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The Eddy Covariance Diagnostic Display screen is a custom utility developed using Matlab GUI capabilities. It allows raw high-frequency data to be displayed on the screen, as it is captured by the computer in the field or as a tool for viewing historical data stored on CDROM. Viewing high-frequency data has been a key factor in maintaining data quality, since it allows rapid diagnosis of problems. The in-built Matlab zooming facility allows high resolution of small plot regions. A linked dialog display allows the gas analyser calibration coefficients to be toggled by the user, and a "delay" editbox can be used to adjust the time-shift between gas analyser and sonic signals caused by sample tube delay. Indicative flux calculations can be carried out on the selected dataset (bottom right).

Figure 1 High frequency CO₂ signal for a 30-minute period, 1230 hrs, Julian day 069, 2000 (see File name box). Calculated CO₂ flux (F_c) is -6.295 mmol/m²/s).

Figure 2 High frequency H₂O signals measured by the LICOR 6262 and KH2O for a 5-minute period, with latent heat fluxes displayed (lower right). This plot format detrends the absolute signals so that KH2O offset drift is eliminated, and the response of both instruments can be compared. A delay of 26 samples allows the KH2O (open-path) and LI6262 (closed path) signals to be overlaid, indicating a travel-time along the sample tube of approximately 2.6 s. When plotted at higher resolution, attenuation of the LI6262 signal becomes apparent due to some smearing along the tube, however the computed fluxes are unaffected.

Figure 3 High frequency sonic anemometer wind speed data for a 1-minute period. The vertical wind speed is w and the two horizontal windspeed components are u and v. Wind statistics are displayed at the base of the graphic. For eddy covariance it is normal to assume zero mean vertical windspeed (wbar).

Figure 4 High frequency CO₂ signal for a 30-minute period just after midnight, showing the sequence of automatic calibration for the LI6262 gas analyser. Calibration is initiated by a CR10X datalogger, which triggers solenoid valves to first send the "zero" gas (dry N₂, 0 ppm CO₂) to the intake end of the sample tube, where it follows the normal path to the LI6262. Secondly a "span" gas" (362 ppm CO₂) is triggered, and finally the normal flow of external air is returned. With the calibration dataset embedded in the raw high frequency data, the calculation of calibration coefficients is carried out at processing time, although the user may adjust the "field" coefficients using the diagnostic display combined with the zero/span GUI (shown here).

Figure 5 Uncorrected fluxes of CO₂ (F_c, mmol/m²/s), sensible heat (H) and latent heat (LE, using both the LI6262 (LI) and the KH2O (KH)) are computed at the end of every half-hour period and stored so that the time-series of fluxes for recent days may be examined during field visits or remotely using pcAnywhere. This plot shows fluxes for a 5-day period.

The next section describes some preliminary results of the research.