Venusian Planetary‐Scale Waves Observed by Akatsuki Longwave Infrared Camera: Coupled Rossby‐Kelvin Waves and Long‐Term Variation

Abstract Planetary‐scale waves are expected to be crucial in driving the Venusian planetary‐scale atmospheric circulation, including the superrotation. To understand the interaction between the waves and the mean flow, we obtained temporal frequency spectra of the cloud‐top brightness temperature using thermal infrared images taken by the Longwave Infrared Camera (LIR) onboard Akatsuki over a period of 10 Venus years. Waves in the equatorial region with periods of around 3.5–4.3 days were identified as Kelvin waves, while waves in the mid‐latitude region with periods of about 5.0–6.0 days were identified as Rossby waves. The mid‐latitude waves with periods 5.0–6.0 days tend to accompany additional local amplitude maxima near the equator, especially when observed at small emission angles. Considering that the contribution function of LIR extends to lower altitudes for smaller emission angles, the result implies that the waves arise from Rossby‐Kelvin instability and the associated Kelvin modes reside below the cloud top. Mid‐latitude peaks are also sometimes seen around periods of 3.5–4.0 days and are coupled with equatorial modes, indicative of Rossby‐Kelvin instability. The coupled Rossby‐Kelvin modes are expected to transport angular momentum equatorward to sustain the superrotation. The mid‐latitude modes decay with altitude. The periods and amplitudes of the waves change with time, and the variations seem to correlate with the background wind in such a way that waves with small intrinsic frequencies are less prominent.