Long‐term variation of the quasi‐five‐day wave in the top layer of Venus clouds

Planetary waves play an essential role in the momentum budget of Venus's atmosphere superrotation. Using the Longwave Infrared Camera (LIR) observations of the Akatsuki and their cloud-tracking wind fields, we study the characteristics of planetary waves and their long-term variations. The dominant planetary wave is a wavenumber 1 wave with a period of 5-6 days (Quasi-Five-Day wave or QFDW) and is symmetrically distributed in the midlatitudes of both hemispheres. The wave phase speed is close to the background zonal wind, indicating the possible existence of a critical level near the LIR emission altitude of 65 km. The QFDW tends to appear at similar to 8:00 a.m. local time, suggesting a modulation by the thermal tide. The QFDW amplitude is correlated with the vertical shear of the background zonal wind modulated by the thermal tide. This indicates some nonlinear interaction between the thermal tide and the QFDW.