Section 7. Installation
Table 53.
WindVector() OutputOpt Options
Option
Description (WVc() is the Output Array)
4
WVc(1): Unit vector mean wind direction (Θ1)
WVc(2): Standard deviation of wind direction σ(Θu). This standard deviation is
calculated using Campbell Scientific's wind speed weighted algorithm. Use of the
resultant mean horizontal wind direction is not recommended for straight-line
Gaussian dispersion models, but may be used to model transport direction in a
variable-trajectory model.
7.9.22.2 Wind Vector Processing
WindVector()
uses a zero-wind-speed measurement when processing scalar wind
speed only. Because vectors require magnitude and direction, measurements at
zero wind speed are not used in vector speed or direction calculations. This
means, for example, that manually-computed hourly vector directions from 15
minute vector directions will not agree with CR1000-computed hourly vector
directions. Correct manual calculation of hourly vector direction from 15 minute
vector directions requires proper weighting of the 15 minute vector directions by
the number of valid (non-zero wind speed) wind direction samples.
Note
Cup anemometers typically have a mechanical offset which is added to each
measurement. A numeric offset is usually encoded in the CRBasic program to
compensate for the mechanical offset. When this is done, a measurement will
equal the offset only when wind speed is zero; consequently, additional code is
often included to zero the measurement when it equals the offset so that
WindVector()
can reject measurements when wind speed is zero.
Standard deviation can be processed one of two ways: 1) using every sample
taken during the data storage interval (enter
0
for the
Subinterval
parameter), or
2) by averaging standard deviations processed from shorter sub-intervals of the
data-storage interval. Averaging sub-interval standard deviations minimizes the
effects of meander under light wind conditions, and it provides more complete
information for periods of transition (see EPA publication "On-site
Meteorological Program Guidance for Regulatory Modeling Applications").
Standard deviation of horizontal wind fluctuations from sub-intervals is calculated
as follows:
where:
is the standard deviation over the data-storage interval, and
are sub-interval standard deviations. A sub-interval is
specified as a number of scans. The number of scans for a sub-interval is given
by:
Desired sub-interval (secs) / scan rate (secs)
For example, if the scan rate is 1 second and the data-output interval is 60
minutes, the standard deviation is calculated from all 3600 scans when the sub-
interval is 0. With a sub-interval of 900 scans (15 minutes) the standard deviation
is the average of the four sub-interval standard deviations. The last sub-interval is
weighted if it does not contain the specified number of scans.
297
Summary of Contents for CR1000
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