Venusian planetary-scale waves observed by Akatsuki LIR: Rossby-Kelvin instability and long-term variation

Planetary-scale waves are expected to be crucial in driving the Venusian planetary-scale atmospheric circulation. 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 with periods of about 5.0–6.0 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 the contribution function of LIR extends to lower altitudes for smaller emission angles, the result implies 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 coupled with equatorial modes, indicative of Rossby-Kelvin instability. 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.

Figure: Latitudinal distributions of the wave amplitudes at the period of (a) 5.04 days and (b) 3.71 days for the emission angle ranges of 30–40 (blue solid), 40–50 (orange dashed), and 50–60 (green dotted).