Operations

Field site

The terrain at the research site was moderately flat. To the northwest, the direction of the prevailing wind, there was approximately 8 kilometers before any significant change of altitude. The ground had a patchy covering of senescent grasses with bare sand exposed in approximately 15% of the area. The vegetation was dominated by sagebrush, one to two meters high, scattered approximately 5 meters apart, and occupying approximately 20% of the area. Figures 3, 4, 5 and 6 show the location and configuration of the ASTER array. The array was established between the 800 meter arc road and the 1600 meter arc road of the Hanford diffusion grid, aligned along an axis bearing 65 degrees west of true north, corresponding to the direction of the prevailing wind.

Stations were marked at 50m, 100m, 175m, 250m, 500m, and 750m from the 400 meter-long SF6 release line. This line was placed 1 meter above the ground at right angles to the prevailing wind direction. It was initially planned to make flux measurements first at the 50m, 100m, 175m and 250m stations during unstable conditions and then at the 50m, 250m, 500m, and 750m stations during stable conditions. Subsequently it was decided to forego the reconfiguration and to maintain the original configuration throughout the entire deployment. Four flux stations were established at 50m, 100m, 175m, and 250m. Each station had a 10m tower equipped with a 3-D sonic anemometer, a fast temperature sensor and a fast response SF6 detector. In addition, at the 50m station there was a wind profile tower and at the 250m station there was a psychrometer profile tower and the radiation and soil heat flux array.

After the array was established, the orientations of the booms of the prop-vanes and of the sonic anemometers were determined using a theodolite set to true north by means of solar sighting and an ephemeris. This defined the true bearing of the prop-vanes as the sensor encoders were accurately aligned with the boom. However for the sonic anemometers the transducer alignment did not accurately correspond to that of the boom. Hence for the sonic anemometers this bearing was used as a nominal value and was later corrected to the true value by comparing the sonic anemometer and the prop-vane data. (see the section on Alignment)

Special deployment aspects

The FOOTPRINT92 deployment offered several special problems. Because of the distributed nature of the deployment, the power distribution and data gathering system had to be modified. The four flux stations each had to be supplied with power and connected by fiber optical cable to the ASTER base. The power was generated at the ASTER base by two 15 KVA diesel generators supplied, fueled and maintained by the Hanford Facility. The generators were refueled daily and sequentially shut down for maintenance every third day for a brief period. Prior to the shut down periods the ASTER power load had to be adjusted, bringing down nonessential computer resources and air conditioning. During the setup phase of the deployment, until the appropriate procedure was established, misbehavior of the Uninteruptable Power Supply led to considerable difficulties. The fiber-optic cable connection from the four flux stations also gave problems until the optical intensity was increased and the discrimination level decreased.

Operational period

The ASTER system was fully operational by 2 June and, for the duration of the field program, data was continuously acquired from all the ASTER-maintained sensors. For the first portion of the field study, during operations 1 and 2 (ops1 and ops2), the flux systems were positioned at a height of 10 meters. During the second portion of the field study (ops3) the flux systems were positioned at a height of 5 meters. The field program was continued until 1 July 1992.

Sensors

Table 1 lists the sensors deployed for FOOTPRINT92.

Sonic anemometers

From the beginning of the deployment until 22 June, jd174, the sonic anemometers operated at 10 m. From that day until the end of the deployment the sonic anemometers operated at 5 m.

The sonic anemometers required attention throughout the deployment. A radical problem to be noted involved the ATI devices. On 15 June, jd 167, the ATI model K at the 50m station, atiK.50.10, was found to be very noisy and the spare sensor, the older ATI model S, atiS.50.10 was substituted. This ATI model S also had problems over the next several days. On 19 June, jd 171, a transducer had to be replaced on the ATI model S and a vertical offset resulted. On 21 June, jd 173, the ATI model S was again replaced by the original ATI model K which had been returned to the manufacturer for repair. The ATI model K was then adjusted to obtain clean signals and a proper zero calibration. This sonic anemometer operated, with occasional problems, for the remainder of the project.

Fast thermometers

From the beginning of the deployment until 22 June, jd174, the fast thermometers operated at 10 m. From that day until the end of the deployment the fast thermometers operated at 5 m.

The fast thermometers performed well throughout the deployment

Krypton hygrometers

Campbell Scientific krypton hygrometers were operated with the flux arrays at the 50m and the 250m flux stations. They were calibrated prior to the deployment and performed well throughout the program.

Prop-vanes

The prop-vanes performed well throughout the deployment. On 6 June, jd 158, the nominal value for their alignment was replaced with the theodolite measured value.

Psychrometers

The psychrometers performed well throughout the deployment. However there was a slight persistent problem with the 3 m humidity value.

Barometers

In general the pressure sensor performed well throughout the deployment. On several occasions the barometer microprocessor went down and ceased to give an output.

Radiation sensors

The thermistor output of the temperature of the case of the down-looking long-wave radiometer, pyg.out.case, yielded suspicious temperature readings. Consequently, the output was ignored and the small correction which normally is applied to the outgoing infra-red radiation was neglected. In addition, gain settings for all four of the Eppley radiometers were corrected on 7June, jd 159.

Soil sensors

The soil temperature probes were connected to the data system via four channel multiplexer housed in a small weather-proof enclosure situated on the soil surface. The electronics associated with the multiplexer were not sufficiently temperature stable and partial function failure occurred several times, most frequently for the Tsand station and occasionally for the Tgrass station. The problem was alleviated by burying the multiplexer at a depth of 25 cm in the soil but the problem persisted when the day-time temperature was high.

SF6 sensors

From the beginning of the deployment until 22 June, jd174, the fast SF6 sensors operated at 10 m. From that day until the end of the deployment the fast SF6 sensors operated at 5 m.

The four fast SF6 sensors operated by WSU were housed in instrument shelters at the base of the towers. A 1/4" teflon tube extended to the instrument from the intake at the sonic anemometer. The time delays introduced by the intake tubing were determined prior to the measurements. The SF6 sensors were not operated continuously but only when the wind direction was expected to be along the axis of the array. During favorable conditions, the SF6 release line was operated and the WSU-maintained SF6 sensors were brought into operation. The SF6 sensor system automatically performed calibrations and zeros every 30 minutes. A total of 220 hours of SF6 release data were acquired. Table 6 summarizes the release period data indicating the 137 hours when the release was coincident with favorable north-west wind episodes.

Operational period

TABLE 2 Field program dates