Dependence Of Tropical Cyclone Genesis And Development On Coriolis Parameter And Their Mechanisms

Tropical cyclones(TC)form over warm oceans.Due to the scarcity of high-resolution observation data and complex multi-scale interactions involved in TC development,the thermodynamics and dynamics of TC genesis and development still remain a challenging and hot topic.In this paper,based on observation reanalysis data and high-resolution numerical model,combined with the theoretical model constructed,we investigate the impact of Coriolis parameter on TC genesis and development as well as their mechanisms.The main conclusions are summarized as follows:(1)A simple theoretical model is formulated,which includes a two-layer free atmosphere and an Ekman boundary layer at the bottom.The connection between the boundary layer and the free atmosphere is through the vertical velocity induced by Ekman pumping in proportion to the vorticity at the top of the boundary layer.The closure of this linear system assumes a calculable relationship between the free atmosphere diabatic heating and the boundary layer moisture convergence.The results show that TCs do not tend to develop under lower or higher Coriolis parameter,but choose an intermediate latitude to achieve the most rapid growth rate.Under a set of realistic atmospheric parameter values,the model indicates that the most optimal latitude for the development of TC is around 5° without considering other factors.In a zeromean flow and a constant SST environment,on the f-plane with different Coriolis parameters,the high-resolution WRF model simulations show that given an initially balanced weak vortex,the TC-like vortex intensifies most rapidly at the reference latitude of 5°.Therefore,the WRF model simulations confirm the f-dependent characteristics of TC intensification rate as suggested by the theoretical model.(2)Based on the TC generated near the equator and the TC generated in major developing regions of the WNP(i.e.,the monsoon trough),the effects of Coriolis parameters are further analyzed dynamically.The results of the high-resolution numerical model show that the Coriolis parameters not only affect the TC growth rate,but also have important effects on the TC structure and size as well as the radius of maximum wind speed.The smaller the Coriolis parameter value,the weaker the inertial stability,the more easily the maximum inflow center penetrates,and the size of TC tends to be smaller.Therefore,the vortex developed in the monsoon trough can obtain stronger intensification rate and bigger size than those near the equator.A vorticity budget analysis reveals that the increases in low-level vorticity during the rapid intensification period are attributed to enhanced horizontal vorticity convergence into the storm core which plays an important role in the formation of vortex.The equivalent Coriolis effect indicates that the monsoon trough region with higher latitude is more conducive to the development of TCs than the near-equatorial region.(3)In a sharp contrast to TCs genesis over the main development region of the western North Pacific,near-equatorial(0°-5°N)TCs exhibit a distinctive annual cycle,peaking in boreal winter and being inactive in boreal summer.The key physical processes of near-equatorial TCs formation and the relative roles of dynamic and thermodynamic background states on them were investigated based on the observational diagnosis of the genesis potential index(GPI)and high-resolution model simulations.It is found that the background vorticity makes a major contribution to the distinctive annual cycle,while mean temperature and specific humidity fields are not critical.Numerical simulations further indicate that seasonal mean cyclonic vorticity in boreal winter has three effects on TC genesis near the equator.Firstly,the environmental cyclonic vorticity interacts with TC vortex to promote a mid-level outflow,which strengthens boundary layer friction induced ascending motion and thus condensational heating.Secondly,it produces an equivalent Coriolis effect(via enhanced absolute vorticity),which strengthens positive feedback between primary and secondary circulation.Thirdly,it helps merge small-scale vortical hot towers into a mesoscale core through vorticity segregation process.However,background vorticity in boreal summer has an opposite effect on TC development near the equator.