  To achieve decarbonization targets, wind turbines are growing in hub height,
rotor diameter, and are being deployed in new locations with diverse
atmospheric conditions not previously seen, such as offshore. Physics-based
analytical wake models commonly used for design and control of wind farms
simplify atmospheric boundary layer (ABL) and wake physics to achieve
computational efficiency. This is done primarily through a simplified model
form that neglects certain flow processes and through parameterization of ABL
and wake turbulence through a wake spreading rate. In this study, we analyze
the physical mechanisms that govern momentum and turbulence within a wind
turbine wake in the stratified ABL. We use large eddy simulation and analysis
of the streamwise momentum deficit and wake-added turbulence kinetic energy
(TKE) budgets to study wind turbine wakes under neutral and stable conditions.
To parse the wake from the turbulent, incident ABL flow, we decompose the flow
into the base ABL flow and the deficit flow produced by the presence of a
turbine. We analyze the decomposed flow field budgets to study the effects of
changing stability on the streamwise momentum deficit and wake-added TKE. The
results demonstrate that stability changes the importance of physical
mechanisms for both quantities primarily through the nonlinear interactions of
the base and deficit flows, with the stable case most affected by higher shear
in the base flow and the neutral case by higher base flow TKE. Buoyancy forcing
terms in the momentum deficit and wake-added TKE budgets are relatively less
important compared to the aforementioned effects. While total TKE is higher in
wakes in neutral ABL flows, the wake-added TKE is higher downwind of turbines
in stable ABL conditions. The dependence of wake-added TKE on ABL stability is
not represented in existing empirical models widely used for mean wake flow
modeling.
