Środowiskowe Seminarium Fizyki Atmosfery

Turbulence is key for the vertical mixing across the planetary boundary layer. However, our understanding of how turbulence interacts with other phenomena, such as density stratification, radiation or clouds, remains limited in crucial ways, particularly at meter and submeter scales. During the last decades, high-resolution simulations have provided further insight into these small-scale processes. In particular, direct numerical simulation has become a powerful complement to large-eddy simulations, laboratory experiments and field observations, since it removes the uncertainty associated with turbulence models and numerical algorithms. In this talk, I will use two examples to illustrate recent developments based on direct numerical simulation. The first example considers the stable boundary layer in the strongly stratified regime. We will see that turbulence collapse in the stable boundary layer can occur intermittently in space without the need of external triggers, such as surface heterogeneity. It suffices that intrinsic large-scale flow structures have space and time to develop. This finding reconciles previous results and explains the difficulty to reproduce this intermittent behavior in simulations, where domains are often too small or grid resolutions too coarse. The second example is cloud-top entrainment in stratocumulus. We will see that centimeter scales are important to faithfully represent the effects of evaporative cooling and gravitational settling on radiative cooling. This finding helps to explain the observed variability across large-scale models when droplet evaporation strongly affects the cloud dynamics, like for buoyancy-reversal conditions. Besides, these results indicate that data from simulations could complement current high-resolution measurements to further advance our understanding of cloud entrainment at meter and submeter scales.