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Wind Resource Mapping for 

United States Offshore Areas 

Preprint 

D. Elliott and M. Schwartz 

To be presented at WindPower 2006 Conference 

Pittsburgh, Pennsylvania 

June 4–7, 2006 

Conference Paper 

NREL/CP-500-40045 

June 2006 
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Wind Resource Mapping for United States Offshore Areas1

Dennis Elliott and Marc Schwartz 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado, United States 

Background 

The United States appears to have vast offshore wind energy potential based on recent 

wind resource maps, measurement data from ocean and Great Lake buoys and automated 

measurement stations, plus estimates of 10-m wind speed and power derived from 

satellite instruments. However, the offshore wind resource areas on the maps produced by 

numerical models thus far have been extensions of the land-based resource estimates and 

not optimized for offshore areas. The U.S. Department of Energy’s (DOE) National 

Renewable Energy Laboratory (NREL) has started a program to produce validated wind 

resource maps for priority offshore regions of the United States. The maps will extend 

from the coastal areas to 50 nautical miles (nm) offshore and have a horizontal resolution 

of 0.2 km. Production of the maps is jointly funded by DOE/NREL, states, and other 

organizations. The maps are scheduled to be completed over a period of several years. 

The first set of regions to be mapped includes the Atlantic Coast from Florida to New 

England, the western Gulf of Mexico, and the Great Lakes. Figure 1 shows the current 

status of offshore mapping. The mapping of the offshore regions of Georgia, Texas, and 

Louisiana is underway. Mapping projects for northern New England and the Great Lakes 

are planned for the near future. 

The methodology that NREL will use to coordinate the development and validation of the 

offshore resource maps is similar to that employed in the validation of the updated landbased state wind resource maps (Elliott and Schwartz 2005). The preliminary offshore 

maps resource will be produced by a private company, AWS Truewind (AWST) based in 

Albany, New York, using its proprietary MesoMap system. AWST’s system uses a 

version of a numerical meso-scale weather prediction model as the basis for calculating 

the wind resource and important wind flow characteristics. An independent validation 

performed by NREL with assistance from regional collaborators will be the basis for 

modifying the preliminary model data and producing the final wind resource map. The 

validation effort will concentrate on producing the final maps of the wind speed and 

power data at 50 m above the surface. This level is considerably lower than the turbine 

hub-heights of modern offshore turbines (80 to 90+ m), but the paucity of 

 

1

 **This work has been authored by an employee of the Midwest Research Institute under Contract No. 

DE-AC36-99GO10337 with the U.S. Department of Energy. The United States Government retains and the 

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a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the publishd form of this 

work, or allow others to do so, for United States Goverment purposes. 

1 
Figure 1. Status of offshore wind-mapping areas 

offshore wind measurements above 10 m make extrapolation to 50 m difficult and 

estimation of the wind resource at turbine hub-heights problematic at best. NREL is also 

planning to estimate the offshore wind potential (installed capacity) in a more systematic 

manner than was done in the past. NREL will build a Geographic Information Systems 

(GIS) database to classify the offshore wind resource by state, water depth, distance from 

shore, and administrative unit. The wind resource data will be based on the available 

offshore wind resource maps. These include the updated regional offshore maps, 

offshore areas completed as part of land-based mapping, and estimates from the “Wind 

Energy Resource Atlas of the United States” (Elliott et al. 1987). The wind potential 

estimates will be updated as the new maps become available. 

Data Sets and Validation 

NREL obtains offshore and coastal meteorological station data for use in validation 

primarily from the National Climatic Data Center, the National Data Buoy Center, and 

some other sources. Coast Guard stations, lighthouses, anchored buoys, coastal marine 

automated network (CMAN) stations, and coastal airports are primary sources for wind 

measurements in coastal and offshore areas. Measurement heights are largely between 5 

m (most buoys) and 20 m. Measurements higher than 20 m are generally limited to some 

taller offshore platforms and CMAN stations. The scarcity of measurements from tall 

structures in most regions causes considerable uncertainty for the offshore map 

validations, especially for estimating the appropriate wind shear to use when 

extrapolating the lower measurements to 50 m and higher. 

