Interfacial layers in clear and cloudy atmospheric boundary layers

This paper reports on some recent advances in the understanding of the behaviour of atmospheric interfacial layers. We focus on those interfaces where a turbulent layer is separated from a quiescent layer by a relatively strong density gradient and study in particlar the entrainment rate, i.e. the rate with which the mixed layer penetrates into the quiescent layer by entraining fluid across the density interface. Making use of massively parallelized supercomputers, we conduct a large number of Direct Numerical Simulations (DNS) for a wide range of conditions and study the impact exerted on the entrainment rate by the Reynolds number, the Prandtl(/Schmidt) number, and the strength of the density jump represented by the Richardson number. We study two cases that are relevant for the atmosphere (/ocean), i.e. I] where turbulence is generated by a surface buoyancy flux and II] where turbulence is generated by shear (surface momentum flux) Of course with DNS one cannot simulate the high Reynolds numbers encountered in atmospheric contexts, but present computer resources do allow faithful simulation of the classical laboratory experiments of these situations and even achieve Reynolds numbers more than ten times larger.