Structure double diffusive des équations de la convection en air humide saturé avec application à l'air nuageux

The ternary mixture `dry air–water vapor–liquid water' is a statically stable medium in general. This property is a consequence of the great number of physical properties of the medium, such as double diffusivity, heterogeneous medium equation of state, and equilibrium of saturation. Convective instability in that medium can be modelled with the help of a Rayleigh number Ra and a moist Rayleigh number Rh . In this paper, we establish necessary conditions for appearance of the instability for a given static distribution of temperature and water vapor concentration, with respect to physical characteristics of the medium. In particular, regimes of stationary instability (`moisture fingers') can exist, even if the dry medium is statically stable.