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Wind Shear Characteristics at 

Central Plains Tall Towers 

Preprint 

M. Schwartz and D. Elliott 

To be presented at the American Wind Energy Association 

WindPower 2006 Conference 

Pittsburgh, Pennsylvania 

June 4–7, 2006 

Conference Paper 

NREL/CP-500-40019 

June 2006 
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Wind Shear Characteristics at Central Plains Tall Towers 

Marc Schwartz and Dennis Elliott 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401, United States 

Background

The object of this study is to analyze wind shear characteristics at tall tower sites in the Central 

Plains of the United States. The hub heights of modern turbines used for wind farm projects are 

now 70 meters (m) to 100 m above ground and some advanced turbines under development for 

deployment during the second half of this decade are rated at 2–5 megawatts of energy generation 

with rotor diameters near 100 m and hub heights of 100–120 m. These advanced turbines will 

take advantage of the higher wind speeds aloft to generate more wind energy. Specific knowledge 

of important wind shear characteristics near and at turbine hub height is needed to optimize 

turbine design and wind farm layout. Unfortunately, wind speed shear measurements at heights 

of 80–120 m were virtually nonexistent a few years ago and are still quite uncommon today. The 

Central Plains is a prime wind energy development region and knowledge about the wind shear 

characteristics will reduce uncertainty about the resource and enhance wind farm design. 

Previous analyses of tall tower data (Schwartz and Elliott, 2005) concentrated on data from 

specific states. 

The wind energy community has recognized the need to fill the gap of direct wind speed 

measurements at levels 70 m and higher above the ground. Programs instituted during the last 5 

years at the state level and supported by the U.S. Department of Energy’s (DOE) State Energy 

Program initiative have placed anemometers and vanes at several levels on existing tall (70 m+) 

communication towers. The Central Plains has a fairly high concentration of tall tower sites. The 

distribution of tall tower sites varies among the states in the Central Plains, because the tall tower 

program is new and the available state and federal funding to establish tall towers is variable. 

Our wind resource assessment group at DOE’s National Renewable Energy Laboratory (NREL) 

has obtained much of these necessary measurement data from both individual state sources and 

regional organizations. Most of the data are available to the public, though data from one tower in 

Colorado are proprietary. We have begun to analyze important wind climate parameters, 

including wind shear from the tall towers. A total of 13 tall towers were used for this study. 

Eleven of the towers had the highest anemometer level between 100 m and 113 m. Two towers 

had the highest measurement level between 70 m and 85 m above ground. The distribution of the 

towers among the states is: two sites in Texas and Oklahoma; six sites in Kansas; and one site 

each in Colorado, South Dakota, and North Dakota. Figure 1 shows the locations and names of 

the thirteen towers. The wind resource at these sites can be classified as ranging from good - to - 

excellent. Eight tall tower sites have Class 3 resource, four sites have Class 4 resource, and one 

has Class 5 resource at 50 m. 

Technical Approach 

The first step for the study was to create “clean” data sets from each tall tower location. This step 

proved to be more problematic than was anticipated. as described later in this paper. The study 

uses the shear exponent α (alpha) from the power law equation (v/vo) = (z/zo)

α as defined in the 

Wind Energy Resource Atlas of the United States (Elliott et al., 1987) to describe the wind shear 

characteristics at the tall towers. Alpha values can vary widely from location to location but 

1
typical values range from 0.10 to 0.30. Alpha doesn’t provide a complete description of the shear 

characteristics (for example, turbulence intensity at different levels) but it does provide basic 

information about the shear and is easy to use in a comparative analysis. 

Figure 1. Locations of tall tower study stations in the Central Plains 

The analysis used wind speed data at each tower from levels at or near 50 m up to its highest 

anemometer for baseline shear information. Each tower had either two or three measurement 

levels for its baseline data. Wind shear characteristics were calculated by averaging all alpha 

values from individual measurements between levels with speeds of at least 3.0 meters per second 

(m/s) occurring at the same time. The criteria of using speeds of at least 3.0 m/s resulted in the 

analysis not using about 9% of the measured data from all of the towers combined. The amount 

of data not used at the individual towers ranged from 6% to 13%. The types of wind shear 

characteristics analyzed included annual average alpha values, the diurnal and seasonal variability 

2
of alpha, the shear variation by prevailing wind directions, and the variation of alpha by height. 

