Clear-air turbulence

Whereas the previous section (7.3,8) was concerned with the calculation of system performance for the vacuum channel, we now turn to the error probabilities for three-frequency nonlinear heterodyne detection for the atmospheric channel. The behavior of the clear-air turbulent atmosphere as a lognormal channel for optical radiation has been well documented both theoretically and experimentally [7.76-78, 80-82], We therefore choose the amplitudes /4i and A 2 to be lognormally distributed, and the phases < j and (j>2 to be uniformly distributed over (0,27t). Since A ocAj and while... 

The region above the atmospheric boundary layer is the free atmosphere. Here, frictional effects are generally negligible except for clear air turbulence caused by shearing instability near atmospheric fronts, cumulus convection, and upward-propagating gravity waves. These are subgiid-scale processes (see Section IV.G). 

Aviation. Perhaps the most obvious industry that relies on the results of meteorology is the airline industry. No flight, whether private, commercial, or military, is undertaken without a clear idea of the predicted weather conditions from point of departure to point of arrival. Changing conditions and unexpected pockets of air turbulence are a constant concern of pilots and their crews, who are trained in methods of analyzing meteorological data while in flight. 

Thermal turbulence is turbulence induced by the stability of the atmosphere. When the Earth s surface is heated by the sun s radiation, the lower layer of the atmosphere tends to rise and thermal turbulence becomes greater, especially under conditions of light wind. On clear nights with wind, heat is radiated from the Earth s surface, resulting in the cooling of the ground and the air adjacent to it. This results in extreme stabihty of the atmosphere near the Earth s surface. Under these conditions, turbulence is at a minimum. Attempts to relate different measures of turbulence of the wind (or stability of the atmosphere) to atmospheric diffusion have been made for some time. The measurement of atmospheric stabihty by temperature-difference measurements on a tower is frequently ntihzed as an indirect measure of turbulence, particularly when climatological estimates of turbulence are desired. 

Other factors to account for topography with regard to valley or hillside sites should include possible inversion and failure to disperse pollutants. Temperature inversion occurs when the temperature at a certain layer of the atmosphere stays constant, or even increases with height, as opposed to decreasing with height, which is the norm for the lower atmosphere. Inversions may occur on still, clear nights when the earth and adjacent air cools more rapidly than the free atmosphere. They may also occur when a layer of high turbulence causes rapid vertical convection so that the top of the turbulent layer may be cooler than the next layer above it at the interface. 

Hexachloroethane released to water or soil may volatilize into air or adsorb onto soil and sediments. Volatilization appears to be the major removal mechanism for hexachloroethane in surface waters (Howard 1989). The volatilization rate from aquatic systems depends on specific conditions, including adsorption to sediments, temperature, agitation, and air flow rate. Volatilization is expected to be rapid from turbulent shallow water, with a half-life of about 70 hours in a 2 m deep water body (Spanggord et al. 1985). A volatilization half-life of 15 hours for hexachloroethane in a model river 1 m deep, flowing 1 m/sec with a wind speed of 3 m/sec was calculated (Howard 1989). Measured half-lives of 40.7 and 45 minutes for hexachloroethane volatilization from dilute solutions at 25 C in a beaker 6.5 cm deep, stirred at 200 rpm, were reported (Dilling 1977 Dilling et al. 1975). Removal of 90% of the hexachloroethane required more than 120 minutes (Dilling et al. 1975). The relationship of these laboratory data to volatilization rates from natural waters is not clear (Callahan et al. 1979). 

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