Atmospheric Circulation of the Extrasolar Giant Planet HD209458b Under Diababtic Heating

A large fraction of the more than 120 extrasolar giant planets currently known has orbits that are very close to their host stars. These close-in planets are likely to be tidally locked and thus continuously heated on the same side. The atmospheric circulation and temperature distribution resulting from such a state is a key issue in the study of extrasolar planets. For one planet, HD209458b, some crucial physical properties (e.g., radius and mass) have been directly measured. Past studies have focused on flows driven by simple, ad-hoc models of radiative heating and cooling. In this work, we drive the flow with heating rates derived from a full radiative transfer model, which has been used in the past to successfully predict Na absorption in HD209458b's atmosphere. The flow model is a high-resolution equivalent barotropic model, capable of resolving small-scale eddies and waves. From our extensive set of simulations, we find that, although the planet rotates slowly (once per ~84 hours), the effects of rotation cannot be ignored: a strong zonal asymmetry is induced by the rotation on a very short timescale (~hours), leading to a complex heat distribution at early evolution times. At long times (~10 rotation periods), the flow evolves to a state marked by a broad, well-homogenized equatorial zone and a coherent circumpolar vortex at each poles, as in the recent adiabatic calculations of Cho et al. (2003). The stability of the 2 to 3 zonal jet circulation pattern found in the adiabatic calculation is also a robust feature of the present diabatic calculation. Wave breaking activity in the equatorial zone and propagation across the zone is enhanced by the strength of the heating. In the near future, some of these findings will be directly tested by observations, such as measurements of day-night temperature difference and temporal variations in the IR flux.