The role of ice nuclei recycling in the maintenance of cloud ice in Arctic mixed-phase stratocumulus

Description

This study investigates the maintenance of cloud ice production in Arctic mixed-phase stratocumulus in large eddy simulations that include a prognostic ice nuclei (IN) formulation and a diurnal cycle. Balances derived from a mixed-layer model and phase analyses are used to provide insight into buffering mechanisms that maintain ice in these cloud systems. Observations of a persistent single-layer Arctic mixed-phase stratocumulus cloud observed near Barrow, Alaska, on 8 April 2008 during the Indirect and Semi-Direct Aerosol Campaign are used for this study. The adjacent Beaufort Sea was generally ice covered during this time, with significant areas of open water observed east of Barrow. A 4 K temperature inversion with inversion base at 1.05 km was observed via a radiosonde at 17:34 UTC; static stability was near neutral within the mixed layer overlying a stable near-surface layer with static stability greater than 2 K/km below 500 m. In this study we demonstrate that sustained recycling of IN through a drying sub cloud layer and additional activation of ice nuclei due to a cooling cloud layer is sufficient to maintain ice production and that these processes regulate liquid production over multiple days in a decoupled AMPS. This study provides an idealized framework to understand feedbacks between dynamics and microphysics that maintain phase partitioning in Arctic mixed-phase stratocumulus. In addition, we show that modulation of the cooling of the cloud layer and the humidity of the sub cloud layer by the diurnal cycle buffers the mixed-layer system from a loss of particles and promotes the persistence of a mixed-phase cloud system. The results of this study provide insight into the mechanisms and feedbacks that may maintain cloud ice in Arctic mixed-phase stratocumulus even when entrainment of IN at the mixed-layer boundaries is weak. While the balance of these processes changes depending upon the specific conditions of the cloud layer, for example whether the cloud layer is coupled to the surface layer, the mechanisms detailed in this paper will manifest to some degree and therefore the current study provides a framework for understanding the role of recycling in maintaining phase partitioning in mixed-phase clouds.