Campbell CR5000, Руководство оператора

Страница: 123 / 238

Section 6. Data Table Declarations and Output Processing Instructions
6-29
When used with polar sensors, the instruction does a modulo divide by 360 on
wind direction, which allows the wind direction (in degrees) to be 0 to 360, 0
to 540, less than 0, or greater than 540. The ability to handle a negative
reading is useful where a difficult to reach wind vane is improperly oriented.
For example, a vane outputs 0 degrees at a true reading of 340 degrees. The
simplest solution is to enter an offset of -20 in the instruction measuring the
wind vane, which results in 0 to 360 degrees following the modulo divide.
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 output period (enter 0 for the Subinterval parameter), or 2)
by averaging standard deviations processed from shorter sub-intervals of the
output period. Averaging sub-interval standard deviations minimizes the
effects of meander under light wind conditions, and it provides more complete
information for periods of transition
1
.
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 output 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.
Measured raw data:
S
i
= horizontal wind speed
Θ
i
= horizontal wind direction
Ue
i
= east-west component of wind
Un
i
= north-south component of wind
N = number of samples
1
EPA On-site Meteorological Program Guidance for Regulatory Modeling
Applications.