ABLE Eddy Correlation Systems

The eddy correlation systems produce 30-minute averages of the surface vertical fluxes of momentum, sensible heat, latent heat, and carbon dioxide representative of the vegetation less than 200 hundred meters upwind of the system. The fluxes are computed from the following directly measured data.

Orthogonal components of the wind velocity, u, v, and w, are measured ten times per second in m s^{-1} by a sonic anemometer. Sonic temperature, which is approximately equal to virtual temperature, is determined ten times per second in degrees K by the sonic anemometer from the speed of sound. Horizontal wind speed is computed ten times per second from the vector sum of the horizontal, orthogonal winds. Water vapor density and carbon dioxide density are measured ten times per second by an open path infrared gas analyzer.

Note: all accuracies below are for a temperature range of 5 to 35 degrees C.

Orthogonal wind velocities u, v, and w

Range: +/- 30 m s^{-1}

Accuracy: +/-1% RMS for speed < 30 m s^{-1}, +/-2% for 30 m s^{-1}< speed < 60 m s^{-1}

Resolution: 0.01 m s^{-1}

Wind direction

Range: 0 to 360 deg

Accuracy: same as for speed

Resolution 0.01 deg

Sonic temperature

Range: -20 to +50 deg C

Accuracy: same as for speed

Resolution: 0.01 deg C

**LI-COR LI-7500 H _{2}O/CO_{2} Infrared Gas Analyzer
**

Water vapor density

Range: 0 to 42 g m^{-3}

Accuracy: +/-0.5 g m^{-3}

Resolution: 0.01 g m^{-3}

Carbon Dioxide Density

Range: 0 to 5148 mg m^{-3}

Accuracy: +/-0.5 mg m^{-3}

Resolution: 0.01 mg m^{-3}

The Gill sonic anemometer makes observations of the orthogonal wind velocities by measuring the travel time of sound with and against the wind and of the temperature by measuring the speed of sound. The infrared gas analyzer makes observations of the water vapor density and carbon dioxide density by measuring the absorption of an infrared light beam. The gas analyzer analog outputs are low-pass filtered to remove high frequency electronic noise.

Data analysis is performed by the VME for 30 minute periods. Vertical fluxes of momentum, sensible heat, latent heat, and carbon dioxide are determined using the eddy-correlation technique (see **Theory of Operation** below). Means, variances, and covariances of the input data are computed and three-dimensional coordinate rotations are applied. The coordinate rotations result in zero mean vertical and transverse wind speeds. AWS meteorological measurements are used to calculate mixing ratio, air density, specific heat of air at constant pressure, and latent heat of vaporization of water from the mean water vapor pressure, air temperature, and barometric pressure. The meteorological measurements and the rotated covariances are used to compute the vertical fluxes of momentum, sensible heat, latent heat, and carbon dioxide.

The infrared gas analyzer measures water vapor density and carbon dioxide density by detecting the absorption of infrared radiation by water vapor or carbon dioxide in the light path. Two infrared wavelength bands are used, centered on bands strongly absorbed by water vapor or carbon dioxide. The sonic anemometer samples the gas analyzer analog outputs ten times per second.

Half hour average ambient air temperature, water vapor pressure and barometric pressure are determined from AWS meteorological measurements and used in the calculations of the sensible and latent heat fluxes.

Flux and spectra data processing is accomplished with a VME-based computer. Half hour data files are stored on the hard disk and later retrieved over phone lines or via cell phone by a remote UNIX workstation.

Momentum flux is determined from the correlation between horizontal and vertical eddy velocities. The eddy velocities are departures from a characteristic mean. The appropriate period for this mean is a function of height and stability. Similarly, the vertical fluxes of sensible heat, latent heat, and carbon dioxide are determined directly from the correlation between departures of the vertical velocity and of temperature, water vapor, and carbon dioxide from their characteristic means.

The characteristic means are estimated from 200 second running means of the turbulent parameters (the 3 orthogonal components of the wind, the computed horizontal wind speed, sonic temperature, water vapor density, and carbon dioxide density). The running means are computed recursively and continuously updated. Data analysis includes computation of means, departures of the input data from their means for the analysis period, and variances and covariances of the departures of the data from the running means. Three dimensional coordinate rotations are applied to the variances and covariances. The rotations result in zero mean vertical and transverse wind speeds.

The mixing ratio, air density, specific heat of dry air at constant pressure, and the heat of vaporization of water are computed from average values of water vapor pressure, air temperature, and barometric pressure. These coefficients are used with the coordinate-rotated covariances to compute the friction velocity, sensible heat flux, latent heat flux, and carbon dioxide flux.

Data Processing and Analysis

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