Double diffusive structure of the equations of convection in moist-saturated air

The ternary mixture 'dry air-water vapor-liquid water' is a statically stab le medium in general. This property is a consequence of the great number of physical properties of the medium, such as double diffusivity, heterogeneo us medium equation of state, and equilibrium of saturation. Convective inst ability in that medium can be modelled with thr help of a Rayleigh number R a and a moist Rayleigh number Rh. In this paper, we establish necessary con ditions for appearance of the instability for a given static distribution o f temperature and water vapor concentration, with respect to physical chara cteristics of the medium. In particular regimes of stationary instability ( 'moisture fingers') can exist, even if the dry medium is statically stable. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevie r SAS.