Geostrophic Wave Circulations
Yong Zhu
Preface to the first edition
It has not been always happy for me to talk about dynamic meteorology since my postgraduate study in the University of Edinburgh, when I had expected that the most fundamental properties of large-scale atmospheric circulations in the extratropical troposphere and stratosphere would be explained if we started from a new point of view, different from the relevant theories before which had failed to persuade me to follow them further in the rest of my life. I knew perfectly what a risk that I would take in the academic would which had already been well organized and disciplined by tremendous work and great reputations, if I was going to carry on my own study. In fact, I had felt the resistance even at the very beginning by receiving various warnings from senior advisors in the academic area. But, I could also not persuade myself to give up the exciting and dangerous exploration that time.
Although I am neither a physicist nor a chemist, I may understand the main principles of physics or chemistry when I read the textbooks simply by comparing with experimental or observational facts. That the main consequences can be verified not only by a theory but also by experiments is a basic property of a mature science. In comparison, what the authors of meteorological dynamics could tell the students was, except the basic principles derived directly from other sciences such as hydrodynamics and thermodynamics, a great amount of hypothetical mechanisms proposed with highly simplified and idealized mathematics and physical models. The verifications were usually not made straightforwardly by comparing with observations, but depended on authors' interpretations supported mostly by numerical experiments. The mechanisms involved in the numerical experiments may be different from those in either the real atmosphere or the theories to which the interpretations were assumed available. For example, none of the numerical models which were able to simulated some features of large-scale low-frequency variabilities in the midlatitude troposphere or the quasi-biennial oscillations in the tropical stratosphere incorporated only the Rossby waves on the spheres, or the Rossby gravity waves and Kelvin waves in the stratosphere, which were supposed to be
responsible for these particular circulation patterns. Thus, it is possible that the acquired results could be produced by others mechanisms in the numerical models.
Since meteorologists have their own ways to prove the values of their theories, the previous experiences and knowledge obtained from other sciences would be partially or totally useless for new students to read the studies in meteorological dynamics. Meteorologists borrowed a number of terms from other scientific areas, and gave them the somewhat different meanings afterwards. The resonance in the atmosphere was explained to be produced by wave reflections, and so was different from that in the classical physics. In thermodynamics entropy is a measure of disorder of a thermal system, but in meteorology it is a conservative quantity in all adiabatic processes. The group velocity of multidimensional waves used by meteorologists is seldom adopted in other sciences. The stationary waves in the atmosphere may produce non-zero group velocity though their phases are zero. Displacement of perturbation due to advection of the mean zonal flow was occasionally called the Doppler shift in some meteorological studies. We can give far more examples like these. In addition, meteorologists have their own products, such as the residual circulation, which cannot be described by either Eulerian or Lagrangian velocities and so is entirely unmeasurable. On the other hand, while, the most fundamental features of the large-scale circulations, like the geographic distributions and seasonal variations of planetary stationary waves and their low-frequency variabilities in both the troposphere and stratosphere, still have not been explained successfully by any of the hundreds mechanisms since their first discoveries. The theoretical meteorologists could be pride of the atmosphere created by themselves which was more obscure and confusing than the real one. Students who were studying the dynamics could forget the real atmosphere but remember what was taught in the classes in order to pass examinations. In fact, many of the numerical experiments were performed by those who never had any training in general meteorology or physics.
Is it true that the large-scale air motions in the Earth's atmosphere may follow some principles different from those described by the classical physics and thermodynamics, or the classical principles are not enough for us to study the atmospheric circulations? These questions should not be ignored again, at least for me in doing the present study. What I found was that mistakes could be produced when I was away from these basic principles. Although this new investigation was taken in a different direction and could be suspected by experienced experts in a previous scientific paradigm, I was encouraged from time to time by the events that the study `predicted' many facts which were not really seen from the observational analyses before.
I
would like to find that more readers may be able to share the ideas employed
in the study. However, I am not confident to convince any
of them. Also, I am looking forward to all the
responses including the most critical comments and challenges
which are constructive to the further approaches.
Yong L. McHall
1992 in Boston