The layout of wind turbines can have an impact on the power production of a wind farm. Design variables that define the layout of wind turbines within a wind farm include: number of turbines; turbine model and diameter; orientation of the rows with respect to the prevailing wind direction (PWD); size and shape of the wind farm; spacing between turbines; and alignment of the turbines (i.e., whether in-line or staggered with one another).
 There are no universal layout recommendations for offshore wind farms, partly because isolating the contribution of each individual design variable is impossible at existing wind farms, where the wind turbines cannot be moved or removed to isolate the effect of individual design variables, and partly because analyzing the sensitivity to design variables requires sophisticated and computer-intensive numerical codes, such as large-eddy simulation (LES). In LES, the large-scale, unsteady, turbulent motions are resolved explicitly, whereas the effects of small-scale motions, as opposed to the motions themselves, are modeled via a subgrid model [Pope, 2000]. LES has been applied successfully to study turbine and array wake losses in recent years [e.g., Calaf et al., 2010; 2011; Lu and Porté-Agel, 2011; Churchfield et al., 2012b], as reviewed recently by Archer et al. . However, only a few LES studies have analyzed the effect of array layout explicitly, although none in a real wind farm. Meyers and Meneveau  found that the optimal spacing in an idealized, infinite, nonstaggered wind farm was much higher than the spacing currently used in wind farms, although trade-offs were identified between the cost of the land and that of the wind turbine. Wu and Porté-Agel  studied the effect of aligned and staggered wind farm layouts with 30 miniature turbines in a small wind tunnel and found that the staggered configuration lead to higher local wind speed and turbine efficiency. Similarly, Stevens et al.  found an increase in power generation up to 40% by staggering wind turbines in an idealized wind farm.
 The National Renewable Energy Laboratory (NREL) developed the only publicly available and open-source LES code that is capable of resolving wind turbine blades as rotating actuator lines (not fixed disks) and does not rely on periodic boundary conditions. This code, named Simulator for Offshore/Onshore Wind Farm Applications (SOWFA; available at http://wind.nrel.gov/designcodes/simulators/sowfa/), has been used successfully in the past for turbulent wake simulations [Churchfield et al., 2010; 2012a; 2012b] and is the model of choice in this study.
 The SOWFA was used to simulate an existing offshore wind farm in Lillgrund (Sweden), consisting of 48 Siemens 2.3-megawatt (MW) turbines with the following specifications: rated power PR = 2300 kilowatts (kW), a diameter (D) of 93 m, a 63.4-m hub height, spacing of 3.2D across and 4.3D along the PWD, and no staggering [Dahlberg, 2009]. This spacing is exceptionally tight; to our knowledge, it is the tightest of all modern wind farms. The prevailing wind direction at Lillgrund is from the southwest [Bergstrom, 2009], just as it is in the summer along most of the U.S. east coast, where several offshore wind farms are planned. Thus, the results of this study are highly applicable to the U.S. While keeping the area and the shape of the farm constant, three critical design variables were varied: number of turbines (NT), turbine spacing (both along and across the PWD), and alignment (in-line or staggered for consecutive rows).
 The outcome of this study is a quantification of the advantages and disadvantages of various array layout choices. These estimates will be useful to the wind industry to optimize a wind project because the effects of alternative layouts can be quantified via array performance triangles with respect to total power, capacity factor, array losses, and number of wind turbines, all of which can ultimately be converted to actual costs or savings.