Roles of the Upper-Level Warm Core in Triggering Tropical Cyclogenesis

Although many processes leading to tropical cyclogenesis (TCG) are well known, we are still searching for the mechanisms by which the surface circulations could be spun up to initiate the wind-induced surface heat exchange (WISHE) process as a route to hurricanes. Previous studies have examined the bottom-up and top-down hypotheses as two of the possible processes leading to TCG from mesoscale convective vortices (MCVs) that often develop in the stratiform region of mesoscale convective systems (MCSs). The bottom-up mechanism was later advanced by introducing the important roles of “vortical hot towers (VHTs)” in TCG. Some recent studies also consider the roles of a protective wave "pouch" in TCG.

In this study, the subject of TCG will be examined by exploring the processes occurring in the upper troposphere, in contrast to those previously discussed in the low- to mid-troposphere, and thermodynamical transformation in the TC vortex. Specifically, we hypothesize that the upper-level subsidence warming resulting from convective detrainments, combined with weak storm-relative flows in the upper divergent outflow layer, is the key to surface pressure falls and the trigger of TCG. The upper-level divergent outflow could play an important role in protecting the warm core from ventilation by environmental flows. This hypothesis has been examined through cloud-permitting simulations of two TCG cases with the finest grid sizes of 1 – 2 km: (i) Typhoon Nari (2001) from an MCS/MCV occurring in a near-barotropic environment with high sea-surface temperatures, and (ii) Typhoon Chanchau (2006) from a westerly wind burst with pronounced deep-layer vertical wind shear. Results show that more significant surface pressure falls occur in coincidence with the increased warming in the upper level, and decreased flows near the storm center with the divergent, anticyclonic flows in the upper outflow layer. Through the simple use of hydrostatic equation, it can be demonstrated that the upper-level warming, especially with stratospheric origin associated with convective bursts, is more effective than lower-level warming in reducing mesoscale sea-level pressure. We argue that only after a surface meso-β-scale low pressure system is established, the bottom-up growth of cyclonic vorticity as well as the protective wave “pouch,” can become operative, due to the associated mass and moisture convergence in the boundary layer. Our results appear to suggest that more attention be paid to the amplitude of storm-relative flows and vertical wind shear in the upper troposphere, rather than just vertical wind shear in the typical 850-200 hPa layer, in order to better understand and predict the genesis of tropical cyclones.