Unfortunately, because of the influence of the equipment on communication towers and perhaps 

the tower itself on wind measurements, we felt that the quality of data at many measurement 

levels was sufficiently compromised to preclude using alpha calculations for this study. 

Tall Tower Data and Tower Effects

The original intent for creating “clean” tall tower data sets was to use data from a set of 

anemometers on the same side of an individual tower to ensure consistency of data quality. The 

appropriate set of anemometers per tower would be judged by overall data quality, data recovery 

rates, and minimal tower flow interference. It was the last condition that proved to be 

troublesome. Upon close examination of the tall tower data, we discovered that tower effects on 

the quality of the data at certain measurement levels posed a greater challenge than anticipated. 

The analysis of wind speed and frequency by direction data from the tall towers provided the best 

method of identifying tower effects in this study. Standard deviation of wind speed by direction 

is another method of detecting tower effects, but we did not have this type of data for all towers. 

The most common pattern observed with possible tower effects was a “notch” in the average 

speed over a 10-or-20 degree segment of wind direction. Average wind speeds on either side of 

the notch are about the same, but the average speed in the notch can be lower by 0.5 m/s or 

greater. Sometimes the notch in speeds is matched by a notch in direction frequency. If a notch 

in wind speed at a particular 10-or-20 degree segment does not appear at all measurement levels, 

or the reduction of wind speed within the notch is different for the measurement levels, then 

tower effects caused by equipment on the tower and/or the tower itself could be compromising 

data quality. Alpha values are very sensitive to tower influence. For example, if the measured 

speed at 50 m is 7.0 m/s and the 80-m speed is 7.6 m/s, the alpha value is 0.175. But, if tower 

effects increase the speed uncertainty by just 0.1 m/s at either anemometer height, then the range 

of possible alpha values is 0.117 to 0.223. We judged the directly measured alpha values of the 

levels used for this study to be within a 0.05 range of accuracy. A measured 0.2 alpha value 

likely represents an envelope of values between 0.175 and 0.225. 

We concluded that eight of the 13 tall towers had at least one level affected by tower flow 

interference. This forced us to make subjective decisions to determine the least biased 

measurement levels to use in the analysis. This was not always easy because some of the tower 

effects appeared to be subtle and the influence on the measured speed hard to define. 

Nevertheless, trends in seasonal, diurnal, and wind direction shear variability at the tall towers 

could still be discerned despite the tower effects. In summary, tower effects tend to lower wind 

speed, are quite sensitive to wind direction, and do not necessarily affect all levels equally. We 

believe that, despite tower effects, the analysis provided good insights into the wind shear 

characteristics in the Central Plains. 

Analysis Results

Table 1 shows the 13 tall towers used in the study, the levels on each tower used for analysis (in 

bold), the periods-of-record, and the annual average alpha value for the study levels. We hoped 

that the highest anemometer level at each tower could be used in the analysis to calculate the 

shear statistics from 50 m to the top of the tower, but tower effects and other data quality issues 

limited the use of the highest measurement level on about half the towers. The periods-of-record 

at 12 of the 13 towers was approximately 2 years. While there is some uncertainty associated 

with the exact shear statistics based on these short periods-of-record, we are confident that the 

trends shown in the data would still be valid for longer measurement periods. 

3
Table 1. Names, anemometer heights, periods-of record, and annual average alpha values 

for tall tower stations. Anemometer heights in bold were used to calculate shear statistics. 

Site Name Anemometer Heights From To Shear (α) 

Lamar, CO (3) 52 113 10-05-2001 09-16-2003 0.150 

Ellsworth, KS 50 (80) 110 04-18-2003 09-02-2005 0.165 

Kearny, KS 50 80 (110) 04-29-2003 09-02-2005 0.138 

Sumner, KS 50 80 (110) 06-11-2003 09-02-2005 0.254 

Jewell, KS 50 (80) 110 04-23-2003 09-04-2005 0.206 

Ness, KS 50 (80) 110 06-04-2003 09-03-2005 0.223 

Logan, KS 50 80 (110) 05-01-2003 09-03-2005 0.179 

Hobart, OK 40 70 (100) 04-01-2002 12-31-2003 0.195 

Elk City, OK (10) 40 70 (100) 10-30-2003 08-31-2005 0.227 

Sweetwater, TX 50 (75) 100 05-17-2003 03-02-2005 0.220 

Washburn, TX 50 75 (100) 09-05-2003 10-03-2005 0.170 

Crow Lake, SD 50 70 12-26-2001 12-31-2005 0.209 

W. Finley, ND (10, 41) 56 85 08-07-2003 04-30-2005 0.200 

Annual Average

The annual alpha values for the levels used at the 13 towers range from 0.138 to 0.254. We have a 

suspicion that the 0.138 alpha at Kearny may be too low because of possible tower effects. 

