EVALUATING METEOROLOGICAL MONITORING SITES USING SIGMA-THETA

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EVALUATING METEOROLOGICAL MONITORING SITESUSING
SIGMA-THETA
Kenneth G. WastrackDoyle E. PittmanTENNESSEE
VALLEY AUTHORITY
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Background

• A 2002 NUMUG presentation discussed using
  Sigma-Theta to determine if trees and tower
  structures were impacting wind data.
• The Impact of Nearby Structures and Trees on
  Sigma Theta Measurements, by Tom Bellinger
• TVA used this approach to examine its
  meteorological monitoring sites.

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Methodology

• TVA sigma measurements.
• Sigma-Y (replaced in 1989).
• Average of 12 5-minute WD Standard Deviations
• Hourly Sigma-Theta.
• Sigma-Theta based on 720 WD readings.
• 15-minute Sigma-Theta.
• RMS of 4 15-minute Sigma-Thetas.
• 15-minute Sigma-Thetas based on 180 WD readings
  each.
• Annual average of 15-minute Sigma-Thetas.
• Did not include cases with WS lt 5 mph.
• Sorted into 10 degree WD sectors.
• Used 25 degrees as arbitrary cut-off in analysis.

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Watts Bar Nuclear Plant15-Minute Sigma-Theta for
wind speeds 5 mph and greater(6-year average,
1997-2002)

• 10-meter values are generally less than 25
  degrees.
• Values above 25 degrees are due to identifiable
  causes.

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Sequoyah Nuclear Plant15-Minute Sigma-Theta for
wind speeds 5 mph and greater(6-year average,
1997-2002)

• More cases with 10-meter values above 25 degrees.
• Values above 25 degrees still due to identifiable
  causes.

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Browns Ferry Nuclear Plant15-Minute Sigma-Theta
for wind speeds 5 mph and greater(6-year
average, 1997-2002)

• Some high cases, but most are below 25 degrees.
• Most values above 25 degrees due to identifiable
  causes.

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Browns Ferry Nuclear Plant15-Minute Sigma-Theta
for wind speeds 5 mph and greater(6-year
average, 1997-2002)
Why is there a dip in 10-meter Sigma-Theta? (Dip
is not consistent with obstructions.)
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Browns Ferry Nuclear Plant10-meter level -
15-Minute Sigma-Theta Recent History(wind speeds
5 mph and greater)

• Dip in Sigma-Theta is consistent over time.
• Dip is not due to a recent change.

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Browns Ferry Environs
0.5 mile
Browns Ferry Nuclear Plant
Meteorological Monitoring Site
Tennessee River (Wheeler Reservoir)
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Meteorological Monitoring Site
Warehouses Training Center
Trees
Target Range
Tennessee River (Wheeler Reservoir)
Except for a few isolated trees, everything meets
110 criteria.
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Browns Ferry Meteorological Monitoring
Site(looking south from tower)
Direction of Sigma-Theta Dip
Target Range Berm
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Effects of Changes

• Reason for dip in Sigma-Theta curve is not
  apparent.
• Need to examine changes.
• Has dip in curve always existed?
• If not, when did change occur?

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Browns Ferry Nuclear PlantHistory of Site
Changes Impacting Sigma-Theta Measurements

• March 1973 Start monitoring at permanent
  meteorological tower.
• October 1977 Clear trees to meet 110 height
  criteria.
• July 1989 Change sigma calculation methodology.
• April 1991 Complete new berms around target
  range adjacent to meteorological tower site.
• February 2000 Change orientation of wind sensor
  mounting arm (old into WNW, new into ENE).
• February 2001 Change to ultrasonic wind sensors
  (included slight changes in wind sensor
  elevations).

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1997-2002 Average 10-meter Sigma-Theta(displayed
on polar coordinates)
Portion that corresponds to dip in line chart
curve.
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Effects of Cutting Trees to Meet 110
Criteria(before trees cut, some high sigma
values)
Trees before cutting
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Effects of Cutting Trees to Meet 110
Criteria(after trees cut, sigma values much
improved)
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Effects of Building Firing Range Berm(before
berm is built, some high sigmas in SW quadrant)
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Effects of Building Firing Range Berm(after berm
is built, sigmas increase in SW quadrant)
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Change in Wind Sensor Orientation (generally no
change, dip is present both before and after)
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Preliminary Results

• 1977-78 Changes in Sigma-Y after trees are cut
  shows general improvement but does not indicate
  possible dip.
• 1989-93 Dip is not apparent both before and
  after target range berm was built.
• 1997-2002 Dip is present both before and after
  change in sensor orientation.

Therefore, Sigma-Theta curve changed between 1993
and 1997.
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Change in Sigma-Theta Curve (Dip not present in
1992-1993. Dip present in 1997-1999)
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Further Analysis

• Dip appears to be related to narrow band of trees
  adjacent to river.
• At 1993 NUMUG meeting, TVA presentation indicated
  that a temperature anomaly was partly due to the
  horizontal extent of a terrain feature.
• Identification and Implications of a Local
  Temperature Anomaly, by Ken Wastrack Norris
  Nielsen
• Therefore, TVA examined not only the presence of
  obstructions, but also their horizontal extent.

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Sigma-Theta and Distance from Tower (Sigma-Theta
scale on left, Distance scale on right)
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Sigma-Theta and Distance from Tower (Add actual
Sigma-Theta)
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Sigma-Theta and Distance from Tower (Add
reservoir, trees, and target range)
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Sigma-Theta and Distance from Tower (Show narrow
band of trees, Remove reservoir to simplify)

• Band of trees less than 100 meters wide.

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Sigma-Theta and Distance from Tower (Examine
portion of curve with dip)

• Dip is related to unobstructed narrow band of
  trees.

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Sigma-Theta and Distance from Tower (What would
curve look like without target range?)

• Expected Sigma-Theta estimated based on tree band
  width.

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Conclusions

• Dip in Sigma-Theta curve is due to an narrow band
  of trees unobstructed by the target range.
• Dip represents relatively unobstructed wind flow.

• Tree band is narrow behind target range, but
  Sigma-Theta values are greater from direction of
  target range.
• Even though it meets obstruction criteria, berm
  still impacts measurements.

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Remember Ruckert's Law.
There is nothing so small that it can't be blown
out of proportion.