Superrotation in a Venus general circulation model

Abstract

[1] A superrotating atmosphere with equatorial winds of $35 m s À1 is simulated using a simplified Venus general circulation model (GCM). The equatorial superrotation in the model atmosphere is maintained by barotropic instabilities in the midlatitude jets which transport angular momentum toward the equator. The midlatitude jets are maintained by the mean meridional circulation, and the momentum transporting waves are qualitatively similar to observed midlatitude waves; an equatorial Kelvin wave is also present in the atmosphere. The GCM is forced by linearized cooling and friction parameterizations, with hyperdiffusion and a polar Fourier filter to maintain numerical stability. Atmospheric superrotation is a robust feature of the model and is spontaneously produced without specific tuning. A strong meridional circulation develops in the form of a single Hadley cell, extending from the equator to the pole in both hemispheres, and from the surface to 50 km altitude. The zonal jets produced by this circulation reach 45 m s À1 at 60 km, with peak winds of 35 m s À1 at the equator. A warm pole and cold collar are also found in the GCM, caused by adiabatic warming in the mean meridional circulation. Wave frequencies and zonal wind speeds are smaller than in observations by cloud tracking but are consistent with a Doppler shifting by wind speeds in the generating region of each wave. Magnitudes of polar temperature anomalies are smaller than the observed features, suggesting dynamical processes alone may not be sufficient to maintain the large observed temperature contrasts at the magnitudes and periods found in this GCM.

Overview of Venus orbiter, Akatsuki

“…Atmospheric general circulation models for Venus-like slowly rotating planets seem to favor this process as the mechanism of maintaining the super-rotation (Del Genio and Zhou, 1996;Takahashi, 2003, 2006;Lee et al, 2007;Kido and Wakata, 2008). However, these models assume either an unrealistically high solar heating rate in the deep atmosphere or an unrealistically large equator-to-pole surface temperature gradient so that a strong Hadley circulation can exist in the deep atmosphere.…”

Zonal winds at high latitudes on Venus: An improved application of cyclostrophic balance to Venus Express observations

“…In this study we investigate the likely cause of these unphysical and inaccurate thermal wind retrievals in the polar regions, using fully self-consistent simulations of Venus atmospheric circulation with a simplified general circulation model (Lee et al 2005;Lee 2006; Lee et al 2007). Likely causes are either due to a breakdown of cyclostrophic balance or the amplifications of observational errors in the assumed lower boundary condition of the cyclostrophic retrieval.…”

“…The latter is derived from the superposition of a global-averaged reference temperature profile, obtained from Pioneer Venus probe data (Seiff et al, 1980), plus a perturbation that is function of latitude and pressure that determines the equator-to-pole thermal contrast at each altitude. This structure is chosen to produce a peak in solar heating within the altitude range of the observed cloud deck (Tomasko et al 1985;Lee 2006;Lee et al 2007). …”

“…The SGCM used here is called the Oxford Planetary Unified (Model) System for Venus (OPUS-V), and obtains a dynamically self-consistent representation of the circulation of the Venus' atmosphere (Lee et al 2005;Lee 2006;Lee et al 2007). …”

“…In the three upper most layers a sponge layer is included, with Rayleigh friction acting to damp horizontal eddy winds to zero (though not the zonal mean flow; Lee 2006;Lee et al 2007). …”