ABLE Eddy Correlation Systems

General Purpose

The ABLE Eddy Correlation Flux Measurement System provides in situ half-hour averages of the surface vertical fluxes of momentum, sensible heat, latent heat, and carbon dioxide. The fluxes are obtained by the eddy-correlation technique, i.e. by correlating the vertical wind component with the horizontal wind component, sonic temperature (which is approximately equal to the virtual temperature), water vapor density, and carbon dioxide density. A 3-dimensional sonic anemometer is used to obtain the orthogonal wind components and the sonic temperature. An open path H₂O/CO₂ infrared gas analyzer is used to obtain the water vapor and carbon dioxide densities. The ECOR system pictured above is located at Smileyberg, KS, and is part of the Ameriflux network. The portable ECOR system is presently installed at a wheat/sorghum field near Brainerd, KS.

Primary Quantities Measured with System

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.

Detailed Description

List of Components

3-D Sonic Anemometer, Gill Instruments Ltd. Omnidirectional Model R3
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 H₂O/CO₂ 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

Description of System Configuration and Measurement Methods

A permanent eddy correlation system is presently deployed at the Smileyberg, KS ABLE site and a portable eddy correlations system is installed near Brainerd, KS. The effective measurement heights of the eddy correlation sensors are 2.1 m at Smileyberg and 3.0 m at Brainerd. Gas analyzer outputs are accepted by the Gill sonic anemometer and are sampled at the same 10 Hz rate as the sonic measurements. Sampling synchronization is controlled by the Gill sonic anemometer. The Gill sonic anemometer outputs all sonic and gas analyzer measurements as a serial data stream; the data stream is sampled by a VME-based data processing unit. The VME unit performs all processing of the eddy correlation data, which is then stored for later retrieval by a remote UNIX workstation.

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.

Assessment of System Uncertainties for Primary Quantities Measured

Wind velocities, sonic temperature, water vapor density, and carbon dioxide density are measured by the sensors and digitally transmitted to the computer. The sensor accuracies specified by the manufacturers (see List of Components above) apply to the primary quantities measured. Air temperature, relative humidity, and barometric pressure measurement accuracies can be found in the documentation on the AWS; the computed fluxes have little sensitivity to errors in these measurements. Interferences may occur within the sound or light paths of the sensors, e.g. liquid or frozen water, soil, plant matter, and insects. Typically, these interferences cause a reduction or incease in the signal; if they occur with sufficient frequency or cause a large enough deviation from the mean, the flux data will be corrupted. Corruption of half hour values or a sensor malfunction can sometimes be determined from spectral analysis of the data .

Description of Observational Specifications

The specifications are given under Primary Quantities Measured with System and further discussed under Assessment of System Uncertainties for Primary Quantities Measured.

Theory of Operation

The Gill sonic anemometer uses three pairs of orthogonally oriented, ultrasonic transmit/receive transducers to measure the transit time of sound signals traveling between the transducer pairs. A pair of measurements are made along each axis ten times per second. The wind speed along each axis is determined from the difference in transit times. The sonic temperature is computed from the speed of sound, determined from the average transit time along the vertical axis.

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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