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. As a consequence, air masses undergo intense transformations. Near the ice edge, strong shear and strong surface heat fluxes result in formation of helical convective rolls and associated cloud streets that extend for tens or even hundreds of kilometers. Further downwind helical convective rolls transition into convective cells forming open cell cloud structures.

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 model, by nesting a high-resolution large-eddy simulation domains within a mesoscale domain. Our coupled mesoscale-microscale WRF setup features a mesoscale domain with horizontal grid cell size of 1050 m coupled online with a cloud-resolving large-eddy simulation domain with horizontal grid cell size of 150 m that stretches from the ice edge to Andenes (~1000 km fetch). Within the cloud-resolving domain are nested two high-resolution large-eddy simulation domains with 30 m grid cells. One of the high-resolution domains is focused on the region convective rolls while the second one is focused on convective cells. Such a configuration enables us to simulate the CAO airmass transformation at high resolution, thus 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.

[1] B. Geerts, and 32 Coauthors, BAMS 103, E1371–E1389 (2022).