1 Introduction

Broadly speaking, atmospheric stability is a measure of the potential for vertical motion in the atmosphere. An atmospheric layer is considered stable when it is stratified, vertical motion is suppressed, and the air is cooled from the bottom up (i.e., the surface heat flux is negative, like at night over land). Conversely, in an unstable layer vertical motion is enhanced (upward or downward), the air is heated from the bottom up (i.e., the surface heat flux is positive, like in the daytime over land), and the layer is mixed or overturned by eddies. Lastly, a layer is neutral when it is adiabatic (i.e., with no net heat exchanges), vertical motion is neither enhanced nor suppressed, and no convection is present [Stull, 1988]. Static stability is the most commonly used measure of atmospheric stability that is not dependent on wind. The atmosphere is defined as statically stable, unstable, or neutral when the vertical gradient of potential temperature (or virtual potential temperature when moisture is nonnegligible) is positive, negative, or zero, respectively [Arya, 1988; Jacobson, 2005]. However, static stability is not sufficient to characterize atmospheric stability properly, especially in convective mixed layers and in the presence of shear‐generated turbulence [Stull, 1988]. Dynamic (i.e., related to wind) information is needed in such cases, in the form of wind shear or turbulent heat and momentum fluxes. The most used parameters are the Richardson number (Ri) and the Obukhov length (L), both of which include static and dynamic terms like turbulent kinetic energy (TKE) production through buoyancy and shear [Stull, 1988]. More details on methods to measure atmospheric stability are presented in section 3.

In the context of wind power, atmospheric stability is important because it impacts the vertical distribution of momentum (e.g., wind shear) and other thermodynamic variables in the atmospheric boundary layer (ABL) and thus affects hub height wind speed, which ultimately determines wind power production. In addition, different stability conditions (stable, unstable, or neutral) are associated with variations in magnitude and distribution of turbulence intensity, thus leading to stresses and fatigue loads on the wind turbines [Eggers et al., 2003; Guo et al., 2015]. Lastly, under stable conditions low‐level jets can form close to the hub height of wind turbines and cause extreme, damaging loads on the wind turbines [Banta et al., 2008; Nunalee and Basu, 2013]. If numerical weather prediction models are used for wind power forecasts, or for long‐term wind resource assessment, or for wind trend analyses, then properly capturing the atmospheric stability conditions is paramount.

At inland wind farms, wind shear and turbulence intensity, which are the two most relevant properties affected by atmospheric stability, have been studied extensively [Eggers et al., 2003; Wagner et al., 2009; Antoniou et al., 2010], but the effect of atmospheric stability, which encompasses both wind shear and turbulence, on wind power generation is still a relatively new topic [Sumner and Masson, 2006; van den Berg, 2008], with contradictory net impacts. For example, Wharton and Lundquist [2012a, 2012b] found that up to 15% more power is generated under stable than unstable conditions for the same hub height wind speed at an inland West Coast North American wind farm, while Vanderwende and Lundquist [2012] found instead higher power production during unstable rather than stable conditions at a wind farm located in the High Plains of central North America.

More studies have been published about stability conditions at offshore research sites or offshore wind farms than inland, but only in Europe [Van Wijk et al., 1990; Barthelmie, 1999; Lange et al., 2004; Barthelmie et al., 2005; Sathe et al., 2011]. Motta et al. [2005] showed that neutral stability, an assumption often used in the absence of stability data [International Electrotechnical Commission, 2009], is not necessarily the dominant condition at offshore wind sites in Denmark. They introduced revised stability corrections to the conventional logarithmic profile to better estimate the vertical distribution of wind speed. Similarly, Kettle [2014] found that nonlogarithmic wind speed profiles, defined as nonmonotonic vertical wind speed profiles with one or more inflection points, are common (~75% of the cases) at the FINO1 offshore research platform in the southern North Sea. Atmospheric stability also impacts wake recovery and wind speed/power deficit; higher deficits were found to be often associated with stable conditions at the Horns Rev offshore wind farm [Hansen et al., 2012]. The offshore boundary layer is further complicated by the presence of waves (via the so‐called “wave pumping” mechanism) and other horizontal heterogeneities [Sullivan et al., 2014; Hara and Sullivan, 2015; Mironov and Sullivan, 2015], which generally contribute to increased surface roughness, enhanced turbulence, and reduced mean wind shear, even in a statically stable ABL.

Although there are not yet offshore wind farms in the U.S. waters, several are scheduled for installation along the East Coast starting in 2016 (Maryland: https://www.doi.gov/news/pressreleases/interior‐auctions‐80000‐acres‐offshore‐maryland‐for‐wind‐energy‐development‐advances‐presidents‐climate‐action‐plan, New Jersey: https://www.doi.gov/pressreleases/interior‐department‐auctions‐344000‐acres‐offshore‐new‐jersey‐wind‐energy‐development, Block Island (RI): http://www.nytimes.com/2015/07/24/business/offshore‐wind‐farm‐raises‐hopes‐of‐us‐clean‐energy‐backers.html?_r=0, and Massachusetts: http://www.boem.gov/Commercial‐Wind‐Leasing‐Offshore‐Massachusetts/). Yet there is no published long‐term study on atmospheric stability in the marine boundary layer offshore of the U.S., only two short‐term field campaigns. CBLAST (Coupled Boundary Layers/Air‐Sea Transfer) investigated air‐sea interactions during a few weeks in the summers of 2002 and 2003 using radiosondes, sodar, and a turbulence flux package [Edson et al., 2007; Helmis et al., 2013, 2015]. POWER (Position of Offshore Wind Energy Resources) was conducted on a cruise along the coast of New England during July–August 2004 and used a high‐resolution Doppler lidar to measure wind profiles aloft in the wind turbine rotor area [Pichugina et al., 2012].

This study is the first to provide a complete stability analysis at a proposed offshore site along the U.S. East Coast, based on two long‐term observational data sets that were collected in the Nantucket Sound. The first data set consists of continuous, multilevel wind observations from a 60 m meteorological mast, near the planned site of the Cape Wind (CW) offshore wind farm. Covering approximately 9 years between 2003 and 2011, this will be referred to hereafter as the “historical data set.” The second was collected during the recent, 2 year IMPOWR (Improving the Modeling and Prediction of Offshore Wind Power Resources) campaign [Colle et al., 2016] and will be referred to hereafter as the “IMPOWR data set.” The IMPOWR campaign was designed to improve our understanding of the marine layer for offshore wind resource assessment.

Given that atmospheric stability has such an important potential impact on the wind resource, the goal of this paper is to provide climatological information on links between stability and wind profiles.