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YEH TU-CHENG AND LI MAI-TSUN(Institute of Geophysics and Meteorology, Academia Sinica)  
In the large-scale motion of the atmosphere there is a so-called geostrophic adaptation. This paper demonstrates that there is also a process of adaptation between the pressure and the wind field in the meso- and small-scale motion.From the scale analysis it is found that on the average in the equation of motion the term of the time derivative is one order of magnitude smaller than the main terms in the meso- and small-scale motion. Therefore it may be concluded that the motion of these scales also proceeds slowly under the condition of quasi-balanced forces just as that the large-scale motion proceeds quasi-geostrophically. If, however, in some way this quasi-balanced condition is disturbed, there must be a mechanism to bring the motion back to the quasi-balanced state. For the meso- and small-scale motion the inertia force (velocity advection), the coriolis force and the force of pressure gradient are in quasi-balanced state.The process to bring the motion to the state of quasi-balance of forces is called adaption process and the motion under quasi-balanced state is called quasi-steady process. The physical nature of these two processes is discussed in this paper. For meso-scale motion it is shown that in the adaption process Dζ, and in quasi-steady process D ≤ ζ, where D and ζ are respectively the divergence and the vorticity.If at certain time the quasi-balanced state is brokendown in certain limited region, further development of this motion may be calculated as an initial value problem. The computation concludes that in the initial short interval of time the meteorological elements undergo vigorous change leading to the reestablishment of the quasi-balanced state and then the motion goes into quasi-steady state.The equations of quasi-balanced condition for the meso- and small-scale motion may be written asThis suggests that for the motion of this scale P-field should be analyzed instead of height or pressure field.
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