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The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, AMS has a membership of more than 14,000 professionals, ...
The direction toward which a fluid is moving, usually implying the horizontal component of the mean direction or direction of the basic current; the opposite of upstream.
Industry:Weather
In general, the change in frequency of a signal reaching a receiver when the receiver and the transmitting source are in motion relative to one another. This phenomenon was first noted for sound waves by the Austrian physicist Christian Johann Doppler (1803–53) in 1842. In meteorology, this effect is successfully employed with remote sensors, such as Doppler radars and Doppler lidars, in which the receiver (collocated with the transmitter) is fixed and only the scatterers (upon which transmitted power impinges and is reradiated) are moving. The frequency shift, f, induced by a scatterer having a radial component of motion vr relative to the radar may be expressed as
where λ is the wavelength of the transmitter, f is positive for motion toward the radar, and, by the usual convention, vr is positive for motion away from the radar. See also Doppler velocity.
Industry:Weather
A very strong, usually gusty, and occasionally violent wind that blows down the lee slope of a mountain range, often reaching its peak strength near the foot of the mountains and weakening rapidly farther away from the mountains. Gust speeds in such winds may exceed 50 m s−1 and occasionally strong vortices capable of doing F1 to F2 damage (see Fujita scale) may occur in association with these winds. Such windstorms are most likely to the lee of elongated quasi-two-dimensional mountain ranges and can be distinguished from gap winds, which are confined to within or downstream of notable gaps or breaks in a mountain barrier and are generally weaker and less gusty. Downslope windstorms of great severity require an upstream mountain range having a crest at least roughly 1 km in height above terrain to its lee, and with a steep leeside slope. Meteorological conditions favoring downslope windstorms are strong synoptic-scale flow across the mountain barrier at the level of its crest, with the cross-range component of the flow either decreasing with height or not increasing too rapidly with height above the crest. Also favorable is high static stability at the level of the mountain crest in the flow approaching the mountain range, decreasing with height above. A mean-state critical level in the middle troposphere, where the flow component across the mountain drops to zero and reverses sign, is often very favorable for downslope windstorms. Downslope windstorms can be considered a gravity wave phenomenon in the sense that vertically propagating gravity waves launched by the passage of stable air over high-amplitude terrain become very steep or break, creating an internal region above the mountain that is characterized by turbulence and a lapse rate approaching the dry adiabatic. Such a region restricts the vertical propagation of energy, allowing the flow near the surface of the mountain to accelerate downslope. Downslope windstorms can also be considered hydraulic jump phenomena in which flow becomes supercritical above and to the lee of a mountain barrier. Downslope windstorms are often known by local names in areas where they occur throughout the world (e.g., the bora along the northeastern shore of the Adriatic Sea and the Taku wind along the Gastineau Channel in southeast Alaska).
Industry:Weather
1. A wind directed down a slope, often used to describe winds produced by processes larger in scale than the slope. Because this flow produces subsidence, downslope winds experience warming, drying, increasing stability, and clearing if clouds are present. 2. Flow directed down a mountain slope and driven by cooling at the earth's surface: a component of the mountain–valley or mountain–plains wind systems; same as katabatic wind. The many synonyms for downslope flow are sometimes used interchangeably, and this gives rise to ambiguity and confusion. Downslope can be used generically to denote any wind flow blowing down a slope, or it is used specifically for katabatic flows on any scale, such as the nocturnal slope-wind component of mountain–valley wind systems or mountain–plains wind systems. See katabatic wind, gravity wind, drainage wind, fall wind, bora, foehn, chinook.
Industry:Weather
The net flux of a variable from regions of high to low concentration, caused by either molecular diffusion or small eddy turbulence. See down-gradient diffusion.
Industry:Weather
A term sometimes applied to the strong downward-flowing air current that marks the dissipating stage of a thunderstorm. Compare uprush.
Industry:Weather
A random molecular process where there is net movement of variables such as chemicals, temperature, moisture, or momentum, from regions of high concentration toward lower concentration. A classic example is heat flowing from the hot end toward the cold end of a metal rod. Gradient refers to the change of mean variable across a distance, such as the temperature change across the metal rod. Down-gradient refers to movement from high to low concentration, such as from hot to cold (as opposed to counter-gradient, meaning from cold to hot). Many turbulence theories such as K-theory and mixing-length theory assume that turbulent transport is caused mostly by small eddies, resulting in a local transport (down-gradient turbulent diffusion). Such small eddy local turbulent transport is not a law, but is a turbulence closure assumption that works well for those atmospheric situations where small eddies dominate (such as in the atmospheric surface layer), but that usually fails for situations where large eddies dominate (such as in the atmospheric mixed layer where large convective thermals cause nonlocal turbulent transport). Compare counter-gradient flux.
Industry:Weather
Small-scale downward moving air current in a cumulonimbus cloud. See also draft.
Industry:Weather
A (usually square) matrix for which the sum of each row and each column equals one. An example is a transilient matrix used to describe vertically nonlocal turbulent mixing in the atmosphere, where the sum of each column must equal one to conserve a state such as heat or moisture, and the sum of each row must equal one to conserve air mass. The largest eigenvector of a doubly stochastic matrix is not larger than unity, which implies absolute numerical stability for any time step and any grid spacing when using the matrix to make forecasts of turbulent mixing.
Industry:Weather
An area of strong, often damaging winds produced by a convective downdraft over an area from less than 1 to 10 km in horizontal dimensions.
Industry:Weather
