Hemispherically Asymmetric Superrotation in a Model Venus Atmosphere with Realistic Topography

We discuss the results of simulations using a Venus atmosphere general circulation model, based on the National Center for Atmospheric Research Community Atmosphere Model. In these simulations we use a high resolution of around 1 degree by 1 degree in latitude and longitude, which allows us to simulate small-scale spatial variations that may be important in Venus' atmosphere. We use a simplified thermal relaxation scheme, similar to that used in other Venus GCMs. Our previous results (Parish et al., 2010) generated cloud level superrotation with mean zonal wind magnitudes comparable to those observed using probes, and around 50 to 60 percent of those measured using cloud-tracking techniques. However, winds at lower altitudes were significantly smaller than those observed. We also found periodic variations in the magnitude of the zonal winds at cloud top heights and below, with a timescale of around 10 years. Here we determine the sensitivity of our results to changes in the atmospheric diffusion coefficient and topography. We find that a reduction in the diffusion coefficient, within limits required to maintain stability, enhances the build up of angular momentum. Zonal wind velocities are found to peak at higher latitudes and lower altitudes with a smaller diffusion coefficient. Spectral analyses show that there is a significant decadal oscillation in these results. However, the influence of decadal oscillations on the wind structure is less apparent due to the larger values of global angular momentum. Oscillations with periods of several years are also found, in particular a six year oscillation. Wind magnitudes in the lower atmosphere, below the cloud levels, are found to be significantly increased, to magnitudes within the range of those observed using probes. Generating winds in the lower atmosphere comparable to those observed has been hard to achieve with many Venus models. However, the simplified thermal relaxation scheme may overestimate heating between the ground and cloud level compared with observations. We find that with realistic topography the angular momentum increases more rapidly over time. However, the maximum angular momentum achieved when equilibrium is reached is smaller. There is a vacillation cycle when topography is included, and the location of the maximum zonal wind alternates between the northern and southern hemispheres over periods of several years. Reference: Parish et al., 2010, doi:10.1016/j.icarus.2010.11.015