As it is described in the previous section, the NEE calculation requires accurate
carbon dioxide storage calculations (Fan et al., 1990) from the concentration
profiles. This is generally calculated using a finite difference approach of
term 1 in the right hand side of equation :
Usually, the profiles are calculated using simple linear interpolation between levels, where the concentration below the lowest level is considered to be constant (K. Davis, pers. comm.).
Similarity theory (see section ) provides a good framework
for the NEE calculations in another way. A more accurate storage calculation
is performed by fitting the concentration data to obtain the concentration profile
using an appropriate similarity function.
In our case the rate of change of the CO storage below the measuring
level is calculated as follows:
1. First, the carbon dioxide profile data are interpolated for each hourly boundary datum using a cubic spline function (Horváth and Práger, 1985) (concentration data are available generally in each 8-10 minutes, which is not neccessarily coincides with the beginning of the hours). As a result, two complete profiles are obtained for the beginning and for the end of a specific 60 min period.
2. A theoretical logarithmic CO profile is constructed for the two
profiles based on the closest available stability parameter (
), friction
velocity (
) and scalar scaling parameter (
) using eq.
.
3. Since the measured CO profile values are considered to be exact,
the theoretical logarithmic profiles are modified to fit the measured data.
Linear interpolation is used between 82 m and 48 m, since the deviation of the
linear profile from the logarithmic profile is slight here. Below 48 m, the
logarithmic profile is forced to fit the measured values at 48 m and 10 m. This
is accomplished by shifting and stretching the ideal profile to fit the data.
Comparison with the rate of change of storage calculated using the linear method shows small difference, but it is well known that slight systhematic errors may grow to cause considerable bias in the integrated net flux (Moncrieff et al., 1996).
As it was described in section , real mean vertical wind
speed is calcualted for our site as part of the wind vector rotation routine.
This value can be utilized to calculate the vertical mass flow term for each
hourly interval (Lee, 1998) as it is performed by e.g. Baldocchi et al. (2000).
However, it was found that this method should not be used for our site.
The reason is that during nighttime the 82 m level gets out of the inversion
layer and completely decouples from the ground. In such a situation, the onset
of a non-zero mean vertical velocity clearly can not advect the whole,
82 m thick air column, but only the upper part of it, which lies above the inversion
with considerably lower CO
concentration. This results in a clearly
different mass flow term compared to the one estimated from the 3rd term in
eq.
. In fact, inclusion of term 3 in the right hand side of eq.
into the calculation of NEE yields ecologically incorrect results:
it causes extremely high or low (depends on the day and hour) respiratory rates
during nighttime.
If the measuring level lies inside the inversion, the mass flow correction may
be more plausible, but the most recent investigations propose that the correction
must be treated with caution, as it was described in section (Finnigan,
1999; Yi et al., 2000). During daytime, when the lower boundary layer is well-mixed,
term 3 in eq.
0 because of the slight difference
between the CO
concentration at 82 m and the average CO
concentration of the air layer below 82 m.
Summarizing the above, NEE is calculated as the sum of the eddy flux measured
at 82 m plus the rate of change of CO storage below 82 m for the
large scale system (NEE=
). For the Japanese system at 3 m,
the eddy flux is considered to be equal to NEE because of the slight storage
change below. The interpretation of the flux data calculated by means of the
similarity theory is descibed in the next section.