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WINDPOWER 2008 | Houston, TX | June 1-4, 2008 | NREL/PO-500-43150 www.windpoweringamerica.gov

Wind Shear and Resources at Elevated Heights:

Indiana and Iowa Case Studies

Wind Shear and Resources at Elevated Heights:

Indiana and Iowa Case Studies

Dennis Elliott, Marc Schwartz, George Scott - Dennis Elliott, Marc Schwartz, George Scott - NREL

National Renewable Energy Laboratory

Carthage, IN – Low/mod roughness, 

prevailing strong winds from S-SW

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Background

• Considerable uncertainty exists in extrapolating wind 

resource information typically available from low 

levels (10–50 m) to turbine hub heights of 80–100 m

• Available wind resource maps for heights of 

80–100 m are largely unvalidated

• Tall-tower wind data are needed to examine the wind 

shear and make more accurate estimates over turbine 

rotor heights, which may extend well above 100 m

Iowa Analysis

• 3 very tall towers with highest anemometers 193–213 m

• Approximately 9 months data (Dec-Aug) at each tower

• Evaluate wind shear and resource variations by height

• Wind speeds of at least 3 m/s required for shear analysis

Objectives

• Analyze wind shear and resource characteristics at tall 

tower sites for some areas of the Great Lakes and 

Midwest 

• Show case studies and comparisons for:

- Indiana towers located in areas of various surface 

roughness

- Iowa towers with heights up to 200+m

Average shear exponents can vary 

from 1/7 to 0.25 or greater, causing 

considerable uncertainty in vertical 

extrapolations of wind resource.

Even if 50-m wind 

resource is known, 

variations in shear 

exponents cause 

considerable 

uncertainty in wind 

resource at heights 

of 80–100 m.

LaGrange, IN – High roughness, 

prevailing strong winds from S-SW

Measured shear exponent at 

Goodland, IN, is 0.23, with 

significantly higher wind resource 

at 90 m than might be estimated.

Geetinsville, IN – Mod/high roughness,

prevailing strong winds from S-SW

Goodland, IN – Low roughness,

prevailing strong winds from S-SW

Iowa Analysis Results

• Diurnal variations in average wind speed increase with height

• Differences in average wind speed among towers decreases 

with height

• Average shear exponent is highest in 100–150 m layer

• Nocturnal shear exponent decreases above 150 m Indiana Analysis

• 4 tall towers in areas with different categories 

of surface roughness

• Highest anemometers 90-100m 

• Approximately one year of data at each tower

• Evaluate wind shear variations by roughness 

category and height

• Wind speeds of at least 3 m/s required for 

shear analysis

Indiana Analysis Results

• Notable variations in average shear 

exponent with highest shear found at towers 

with highest roughness

• Roughness effects on shear extend well 

above 50m and are apparent in both daytime 

mixed and nighttime stable layers

• Differences in average wind speed among 

towers decreases with height

Conclusions

• Surface roughness effects on wind shear and resource at turbine hub heights can be 

significant

• Wind shear exponents at heights of 100–150 m can exceed those at heights of 

50–100 m

• Diurnal variations of wind speed increase with height above 50 m

• Average wind resource variability among stations decreases with height

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