| Section M | M index | 221-229 of 651 terms |
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mei-yu front—(Also called baiu front). A quasi-persistent, nearly stationary, east–west-oriented weak baroclinic zone in the lower troposphere that typically stretches from the east China coast, across Taiwan, and eastward into the Pacific, south of Japan. The term “mei-yu” is the Chinese expression for “plum rains.” The mei-yu front generally occurs from mid- to late spring through early to midsummer. This low-level baroclinic zone typically lies beneath a confluent jet entrance region aloft situated downstream of the Tibetan Plateau. The mei-yu/baiu front is very significant in the weather and climate of southeast Asia as it serves as the focus for persistent heavy convective rainfall associated with mesoscale convective complexes (MCCs) or mesoscale convective systems (MCSs) that propagate eastward along the baroclinic zone. The moisture source is typically the South China Sea and sometimes the Bay of Bengal. The usual lifting mechanism is low-level warm-air advection in association with a low-level jet on the equatorward flank of the baroclinic zone. Deep ascent and resulting organized MCCs/MCSs are especially favored when the low-level warm-air advection is situated beneath the favorable equatorward jet entrance region aloft.
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Mellor–Yamada parameterization—A parameterization of a complex model for turbulent flows in the planetary boundary layer. A series of simplifications of the full turbulence model to remove complex terms and form a closed set of equations leads to a hierarchy of so-called closure models of decreasing complexity labeled level 4 through level 1. The level 2.5 model is widely used; it incorporates only one additional equation and produces conventional turbulent fluxes.
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melt pond—A pond of liquid water (mostly from melted snow) on the surface of sea ice, usually occurring in the spring. Melt ponds are common in the Arctic but less so in the Antarctic.
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meltém—1. (Also spelled meltémi.) A strong wind from the northeast or east that often sets in suddenly and blows during the day in summer on the Bulgarian coast and in the Bosporus. 2. Same as monsoon.
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melting layer—The altitude interval throughout which ice-phase precipitation melts as it descends. The top of the melting layer is the melting level. The melting layer may be several hundred meters deep, reflecting the time it takes for all the hydrometeors to undergo the transition from solid to liquid phase. The temperature of the melting layer is typically 0°C or slightly warmer. See bright band.
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melting point—The temperature at which a solid substance undergoes fusion, that is, melts, changes from solid to liquid form. The melting point of a substance should be considered a property of its crystalline form only. At the melting point the liquid and solid forms of a substance exist in equilibrium. All substances of crystalline nature have their characteristic melting points. For very pure substances the temperature range over which the process of fusion occurs is very small. The melting point of a pure crystalline solid is a function of pressure; it increases with increasing pressure for most substances. However, in the case of ice (and a few other substances) the melting point decreases with increasing pressure (see regelation). Under a pressure of one standard atmosphere, the melting point of pure ice is the same as the ice point, that is, 0°C. Compare freezing point.
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meniscus—The crescent-shaped upper surface of a column of liquid. The curvature of this surface is dependent upon the cross-sectional area of the liquid and the relative ability of the liquid to wet the walls of the enclosure. For liquids that wet the walls of the enclosure, the curvature of the surface is concave. In the case of a mercury column enclosed in a glass container, the surface is convex, since mercury does not wet glass.
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