2 
Complete validations of offshore mapping areas depend on using data sets that can 

supplement direct observations. Ocean wind speed estimates derived from satellite 

instruments are the most important of the supplemental data. The state of the ocean (or 

lake) surface is measured by a microwave scatterometer, and that information is 

converted by algorithms into an estimate of the 10-m wind speed. Satellite estimates 

have the advantage of having uniform data points (about 25 km for most data sets) and 

can show details of wind speed gradients that might be present. However, satellite 

measurements are more inaccurate closer to land because the state of the ocean surface is 

affected, in these areas, by more than wind speed. Local currents and ocean/coast 

topography effects on the ocean surface can result in misleading wind speed estimates. 

Precipitation and low clouds can cause degradation to the scatterometer signal and wind 

speed estimates. There are several satellite data sets available for use in the offshore 

validation. Average wind speeds for specific locations can differ up to 0.5 m/s or more 

among the data sets, so no one satellite data set will be relied on for the validation. Other 

remote sensing data that, in theory, could be useful for offshore validation, such as sodar, 

lidar, and radar wind measurements, have very limited availability and have not been 

evaluated for this purpose. The future integration of these data sets into NREL’s offshore 

validation remains an open question. Two other data sets – upper-air observations made 

by weather-balloon instruments and reanalysis upper-air data (derived by numerical 

models) – will be used in the offshore validation. The upper-air data sets will be used to 

identify large-scale wind characteristics in the mapped offshore areas and to evaluate the 

quality of the measurement data and satellite estimates. 

Georgia Offshore Wind Mapping 

The Georgia offshore area is the first region to be mapped under this program. This 

effort is jointly funded by the Georgia Environmental Facilities Authority and 

DOE/NREL. Figure 2 shows the preliminary estimates of the 90-m wind speed as 

produced by AWST’s MesoMap system. 

NREL, in collaboration with the Georgia Institute of Technology and the Skidaway 

Institute of Oceanography, is analyzing the available measurement data for the validation 

of the preliminary map estimates. 

Figure 3 shows the locations of the towers, buoys, Coast Guard stations, and coastal 

airports that will be part of the validation effort. Anemometer heights at the buoys are 

only 5 m, but several offshore stations in this region have anemometer heights above 30 

m: the Savannah Light Station (a CMAN station) and the four Navy Towers (part of the 

South Atlantic Bight Synoptic Offshore Observational Network – SABSOON). The 

anemometer height at Savannah Light is 33 m above the ocean, and the anemometer 

heights at the Navy Towers range from 34 m to 50 m. Pictures of two measurement 

stations – Savannah Light and Navy Tower R4 (50-m anemometer height) – are shown in 

Figure 4. 

NREL has started to analyze data from the measurement sites. Monthly and directional 

wind characteristic data from Savannah Light are presented in Figures 5 and 6, and the 

diurnal pattern is shown in Figure 7. The data indicate that Class 4 resource is present at 

the anemometer height level. Wind power peaks between October and March, with 

3 
October and February having slightly higher speeds and power than does the mid-winter 

period. Summer is the lowest wind resource season, though the average wind speeds are 

still greater than 6 m/s. The strongest winds at Savannah Light blow from the northeast 

at around 9 m/s. The overall frequency of northeast and south winds is about the same, 

but the speeds of the south winds are about 2 m/s lower than the northeast winds. 

Figure 2. Preliminary 90-m wind-speed map for Georgia offshore region. The solid 

dark lines are the current proposed boundaries for the Georgia offshore 

administrative area. The boundaries were developed by the Mineral Management 

Service of the U.S. Department of the Interior. 

4 
Figure 3. Wind-measurement locations in the Georgia offshore region. The solid 

dark lines are as described in Figure 2. 

5 
Figure 4. Pictures of the Savannah Light Station (left) and U.S. Navy R2 

Tower (right) 

Figure 5. Wind speed and power density by month for Savannah Light Station 

6 
Figure 6. Wind speed and frequency by direction for Savannah Light Station 

Figure 7. Wind speed by time of day for Savannah Light Station for 4 months 

and the annual average. Time of day is in Local Standard Time. 

