The making and breaking of a cloud: How microphysical and dynamical timescales define stratocumulus structure
Strikingly different formations of stratocumulus clouds can occur within nearly identical environments, 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 formidable challenge for climate models striving to correctly predict these clouds with markedly different abilities to reflect solar radiation. In this presentation, a conceptual model for the transition between closed and open cloud cells is presented based on Edwin Kessler’s considerations for the continuity of water substance in atmospheric circulations. 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 disturb that uniformity. When sufficient precipitation forms within updrafts, cloud water in the outflow is depleted to the point that an overcast cloud layer cannot be sustained. Intensities of these processes are 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.
Dr. Ovchinnikov earned an M.S. degree in applied mathematics from the Moscow Institute of Physics and Technology in 1989 and a Ph.D. degree in meteorology from the University of Oklahoma in 1997. He worked as a research scientist at the Central Aerological Observatory in Moscow, Russia (1988-1992) and then as a research assistant and later scientist at the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS, 1992-2000) before joining the Pacific Northwest National Laboratory (PNNL) in Richland, WA in 2000 as a senior research scientist. Dr. Ovchinnikov’s scientific interests include a wide range of topics in cloud physics, as well as cloud-related aspects of remote sensing, radiation, and atmospheric dynamics. His research, funded by DOE, NASA, and NOAA projects, has three major thrusts: 1) studying individual cloud and aerosol processes, 2) retrieving cloud and aerosol properties using remote sensors, and 3) developing, testing, and improving parameterizations for atmospheric models from cloud to global scales.