Progressive emergence of canopy gravity waves during the afternoon–evening transition

Abstract The afternoon–evening transition (AET) of canopy turbulence marks a fundamental shift from the daytime convective regime to nocturnal stable conditions. Despite its importance for canopy–atmosphere exchanges of momentum, heat, and scalars, this transition remains insufficiently understood, partly because resolving the coupled canopy–atmospheric boundary‐layer dynamics across evolving unstable‐to‐stable stratification remains challenging for large‐eddy simulations (LES). Here, LES are used to investigate the evolution of turbulence during the AET over an idealized forest of 32‐m height () subjected to a weak geostrophic forcing. Simulations revealed a progressive shift from daytime turbulence‐dominated to evening oscillatory flow dynamics, as surface cooling increases stability and near‐canopy winds weaken. The oscillatory motions, known as canopy waves, remained confined to the vicinity of the canopy top and exhibited wavelengths of approximately four to six canopy heights and periods consistent with the local buoyancy time‐scale (1–2 min). They propagated at a speed comparable with the mean wind velocity around , similar to canopy‐top coherent turbulent motions in near‐neutral conditions. Their contribution to vertical momentum and scalar transport is weak because vertical velocity and temperature fluctuations become increasingly out of phase. The transition toward oscillatory dynamics occurred progressively rather than abruptly, beginning approximately when the gradient Richardson number at the canopy top became positive and lower than . Although obtained under idealized conditions, these simulated wave characteristics were qualitatively consistent with observations from a field experiment conducted over a comparable forest canopy. These results provide new insight into the evolution of canopy‐scale motions during the AET and highlight the increasing role of wave dynamics as turbulence collapses under stable conditions.