Short-period planetary-scale waves in a Venus general circulation model: Rotational and divergent component structures and energy conversions

Abstract

To elucidate the generation and coupling of the short-period waves in the Venus atmosphere, we investigated the three-dimensional wave structure in an AORI (Atmosphere and Ocean Research Institute, University of Tokyo, Japan) Venus general circulation model. The most predominant waves are 7.5-day waves with zonal wavenumber 1 and are slower than observed planetary-scale 5.5-day waves around the cloud bottom (∼50 km), associated with a slower zonal flow in the model. The 7.5-day waves comprise three types: Type I, a Rossby wave in the upper cloud layer; Type II, a Rossby wave around the polar tropopause (∼50 km); and Type III, an equatorial Kelvin-like wave around and below the cloud bottom.Around the cloud top (65–70 km), the Rossby wave (Type I) has a rotational eddy structure and is produced by potential-energy conversion from zonal mean to eddy, suggesting baroclinic instability in the cloud layer. The geopotential of the Rossby wave in the upper cloud layer vertically connects to that of the equatorial Kelvin-like wave (Type III) across the critical line at ∼55 km and 30° latitudes. At the cloud bottom (∼50 km), the Kelvin-like wave and Rossby wave (Type II) are separated at a critical latitude around the cloud bottom, but are horizontally jointed with the stream function and velocity potential at the critical latitude. These two waves are major equatorward momentum transporters and are generated by horizontal shear (or barotropic) and baroclinic instabilities associated with energy conversion at the critical line. For high-latitude Rossby waves around the polar tropopause (Type II), the positive kinetic-energy conversion by vertical momentum transport is predominant, tending to relax the vertical shear of the zonal flow, leading in turn to relaxation of the horizontal temperature gradient under the cyclostrophic thermal wind. In contrast, the negative potential-energy conversion tends to produce the horizontal temperature gradient in the lower atmosphere where the solar insolation is very weak.The equatorial jet core rotating with a 7.5-day period is composed of both divergent and rotational wind components of the equatorial Kelvin-like wave at the cloud bottom. Above the critical level of the Kelvin-like wave, the equatorial Rossby wave (downward and equatorward flank of the Type I wave) appears at around 54 km and is produced by the barotropic energy conversion.