ATMOSPHERIC DYNAMICS OF BROWN DWARFS AND DIRECTLY IMAGED GIANT PLANETS

Abstract

A variety of observations provide evidence for vigorous motion in the\natmospheres of brown dwarfs and directly imaged giant planets. Motivated by\nthese observations, we examine the dynamical regime of the circulation in the\natmospheres and interiors of these objects. Brown dwarfs rotate rapidly, and\nfor plausible wind speeds, the flow at large scales will be rotationally\ndominated. We present 3D, global, numerical simulations of convection in the\ninterior, which demonstrate that, at large scales, the convection aligns in the\ndirection parallel to the rotation axis. Convection occurs more efficiently at\nhigh latitudes than low latitudes, leading to systematic equator-to-pole\ntemperature differences that may reach ~1 K near the top of the convection\nzone. The interaction of convection with the overlying, stably stratified\natmosphere will generate a wealth of atmospheric waves, and we argue that, as\nin the stratospheres of planets in the solar system, the interaction of these\nwaves with the mean flow will cause a significant atmospheric circulation at\nregional to global scales. At large scales, this should consist of stratified\nturbulence (possibly organizing into coherent structures such as vortices and\njets) and an accompanying overturning circulation. We present an approximate\nanalytic theory of this circulation, which predicts characteristic horizontal\ntemperature variations of several to ~50 K, horizontal wind speeds of ~10-300\nm/sec, and vertical velocities that advect air over a scale height in\n~10^5-10^6 sec. This vertical mixing may help to explain the chemical\ndisequilibrium observed on some brown dwarfs. Moreover, the implied large-scale\norganization of temperature perturbations and vertical velocities suggests\nthat, near the L/T transition, patchy clouds can form near the photosphere,\nhelping to explain recent observations of brown-dwarf variability in the\nnear-IR.\n