Post project analysis of sonic anemometer data

The post-project analysis of the sonic anemometer data to correct for bias and tilt was carried out using the two functions: fun.sonic.bias.plot and fun.sonic.tilt.plot which were composed by T. Horst and subsequently modified by G. Maclean. These functions are available on request.

Specified values for parameters which were utilized in this analysis were:

prop.corr = 65.75 degrees for ops1

flagmax = 0.1 for ops1, 2, and 3

wmax = 1.0 ms-1 for ops1, 2, and 3.

(With the exception of ops2, atiS.50.10m, 172:174 when a value of wmax = 5.0 ms-1 was used.)

prop.corr is the correction applied to the geographic bearing of the prop-vanes. During ops1 prior to the theodolite sighting, a default bearing of 0 degrees = true north was used in the prop-vane microprocessor. For ops2 and ops3 the true bearing was incorporated in the microprocessor.
flagmax is an indication of the extent of sonic anemometer spiking. The sonic anemometer data is despiked using a statistical screening procedure derived by Jorgen Hostrup. (Hostrup, J., 1993, A statistical data screening procedure, Meas. Sci. Technol., 4, 153-157). Individual wind component values lying outside their predicted range are replaced by forecasted values. When the fraction of thus-corrected individual points within a five minute covar interval exceeds the value of flagmax, then the entire five minute covar period is rejected.
wmax is the maximum acceptable indicated vertical air motion offset.

Operations: ops1, ops2, and ops3

Three configuration were specified during the FOOTPRINT92 deployment.

ops1 from jday 154:157
- all sonics at 10m height on the four towers
  - atiK.50.10m, uw.100.10m, uw.175.10m, atiK.250.10m
- prop vane at 10 m at 50m station used to define the true wind direction.
- prop vane azimuth not adjusted and a prop vane correction of 65.75 degrees needed
ops2 from jday 157:174 (Although this was designated as a single ops, two different sonics were deployed on the 50m tower; the atiK from jday 157:163 and the atiS from jday 167:174)

N.B. atiS.50.10m underwent a radical change on jday 171 when a transducer failed and was replaced. The period 167:174 as been split into two periods, 167:170 and 172:174
- all sonics at 10m height on the four towers
  - atiK.50.10m, uw.100.10m, uw.175.10m, atiK.250.10m (jday 157:163)
  - atiS.50.10m, uw.100.10m, uw.175.10m, atiK.250.10m (jday 167:174)
- prop-vane at 10 m at 50m station used to define the true wind direction.
- prop-vane azimuth correctly entered in microprocessor and no prop vane correction needed
ops3 from jday 174:181
- all sonics at 5m height on the four towers
  - atiK.50.5m, uw.100.5m, uw.175.5m, atiK.250.10m
- prop-vane at 5 m at 50m station used to define the true wind direction.
- prop-vane azimuth correctly entered in microprocessor and no prop vane correction needed.

Alignment

These routines use linear regression.

For all sonic anemometers for each ops the sonic anemometer azimuths were determined with a theodolite. These values of sonic.azm are the nominal values. The first step in processing the sonic anemometer data was to determine the true values for sonic.azm. This was done by comparing the difference between the prop-vane and the sonic anemometer as the wind direction varied. The value of the function,

was plotted as a function of the assumed azimuth of the sonic. A minimum in the curve was assumed to correspond to the true azimuth of the sonic anemometer.These true sonic.azm's were then used as input to the bias program which plots u and v wind components as measured by the sonic anemometer and the prop-vane. The gains and intercepts of these lines are given as:

u.gain, v.gain, u.off, and v.off.

These gains and intercepts were then used as input to the tilt program, which calculates for each sonic for each ops the offset, pitch and roll.

Table 3 summarizes the sonic anemometer data for FOOTPRINT92.

where

sonic.azm is the true azimuth of the sonic anemometer.
prop.corr is the correction applied to the prop-vane azimuth.