CLOUD-LEVEL PENETRATIVE COMPRESSIBLE CONVECTION IN THE VENUS ATMOSPHERE

A two-dimensional, nonlinear, fully compressible model of a perfect ga s is used to simulate cloud-level penetrative convection in the Venus atmosphere from 40 to 60 km altitude. Three cases with different amoun ts of solar heating are considered: 60%, 80%, and 100% subsolar heatin g conditions corresponding to maximum internally heated Rayleigh numbe rs of 4.0 x 10(6), 5.4 x 10(6), and 6.8 x 10(6), respectively. Cloud-l evel convection is characterized by cold, narrow downwellings that dee ply penetrate (similar to 5 km) the underlying stable layer. The horiz ontal spacing of the downwellings is 15-30 km, an order of magnitude s maller than observed cloud-top cells in ultraviolet images. The penetr ating head of the downflow is both mechanically forced upward and comp ressionally heated by the underlying stable layer. The local compressi onal heating rate induced by penetration is four orders of magnitude l arger than the solar heating rate. Although slightly larger in magnitu de, the calculated vertical velocities at 54-km altitude are consisten t with Vega balloon measurements. The computations show that the Vega balloons drifted in a relatively quiescent part of the convection laye r. Vertical velocities are three to five times larger in the lower par i of the convection layer than in the upper part of the layer because of the dominance of convection by intense downwellings that acquire th eir highest speeds as they penetrate the underlying stable region. Mix ing length theory underestimates the vertical velocity of convection b y 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 the ory, 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 wavelen gths of 5-30 km and intrinsic horizontal phase speeds comparable to co nvective velocities.