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Cross-spectral analysis on Net Ecosystem Exchange: Dominant timescale and correlations among key
ecosystem variables over the Ameriflux Harvard forest site
ATMO 529 class project
Koichi Sakaguchi
LTER photo galleryhttp://savanna.lternet.edu/gallery/albums.php
Objectives
1. Find major frequencies (in days ~ inter-annual time scale) that explains large fraction of the variance in Net Ecosystem Exchange (CO2 flux)
2. Find major frequencies in which NEE and another variable change together
These knowledge will be helpful to infer important variables and processes that control ecosystem-level carbon cycle
Data description• Time series measured at the Harvard forest, MA
-Temperate deciduous forest-Annual precipitation:756-1469 mm-Mean air temperature 6.46 °C
•Eddy covariance measurement-Covariance of vertical wind and temperature or mass concentration fluctuation = vertical fluxes-Daily average values from level 4 data for 10 years period (1992 ~ 2001)
Good summary by Baldocchi, 2003 in GCB
Data description•Net Ecosystem Exchange (net CO2 flux between the atmosphere and land surface)
http://www.atm.helsinki.fi/mikromet/
NEE = - (photosynthesis - plant respiration - soil respiration)
Sign convention : downward positive
Methods
1. Spectrum analysis on ecosystem CO2 flux (Net Ecosystem Exchange) time series
2. Cross-spectrum analysis on NEE and other variables: - Surface air temperature - Vapor pressure deficit - Latent heat flux - Sensible heat flux to find timescales of high correlations (Spectral
Coherence) with NEE
I’m having a trouble with this !!!
Data preparation1. Gap-filling: Gaps have to be filled for harmonic analysis (Discrete
Fourier transforms)(Stull, 1988)!
There are about 50 days of continuous gaps in daily average data in 1992. Mean values from other 9 years are placed on this period.
2. Hanning window :Box car window is for amateurs!
Fig. 6.15 in Hartmann’s note
- decrease distortion in power spectrum from the side lobes.
NEE time series
Mean: -0.53 gC/m2/daySTD: 3.1 gC/m2/dayLag-1 autocorrelation: 0.88
Harmonic Analysis
For a particular frequency k, Ck2 / 2
represents the fraction of the variance explained at that frequency
From Hartmann’s note
€
Ck2 = (Ak + iBk )(Ak − iBk ) = Ak
2 + Bk2
“Spectral Power”
By using Fourier series in least-squares fit, we have
Power spectrum: NEE
341 ~ 409 days
178 ~ 194 days
No spectral averaging or smoothing
Red line : Red noise spectrum
Dashed line: statistically significant threshold with 90 & 95 % confidence
~90 days period
Power spectrum: NEE
341 ~ 409 days
178 ~ 194 days
Top: linear scaleBottom: semi-log plot(x-axis in log scale)
No spectral averaging or smoothing
Red line : Red noise spectrum
Dashed line: statistically significant threshold with 90 & 95 % confidence
~90 days period NEE varies largely in annual & seasonal scale… not too exciting
Look at NEE anomaly
Subtract 10 years mean from each daily average value.Mean: 0 gC/m2/daySTD: 1.48 gC/m2/dayLag-1 autocorrelation: 0.51Large variation concentrated in growing season
NEE anomaly power spectrum
Again most of the variance is in lower frequency.Annual ~ seasonal time scale still dominate!
Focused on lower frequencies
Top: linear scale, no smoothing
Bottom: linear scale, smoothed by 5-points running mean
NEE anomaly explained variances
1.2%
340~510
180~220
98~10549~51
45~45.5
1.3% 0.8% 0.7% 0.2%
Significant time scale (period in days) with 95% confidence
% variance explained by each frequency range
Similar magnitudes with SSA analysis on coniferous forest in Germany (Mahecha et al., 2007)
Cross-spectrum analysisAnalyze the power spectrum of different variables together:See how they are related in different temporal scale (covariance explained at each frequency)
Explained covariance at a particular frequency, k is related to:
€
Cxy,k2
=Ax,k + iBx,k( ) Ay,k − iBy,k( )
2
€
Cx,k2 = (Ax,k + iBx,k )(Ax,k − iBx,k ) = Ax,k
2 + Bx,k2
For two variables x(t), y(t), and their spectral power
€
Cy,k2 = (Ay,k + iBy,k )(Ay,k − iBy,k ) = Ay,k
2 + By,k2
“Cross spectrum” between x and y
From Stull, 1988
Example: NEE & air T
Intensity of in-phase signal ~ covariance
~ 90°-out-of-phase- kind-of covariance
~ Correlation
Phase difference
WHY?
Example: NEE & LH
~ Covariance
~ 90°-out-of-phase- kind-of covariance
~ Correlation
Phase difference
Tool for comparing different variables and for statistical significance
Conclusion•Processes in annual (340 ~ 400 days) and half-annual (178~194 days) time scale controls most of the variance of NEE
•The variance of NEE anomaly are distributed more evenly, but still large fraction is associated with period greater than 20 days. Statistically significant periods are 340~510, 180~220, 98~105, and 49~51 days, together explains about 4% of the total variance of the anomaly.
•It is demonstrated that NEE and surface air T (and LH) anomaly seem to be correlated in annual, half-annual, and 50 days periods, but statistical significance analysis needs further understanding of the speaker on cross-spectral analysis.
Future work (in one week)
•Spectral coherence analysis of NEE with other variables (85%)
• Temporal correlation & cross-spectral analysis of observed NEE with modeled NEE by NCAR CLM3.5 (55%)
•Similar analysis on other ecosystems - arid grass-shrub land & tropical forests (40%)
•Temporal correlation & cross-spectral analysis of simulated NEE with other variables from model simulation (25%)
•SSA analysis on the NEE (5%)
The numbers in ( ) represents the probability of finishing before the write-up due date with 95% confidence.
Supplemental material AData for cross-spectrum analysis
General statistics:
Row time series
Anomaly
variable mean std varlag1 auto-correlation
linear correlation with NEE
explained variance of
NEE
NEE -0.528 3.106 9.643 0.876
airT 8.421 9.295 86.405 0.945 -0.661 0.437
VPD 0.825 0.344 0.118 0.800 0.067 0.005
LH 35.931 36.466 1329.800 0.777 -0.771 0.594
SH 32.860 38.580 1488.400 0.586 -0.037 0.001
variable mean std varlag1 auto-correlation
linear correlation with NEE
explained variance of
NEE
NEE 0.000 1.478 2.186 0.507
airT 0.000 3.938 86.405 0.722 0.039 0.002
VPD 0.000 0.328 0.108 0.801 0.097 0.010
LH 0.000 21.369 456.650 0.414 -0.385 0.148
SH 0.000 29.896 893.792 0.373 -0.136 0.019
ReferencesBaldocchi, D.D, 2003. Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present, and future. Global Change Biology,9, p479-492
Mahecha, M.D., M.Reichstein, H.Lange, N.Carvalhais, C.Bernhofer, T.Grunwald, D.Papale, and G.Seufert, 2007. Characterizing ecosystem-atmosphere interactions from short to interannual time scales. Biogeosciences, 4, p743-758.
Stull, R.B. An introduction to Boundary Layer Meteorology, 1988. Kluwer Academic Press, MA, USA.