The phase of Venus’s surface thermal tide simulated with a Venus PCM

Venus’s atmosphere is characterized by pronounced thermal tides, which have been extensively studied at the cloud level through direct observations and numerical simulations. There, a semi-diurnal tide dominates the equatorial region and a diurnal signal is more prominent at high latitudes. Due to the lack of measurements at the lower levels of the atmosphere, less attention has been given to the significance of the thermal forcing close to the surface. However, understanding the surface thermal tide has significant implications for Venus's atmospheric dynamics, and for future exploration of Venus's atmosphere, for example, via the effect of the surface pressure variations on the atmospheric loading on the gravity field.
Using idealized simulations from a Venus PCM - LMDZ (Lebonnois et al. 2010), we examine surface thermal tides and planetary-scale wave activity in the lower atmosphere, a region that remains largely unconstrained by direct observations. The analysis focuses on the surface pressure anomalies induced by solar forcing, which exhibit a systematic phase lag relative to the sub-solar point. 
Here we find that the phase lag’s dependence on solar day length across simulations reveals a consistent radiative timescale governing the thermal response of the near-surface atmosphere.
As the solar day shortens, the pressure response transitions from a predominantly diurnal to a semi-diurnal structure, suggesting a resonance between the forcing frequency and the thermal response of the lower atmosphere. In addition to the tidal signal, a large-scale wave mode emerges in the simulations, confined vertically below the cloud layer around 15 km altitude. This wave displays properties consistent with an internal gravity wave, including westward propagation, and peak amplitude in a stably stratified layer. Its period remains largely insensitive to the imposed solar day, suggesting it is governed by intrinsic atmospheric structure rather than external forcing.