Large-eddy simulation of a nocturnal stratocumulus-topped marine atmospheric boundary layer: An uncertainty analysis

A large-eddy simulation (LES) model has been used to study a nocturnal stra tocumulus-topped marine atmospheric boundary layer. The main objectives of our study have been first to investigate the statistical significance of LE S-derived data products. Second, to test the sensitivity of our LES results with respect to the representation of subgrid-scale mixing and microphysic al processes, and third to evaluate and to quantify the parametric uncertai nty arising from the incomplete knowledge of the environmental parameters t hat are required to specify the initial and boundary conditions of a partic ular case study. Model simulations were compared with observations obtained in solid stratocumulus during the third flight of the first 'Lagrangian' e xperiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). Base d on these simulations the following conclusions could be drawn. Resolution (50 x 50 x 25 m(3)) and domain size (3.2 x 3.2 x 1.5 km(3)) of the LES cal culations were adequate from a numerical point of view to represent the ess ential features of the stratocumulus-topped boundary layer. However, the en semble runs performed in our study to investigate the statistical significa nce of LES-derived data products demonstrate that the area-time averaging p rocedure for the second-order moments produces only a low degree of statist ical reliability in the model results. This illustrates the necessity of ha ving LES model results that are not only of adequate resolution but also of sufficiently large domain. The impact of different subgrid schemes was sma ll, but the primary effects of drizzle were found to influence the boundary -layer structure in a climatologically significant way. The parametric unce rtainty analysis revealed that the largest contribution to the variance of the LES-derived data products is due to the uncertainties in the cloud-top jump of total water mixing ratio and the net radiative forcing. The differe nces between the model and measurements for most of the simulated quantitie s were within the modelling uncertainties, but the calculated precipitation rate was found to differ significantly from that derived in the observatio ns.