Cellular convection in the atmosphere of Venus
Cellular convection in the atmosphere of Venus
复制标题
金星大气层中的细胞对流
作者：
R. D. Baker;G. Schubert
金星大气层最有趣的特征之一是在副极点附近和顺风处存在细胞结构。水手 10 号和先锋金星任务在 60-70 公里云顶高度拍摄的紫外图像表明，这些细胞主要只在下午在低纬度地区发现，黑边细胞的水平尺度约为 500-1,000 公里，亮边缘细胞的水平尺度约为 200-500 公里（参考文献 1、2）。最近伽利略飞越金星时拍摄的太阳下区域图像证实了这些早期观测结果——水平尺度为 250-1,000 公里的细胞特征已被观测到3（图1a、b）。有人认为这些结构是大气对流细胞2-5，事实上，三维可压缩对流的数值模拟产生的特征让人想起在太阳下金星大气中发现的细胞结构（图1c）6。然而，计算这些结构的大小存在一些困难。对流单元被认为位于中性稳定云层中，距地表约 50 公里，厚度仅为几公里。如此宽和薄的对流室将对对流动力学构成严峻的挑战。在这里，我们提出，进入中性稳定云层上方和下方稳定区域的强穿透对流，加上来自表面的穿透对流，增加了单元的垂直尺寸，从而有助于解释它们的大水平范围。
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 cells2–5, 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.