Atmospheric turbulence

Laser communication systems based on free-space propagation through the atmosphere suffer drawbacks because of factors like atmospheric turbulence and attenuation by rain, snow, haze, or fog. Nevertheless, free-space laser communication systems were developed for many appHcations (89—91). They employ separate components, such as lasers, modulators, collimators, and detectors. Some of the most promising appHcations are for space communications, because the problems of turbulence and opacity in the atmosphere are absent. 

Early models used a value for that remained constant throughout the day. However, measurements show that the deposition velocity increases during the day as surface heating increases atmospheric turbulence and hence diffusion, and plant stomatal activity increases (50—52). More recent models take this variation of into account. In one approach, the first step is to estimate the upper limit for in terms of the transport processes alone. This value is then modified to account for surface interaction, because the earth s surface is not a perfect sink for all pollutants. This method has led to what is referred to as the resistance model (52,53) that represents as the analogue of an electrical conductance... 

Wind speed has velocity components in all directions so that there are vertical motions as well as horizontal ones. These random motions of widely different scales and periods are essentially responsible for the movement and diffusion of pollutants about the mean downwind path. These motions can be considered atmospheric turbulence. If the scale of a turbulent motion (i.e., the size of an eddy) is larger than the size of the pollutant plume in its vicinity, the eddy will move that portion of the plume. If an eddy is smaller than the plume, its effect will be to difhise or spread out the plume. This diffusion caused by eddy motion is widely variable in the atmosphere, blit even when the effect of this diffusion is least, it is in the vicinity of three orders of magnitude greater than diffusion by molecular action alone. 

A transmissometer is similar to a telephotometer except that the target is a known light source. If we know the characteristics of the source, the average extinction coefficient over the path of the beam may be calculated. Transmissometers are not very portable in terms of looking at a scene from several directions. They are also very sensitive to atmospheric turbulence, which limits the length of the light beam. 

The final effective plume height H, in m, is stack height plus plume rise. Where buoyancy dominates, the horizontal distance Xf from the stack to where the final plume rise occurs is assumed to be at 3.5 , where x is the horizontal distance, in km, at which atmospheric turbulence begins to dominate entrainment. 

Briggs, G. A., "Diffusion Estimation for Small Emissions." Atmospheric Turbulence and Diffusion Laboratory, Contribution File No. 79. (draft). Oak Ridge, TN, 1973. 

Long, P. E., and Pepper, D. W., A comparison of six numerical schemes for calculating the advection of atmospheric pollution, in "Proceedings of the Third Symposium on Atmospheric Turbulence, Diffusion and Air Quality." American Meteorological Societv, Boston, 1976, pp. 181-186. 

Huber, A. H., and Snyder, W. H., Building wake effects on short stack effluents, pp. 235-242 in Preprints, Third Symposium on Atmospheric Turbulence, Diffusion and Air Quality. October 19-22, 1976, Raleigh, NC. American Meteorological Society, Boston, 1976. 

The vertical motion of the plume to the height where it becomes horizontal is known as the plume rise, (refer back to Figure 1). The plume rise is assumed to be a function primarily of the emission conditions of release, (i.e. velocity and temperature characteristics). A velocity in the vertical plane gives the gases an upward momentum causing the plume to rise until atmospheric turbulence disrupts the integrity of the plume. At this point the plume ceases to rise. This... 

The energy of large and medium-size eddies can be characterized by the turbulent diffusion coefficient. A, m-/s. This parameter is similar to the parameter used by Richardson to describe turbulent diffusion of clouds in the atmosphere. Turbulent diffusion affects heat and mass transfer between different zones in the room, and thus affects temperature and contaminant distribution in the room (e.g., temperature and contaminant stratification along the room height—see Chapter 8). Also, the turbulent diffusion coefficient is used in local exhaust design (Section 7.6). 

The above discussion holds for dispersion by atmospheric turbulence. In addition, a momentum release of fuel sometimes generates its own turbulence, e.g., when a fuel is released at high pressure in the form of a high-intensity turbulent jet. Fuel mixes rapidly with air within the jet. Large-scale eddy structures near the edges of the jet entrain surrounding air. Compositional homogeneity, in such cases, can be expected only downstream toward the jet s centerline. 

Lumley, J. L., and H. A. Panofsky. 1964. The Structure of Atmospheric Turbulence. New York John Wiley and Sons. 

Keywords atmosphere, turbulence, Kolmogorov model, phase stmcture function, transfer... 

