Transition to turbulence in a conditionally unstable moist convection layer

The transition to turbulence in a conditionally unstable moist convection layer is studied in a low-dimensionsional Galerkin model. Such a model allows to probe the five-dimensional physical parameter space of the present moist convection model, which is spanned by the Prandtl number, the dry and moist Rayleigh number as well as two further parameters. They determine the degree of saturation of air parcels at the bottom of the convection layer and the amount of liquid water that can be formed during an adiabaitc ascent. The turbulence transition has to be initiated by a finite amplitude perturbation. Depending on the amplitude of the perturbation, the particular shape and the dry Rayleigh number, we observe either an immediate decay of the perturbation, a transient turbulent state or a fully developed turbulent state. We detect no sharp boundary for the transition to turbulence. The findings are similar to the turbulence transition in plane shear flows or some cases of doubly-diffusive convection. A lifetime statistics is performed which indicates that rather a chaotic repellor than an attractor is the phase space structure in which the trajectory of the present system is trapped. Our results are also supported by direct numerical simulations at larger Rayleigh numbers.