Modelling of the atmosphere of Venus


A General Circulation Model of the atmosphere of the planet Venus is developed using a climate model developed by the UK Meteorological Office. The model is adapted to usea 5×5 horizontal resolution covering the entire horizontal domain with 33 levels extending from the surface to 90km altitude, with a maximum vertical spacing of 3km. Modifications are made to provide a regime appropriate to the atmosphere of Venus. The rotation and orbital periods are changed to the measured values for Venus, and a realistic temperature profile is used to provide a suitable equilibrium temperature. Radiative and frictional forcing schemes are linearized and adapted to provide a plausible climate while keeping the parameterizations as simple as possible. A super–rotating atmosphere is found in the simplest model configuration with no diurnal or seasonal cycles, with horizontal equatorward transport of momentum, at altitudes be- tween 40km and 80km, as the mechanism which maintains the equatorial super–rotation. Equatorial Kelvin waves with a period of 9.5 days and Mixed–Rossby–gravity (MRG) waves with periods of 30 days are spontaneously produced in the same model, and the MRG modes are found to contribute significantly to the maintenance of the equatorial super–rotation. The sensitivity of the model to changes in the forcing pa- rameterization is tested and the presence of the equatorial super–rotation is found to be a stable feature for a range of values. The polar region of themodel qualitatively reproduces the observed ‘cold collar’ surrounding the middle atmosphere pole, and the ‘warm pole’ in the upper atmosphere. A passive tracer scheme is implemented in themodel and is used to investigate the effect of the atmospheric circulation on simple clouds in the atmosphere. Precipitation, evaporation, and condensation are implemented to provide a source and sink for the volatile tracer. Large scale structures are found in the cloud, similar to the “Y” shape and reversed ‘C’ shape features observed in the atmosphere of Venus. Depletion of the cloud phase of the volatile is seen in the polar regions of the model, as a result of the downwelling in the upper branch of the meridional circulation. A Monin–Obukhov boundary layer parameterization is integrated into the model and the effect of topography is tested using realistic topography of Venus. No significant dif- ferences are found in the large scale circulation or magnitude or form of the equatorial super–rotation. However, the surface temperature and pressure are both negatively correlated with topographic height, leading to cold mountains and warm valleys, as would be expected for an optically thick atmosphere. An idealised diurnal cycle is included in the thermal relaxation field to examine the re- sponse of the model to a time–varying heating. The diurnal and semi–diurnal tides are found in the circulation and temperature fields of the model atmosphere, with the tides contributing to the equatorward momentum transport, maintaining an equatorial super– rotation in the model and producing a larger global super–rotation than in the model with zonally symmetric forcing.

University of Oxford