A Numerical Investigation of the Atmospheric General Circulation and Stratospheric-Tropospheric Mass Exchange : II. Lagrangian Motion of the Atmosphere

[摘要] A numerical simulation of Stratospheric-Tropospheric mass exchange was performed by tracing a large number of air particles of the stratospheric origin with a simplified general circulation model. It shows that the most plausible mechanism of outflow of the stratospheric air to the troposphere is cross-tropopause motion associated with the mid-latitude cyclone.The individual process of air particle's movement is that the lowest stratospheric air particles in the rear of the cyclone and around at 60 degrees of latitude most-effectively move downward and equatorward and arrive around at 40 degrees of latitude in the middle troposphere, while many other tratospheric air particles merely flow eastward with north and south meander in the westerlies. Some air particles of the stratospheric origin intruding into the troposphere return toward the original levels to the east side of the cyclone. In spite of the return motion, they eventually descend and merge in the troposphere, since the net downward movement of air particles for a long time is induced by dynamical effects of the cyclone.On the other hand, the model shows the quasi-horizontal outflow of the stratospheric air through the tropopause gap in the subtropics does not take place.Possible general circulation of air particles was discussed and presented in the model.It will be referred to as Lagrangian general circulation of the atmosphere. Its structure shows one cell over a hemisphere with downward and upward branches in higher and lower latitudes, respectively, and coincides with the so-called Brewer-Dobson circulation in the lower stratosphere. Therefore, it may be considered that the individual short-range and statistical long-range mechanisms of Stratospheric-Tropospheric mass exchange are the preferential descending motion in the rear of the cyclone and the Brewer-Dobson type meridional circulation, respectively. The experiment shows that Lagrangian general circulation has the vertical downward velocity of about 100 mb/month at a level of the mid-latitude tropopause, indicating that the magnitude of the residence time needed for the stratospheric air to be completely replaced is of the order half a year. However, since there are some reasons to consider that this value is underestimated by a factor of 2-3, it may be inferred that its magnitude is of the order 1-2 years in the real atmosphere, being in agreement with the results of observational studies.

[学科分类] 大气科学