Perturbed Physics Ensemble Simulation of Post-Cold-Frontal Clouds at the ASR/ARM East North Atlantic Site

Most general circulation models and reanalysis products underestimate the amount of clouds over the midlatitude oceans. This cloud bias occurs within extratropical cyclones, with the largest bias in the post-cold frontal (PCF) regions. The current study explores the physics of the PCF clouds using satellite data, observations from the ASR/ARM East North Atlantic (ENA), and a perturbed physics ensemble generated using the Weather Research Forecast (WRF) model. The ensemble is generated using combinations of three distinct microphysics schemes, three distinct boundary layer schemes, and four distinct convection schemes. Because of the differences in parametrized vertical transport from the different schemes a wide range of performance levels in simulating the PCF clouds and the boundary layer occur. Also, the cloud cover in the PCF region is found to be sensitive to large-scale metrics and mainly sensitive to changes in the convection and PBL schemes. The comparison between the observations and numerical modeling provides a better understanding of the behavior of post-cold-frontal low-level marine clouds at the ENA site. The comparison revealed a consistent bimodal distributions of cloud fraction and the instability with a transition between cumulus and stratocumulus clouds during the evolution of the PCF boundary layer. The PCF clouds are found to be substantially different from non-PCF clouds located in a different subsidence region.