Can Today’s Models Reproduce Decoupling during the Lagrangian Evolution of a Highly Supercooled CAO?

The Arctic is experiencing a warming trend unlike any other region on the Earth, with global climate models (GCMs) underestimating the intensity of warming. Cold-air outbreaks (CAOs) are one of the most important weather phenomena that occur in the Arctic and sub-Arctic. CAOs are characterized by drastic airmass transformations by which cold air, originating over an ice shelf or land mass, advects equatorward over the relatively warm open ocean. Intense surface-atmosphere exchanges ensue, leading to the rapid development of a convective boundary layer and mixed-phase clouds (MPCs). These clouds often undergo a transition, which may include some combination of rolls, closed cells, and open cells depending upon a combination of factors including, but not limited to, air-sea coupling, aerosols, precipitation, wind shear, and subsidence. CAOs therefore represent a challenge to understand and accurately simulate interactions between dynamics, turbulence, and microphysics in both GCMs and higher-resolution models with more detailed aerosol and cloud microphysics.

To better understand aerosol-cloud interactions in CAO MPCs and provide a testbed for GCM development, our team is leading an international model-observation intercomparison project (MIP) based on a CAO event observed on 13 March 2020 during the DOE ARM Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) field project. We have developed the COMBLE-MIP specification by combining successful aspects from previous Arctic MIPs, including a quasi-Lagrangian approach (i.e., “airmass-following”), a set-up that is suitable for both large-eddy simulations (LES) and single column models (SCMs), and sensitivity tests with increasing complexity of surface flux, aerosol, and microphysics treatments. There are a total of 10 LES and 7 SCM models participating in COMBLE-MIP. One unique aspect of COMBLE-MIP compared to previous MPC MIPs is a multi-modal aerosol specification that is sufficiently complete to prognose droplet activation, heterogeneous ice nucleation, and sea spray surface fluxes. This presentation will first focus on assessing physical processes responsible for the Lagrangian evolution of MPCs as depicted by the LES models. We will then use the LES results to identify shortcomings in the SCMs related to the representation of MPC macrophysical properties. Observational constraints are used to show that the LES models can generally capture the evolution of surface fluxes, liquid water path, and cloud top height and temperature in the baseline sensitivity tests, when in-cloud ice number concentrations are fixed. Toward the end of the simulation, SCMs begin to diverge with respect to their representation of cloud properties, associated with the formation of a decoupled cloud layer.