Open Cell Convection What is it? • Three-dimensional convective circulation • Occurs when cold air moves over warmer water. • Hexagonal rings of convective clouds surrounding clear air. • Cross section view: • Thought to be an atmospheric analogue to Rayleigh-Benard convection, a hexagonal vertical circulation that occurs when a fluid is heated at the bottom. Open cell convection over the ocean. Rayleigh-Benard cells in cylindrical vessel. • Cold air moves over warmer water behind cold fronts, west and south of low pressure centers. • Generally found in mid- to-high latitude locations over the oceans, usually during cold season. • In GOES imagery of the northern hemisphere, we are most likely to see them over the North Atlantic and North Pacific (not at subtropical latitudes!). • Typical open cell pattern in the Pacific—it is mainly a cold-season phenomenon. • Closer view shows that some cells have cirrus anvils. They can bring showery weather, thunder, and small hail, but their tops are low (20-25K ft.) compared to summer t-storms. • Open cells occur with showery blustery weather: Sfc analysis for previous slides: open cells are in northerly flow on west side of low pressure center, behind cold front. 300 mb jet stream pattern: trough illustrates that cold air has come southward off west coast. • Notice open cells forming only over Hudson Bay where water warmer than air has destablized lower atmosphere. Open vs. Closed Cells • Closed cells occur where there is not as much temperature difference between the water and the air. • They may be relatively lower level stratocumulus clouds. • Open cells occur in a more unstable atmosphere; they are more convective. • Open cells are rings of clouds surrounding clear airpattern is more black than white. • Closed cells are rings of clear air surrounding cloudsmore white than black. • Open cells common in high latitudes. • Closed cells common in low latitudes. • Transition from open to closed can mark the position of jet stream…see following slides. jet stream “open” cells (more black than white) Jet stream axis “closed” cells (more white than black) Documentation of jet stream.