Central California Ozone Study (CCOS) - Desert Research Institute

CCOS Field Operations Plan Version 2: 5/31/00
4.5.3 Sodars
Sodar performance audits depend, to some extent, on the type of sodar being employed.
For most sodars, the audit is conducted by comparing the sodar data with simulated winds from
an Acoustic Pulse Transponder (APT). Unlike conventional sensors where known wind speeds
and directions can be input directly to the sensor through various rotational methods, the acoustic
system relies on the measurement of time and frequency shift of the backscattered acoustic pulse.
The only means of truly providing a known input is through the introduction of fixed audio
frequencies at known times. The frequency shift will correspond to a simulated Doppler shift
introduced by winds to or from an antenna. The timing of the simulated return will represent a
known altitude based on the speed of sound. These simulations of the Doppler shifted signal will
be performed with an (APT). However, if the sodars are of the phased array variety, the audit is
conducted by comparing the sodar data with independent wind measurements. This comparison
verifies that the beam steering is appropriate by assessing the reasonableness of the data. The
collocated data will most likely be collected using an anemometer kite system.
All wind speed data will be evaluated in resultant vectors as well as converted to
components along the sodar antenna axes. This is consistent with the newest EPA guidance and
will help identify component related problems that may be missed by looking at the resultant
data alone.
As a final check of the sodar data, data collected during several days prior to the audit is
reviewed to establish the internal consistency of the values. As this is a qualitative check, there
are no fixed evaluation criteria. The goal is to evaluate the following:
• Data reliability or quality codes for consistency
• Measured vertical intensity values for detection of potential fixed echoes
• Vertical profile of the individual wind components for detection of potential fixed echoes
and consistency
• Vertical profile of the calculated vector winds for internal consistency
• Methods used to create hourly values from sub-hourly intervals
4.5.4 Surface Meteorological Measurements
Performance audit procedures and criteria are those recommended in the U.S. EPA
Handbook for Air Pollution Measurement Systems, Volume IV (EPA, 1995). The audit
standards used in the audits, audit standard accuracies and precision, as well as the audit criteria,
are detailed in Table 4.5-1.
Wind Speed. The wind speed audit begins with the inspection of the wind speed cups or
propeller(s) to ensure that they are intact. The cups are then removed to produce a zero point.
Next, the R.M. Young selectable speed anemometer drive is connected to the sensor shaft to
simulate wind speeds of approximately 5, 15 and 35 m/s. Actual values depend on the sensor
model and are determined by multiplying the motor speed by a cup or propeller transfer
coefficient supplied by the manufacturer. The data logger responses are compared to the
calculated actual values and the differences compared to the audit criteria.
Chapter 4: QUALITY ASSURANCE 4-16