BFN Sigmas1 EVALUATING METEOROLOGICAL MONITORING SITES USING SIGMA-THETA Kenneth G. Wastrack Doyle E. Pittman T ENNESSEE V ALLEY A UTHORITY

BFN Sigmas2 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.

BFN Sigmas3 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 < 5 mph. Sorted into 10 degree WD sectors. Used 25 degrees as arbitrary cut-off in analysis. Methodology

BFN Sigmas4 Watts Bar Nuclear Plant 15-Minute Sigma-Theta for wind speeds 5 mph and greater (6-year average, ) 10-meter values are generally less than 25 degrees. Values above 25 degrees are due to identifiable causes.

BFN Sigmas5 Sequoyah Nuclear Plant 15-Minute Sigma-Theta for wind speeds 5 mph and greater (6-year average, ) More cases with 10-meter values above 25 degrees. Values above 25 degrees still due to identifiable causes.

BFN Sigmas6 Browns Ferry Nuclear Plant 15-Minute Sigma-Theta for wind speeds 5 mph and greater (6-year average, ) Some high cases, but most are below 25 degrees. Most values above 25 degrees due to identifiable causes.

BFN Sigmas7 Browns Ferry Nuclear Plant 15-Minute Sigma-Theta for wind speeds 5 mph and greater (6-year average, ) Why is there a dip in 10-meter Sigma-Theta? (Dip is not consistent with obstructions.)

BFN Sigmas8 Browns Ferry Nuclear Plant 10-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.

BFN Sigmas9 Browns Ferry Environs Tennessee River (Wheeler Reservoir) Browns Ferry Nuclear Plant Meteorological Monitoring Site ~0.5 mile

BFN Sigmas10 Meteorological Monitoring Site Trees Warehouses & Training Center Tennessee River (Wheeler Reservoir) Target Range Except for a few isolated trees, everything meets 1:10 criteria.

BFN Sigmas11 Browns Ferry Meteorological Monitoring Site (looking south from tower) Target Range Berm Direction of Sigma-Theta Dip

BFN Sigmas12 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?

BFN Sigmas13 Browns Ferry Nuclear Plant History of Site Changes Impacting Sigma-Theta Measurements March 1973 – Start monitoring at permanent meteorological tower. October 1977 – Clear trees to meet 1:10 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).

BFN Sigmas Average 10-meter Sigma-Theta (displayed on polar coordinates) Portion that corresponds to dip in line chart curve.

BFN Sigmas15 Effects of Cutting Trees to Meet 1:10 Criteria (before trees cut, some high sigma values) Trees before cutting

BFN Sigmas16 Effects of Cutting Trees to Meet 1:10 Criteria (after trees cut, sigma values much improved)

BFN Sigmas17 Effects of Building Firing Range Berm (before berm is built, some high sigmas in SW quadrant)

BFN Sigmas18 Effects of Building Firing Range Berm (after berm is built, sigmas increase in SW quadrant)

BFN Sigmas19 Change in Wind Sensor Orientation (generally no change, dip is present both before and after)

BFN Sigmas20 Preliminary Results [ ] Changes in Sigma-Y after trees are cut shows general improvement but does not indicate possible dip. [ ] Dip is not apparent both before and after target range berm was built. [ ] Dip is present both before and after change in sensor orientation. Therefore, Sigma-Theta curve changed between 1993 and 1997.

BFN Sigmas21 Change in Sigma-Theta Curve (Dip not present in Dip present in )

BFN Sigmas22 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.

BFN Sigmas23 Sigma-Theta and Distance from Tower (Sigma-Theta scale on left, Distance scale on right)

BFN Sigmas24 Sigma-Theta and Distance from Tower (Add actual Sigma-Theta)

BFN Sigmas25 Sigma-Theta and Distance from Tower (Add reservoir, trees, and target range)

BFN Sigmas26 Sigma-Theta and Distance from Tower (Show narrow band of trees, Remove reservoir to simplify) Band of trees less than 100 meters wide.

BFN Sigmas27 Sigma-Theta and Distance from Tower (Examine portion of curve with dip) Dip is related to unobstructed narrow band of trees.

BFN Sigmas28 Sigma-Theta and Distance from Tower (What would curve look like without target range?) Expected Sigma-Theta estimated based on tree band width.

BFN Sigmas29 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.

BFN Sigmas30 Remember Ruckert's Law. There is nothing so small that it can't be blown out of proportion.