| Section C | C index | 561-569 of 1157 terms |
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coagulation—1. In cloud physics, an obsolete term denoting any process that converts the numerous small cloud drops into a smaller number of larger precipitation particles. When so used, the term is employed in analogy to the coagulation of any colloidal system. The process can take place at temperatures both above and below 0°C for supercooled drops. See coalescence. 2. Similar to accretion. 3. The process whereby aerosol or colloidal particles collide with each other by Brownian motion and coalesce (liquid) or aggregate (solid).
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coalescence efficiency—The fraction of all collisions between water drops of a specified size that results in actual merging of the two drops into a single larger drop. In discussing the details of the growth of raindrops by collision and coalescence, it is important to distinguish clearly the terms coalescence efficiency, collision efficiency, and collection efficiency, the last being equal to the product of the first two.
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coalescence—In cloud physics, the merging of two water drops into a single larger drop after collision. Coalescence between colliding drops is affected by the impact energy, which tends to increase with the higher fall velocities of larger drops. Colliding drops having negligible impact energy compared to their surface energy behave as water spheres that collide with a collision efficiency (the fraction of small drops that collide with a large drop within the geometric collision cross section) predicted by the theory for falling spheres. The result of increasing impact energy is to flatten the colliding drops at the point of impact, impeding the drainage of the air and delaying contact between them. As the distortion relaxes, the drops rebound, reducing the coalescence efficiency for cloud drops and drizzle drops colliding with smaller drops. At larger impact energy, separation will occur if the rotational energy (fixed by conservation of angular momentum) is higher than the surface energy of the coalescing drops. This phenomenon, termed temporary coalescence, can result in satellite droplets considerably smaller than either of the parent drops. This phenomenon is also called partial coalescence because the large drop may gain mass as a result of the higher internal pressure in the small drop. At still larger impact energy, drop breakup occurs for the smaller drop. About 20% of the high-energy collisions between large raindrops (d > 3 mm) and drizzle drops (d > 0.2 mm) result in the disintegration of both drops. Other factors that affect coalescence are electric charge and electric field, both of which promote coalescence, leading to earlier onset of coalescence during an interaction so that coalescence efficiencies are increased by suppression of rebound and temporary coalescence. All of these processes are important in formation of precipitation in all liquid clouds both above and below 0°C. See collision– coalescence process.
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coarse-mesh grid—A nonspecific term indicating a grid that has a relatively low resolution, that is, its grid points are relatively far apart. The term is used to contrast a grid with another that has significantly higher resolution. See fine-mesh grid.
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coastal front—A shallow (typically < 1 km deep) mesoscale frontal zone marked by a distinct cyclonic windshift in a region of enhanced thermal contrast (≈ 5°–10°C/10 km). These fronts typically develop in coastal waters or within 100–200 km of the coast during the cooler half of the year when the land is cold relative to the ocean. In the United States coastal fronts are most frequent in New England, the Middle Atlantic states, the Carolinas, and Texas. The typical coastal front is oriented quasi-parallel to the coast and may extend for several hundred kilometers. During the winter, the coastal front may mark the boundary between frozen and nonfrozen precipitation. Given that coastal front development usually precedes synoptic-scale cyclogenesis and marks an axis of enhanced thermal contrast and a maximum in cyclonic vorticity and convergence, the coastal front often serves as a boundary along which intensifying synoptic- scale cyclones move poleward. Surface coastal front development typically occurs beneath the forward side of advancing troughs following the passage of the ridge axis aloft. Coastal fronts most frequently form equatorward of cold anticyclones where a warmer onshore flow encounters a colder continental air stream. Damming of cold air on coastal orographic barriers such as the Appalachians often appears to play an important role in coastal front development. Coastal thermal contrasts are augmented by differential diabatic heating where the onshore flow has passed over oceanic thermal boundaries such as the Gulf Stream and the adjacent continental airstream has passed over snow-covered land. Coastal fronts may form independently of cold anticyclones and associated cold air damming. In situ coastal front developments can occur near mountain barriers where upslope flow results in differential airmass cooling and stabilization and where offshore troughs form due to differential heating across oceanic thermal boundaries. Coastal front dissipation typically occurs with the cessation of onshore flow following cyclone passage.
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