Evaluating the effect of coupled-to-decoupled transition of the atmospheric boundary layer on the change of phase partitioning in the mixed-phase stratiform clouds

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Rapid change of phase partitioning between supercooled water and ice particles in the mixed-phase stratocumulus clouds, important for cloud precipitation and radiative forcing, is often observed but the mechanism is not well understood [1]. A case is studied where a rapid increase of ice water content (ice water path quadruples within an hour) was observed in a mixed-phase stratiform cloud from the Second ARM Mobile Facility at McMurdo Station, Antarctic, during the ARM West Antarctic Radiation Experiment (AWARE) field campaign. Sounding profiles on that day suggest that the cloud-driven mixing layer is coupled to the surface before the fast transition and is decoupled from the surface afterwards. This raises a question of whether the rapid change of phase partitioning is mainly due to the change of ice nucleating particle concentration [2], or due to the change of the boundary layer structure proposed in Ref [3]. This study uses idealized large eddy simulations to investigate the effect of coupled-to-decoupled transition of the atmospheric boundary layer on the change of phase partitioning in the mixed-phase stratocumulus clouds. Our preliminary results show that the mixed-phase clouds respond quickly to a change of ice nucleating particle concentrations but respond slowly to the change of the boundary layer structure, suggesting that the coupled-to-decoupled transition of the atmospheric boundary layer is unlikely to be the main cause of the observed fast change of phase partitioning. Reference: [1]. Kalesse, Heike, Gijs de Boer, Amy Solomon, Mariko Oue, Maike Ahlgrimm, Damao Zhang, Matthew D. Shupe, Edward Luke, and Alain Protat. "Understanding rapid changes in phase partitioning between cloud liquid and ice in stratiform mixed-phase clouds: An arctic case study." Monthly Weather Review 144, no. 12 (2016): 4805-4826. [2]. Solomon, Amy, Gijs de Boer, Jessie M. Creamean, Allison McComiskey, Matthew D. Shupe, Maximilian Maahn, and Christopher Cox. "The relative impact of cloud condensation nuclei and ice nucleating particle concentrations on phase partitioning in Arctic mixed-phase stratocumulus clouds." Atmospheric Chemistry and Physics 18, no. 23 (2018): 17047-17059. [3]. Yang, Fan, Mikhail Ovchinnikov, and Raymond A. Shaw. "Long‐lifetime ice particles in mixed‐phase stratiform clouds: Quasi‐steady and recycled growth." Journal of Geophysical Research: Atmospheres 120, no. 22 (2015): 11-617.