- 行业: Weather
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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, ...
1. The covariance between two variables, associated with turbulent motions. For example, if the overbar represents a mean value, and the prime denotes a deviation from the mean, then
is the eddy correlation of vertical velocity w and potential temperature θ, where i is a data index and N is the total number of data points. If one of the two variables is a velocity, such as in the example here, then the eddy correlation represents a kinematic flux associated with turbulence. In this example, the correlation is a vertical kinematic heat flux (units of K m s−1), which can be transformed into a dynamic flux (W m−2) by multiplying by air density times specific heat at constant pressure. 2. A method of measuring the flux densities of mass, heat, and momentum across a plane at a point in turbulent flow. For vertical fluxes, the fluxes are found as the covariance of the fluctuations in the vertical wind velocity with local variations in concentration, heat content, or horizontal wind velocity, respectively. For conservative quantities in the atmospheric surface layer, such vertical fluxes are virtually equal to the fluxes at the surface of the earth. Measurements from towers require sensors with a speed of response that is typically no larger than about 1 s, but the required response varies with height, wind speed, and amount of buoyancy-induced mixing. Observations from eddy correlation systems on fixed-wing aircraft in the planetary boundary layer typically require a response that is about ten times faster than stationary systems.
Industry:Weather
The most direct approach for determining a vertical turbulent flux. The flux is calculated as the average of the instantaneous product of vertical velocity and a scalar quantity (such as a concentration).
Industry:Weather
In general, the continuum hypothesis used to generalize Newton's laws for point masses to continuous media such as fluids or gases. At any point in the continuous media it must be possible to identify volumes small enough that derivatives exist, yet large enough to contain sufficient particles that the average macroscopic property (such as momentum, mass, or temperature) may be defined. The eddy continuum hypothesis is used to extend the concept of a fluid continuum to a turbulent flow in which small- scale eddies are substituted for fluid particles. To apply the eddy continuum hypothesis, a scale analysis must be performed to verify that the averaging periods are sufficiently long that enough small-scale turbulent eddies are sampled that their dynamical effect on the mean flow is captured. If the eddies' length scales are comparable to the mean flow length scales, the eddy continuum most likely can be defined. The eddy continuum hypothesis is often applied in the study of large- scale organization (of the smaller-scale turbulent eddies) in the planetary boundary layer turbulence.
Industry:Weather
Transport of a fluid property by eddies too large to be modeled by eddy diffusivity in a turbulent flow. Examples are the transport by organized large eddies in the boundary layers of the atmosphere and the ocean. These large eddies have dimensions comparable to the height of the boundary layers.
Industry:Weather
A technique for estimating vertical turbulent trace gas fluxes that does not require fast response sensors. Air is collected in two reservoirs, one when the vertical velocity is positive and the other when the vertical velocity is negative. The volume rate at which the sample is collected is proportional to the magnitude of the vertical velocity. The flux is estimated from the difference in mass collected in each reservoir divided by the collection time.
Industry:Weather
1. By analogy with a molecule, a “glob” of fluid within the fluid mass that has a certain structure and life history of its own, the activities of the bulk fluid being the net result of the motion of the eddies. The concept is applied with varying results to phenomena ranging from the momentary spasms of the wind to storms and anticyclones. 2. Any circulation drawing its energy from a flow of much larger scale, and brought about by pressure irregularities, as in the lee of a solid obstacle. 3. In studies of the general circulation, departures of a field (e.g., temperature or relative vorticity) from the zonal mean of that field. 4. A closed circulation system produced as an offshoot from an ocean current. Eddies are the result of the turbulence of the oceanic circulation and are common throughout the World Ocean. The corresponding features in the atmosphere are the wind currents around high and low pressure disturbances. Oceanic cyclonic eddies have a shallow thermocline at the center and are therefore also known as cold-core eddies; anticyclonic eddies are associated with a depressed thermocline in the center and are also known as warm-core eddies. The most prominent eddies are those shed by western boundary currents, also known as rings; they are about 200 km in diameter and reach beyond a depth of 1500 m. Another class of eddies is produced by shear between currents flowing in opposing directions. These eddies tend to be smaller (10–50 km in diameter) and shallower.
Industry:Weather
1. Of or relating to the soil. 2. Resulting from or influenced by the soil rather than the climate.
Industry:Weather
In ecology, a zone of transition from one major plant community to another. For example, the forest–tundra ecotone in high northern latitudes is a zone of patchy and often stunted tree growth intermixed with areas of tundra.
Industry:Weather
Organisms and the environment in which they interact. Sometimes the definition is extended to include the processes of interaction. See ecology.
Industry:Weather
