PNNL

Abstract

Gravity waves are a common occurrence in the atmosphere; however, their impact on wind farms has only recently gained attention with most studies focused on wind farm induced gravity waves. In this study, simulations are performed of an observed atmospheric bore with its associated gravity waves impacting a wind farm as part of the American Wake Experiment (AWAKEN) in the U.S. Southern Great Plains. The atmospheric bore is determined to be driven by a thunderstorm downburst from a nocturnal mesoscale convective system (MCS). We use large-eddy simulation and a novel two-domain nested setup (?x = 300 m and 20 m) in the Weather Research and Forecasting model forced with data from the High-Resolution Rapid Refresh model to capture the necessary dynamics for resolving the gravity waves while including a generalized actuator disk wind turbine parameterization on the finest domain to simulate turbine wakes. On the coarser domain, the MCS is resolved and the structure of the simulated gravity waves is found to be especially sensitive to cloud and precipitation processes. On the domain with finer grid resolution, the gravity waves have very strong effects on the flow in the upper atmosphere; however, closer to the surface where there is additional ambient turbulence and turbulence generated by wakes, the effect of the waves is more nuanced. Notably, the waves induce local wind direction variations correlated with fluctuations in pressure, which leads to fluctuations in the power output as various turbines within the farm are subjected to waking from nearby turbines. Additionally, the gravity waves modulate the mesoscale environment by weakening an existing low-level jet, which reduces hub-height wind speed and, in-turn, power output.