SECTION 11. OUTPUT PROCESSING INSTRUCTIONS

*** 69 WIND VECTOR ***

FUNCTION

Instruction 69 processes the primary variables of wind speed and direction from either polar (wind speed and direction) or orthogonal (fixed East and North propellers) sensors. It uses the raw data to generate the mean wind speed, the mean wind vector magnitude, and the mean wind vector direction over an output interval. Two different calculations of wind vector direction (and standard deviation of wind vector direction) are available, one of which is weighted for wind speed.

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 in an example where a difficult to reach wind vane is improperly oriented and 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 instruction 89 can be used to check wind speed, and if less than the threshold, Instruction 30 can set the input location equal to 0.

Standard deviation can be processed one of two ways: 1) using every sample taken during the output period (enter 0 for parameter 2), 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 transition1.

Standard deviation of horizontal wind fluctuations from sub-intervals is calculated as follows:

σ(Θ)=[((σΘ1)2+(σΘ2)2 ...+( σΘM)2)/M]1/2

where σ(Θ) is the standard deviation over the output interval, and σΘ1 ... σΘM are sub-interval standard deviations.

PARAM.

DATA

 

 

NUMBER

TYPE

DESCRIPTION

01:

2

Repetitions

02:

4

Samples per sub-

 

 

interval (number of

 

 

scans, enter 0 for no

 

 

sub-interval)

03:

2

Sensor/Output 2 digits:

 

 

AB

 

 

 

A

Sensor type:

 

 

0

= Speed and

 

 

 

Direction

 

 

1

= East and North

 

 

B

Output option:

 

 

0

S, Θ1, σ(Θ1)

 

 

1

S, Θ1

 

 

2

S, U, Θu, σ(Θu)

04:

4

First wind speed input

 

 

location no. (East wind

 

 

speed)

05:

4

First wind direction

 

 

input location no.

 

 

(North wind speed)

Outputs Generated: 2-4 (depending on output option) for each repetition

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)

1EPA On-site Meteorological Program Guidance for Regulatory Modeling Applications.

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