Campbell Scientific CR6 CR6 Measurement and Control System - Page 299
Wind Vector Processing
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Section 7. Installation 7.9.21.2 Wind Vector Processing WindVector() uses a zero-wind-speed measurement when processing scalar wind speed only. Measurements at zero wind speed are not used in vector speed or direction calculations (vectors require magnitude and direction). This means, for example, that manually-computed hourly vector directions from 15 minute vector directions will not agree with CR6-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 datastorage 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: 7.9.21.2.1 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 subinterval 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. The EPA recommends hourly standard deviation of horizontal wind direction (sigma theta) be computed from four fifteen-minute sub-intervals. 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 299