Atmospheric tide
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Atmospheric tide, air tide, or atmospheric oscillation is a periodic atmospheric motion with a size scale of the Earth, with periods of 24, 12, 8, or 6 hr (24/n hr, n=1,2,3,... = the number of maxima per day). It is usually measured in mbar or µbar (pressure). Wind and temperature measurements show that atmospheric tides often govern the dynamics of the mesosphere and lower thermosphere(~80-120km). The use of the word tide is only by analogy to water tide, and the mechanics and effects are quite different. [1]
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[edit] Solar and thermal atmospheric tide
Atmospheric tide is dominated by the solar atmospheric tide, which is due to the thermal and gravitational action of the Sun. The solar atmospheric tide is not stopped by terrain and propagates westward with the apparent motion of the Sun around the entire Earth. The thermal tide is the variation in atmospheric pressure due to the diurnal differential heating of the atmosphere by the Sun, that is, daytime warming and nighttime cooling. This means compressibility (or expansion) is important in atmospheric tides. [2] Gravitational tide is an atmospheric tide due to the periodic gravitational (and centrifugal) forcing due to the sun or moon. Lunar (gravitational) tide contributions are much weaker (less than 60 µbar or so). The 12 hr component (or contribution) of the solar tide has by many times the greatest global amplitude of any atmospheric tidal component, typically 0.5 mbar in mid-latitudes and about 1.5 mbar in the tropics (comparable to 0.5 and 1.5 cm of water height). This relatively large amplitude is often explained as a resonance effect. The 12-hour or semidiurnal solar atmospheric tide (labeled S2) is both gravitational and thermal in origin. The 24 hr component or diurnal solar atmospheric tide (labeled S1) is a thermal tide with great local variability, reaching amplitudes of 1.5 mbar over some tropical land areas.
[edit] Migrating tide model
The previous section looked at the atmospheric tides moving over the Earth. This section reverses the point of view, looking at the Earth as it rotates and keeping the tides fixed.
To a large extent, particularly S2, the atmospheric solar tides do not move relative to the Sun. They appear as a steady influence on the atmosphere, not changing over daily and seasonal periods as observed from the Sun. This is called a mechanistic migrating tide model. Just as an observer at a fixed place on the Earth will see the Sun move across the sky, the observer will measure variations of 24 hr period (or harmonics) in solar tide resulting from the Earth's rotation through the fixed pattern of variations in atmosphere. Similarly, seasonal variations in the solar tides occur as the Earth tilt varies within this pattern. The periodic change of the Earth in this fixed pattern means that time periodicity coincides with spatial(zonal) periodicity. In particular, for a fixed tidal component S1, S2, ..., the number of maxima per day n=1,2,..., (respectively), will be the same as the number of maxima counted at a fixed latitude going around the Earth. Additionally the tidal patterns along a fixed longitude repeat after one period. The mechanistic migrating tide models predict the important wind and temperature patterns in the mesosphere and lower thermosphere. [3] This also means that migrating atmospheric tides do not depend much on the non-periodic weather experienced at ground level. Although there is a periodic absorption of solar radiation throughout the atmosphere, the main contributions to tide are absorption of ultraviolet radiation by stratospheric ozone and of infrared by water and water vapor in the troposphere. [4]
[edit] Non-Migrating influences
Influences and forcing from other sources than in the migrating tidal model may be added for better predictions and explanation.
Influences which are fixed in time or very slowly changing are empirical background climatologies of zonal mean temperature, neutral density, background winds, and latitudinal gradients in the background atmosphere. [5] Background concentrations of water vapor and ozone also vary with altitude and location on the globe. The background atmospheric motion includes geostrophic wind jets caused by equilibrium with meridional energy exchange balancing differences in global heating between the northern and southern hemispheres.
Another thermal forcing is the latent heat which is stored in water vapor and transported by complex meteorological activity, to be released again in other regions of the globe when the vapor precipitates.
Finally the S1 component has large amplitudes due large warming effects over equatorial Africa, especially in the east, northern South America, especially eastward of the Andes and eastern Brazil, and northern Australia.
[edit] Dissipation
Atmospheric tidal dissipation may come from many mechanisms.
Short-term or small-scale waves are generated over terrain (mountains, for example) and called gravity waves. Gravity waves increase in amplitude with altitude due to the decreasing density of the atmosphere and above 80km the gravity waves begin to break. This disrupts tidal components by causing turbulence, resulting drag on the tidal flow and also causing eddy currents that disrupt the energy propagation.
Upward propagating tidal amplitudes also increase with altitude as the density decreases, however wave dissipation effects become important in the middle and upper atmosphere. Above about 90-100km in the lower thermosphere, the decreasing density of the air causes energy transfer by wave motion becomes less efficient due to molecular diffusion, further damping the tides. Above the thermosphere, tides are dissipated by the radiation of energy into space and drag caused by charged particles in the ionosphere.
[edit] Effects of atmospheric tide
Atmospheric tides affect gravity. They cause very clear perturbations in the orbits of artificial satellites, and for this reason the tides must be accurately modeled. [6]
At some quiet weather stations in the tropics, the atmospheric tide is sometimes the dominant signal in the barometric records.
[edit] See also
[edit] Notes and references
- ^ Atmospheric tide definitionfrom the AMS; based on S. Chapman, 1951: Compendium of Meteorology, 510-530. Tides will always refer to atmospheric tides in this article
- ^ This is unlike water tide fluid mechanics which assumes incompressibility. The vertical tidal accelerations, however, are still negligible in air.
- ^ Salient features of signatures from ground-based and satellite-borne wind and temperature measurements agree well with predictions of the mechanistic migrating tide models for the mesosphere and lower thermosphere.
- ^ Global Scale Wave Model UCAR. It should not be concluded that within this relatively stationary pattern that there no wind; the wind pattern stays the same.
- ^ Background, average, unperturbed and so forth are used to describe a time invariant (or varying only seasonally) atmospheric pattern that stable and has no tidal effects.
- ^ Barometric Pressure Tide from R. D. Ray and R. M. Ponte, Barometric tides from ECMWF operational analyses, Annales Geophysicae 21: 1897-1910 (2003), European Geosciences Union.
