The role of diabatic heating, torques and stability in forcing the meridional circulation within cyclones

Simulations of meridional motion within cyclones are carried out based on a linear diagnostic equation of azimuthally-averaged mass-weighted meridional circulation within translating cyclones. The consistency between transport processes and physical processes, and the contribution of various physical processes to the forcing of the meridional circulation are investigated. With the use of the linear diagnostic equation, the relative importance of several physical processes is discussed for different cyclones and different stages of development.; Three extratropical cyclones and one typhoon were selected for the diagnostic study. The results show that intensification of these four cyclones is accompanied by strengthening of the meridional mass circulation. The development of cyclones requires: (1) mass and mean angular momentum transport forced by torques and inward eddy angular momentum transport, and (2) vertical transport of angular momentum associated with diabatic heating and/or angular momentum transfer across inclined isentropic surfaces through pressure and inertial torques.; The dominant factors for forcing the meridional circulation within the extratropical cyclones are those associated with the baroclinic/barotropic structure within the vortex and its environment, and the effect of topography. The relative importance of these factors vary from cyclone to cyclone, particularly in the early stages. For the typhoon over the South China Sea, the main factors are associated with latent heat release and weak hydrodynamic stability.; This investigation confirms results from previous studies of cyclone development and demonstrates the importance of cyclone translation on forcing the meridional motion within cyclones. The results for these four cyclones show that the simulation of meridional motion within cyclones agrees quite well with the observed meridional circulation. The excellent agreement indicates that the diagnostic equation, simplified under the assumptions of hydrostatic and gradient balances, provides: (1) a systematic assessment of the distinct physical processes responsible for the meridional motion which is related to cyclone development, and (2) an unique unified approach for investigating the forcing of the systematic meridional circulation within both extratropical and tropical cyclones.