Cloud-Level Penetrative Compressible Convection in the Venus Atmosphere

Abstract

A two-dimensional, nonlinear, fully compressible model of a perfect gas is used to simulate cloud-level penetrative convection in the Venus atmosphere from 40 to 60 km altitude. Three cases with different amounts of solar heating are considered: 60%, 80%, and 100% subsolar heating conditions corresponding to maximum internally heated Rayleigh numbers of 4.0 × 106, 5.4 × 106, and 6.8 × 106, respectively. Cloud-level convection is characterized by cold, narrow downwellings that deeply penetrate (∼5 km) the underlying stable layer. The horizontal spacing of the downwellings is 15–30 km, an order of magnitude smaller than observed cloud-top cells in ultraviolet images. The penetrating head of the downflow is both mechanically forced upward and compressionally heated by the underlying stable layer. The local compressional heating rate induced by penetration is four orders of magnitude larger than the solar heating rate. Although slightly larger in magnitude, the calculated vertical velocities at 54-km altitude are consistent with Vega balloon measurements. The computations show that the Vega balloons drifted in a relatively quiescent part of the convection layer. Vertical velocities are three to five times larger in the lower part of the convection layer than in the upper part of the layer because of the dominance of convection by intense downwellings that acquire their highest speeds as they penetrate the underlying stable region. Mixing length theory underestimates the vertical velocity of convection by a factor of 3 or more because kinetic energy in the convection layer is balanced not only by buoyancy work as assumed by mixing length theory, but also by pressure work and viscous work. A transfer of energy from low-frequency convective modes to higher-frequency “interfacial” penetrative modes occurs in the penetrative region. Internal gravity waves are also generated in the stable layers with horizontal wavelengths of 5–30 km and intrinsic horizontal phase speeds comparable to convective velocities.