Campbell Scientific CR7 Measurement and Control System - Manual (Page 114)

SECTION 11. OUTPUT PROCESSING INSTRUCTIONS
11-2
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.
There are three Output Options, which specify
the values calculated.
Option 0:
Mean horizontal wind speed, S.
Unit vector mean wind direction,
θ1.
Standard deviation of wind direction,
σ(θ1).
Standard deviation is calculated using the
Yamartino algorithm. This option complies
with EPA guidelines for use with straight-
line Gaussian dispersion models to model
plume transport.
Option 1:
Mean horizontal wind speed, S.
Unit vector mean wind direction,
θ1.
Option 2:
Mean horizontal wind speed, S.
Resultant mean wind speed, U.
Resultant mean wind direction,
θu.
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.
Measured raw data:
Si = horizontal wind speed
θi = horizontal wind direction
Uei = east-west component of wind
Uni = north-south component of wind
N = number of samples
Calculations:
Scalar mean horizontal wind speed, S:
S=(
ΣSi)/N
where in the case of orthogonal sensors:
Si=(Uei2+Uni2)1/2
Unit vector mean wind direction,
θ1:
θ1=Arctan (Ux/Uy)
where
Ux=(
Σsin θi)/N
Uy=(
Σcos θi)/N
or, in the case of orthogonal sensors
Ux=(
Σ(Uei/Ui))/N
Uy=(
Σ(Uni/Ui))/N
where Ui=(Uei2+Uni2)1/2
Standard deviation of wind direction,
σ(θ1),
using Yamartino algorithm:
σ(θ1)=arc sin(ε)[1+0.1547 ε3]
where,
ε=[1-((Ux)2+(Uy)2)]1/2
and Ux and Uy are as defined above.
Resultant mean horizontal wind speed, U:
U=(Ue2+Un2)1/2
where for polar sensors:
Ue=(
ΣSi Sin θi)/N
Un=(
ΣSi Cos θi)/N
or, in the case of orthogonal sensors:
Ue=(
ΣUei)/N
Un=(
ΣUni)/N
Resultant mean wind direction,
θu:
θu=Arctan (Ue/Un)
Standard deviation of wind direction,
σ(θu),
using Campbell Scientific algorithm:
σ(θu)=81(1-U/S)1/2