Simulations and Observations of Summertime Frontal Clouds Over the Eastern North Atlantic

Description

A wide body of literature exists detailing the physical processes that maintain and modify existing marine boundary layer clouds, but a limited number of studies that have attempted to relate process-level marine cloud structure to larger scale features such as fronts. The summertime marine boundary layer cloud system over the Eastern North Atlantic is characterized by an array of cloud formations, many of which are the direct consequence of frontal forcing, which complicate interpretation of the observations collected at ENA. Compounding this issue is a lack of fundamental understanding of the role played by this forcing in shaping the boundary layer thermodynamic, cloud, and precipitation structure in this region and beyond. To increase understanding of role of frontal dynamics in modulating summertime marine boundary layer structure, we performed three multi-day simulations over the ENA region using the Weather Research and Forecasting (WRF) model on the NCAR’s Cheyenne Supercomputer. These simulations feature 1.3 km horizontal resolution and high resolution vertical resolution in the marine boundary layer. After some experimentation with various combinations of parameterizations, we were able to reproduce many aspects of the observed cloud and boundary layer structure. Particularly noteworthy was the model’s ability to represent transitions from a semi-continuous layer of post-cold-frontal stratocumulus to individual boundary layer mesoscale structures that were characterized by cumulus rising into stratocumulus, thereby suggesting a relationship between frontal dynamics and cloud structural transitions. Two of the three simulations exhibited somewhat similar cloud-life cycles associated with post-cold-frontal cloud transitions, while the third produced a different evolution. Frontal kinematics are analyzed in the context of observed cloud and thermodynamic structure, and composite structures of pre-, frontal, and post-frontal cloud conditions are presented. The life cycle of cloudiness at ENA in these frontal environments is also examined.