7 
The highest wind speeds occur at night and the lowest at midday, a typical pattern for 

marine locations. The highest amplitude of the diurnal pattern is in summer because of 

the significant difference (2.5 m/s) between the midday and the evening-to-night wind 

speeds. The lower wind speeds during early morning to midday hours in summer are 

responsible for that season’s wind resource minimum. Highest average wind speeds in 

summer occur from about 4 p.m. to midnight and are comparable to average wind speeds 

during those hours in the other seasons. 

Similar analyses of data at other measurement locations and study of the satellite 

estimates and the upper-air data will be used to derive an appropriate wind shear to use to 

extrapolate measurement data to 50 m. The validation will also compare the 50-m wind 

speed and power from the model estimates to the extrapolated 50-m values from the 

measurement stations. The validation results will be used to revise the preliminary wind 

resource estimates and produce the final Georgia offshore map. 

Offshore Wind Potential 

An important parallel task to the offshore mapping activity is a systematic estimation of 

the offshore wind potential. NREL has developed a methodology to do this using a GIS 

database that will allow the offshore wind resource to be classified by a number of 

criteria, including wind power class, water depth, distance from shore, and offshore 

administrative unit. A variety of data sources will be used to construct the database. 

Wind power class values will be obtained from available offshore resource maps. At 

first, we will integrate data from offshore maps that were created by different methods 

and not strictly for offshore wind assessment. As the new offshore maps are completed, 

these data will be inserted into the database. Data produced by the National Oceanic and 

Atmospheric Administration (NOAA) and the Department of Interior’s Mineral 

Management Service (MMS) will be key components the database. NOAA’s information 

includes bathymetry and shorelines in a GIS format. The shoreline identification is an 

important baseline to calculate offshore distance. The MMS has created GIS files that 

identify important economic zones, such as the 3-nm and 6-nm zones from shore and 

proposed state jurisdiction boundaries. The MMS periodically updates its data, and we 

will use the latest version in our GIS database. 

Each offshore wind resource grid cell (0.2 km by 0.2 km) will be classified by GIS 

database element. The standard final products will be tables of the offshore wind 

resource classified by state and GIS data. The flexibility of the database will also 

accommodate special wind potential requests and projects. NREL will document and 

publish the relevant offshore potential assumptions and summaries. 

Conclusions 

NREL has started a program to produce and validate offshore wind-resource maps for 

many areas of the United States. The first mapping region is the Georgia offshore area. 

The western Gulf of Mexico off the coast of Texas and Louisiana is the next mapping 

region. NREL plans to map most of the Atlantic coast and the Great Lakes in the first 

phase of this project. The preliminary maps will be validated using many types of 

meteorological data. Offshore validation has more uncertainty than land-based validation 

8 
because of the scarcity of direct wind measurements higher than 20 m above the water 

surface. A parallel activity to the mapping is the systematic estimation of the offshore 

wind potential. A GIS database will enable the offshore resource to be classified by a 

number of attributes. Together, the mapping and the wind potential activities will impart 

offshore wind resource information to the wind energy community and help develop 

future projects. 

Acknowledgments 

We thank George Scott, Steve Haymes, and Donna Heimiller of the NREL wind resource 

assessment group for their support in data processing and GIS development. We also 

thank the staff of AWS Truewind for their part in producing the model data for offshore 

regions. This paper was written at NREL in support of DOE under contract number DEAC36-99-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, CO: Solar 

Energy Research Institute. 

Elliott, D.; Schwartz, M. (2005). Development and Validation of High-Resolution State 

Wind Resource Maps for the United States. NREL/TP-500-38127. Golden, CO: 

National Renewable Energy Laboratory. 

9 
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Wind Resource Mapping for United States Offshore Areas: Preprint 

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The U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) is producing validated wind 

resource maps for priority offshore regions of the United States. This report describes the methodology used to 

validate the maps and to build a Geographic Information Systems (GIS) database to classify the offshore wind 

resource by state, water depth, distance from shore, and administrative unit. 

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wind energy; offshore wind development; wind resources; wind resource mapping; geopgraphic information systems 

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