A DIAGNOSTIC STUDY OF AN EXPLOSIVELY DEEPENING OCEANIC CYCLONE OVER THE NORTHWEST PACIFIC

The vertical motions and secondary circulation of an explosively deepening oceanic cyclone,which occurred over the Northwest Pacific Ocean and was in conjunction with 200 hPa-level jet stream and has central pressure falls of 33.9 hPa/24h,have been computed from seven-level nonlinear balance model and Sawyer-Eliassen-Shapiro equation for the transverse ageostrophic circulation.The vertical motions are partitioned into contributions from large-scale latent heat release,effect of cumulus heating,thermal advection,differential vorticity advection,etc.,while the secondary circulation stream function is partitioned into contributions from geostrophic deformation,transfer of momentum and heat in the area of cumulus and diabatic heating.The principal results are the following.Large-scale latent heat release is very crucial to the explosive development of cyclones.If there is enough transfer of moisture,the positive feedback process between ascent of air and large-scale heating would work.The cumulus heating and the transfer of momentum and heat in the area of cumulus play an important role during the explosively deepening stage.Thermal advection is the initial triggering condition for large-scale heating and the conditional instability for the convection of cumulus.