Inner core asymmetric structures and tropical cyclone intensity

The influence of large-scale environmental flow on tropical cyclone (TC) intensification is investigated through observational analysis and numerical simulation. The favorable large-scale environmental flow patterns for TC intensification are identified from observation first and then incorporated into idealized numerical experiments to unveil the underlying physical mechanisms.; The large-scale environmental flow patterns are investigated by examining the spatial distribution of the tropical storm formation frequency and corresponding environmental flow shear structures over the tropical Western North Pacific (WNP). The most favorable pattern is the combination of low-level cyclonic shear, upper-level anticyclonic horizontal shears and weak vertical shear.; Since TC intensity is closely associated with the TC inner core structure, the numerical simulations are focused on the changes in the TC inner-core asymmetric and symmetric structures, which are either generated by internal dynamics or affected by the environmental forcing. The effects of internally-generated asymmetries on TC intensity are investigated through a comparative study between a triply nested three-dimensional (3D) TC model (TCM3) and its axisymmetric version. It is found that eddy transport in 3D is important for reducing the eyewall tilt, which in turn limits the drying and cooling effects of downdrafts in the subcloud layer, and reducing the air-sea entropy deficit at the ocean surface under the eyewall, and thus weakening TCs.; The large scale environmental flow affects TC intensity through generating asymmetric structures at the TC inner core. For a weak vortex embedded in the zonal flow with low level cyclonic and upper level anticyclonic shears, the environmental forcing continuously excites radially propagating shear waves in the TC inner core. The axisymmetrization of these inner core asymmetries, as coupled with moist and boundary processes in a conditional unstable atmosphere, provides a trigger for the eyewall contraction, making inner convection dominant and leading to rapid intensification.; Sensitivity experiments demonstrate that both low-level cyclonic shear and upper-level anticylconic shear contribute to TC intensification. The impact of these shears on TC intensification decreases as the initial vortex becomes strong because of the associated high inertial stability.