The origin of atmospheric turbulence is diurnal heating of the Earth s surface, which gives rise to the convection currents that ultimately drive weather. Differential velocities caused perhaps when the wind encounters an obstacle such as a mountain, result in turbulent flow. The strength of the turbulence depends on a number of factors, including geography it is noted that the best observation sites tend to be the most windward mountaintops of a range— downwind sites experience more severe turbulence caused by the disturbance of those mountains upwind. 

Evolution in telescope making since the Palomar has not been limited to the area of optical production. The alt-azimuthal mount has become the established solution since the Bolshoi 6 m telescope, for its superior mechanical performance and the compact, cost-efficient enclosure design it allows. Better understanding of the properties of atmospheric turbulence allowed a more accurate characterization of a telescope properties, a more balanced approach towards specifications and error budgeting and a better understanding of the utmost importance of site selection. Any ground-based telescope of appreciable size will be primarily limited by the effect of atmospheric turbulence, not to mention the proportion of photometric nights allowed by weather conditions. 

The first example is the 4-m class William Herschel telescope, at la Pakna, whose optical specifications, drafted by D. Brown, were expressed in terms of allowable wavefront error as a function of spatial frequencies matching those of atmospheric turbulence. 

Adaptive optics. atmospheric turbulence, residuals Metrology Wavefront sensor(s)... 

Adaptive optics (AO) undoubtedly constitutes the most daring challenge. It is also absolutely essential seeing-limited observations with a 100-m class telescope would imply impossibly short focal ratio of the instrumentation, and immediate saturation by sky background. A smaller on-sky resolution element is the only way out, which implies at least some degree of adaptive compensation for atmospheric turbulence. 

Atmospheric turbulence is a dynamic process and the wavefront aberrations are constantly changing. A characteristic timescale may be defined as the time over which the mean square change in wavefront error is less than 1 rad. If the turbulence were concentrated in a single layer with Fried parameter ro moving with a horizontal speed of v ms then the characteristic time, ro, is given by... 

Spot size. The size of the LGS is a critical issue, since it dehnes the saturation effects of the laser and the power needed to reach a given system performance, and also the quality of the wavefront sensing. There is an optimum diameter of the projector, because, if the diameter is too small, the beam will be spread out by diffraction and if it is too large it will be distorted due to atmospheric turbulence. The optimum diameter is about 3ro, thus existing systems use projection telescopes with diameters in the range of 30-50 cm. 

For large telescope apertures, Na LGS offer improve sampling of the atmospheric turbulence due to their much higher altitude. Single beam systems are now being developed for and deployed on 8-10 m class telescopes. Since resonant backscattering from the mesospheric Na layer is the method chosen for most LGS projects, we will concentrate mostly on this technique. 

The term Pr d is the key to solving an underdetermined system of equations. Given two possible solutions which would produce exactly the same measurements, we use Pr d to select the most likely. In the case of a wave-front aberration caused by atmospheric turbulence the estimated coefficients. 

As expecfed the optimal estimator is indeed a function of both the covariance of the atmospheric turbulence and the measurement noise. A matrix identity can be used to derive an equivalent form of Eq. 16 (Law and Lane, 1996)... 

Noll, R., 1976, Zernike Polynomials and atmospheric turbulence, JOSA.A 56, 207... 

Wind vectors (speed and direction), and Dispersive effects (intensity of atmospheric turbulence). 

G. A. Briggs, Diffusion Estimation for Small Emissions, Report ATDL-106 (Washington, DC Air Resources, Atmospheric Turbulence, and Diffusion Laboratory, Environmental Research Laboratories, 1974). 

General References Britter and McQuaid, Workbook on the Dispersion of Dense Gases, Health and Safety Executive Report 17/1988, Sheffield, U.K., 1988. Mannan, Lees Loss Prevention in the Process Industries, 3d ed., Chap. 15, Elsevier Butterworth-Heinemann, Oxford, U.K., 2005. Panofsky and Dutton, Atmospheric Turbulence, Wiley, New York, 1984. Pasquill and Smith, Atmospheric Diffusion, 3d ed., Ellis Horwood Limited, Chichester, U.K., 1983. Seinfeld, Atmospheric Chemistry and Physics of Air Pollution, Chaps. 12-15, Wiley, New York, 1986. Turner, Workbook of Atmospheric Dispersion Estimates, U.S. Department of Health, Education, and Welfare, 1970. 

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