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

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.

Circulation in upper layers of southern North Atlantic deduced with use of isentropic analysis

“…Quantitative treatment of many phases is sorely needed, especially determination of lateral mixing coeffcients, as Grimminger (1938) has done for isentropic charts of the atmosphere, and their application, together with a suitably chosen velocity distribution, to the task of testing whether the flow patterns which have been derived conform to continuity of mass and salt.…”

NUMERICAL STUDIES OF EFFECTS OF SURFACE FRICTION ON LARGE-SCALE ATMOSPHERIC MOTIONS1

“…The value K = 4 X lo9 em2 sec-l is used. This lies within the range of values computed by Grimminger (1941) and Murgatroyd (1969). The frictional terms in equations (9) and (10) are computed from data at the previous rather than the current time step to avoid computational instability.…”

Solenoidal circulations resulting from lateral mixing

“…No attempt will be made here to discuss numerical values of this speed. It is sufficient to point out that calculations such as those of Grimminger [1938] are necessary before the problem of the general circulation can be attacked numerically by the method outlined in this paper. A value for the lateral eddy-viscosity coefficient of about 10 7 CGS units, which falls within the limits found by Grimminger, seems to give a reasonable rate for the transversal circulation analysed above.…”