Specifications

Section 11. Programming Resource Library
11-28
11.5.2 Wind Vector Processing
CR1000 WindVector instruction processes wind speed and direction from
either polar (wind speed and direction) or orthogonal (fixed East and North
propellers) sensors. It uses raw data to generate mean wind speed, mean wind
vector magnitude, and mean wind vector direction over a data storage interval.
Two different calculations of vector direction (and standard deviation of vector
direction) are available, one of which is weighted for wind speed.
When a wind speed sample is 0, the instruction uses 0 to process scalar or
resultant vector wind speed and standard deviation, but the sample is not used
in the computation of wind direction. The user may not want a sample less
than the sensor threshold used in the standard deviation. If this is the case,
write the datalogger program to check wind speed, and if it is less than the
threshold set the wind speed variable equal to 0 prior to calling the data table.
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
3
.
Standard deviation of horizontal wind fluctuations from sub-intervals is
calculated as follows:
σ(Θ)=[((σΘ
1
)
2
+(σΘ
2
)
2
...+( σΘ
M
)
2
)/M]
1/2
where σ(Θ) is the standard deviation over the data storage interval, and σΘ
1
...
σΘ
M
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 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.
3
EPA On-site Meteorological Program Guidance for Regulatory Modeling
Applications.