摘要
Observational evidence is presented that during Extratropical Transition(ET) of Tropical Cyclones(TCs),Downstream Development(DD) is frequently underway.We show that DD results in rapid changes to the environment.A critical flow change is the development of a low-level trough,sandwiched between two developing anticyclones.The trough appears to merge with the storm,seemingly holding it upright and allowing it to withstand the damaging effects of wind shear.In this way the storm can eventually reach the favourable equatorward entrance region of the upper jet.To evaluate the mechanism,two sets of simulations have been run:one using high-resolution,full physics integrations and another using coarse-resolution with dry physics and the TC removed from the initial condition.We show that the dry dynamics can establish the large scale environment to enable the transition to proceed.The process can produce:(a) the deep,vertically-aligned,low-level pressure trough that merges with the storm,and(b) a partial inhibition through subsidence to embedded convection,allowing the boundary layer to moisten via(i) sustained surface fluxes,and(ii) enhanced horizontal moisture flux convergence from the environmental flow changes.This produces potential for more intense convective activity and vortex resiliency even in relatively strong,deep vertical wind shear.
