Comparative study of vertical motions in the global atmosphere evaluated by various kinematic schemes

Large-scale vertical motions estimated by various classical kinematic metho ds are compared with those of NCEP-NCAR and ECMWF reanalyses. The classical kinematic methods include the central difference method, plane fitting met hod, and spectral method. In addition, a normal mode method is proposed in this study as a new approach to the computation of the vertical motion. In this method, the erroneous high frequency divergence is controlled by filte ring the higher order gravity modes while the low-frequency Rossby modes ar e retained. According to the result, ECMWF reanalysis contains large amount of small-sc ale eddies, whereas the NCEP-NCAR reanalysis is smoother, reflecting the di fferent spectral truncations. The normal model method provides a reasonable distribution of vertical motion, and the problem in the spectral method ov er the complex terrain is solved by this method. It is demonstrated that th e normal mode method may be useful for the diagnostic studies of the genera l circulation of the atmosphere.