Observations and Simulations of Shallow Convection during a Cold-Air Outbreak Along Alaska’s North Shore

Mixed-phase boundary-layer convective clouds are common in high latitudes during cold-air outbreaks (CAOs) over open water. The microphysics of the CAO cloud regime are poorly characterized and understood. These BL clouds generally are mixed-phase and can produce rather heavy precipitation. They occur rather frequently in autumn north of Alaska’s north coast. For more than a decade the U.S Atmospheric Radiation Measurement (ARM) program has been operating a rich array of profiling and scanning radars, lidars and other sensors at Point Barrow. We use these data to document the observed vertical structure of the marine BL, the clouds and precipitation of one persistent CAO episode at this site.

The CAO cloud regime is not represented well in global and even regional numerical models, because most shallow convection parameterizations do not represent the sub-grid scale effects of mixed-phase microphysics, cloud-radiation interaction and precipitation. To illustrate this, we ran WRF simulations of this CAO episode, and compare a high-resolution (900-m in horizontal) run with convection represented explicitly and three coarser resolution (13.5-km) runs with different shallow convection schemes. While the 900-m WRF simulation captures the observed BL thermodynamic and wind profiles, cloud macro-structure, and vertical transects of vertical velocity, reflectivity and liquid water path, the coarse simulations with shallow convection parameterizations differ significantly in BL and cloud characteristics, compared not only to the 900 m explicit run, but also to each other.

To further our understanding of the CAO cloud regime, ARM will be deploying an ARM Mobile Facility in early 2020 in the far North Atlantic, where CAO conditions are far more common. The talk will end by highlighting this field campaign, referred to as COMBLE (Cold Air Outbreaks in the Marine Boundary Layer Experiment)