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Break in the clouds: How dynamical and microphysical timescales define stratocumulus structure

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Monday, November 18, 2013

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Mikhail Ovchinnikov
Research Scientist
Pacific Northwest National Laboratory

Strikingly different formations of stratocumulus clouds can occur within nearly the same environment, as evidenced by the common emergence of honeycomb-like "open" cells surrounded by overcast, or "closed"-cell areas. These intriguing features not only provide for spectacular satellite images, but also present a challenge for climate models striving to correctly predict clouds with markedly different abilities to reflect solar radiation. In this presentation, a new conceptual model for the transition between closed and open cloud cells is presented. It postulates that the resulting cloud structure reflects the balance between two competing processes: mixing by turbulent eddies, which push the layer toward a vertically and horizontally uniform distribution of total (vapor + condensate) water, and precipitation, which tends to disturbs that uniformity. When sufficient drizzle forms within updrafts, cloud water in the outflow is depleted enough that an overcast cloud cannot be sustained. The intensity of these processes is related to two timescales, an updraft timescale and a rain initiation timescale, which in turn are linked to three observable parameters (droplet number concentration, cloud depth, and updraft speed) using a cloud parcel model. An approximate but insightful analytical criterion for the transition from closed to open cells is also derived, quantifying the interplay between the dynamical and microphysical factors in controlling the cloud structure. The model demonstrates remarkable skill in categorizing several observed cases of open and closed cell clouds in different environments and provides critical new insights for improving their representations in climate models.

Host: Raymond Shaw

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