Moist turbulent Rayleigh-Benard convection with Neumann and Dirichlet boundary conditions

TL;DR

This study uses direct numerical simulations to compare moist turbulent Rayleigh-Benard convection under Neumann and Dirichlet boundary conditions, revealing how boundary conditions influence turbulence, cloud formation, and stratification at high Rayleigh numbers.

Contribution

It provides a detailed comparison of moist turbulent convection with different boundary conditions, highlighting the effects on turbulence and stratification at high Rayleigh numbers.

Findings

Differences in velocity fluctuations decrease with increasing Rayleigh number.

At high Rayleigh numbers, a two-layer regime forms with a dry, stable layer beneath a saturated, cloudy layer.

The inversion between layers becomes narrower as Rayleigh number increases.

Abstract

Turbulent Rayleigh-Benard convection with phase changes in an extended layer between two parallel impermeable planes is studied by means of three-dimensional direct numerical simulations for Rayleigh numbers between 10^4 and 1.5\times 10^7 and for Prandtl number Pr=0.7. Two different sets of boundary conditions of temperature and total water content are compared: imposed constant amplitudes which translate into Dirichlet boundary conditions for the scalar field fluctuations about the quiescent diffusive equilibrium and constant imposed flux boundary conditions that result in Neumann boundary conditions. Moist turbulent convection is in the conditionally unstable regime throughout this study for which unsaturated air parcels are stably and saturated air parcels unstably stratified. A direct comparison of both sets of boundary conditions with the same parameters requires to start the turbulence simulations out of differently saturated equilibrium states. Similar to dry Rayleigh-Benard convection the differences in the turbulent velocity fluctuations, the cloud cover and the convective buoyancy flux decrease across the layer with increasing Rayleigh number. At the highest Rayleigh numbers the system is found in a two-layer regime, a dry cloudless and stably stratified layer with low turbulence level below a fully saturated and cloudy turbulent one which equals classical Rayleigh-Benard convection layer. Both are separated by a strong inversion that gets increasingly narrower for growing Rayleigh number.