Calculating turbulent fluxes for a system composed of an LI-7200/RS and LI-7700

In this situation, corrected fluxes are calculated from uncorrected (fluxes as F₀) with the following procedure:

Level 1 fluxes: application of spectral corrections

As a first step, uncorrected methane fluxes from the LI-7700 are corrected for spectral attenuation, as explained in Calculating Fluxes for Open Path Analyzers:

6‑106F_1,ch4 = F_0,ch4 · SCF_w,ch4

Momentum flux and friction velocity are also spectrally corrected at Level 1, similarly:

6‑107T₁ = T₀ · SCF_u,w

6‑108u_*1 = u_*0 · SCF_u,w

Fluxes from the LI-7200 are not spectrally corrected at this stage, because the application of correction for air density fluctuation effects to concentrations calculated with closed path systems requires using uncorrected fluxes.

Level 2 and 3 fluxes

Whenever possible, air density fluctuation effects in closed path systems are addressed by converting raw gas concentration measurements into mixing ratios (see Converting to Mixing Ratios). However, when this is not possible due to unavailability of necessary raw information, the correction is implemented following the WPL approach, as refined by Ibrom et al. (2007b). As a first step, evapotranspiration flux is corrected for the WPL term using the formulation proposed in Webb et al. (1980, eq. 42b), properly modified to account for the fact that the relevant sensible heat flux is measured inside the cell of the LI-7200 (and not in ambient conditions):

6‑109

where and are covariances of w and cell temperature pressure respectively, evaluated after dislodging the cell temperature time series using the time lag of CO₂. If covariance or are not available, they are assumed negligible and E₁ is corrected only for the dilution term due to water vapor:

6‑110E₂ = (1 + μσ)E₁

Fully corrected (level 3) evapotranspiration flux can now be calculated from E₂ by applying the proper spectral correction factor:

6‑111E₃ = E₂ · SCF_w,h2o

Fully corrected latent heat and water vapor flux can now be calculated:

6‑112F_3,h2o = E₃ · 10^-3·M_h2o

6‑113LE₃ = λ · E₃

Now, ambient sensible heat flux is corrected for humidity effects following van Dijk et al. (2004), revising Schotanus et al. (1983) as explained in Calculating Fluxes Level 1 2 and 3.

6‑114

Then it is spectrally corrected to get fully corrected sensible heat flux:

H₃ = H₂ · SCF_w,Ts

Next, level 2 CO₂ flux is calculated by applying the correction for air density fluctuations. EddyPro uses a formulation similar to Webb et al. (1980 eq. 24), where, however, sensible heat flux and latent heat flux are evaluated in the cell and using the time lag of CO₂:

6‑115

where E_cell and H_cell are calculated as:

6‑116

6‑117

and the subscript τ_co₂ indicates that covariances are evaluated at time lags of CO₂. The factor v_c / v_a is needed to compute ambient quantities from cell quantities.

Methane fluxes are evaluated similarly to in a manner similar to fluxes for the LI-7500 and LI-7700 analyzers:

6‑118

where now, the spectrally corrected, not-WPL-corrected evapotranspiration flux is calculated as:

6‑119

Finally, CO₂ fluxes are corrected for spectral attenuations:

6‑120