Fluid mechanics applied to the study of atmospheric circulations. I. A study of flow patterns with the aid of isentropic analysis.

A. On the maintenance of the westerlies south of the polar front. B. Technique and examples of isentropic analysis. C. Isentropic analysis of a case of anticyclogenesis.This paper constitutes Part I of a report on certain investigations which have been in progress at the Massachusetts Institute of Technology during the past few years and which have been supported in part with funds provided by the Weather Bureau of the U. S. Department of Agriculture under the Bankhead-Jones Special Research Fund. The ultimate purpose of these investigations is to develop a sound physical model of the general circulation of the atmosphere, in the hope that an improved understanding of this process eventually may furnish valuable clues as to how the time range of our present daily weather forecasts may be extended and their quality be improved. In the past, the interpretation of the large-scale circulations of the atmosphere with the aid of the tools of classical hydrodynamics has suffered from the fact that these tools were designed for the study of thermodynamically inactive fluids, in which, furthermore, viscous or eddy stresses could be neglected. Through the work of V. Bjerknes and his students a good start has now been made towards the development of a science of hydrodynamics applicable also to thermodynamically active fluids, in which density changes are taking place as a result of non-adiabatic temperature changes. The removal of the second restriction-i.e., the development of hydrodynamic tools adapted to the study of fluids in which eddy stresses playa dominant role-has been accomplished mainly through the investigations of the Göttingen school of fluid mechanics. As yet, no synthesis of these two modern developments has been accomplished, although it is becoming increasingly clear that such a synthesis is needed before any headway can be made with the interpretation of the behaviour of the atmosphere. There has been a tendency on the part of meteorologists to assume that the effects of eddy stresses are restricted to a layer near the ground, and that the atmosphere above this layer behaves approximately as an ideal fluid. Even fairly elementary considerations show that a real understanding of atmospheric circulations becomes absolutely impossible on the basis of this assumption. A modest first attempt towards such a synthesis of the Norwegian and German developments will be attempted in these reports. It will be shown that the movements in the free atmosphere above the ground friction layer are affected by large-scale lateral mixing processes which produce shearing stresses acting across vertical planes, and one or two examples will be given to demonstrate that reasonable steady state solutions for the atmosphere can be obtained by taking this internal stress distribution into account. It will be shown, moreover, that the distribution of cold sources and heat sources in the free atmosphere is at least in part controlled by the stress distribution, which regúlates the location of ascending and descending movements