ], T, u, v, w, T, tower B with CO 2 -fluxes below canopy, tower C and main tower with CO 2 -fluxes above canopy 60m Why care about advection? Eddy covariance theory is derived from tracer conservation equation with many simplifications which are only valid under homogeneous conditions Flux Calculation All fluxes are calculated in the following steps: Calculation of planar fit planes (Wilzcak et al., 2001) comparison with 2D-rotation Determination of CO 2 - and H 2 O-lags for closed path analysers Determination of H 2 O-lag dependency on VPD modelling Determination of spectral correction according to Eugster & Senn (1995) for closed path analysers, Webb et al. (1980) correction for open path analysers Data gaps are due to Maintenance interruptions, power failures, ice coating Instrumental problems Non-turbulent conditions Unfavoured wind directions (tower effects, heterogeneous terrain) Precipitation, fog events (open path analyser) high wind speeds Wetzstein, main tower data gaps (closed-path analyser) Jan 1 Aug 24, 2006 Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 data gaps caused by maintenance, power failures etc. stepMain tower (TM) Tower C (TC) 1 (maintenance etc) 3.6%4.8% 2 (after pre-selection) 3 (after stationarity test 1) Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 time series of CO 2 -fluxes after pre-selection (eg Vickers & Mahrt 1997, JAOT14) Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 data gaps after pre-selection stepMain tower (TM) Tower C (TC) 1 (maintenance etc) 3.6%4.8% 2 (after pre-selection) 4.5%30.2% 3 (after stationarity test 1) 30.2% Wetzstein, main tower and tower C which data are rejected in case of open path-analyser? 24 of 624 half-hours (3.8%) rejected April 30 May 12, 2006, dry period Wetzstein, main tower and tower C which data are rejected in case of open path-analyser? 263 of 630 half-hours (41.7%) rejected!!! May 13 28, 2006, rainy period Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 consequences for dependencies on meteorological variables Michalis-Menten-relationship: see Falge et al. 2001, AFM107 NEE: net ecosystem exchange (mol CO 2 m 2 s 1 ) PPFD: photosynthetic photon flux density (mol quantum m 2 s 1 ) a: ecosystem quantum yield (mol CO 2 ) / (mol quantum) F GPP,sat : gross primary productivity at saturating light (mol CO 2 m 2 s 1 ) R day : ecosystem respiration during the day (mol CO 2 m 2 s 1 ) TMTC a F GPP,sat R day r2r Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 consequences for dependencies on meteorological variables Michalis-Menten-relationship: see Falge et al. 2001, AFM107 NEE: net ecosystem exchange (mol CO 2 m 2 s 1 ) PPFD: photosynthetic photon flux density (mol quantum m 2 s 1 ) a: ecosystem quantum yield (mol CO 2 ) / (mol quantum) F GPP,sat : gross primary productivity at saturating light (mol CO 2 m 2 s 1 ) R day : ecosystem respiration during the day (mol CO 2 m 2 s 1 ) TMTM, TC av TC a F GPP,sat R day r2r Wetzstein, main tower and tower C time series of CO 2 -fluxes with stationarity tests May 8 14, 2006 Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 When do instationaries occur? Instationarities occur mainly at low or zero radiation conditions Wetzstein, main tower and tower C Apr 11 Jun 19, 2006 data gaps summary stepMain tower (TM) Tower C (TC) 13.6%4.8% 2 (after pre-selection) 4.5%30.2% rainy, moist conditions 3 (after stationarity test 1) 9.8%33.2% Low radiation conditions Do we have perfect data now? Are these data reliable as input for gap filling procedures? Still missing: advective processes night flux treatment reliability check Hainich Drainage/advective fluxes Data from W. Kutsch Night-flux problem Weak turbulence Instrumental problems, large footprints, gravity waves Turbulent flux is influenced by other transport/storage processes Site dependent see eg: Lee, 1998 Aubinet et al, 2003, 2005 Staebler and Fitzjarrald, 2004 Feigenwinter et al, 2004 Night-flux corrections Empirical: Separate calm and turbulent periods, remove calm periods, fill the gap u*-criterion mostly used Aubinet et al. AER30, 2000 NEE night versus u* Wetzstein NEE 2005, unrealistic high night-time fluxes Wetzstein when do high fluxes occur? u*>0.4m s-1 wind direction between 200 and 280 or 30 and 40 neutral atmospheric conditions: stability parameter: