LES sensitivity experiments of the EUCLIPSE stratocumulus to cumulus transition based on ASTEX

A large-eddy simulation model is used to simulate the stratocumulus to cumulus transition as was observed during ASTEX. A prognostic equation for the tendency of the stratocumulus layer depth is derived, using the budget equations for humidity and temperature and the equation for the inversion height tendency. In this way, the relative importance of physical processes, such as radiation, precipitation and entrainment, for the transition can be individually analysed. The effects of the processes balance to a great extent, resulting in a very small net tendency, even though the contribution of each of the individual terms is large.

Additional simulations are performed to investigate the sensitivity of the transition to changes in 1) the cloud droplet number concentration and 2)the large scale divergence . The change in precipitation rate due to the change in the number of cloud droplets has a significant influence on the liquid water path during the first night of the transition, but does not influence the pace of the transition. Lowering the divergence is shown to increase boundary height significantly, causing the boundary layer to be more decoupled. Because of this effect, the drying and warming effect of entrainment decreases, prolonging the life-time of the stratocumulus cloud layer.