Buoyancy flux statistics and cloud patterns in shallow moist Rayleigh-Benard convection

Three-dimensional direct numerical simulations of shallow moist non-precipitating turbulent Rayleigh-Benard convection are presented. The thermodynamics of phase changes is linearized close to the phase boundary between gas and liquid. In the present model, a local thermodynamic equilibrium is assumed which permits supersaturation and fall out of rain. It can be considered as a first significant extension of dry Rayleigh-Benard convection in the Boussinesq approximation and allows to study the Rayleigh number dependence of transport properties in a limited range. The simulations confirm the connection between enhanced buoyancy flux, asymmetric vertical velocity fluctuations and the cloud cover in the layer. A geometric analysis of the cloud patterns in extended layers reproduces perimeter-to-area scaling dimensions that have been observed in large eddy simulations of cumulus convection. A multifractal analysis of the upward buoyancy flux demonstrates the increasing small-scale intermittency of the turbulent transport with growing Rayleigh number. Our study indicates that the moist Rayleigh-Benard problem offers a practical framework for the development and evaluation of parameterizations for atmospheric convection.