LES results of the EUCLIPSE Lagrangian stratocumulus to shallow cumulus transition cases

As part of the EUCLIPSE project five LES models were used to simulate four different Lagrangian stratocumulus to shallow cumulus transition cases. For each case the models show a gradual increase in the boundary layer depth. The stratocumulus cloud layer also rises with time, whereas the temporal change in cumulus cloud base height is rather modest. The degree of decoupling, as quantified by the difference in the mean subcloud and cloud layer total specific humidity and liquid water potential temperature is compared to results from aircraft observations as reported by Wood and Bretherton (2004). Like the observations, the LES results exhibit a larger difference for deeper boundary layers, although the differences are somewhat larger in the LES fields. The dynamics in the decoupled subcloud layer are found to be similar to the dry convective boundary layer. A clear diurnal cycle in the shallow cumulus moisture transport is found. During the day moisture builds up in the subcloud layer, which is being redistributed by the cumuli towards the stratocumulus layer above during the night. Overall, the mean humidity transport at the top of the subcloud layer is only slightly smaller than the surface evaporation, thus being close to a zero-flux divergence. Last, it is found that during the night the boundary layer is close to a quasi-steady state.