Nonetheless, even accepting the 0.138 value at Kearny, 12 of the 13 stations had alpha values 

above 0.143 (1/7), a shear value often used to extrapolate measured data at 50 m to the hub 

heights of modern turbines. It appears that for the Central Plains, the 1/7 shear assumption is too 

conservative and that the wind speed can increase with an exponent as high as 0.2 or 0.25. The 

shear pattern was quite interesting at the six Kansas towers. The highest measured wind speeds at 

50 m were associated with the lowest shears, and towers with the lowest wind speed had the 

highest shears. This indicates that the wind speed tends to become more uniform in Kansas as 

low as 110 m above the ground. Also, data from 50-m, 80 m, and 110-m levels in the state show 

there is relatively little change of shear between 50-80 m and 80-100 m. There was insufficient 

data to come to any conclusions about the variation of shear with height in other parts of the 

Central Plains. 

Diurnal Variability

Figure 2 shows the annual diurnal pattern of alpha values for four of the tall tower stations. 

These stations, which cover the Plains from Texas to South Dakota, show the diurnal variability 

that is typical for all 13 towers. The alpha values are lowest (near or just below 0.10) during the 

middle of the day and highest at night. Sumner, Kansas, is noteworthy because its high annual 

average alpha value of 0.254 is produced primarily by the extremely strong annual nighttime 

shear that is close to 0.40 for several hours. The other stations exhibit less extreme nighttime 

shear values but are still between 0.25 and 0.30. There are some notable seasonal diurnal pattern 

differences among the stations. Figures 3 and 4 show the diurnal shear pattern at Lamar, 

Colorado, for the 52-113-m levels, and Washburn, Texas, for the 50-75-m levels for 4 individual 

months, as well as the annual diurnal pattern. 

4
Figure 2. Annual diurnal wind shear exponent pattern for four tall tower stations. Annual 

average alpha values are in parentheses. 

Figure 3. Diurnal wind shear exponent pattern at Lamar, CO, for 52-113-m levels. 

5
Figure 4. Diurnal wind shear exponent pattern at Washburn, TX, for 50-75-m levels.

The amplitude of the diurnal pattern in July at Lamar is quite large. The daytime alpha values are 

below 0.1, while the nighttime values peak at 0.4. In contrast, Washburn, Texas, has little 

seasonal variation of its nighttime alpha values between 0.25 and 0.30. Another station at Crow 

Lake, South Dakota (not shown), has a moderate spread of its nighttime shear. January values are 

about 0.2 while April and July values are 03.-0.35. 

Seasonal Variability

The seasonal variability of alpha values at the 13 towers can be grouped into two patterns. 

Stations in northwest Texas, Oklahoma, and Kansas (nine stations) exhibited a flat seasonal 

pattern of alpha values. There was a slight tendency for maximum values to occur in early 

autumn and lowest values in late winter, but this tendency could have been due to the short 

periods-of-record at these sites. The other four towers in central Texas, Colorado, and the 

Dakotas showed a distinct maximum of alpha values from July to October and a minimum of 

shear from January through March or April. Figure 5 shows the pattern for three of the four 

tower locations. The pattern on the figure is based on an approximate 5-week running average of 

alpha values. 

6
Figure 5. Season variability of alpha values for Sweetwater, W. Finley, and Lamar. The 

tick marks represent the first day of the month shown on the bottom of the figure. Annual 

average alpha values are in parentheses. 

Shear Variability by Wind Direction

Figure 6 shows the prevailing wind directions in the Central Plains. The prevailing wind 

directions have a southerly and northerly component. The southerly winds prevail during the 

warm season and the north winds prevail in the cool season. In addition, westerly winds in the 

cool season play a significant role in the wind climate of eastern Colorado. The southerly winds 

predominate in the southern Central Plains. The northerly winds occur a bit more frequently than 

the southerly winds in the Dakotas. 

