ABLE Automatic Weather Station (AWS)

General Purpose

The Automatic Weather Station (AWS) mostly uses conventional in situ sensors to obtain one-minute and thirty-minute averages of surface wind speed, wind direction, air temperature, relative humidity, precipitation, and barometric pressure, the latter only at surface characterization sites and the central site.

Primary Quantities Measured with System

The AWS stations directly measure:

Wind speed at 10 m, Precision: 0.01 m/s; Uncertainty: +/-1% for 2.5 to 30 m/s, increases to +/-0.5 m/s when 0.5 m/s is reported.

Wind direction at 10 m, Precision: 0.1 deg; Uncertainty: +/-5 deg

Air temperature at 2 m, Precision: 0.01 C; Uncertainty: +/-0.7 C (This will eventually improve when an aspirated reference is obtained.)

Relative humidity at 2 m, Precision: 0.1% RH; Uncertainty: +/-2.06% RH (0% to 90% RH), +/-3.04% RH (90% to 100% RH)

Barometric pressure at 1 m, Precision: 0.01 kPa; Uncertainty: +/-0.035 kPa

Precipitation, Precision: 0.254 mm; Uncertainty: +/-0.254 mm (unknown during strong winds and for snow)

Detailed Description

List of Components

Wind speed and direction sensor: Propellor anemometer and wind vane, R. M. Young Model 05103 Wind Monitor

Temperature and relative humidity sensor: Thermistor and Vaisala RH, Campbell Scientific Model HMP45A Temperature and Relative Humidity Probe

Barometric pressure sensor: Digital barometer, Vaisala Model PTB201A

Precipitation: Electrically heated, tipping bucket precipitation gauge, MetOne Model 385 Rain/Snow Gage

Data logger: Campbell Scientific Model CR10X-1M Measurement & Control Module with 1 MByte memory; Precision: A function of input type and range, Uncertainty: 0.2% of Full Scale Range for Analog Inputs

Description of System Configuration and Measurement Methods

The AWS sensors are mounted on a 10 meter, triangular tower, except for the rain gauge.

The wind monitor propeller anemometer produces a magnetically controlled AC output whose frequency is proportional to the wind speed. The Wind Monitor direction vane drives a potentiometer, which is part of a resistance bridge. The Wind Monitor is mounted on a cross-arm at a height of 10 m.

The T-RH probe temperature sensor, a thermistor, is connected into a resistance bridge. The Vaisala RH circuitry produces a voltage that is proportional to the capacitance of a water vapor absorbing, thin polymer film. The T-RH probe is mounted in an R. M. Young Mode l43408 Gill Aspirated Radiation Shield. The Radiation Shield is mounted at a height of 2 m on the southern face of the tower.

The barometric pressure sensor uses a silicon capacitive pressure sensor and is housed in a weatherproof enclosure along with a data logger, a storage module, and serial communications equipment, all mounted on the tower at a height of 1 m.

The rain-snow gauge has a 12-inch orifice and is located near the tower. A thermostatically controled heater melts frozen precipitation. The water is funneled to a tipping bucket, which triggers a magnetic reed switch. An Alter Shield is used to increase the reliability of rain collection in high winds and of snow collection.

The data logger measures each input once per second except for barometric pressure, which is measured once per minute. The vapor pressure is computed from the air temperature and relative humidity. The data logger produces 1- and 30-min averages of wind speed, vector-averaged wind speed, vector-averaged wind direction, air temperature, relative humidity, and vapor pressure. The standard deviation of the wind direction is computed by an algorithm. The 1-min output includes the barometric pressure reading and total precipitation during the minute. The 30-min output includes the battery voltage and 30-min-total precipitation. The 30-min output also includes the standard deviations of wind speed, temperature, relative humidity, vapor pressure, and barometric pressure.

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Atmospheric Boundary Layer Experiments - Argonne National Laboratory, Argonne, IL 60439