Comparative Study of Vertical Motions in the Global Atmosphere Evaluated by Various Kinematic Schemes

Abstract

Large-scale vertical motions estimated by various classical kinematic methods are compared with those of NCEP-NCAR and ECMWF reanalyses. The classical kinematic methods include the central difference method, plane fitting method, 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 filtering the higher order gravity modes while the low-frequency Rossby modes are retained. According to the result, ECMWF reanalysis contains large amount of small-scale eddies, whereas the NCEP-NCAR reanalysis is smoother, reflecting the different spectral truncations. The normal model method provides a reasonable distribution of vertical motion, and the problem in the spectral method over the complex terrain is solved by this method. It is demonstrated that the normal mode method may be useful for the diagnostic studies of the general circulation of the atmosphere.