Coupled Mesoscale to Microscale Simulations of Mixed-Phase Convective Clouds Observed during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE)

Equatorward excursions of cold polar air masses during cold air outbreaks (CAOs) result in the development of mesoscale convective circulations that significantly affect surface fluxes. Air masses undergo intense transformations as they transition from the ice to the warmer ocean.  Initially strong surface heat fluxes and strong shear result in the formation of helical convective rolls and associated cloud streets that can extend for hundreds of kilometers. Further downwind helical convective rolls evolve into convective cells forming open cell clouds.

We study an intense CAO observed on 13 March 2020 during Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) [1]. COMBLE deployed the Department of Energy Atmospheric Radiation Measurement (ARM) Mobile Facility 1 (AMF1) at Andenes, Norway to observe a range of CAO conditions. We simulate the evolution of a CAO using coupled mesoscale to microscale simulations with the Weather Research and Forecasting (WRF) model. The coupled simulation using WRF includes a mesoscale domain with 1050 m horizontal grid cell coupled online with a cloud-resolving LES domain with horizontal grid cell size of 150 m that stretches through the full ~1000 km extent of a CAO, from the ice edge to Andenes. Within the cloud-resolving domain are nested two LES domains with 30 m grid cells. One of these domains is focused on the region of convective rolls while the other one is focused on convective cells. This configuration enables us to study the transformation of airmass at high resolution, providing unprecedented insight into the mixed phase cloud (MPC) transition from rolls to cells. We study the interaction between large-scale forcing, surface fluxes, radiative transfer, and cloud processes in the formation and evolution of mesoscale organization and MPCs. As part of this effort, we utilize the Cloud Resolving Model Radar Simulator (CR-SIM) to compare WRF more directly to the measurements. Our CR-SIM analysis suggests that convective cell structures and properties are well modeled at the AMF1 site when using turbulence-resolving resolutions.