The intensity of lateral mixing in the atmosphere as determined from isentropic charts

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Abstract
Since the inauguration of the analysis of isentropic charts at the Massachusetts Institute of Technology it has been found that certain well‐defined properties are exhibited by these charts, the most important of which is the occurrence of well‐marked currents of moist air which can be traced in their course across the whole United States and which have a characteristic distribution of specific humidity q. In the center of the moist currents, the specific humidity is a maximum where the current first appears on the chart and slowly decreases downstream while at the same time there is a gradual lateral spreading apart downstream of the q‐lines normal to the axis of the current. This distribution of specific humidity is about what one might expect to find when there is a transport of moisture from a moist current to the surrounding drier atmosphere brought about by turbulent mixing, or diffusion. As Rossby has already pointed out [see 1 of “References” at end of abstract] and as can be seen from the daily isentropic charts, the currents of maximum moisture advance less rapidly than would be expected from the observed wind‐distribution, thus indicating lateral, isentropic mixing with the environment. It thus appears that the observed distribution of specific humidity in well‐marked currents on the isentropic chart must be largely the result of an isentropic mixing‐process. Once this is recognized it is then possible to arrive at the intensity of the mixing expressed as a coefficient of eddy‐diffusion.

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