Cellular convection in the atmosphere of Venus

Abstract

AMONG the most intriguing features of the atmosphere of Venus is the presence of cellular structures near and downwind of the subpolar point. Ultraviolet images from the Mariner 10 and Pioneer Venus missions at cloud-top heights of 60–70 km indicate that these cells are chiefly found only during the afternoon at low latitudes, with dark-rimmed cells having horizontal scales of roughly 500–1,000 km, and bright-rimmed cells having horizontal scales of roughly 200–500 km (refs 1, 2). Images of the subsolar region from the recent Galileo flyby of Venus confirm these earlier observations—cellular features with horizontal scales of 250–1,000 km have been observed³ (Fig. la,b). It has been suggested that the structures are atmospheric convection cells^2–5, and, in fact, numerical simulations of three-dimensional compressible convection produce features reminiscent of the cellular structures found in the subsolar Venus atmosphere (Fig. 1c)⁶. There has been some difficulty, however, in accounting for the sizes of these structures. The convection cells were thought to be located in a neutrally stable cloud layer, roughly 50 km above the surface, with a thickness of only a few kilometres. So broad and thin a convection cell would pose a severe challenge to the dynamics of convection. Here we propose that strongly penetrative convection into the stable regions above and below the neutrally stable cloud layer coupled with penetrative convection from the surface increases the vertical dimensions of the cells, thereby helping to explain their large horizontal extent.