Simulating Observed Cloud Transitions in the Northeast Pacific during CSET.

The goal of this study is to challenge a large-eddy simulation model with a range of observations from a modern field campaign and to develop case studies useful to other modelers. The 2015 Cloud System Evolution in the Trades (CSET) field campaign provided a wealth of in situ and remote sensing observations of subtropical cloud transitions in the summertime northeast Pacific. Two Lagrangian case studies based on these observations are used to validate the thermodynamic, radiative, and microphysical properties of large-eddy simulations (LES) of the stratocumulus to cumulus transition. The two cases contrast a relatively fast cloud transition in a clean, initially well-mixed boundary layer versus a slower transition in an initially decoupled boundary layer with higher aerosol concentrations and stronger mean subsidence. For each case, simulations of two neighboring trajectories sample mesoscale variability and the coherence of the transition in adjacent air masses. In both cases, LES broadly reproduce satellite and aircraft observations of the transition. Simulations of the first case match observations more closely than for the second case, where simulations underestimate cloud cover early in the simulations and overestimate cloud top height later. For the first case, simulated cloud fraction and liquid water path increase if a larger cloud droplet number concentration is prescribed. In the second case, precipitation onset and inversion cloud breakup occur earlier when the LES domain is chosen to be large enough to support strong mesoscale organization. Significance Statement: Low-lying clouds over the ocean are difficult to represent in global climate models and contribute to uncertainty in climate predictions. To improve understanding and simulation of these clouds, an intensive airborne measurement campaign in 2015 over the northeast Pacific Ocean sampled these clouds and the surrounding air mass as the trade winds carried them toward Hawaii. In this paper, we simulate two contrasting case studies from this campaign with a high-resolution model that captures cloud-scale motions and processes. The observations test the model's fidelity in representing the transition from widespread to broken cloud cover, while the model suggests that this transition is accelerated by weather conditions promoting unusually weak subsidence and by the onset of drizzle. [ABSTRACT FROM AUTHOR]