Cloud ice could impact cloud patterns in mixed-phase stratocumulus

Figure [Courtesy Eirund et al (2019)]: Snapshot of vertically integrated liquid water path (LWP) and ice water path (IWP) of mixed phase clouds simulated within the control simulation (frame 1), a simulation with enhanced ice formation (frame 2), and a simulation of enhanced liquid precipitation (frame 3). Grey to white contours correspond to increased amounts of ice or liquid water, whereas the ocean surface (dark blue) is visible in regions of clear sky. In both perturbed simulations the cloud deck breaks up and the cloudy cell rings are spaced further apart.

Cloud-ring structures, or so-called open cells, are characterised by a ring of thick cloud surrounding a region of either thin cloud or clear-sky in between. They have been extensively studied in the subtropics for many decades. In this climate zone these clouds only consist of liquid drops and water vapour. In the extratropics of the northern hemisphere during the winter months, in the Southern Ocean all year around, and in the lower Arctic, these open-cell cloud systems do not only contain liquid drops and water vapour, but also small amounts of ice crystals and snow.

The dynamics of these systems and the spatial scale of the cloud pattern is predominantly driven by a cycle of precipitation formation, precipitation fall-out, and cooling below the cloud as the precipitation evaporates (liquid) or sublimates (ice) before reaching the surface. This cooling in turn leads to the formation of so-called cold pools that displace the warmer surrounding air, which converges along the edges of colliding cold pools where they generate medium-sized updrafts that induce new cloud formation and consequently precipitation formation and closes the cycle.

Thus the pattern itself as well as the its spatial scale are strongly governed by this cycle of precipitation, which in turn governs the cold pool strength and cold pool size. Deeper clouds can produce more precipitation and thus, deeper boundary layers are associated with larger cells. Mixed-phase clouds, as opposed to pure liquid clouds, have two mechanisms of generating precipitation: the ice and the liquid phase.

In this proof-of-concept study Eirund et al (2019) explore whether the formation of ice inside mixed-phase clouds can affect the precipitation and spatial statistics of these clouds. In perturbation experiments performed in a high-resolution numerical model, we show that mixed-phase clouds with an ice water to liquid water path ratio of 1:2 are associated with larger cell diameters when compared to the corresponding super-cooled liquid cloud scenario.

Mixed-phase clouds where the liquid precipitation was enhanced, also displayed this increase in cell size. However, the duration of the increased characteristic pattern scale was of a relatively short time-period. Meanwhile these simulations indicate that a prolonged increase on cell size is plausible in mixed-phase open-cell clouds associated with substantial ice-phase precipitation.

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