From AMS Glossary
The exchange of airmass properties and the associated momentum and energy transports produced on a worldwide scale by the migratory large-scale disturbances of middle latitudes.
When the atmospheric circulation is regarded as a large-scale turbulence process, the cyclones and anticyclones are considered to be eddies superposed on the average zonal wind currents. The mixing length, that is, the average distance over which these traveling eddies maintain the characteristics of their original environment, has a value near 108 cm. The coefficient of turbulent mass exchange or the exchange coefficient, which is a measure of the intensity of the large-scale exchange processes and hence of the intensity of the general circulation, has a magnitude of between 108 and 106 gm cm-1s-1 (compared with a value of about 102 gm cm-1s-1 for small-scale turbulence). On the basis of results obtained so far, there is considerable doubt whether turbulence concepts can be applied meaningfully to the large-scale features of the atmosphere.
Haurwitz, B. 1941. Dynamic Meteorology. 265–267.