The shear variability by wind direction was similar throughout the Central Plains from Texas to 

North Dakota. Figure 7 shows the wind shear variability by direction for five tall towers. The 

shaded areas represent the prevailing wind directions from the south and north. The curves on the 

graph are a 30 degree running average of alpha values. The southerly winds at all towers had 

alpha values between 0.2 and 0.3, with most values in the prevailing wind direction around 0.25. 

In contrast, the alpha values for the northerly winds were much lower, averaging between 0.1 and 

0.2. The high alpha value at Crow Lake, South Dakota for north-northeast winds (20 degrees) 

reflects only a few observations, the prevailing direction there is from the northwest (320-330 

degrees). The west winds at Lamar, Colorado, also had low alpha values between 0.1 and 0.15. 

Summary and Conclusions 

A study of alpha values at tall towers in the Central Plains revealed some distinct features of wind 

shear climatology. The annual average wind shear is generally between 0.15 and 0.25, greater 

than the 1/7 power law alpha value of 0.143. There was greater variation of shear between towers 

within a region (exemplified by Kansas) than there was between the southern and northern 

7
Figure 6. Prevailing wind directions in the Central Plains. Red arrows represent wind 

directions in the warm season. Blue arrows represent the wind directions in the cool season. 

Central Plains. Limited data from Kansas also indicated that the alpha value did not change much 

with height. 

The diurnal shear pattern was similar throughout the region. Daytime alpha values were 

generally near or slightly below 0.1, and nighttime values were between 0.25 and 0.4. There was 

some variation in the seasonal diurnal pattern among the towers with Lamar, Colorado, exhibiting 

tremendous variation in July, while Washburn, Texas, showed little seasonal variation in its 

pattern. 

8
Figure 7. Alpha values by wind direction for five tall towers. The shaded areas represent 

the prevailing wind directions in the Central Plains. Annual average alpha values are in 

parentheses. 

Winds from the south exhibited higher shear than winds from the north throughout the Central 

Plains. The alpha value for southerly winds was between 0.2 and 0.3, while northerly winds had 

values from 0.1 to 0.2. 

The seasonal variability of alpha values was grouped into two distinct patterns. Towers in west 

Texas, Oklahoma, and Kansas had a flat seasonal pattern with no distinct change of alpha values. 

Stations in central Texas, Colorado, and the Dakotas had a distinct maximum of alpha from July 

to October and minimum values from January through March and April. One hypothesis for this 

difference in patterns is that the area of west Texas, Oklahoma, and Kansas is subject to more

frequent high-shear southerly winds in winter than areas to their south and north because of the 

development of surface low pressure areas to the lee of the central Rocky Mountains. 

A major problem to overcome when collecting and analyzing tall communication tower data is 

wind flow interference caused by equipment on the structure and the tower itself. Both elements 

can affect the wind flow and raise the uncertainty of the measurement data. Organizations that 

collect tall tower measurements on communication towers should attempt to minimize tower 

effects, but it is unlikely that all effects can be eliminated. Therefore, it is important not to take 

wind shear information at face value but to carefully analyze and identify tower effects. This is 

not always an easy task. We believe that accurate wind direction data are essential for identifying 

tower effects. In this study however, only about 60% of the measurement levels across the 13 

towers had high-quality direction data. Nevertheless, we believe tall tower data will continue to 

be an important source of wind speed and shear data for levels near and above turbine hubheights across the United States. 

9
Acknowledgments 

We would like to thank George Scott and Steve Haymes of the NREL wind resource assessment 

group. George for his tireless efforts to process, clean, and manipulate the tall tower data in 

support of this analysis, and Steve for his production of the maps used in this paper. We would 

also like to thank the numerous organizations that have operated, collected, and distributed data 

from the tall towers, sometimes under trying conditions. This paper was written at NREL in 

support of DOE under contract number DE-AC36-99-GO10337. 

References 

Elliott, D.; Holliday, C.; Barchet, W.; Foote, H,; Sandusky, W. (1987). Wind Energy 

Resource Atlas of the United States. DOE/CH 10093-4, Golden, Colorado: Solar Energy 

Research Institute, 210 pp. 

Schwartz, M.; Elliott, D. (2005). Towards a Wind Energy Climatology at Advanced 

Turbine Hub-Heights. NREL/CP-500-38109. Golden, Colorado: NREL, 8 pp.

10
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