On the Stability of Eddy Motion and Stratification of the Atmosphere

[摘要] It is well known as an observational fact that every locality has a proper surface wind direction of the maximum frequency on account of the peculiar topographic effect. Hence the angle between the directions of surface and gradient wind is always subject to a considerable fluctuation accompanied by the change of gradient wind direction. As an extreme case of such a procedure we can imagine an oblate Ekman's spiral. In this case the fundamental equations of motion based on the principle of vorticity transport are, in customary notation, We put herethen we readily see that the eddy motion is stable or unstable according asThus the critical state is characterized by the relation 2a2=1.Adopting Dobson's observation numerical computation has been performed and we see that the result once published by the present author obeys this stability condition.Next the thermal stability of atmosphere is discussed. According to the usual conception the stability of atmospheric stratification is determined by the lapse-rate of temperature and it is stable or unstable according as the lapse-rate is lower or higher than the adiabatic lapse-rate.But this is not always the case with the recent aerological observation so far as it concerns the lower atmosphere of 1 kilometer level, even if the atmosphere is considerably stable as revealed out by the observation of cloud form in the lower stratum.Namely we often find the case of super-adiabatic lapse-rate in which the stratification is comparatively stable.As one plausible explanation of this fact the theory of the thermal stability of viscous fluid has been applied and the recent study of A. L. Hales also supports this view.

[学科分类] 大气科学