Troposphere vertical eddy diffusion

Mechanisms and rates of transport of nuclear test debris in the upper and lower atmosphere are considered. For the lower thermosphere vertical eddy diffusion coefficients of 3-6 X 106 cm.2 sec. 1 are estimated from twilight lithium enhancement observations. Radiochemical evidence for samples from 23 to 37 km. altitude at 31° N indicate pole-ward mean motion in this layer. Large increases in stratospheric debris in the southern hemisphere in 1963 and 1964 are attributed to debris from Soviet tests, transported via the mesosphere and the Antarctic stratosphere. Most of the carbon-14 remains behind in the Arctic stratosphere. 210Bi/ 210Pb ratios indicate aerosol residence times of only a few days at tropospheric levels and only several weeks in the lower stratosphere. Implications for the inventory and distribution of radioactive fallout are discussed. 

The large fluctuations with altitude that are frequently observed in tropospheric measurements of ozone and water number densities suggest that vertical eddy diffusion with a constant diffusion coefficient is not adequate. 

Table 1-7 summarizes vertical eddy diffusion coefficients for the troposphere and the lower stratosphere as derived from various observations, mainly of tracers. For the troposphere the Kz data of Davidson et al. (1966) are the least trustworthy, because they are based on tracers originating in the stratosphere. The best values probably are those of Bolin and Bischof (1970), derived from the seasonal variation of C02. Above the tropopause... 

While molecular diffusivity is commonly independent of direction (isotropic, to use the correct expression), turbulent diffusivity in the horizontal direction is usually much larger than vertical diffusion. One reason is the involved spatial scales. In the troposphere (the lower part of the atmosphere) and in surface waters, the vertical distances that are available for the development of turbulent structures, that is, of eddies, are generally smaller than the horizontal distances. Thus, for pure geometrical reasons the eddies are like flat pancakes. Needless to say, they are more effective in turbulent mixing along their larger axes than along their smaller vertical extension. 

As the plane continues to climb, the ride becomes smooth. Coffee served in open cups does not spill, providing testimony that in the tropopause [at approximately 10,000 m (33,000 ft)] and the still higher stratosphere (Fig. 4-1), the size and energy of atmospheric eddies decrease. Weather phenomena are confined almost entirely to the troposphere. Being at the edge of the stratosphere is comparable to being in the thermocline of a stratified lake (Section 2.2.2) turbulent diffusion is suppressed, and vertical Fickian transport is slowed. Chemicals released into the air near Earth s surface may mix... 

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