The dynamics of the Venusian mesosphere and thermosphere

• Main Author: Tingle, Susannah
• Other Authors: Müller-Wodarg, Ingo
• Published: Imperial College London 2011
• Subjects: 520
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.542167

We present the first circulation model of Venus' atmosphere to couple the super-rotating cloud tops and upper thermosphere. To drive these simulations, we formulate the first continuous semi-empirical model of atmospheric structure between 60-250 km. Our model hydrostatically links the VIRA and VTS3 models. Our approach is validated by comparisons with observations where we find a good agreement with data. We base our dynamic model on the Müller-Wodarg et al. [2003] general circulation model (GCM) of Titan's thermosphere. Our simulations solve the full non-linear Navier-Stokes momentum equation assuming a realistic thermal structure and lower boundary super-rotation. We find our derived winds are consistent with much of the data between 70-120 km. Solving the full momentum equation we find dynamics below 80km are predominately cyclostrophic. Near 75km we find a good agreement between our GCM, cyclostrophic and cloud tracked winds between 45-85⁰ latitude. Equatorward of 30⁰ cyclostrophic winds decrease steeply with latitude. This is not seen in our GCM winds, which are sustained by an equatorward transport of momentum, neglected in the cyclostrophic approximation. Above 80km we find a balance of advection and pressure gradients replaces cyclostrophic balance poleward of (50-60⁰). Above 75km a pole-to-equator temperature gradient drives equatorward winds with peak speeds of 100ms⁻1 near 95 km, and zonal winds decrease with height. Zonal forcing above 90km drives a reversal in dayside meridional winds and accelerates a subsolar-to-antisolar flow. We find the winds between 90-150km are not characterised by a simple balance of accelerations. Above 150km we find a symmetric subsolar-to-antisolar flow, characterised by a balance of horizontal pressure gradients and viscosity with 200 ms⁻1 cross terminator winds. Our simulations address the origin of the thermospheric super-rotation. We find the cloud top super-rotation does not propagate above 100 km, nor is a super-rotation above 150km driven in situ by our pressure gradients.