Impacts of ice phase processes on tropical cyclone development

Idealized cloud-resolving simulations with the horizontal resolution of 2 km are carried out to investigate effects of ice-phase processes on the development and structures of tropical cyclones (TCs). A comparison between cold-rain and warm-rain simulations shows that ice-phase processes delay the TC organization, and decrease the area-averaged kinetic energy. The ice-phase processes also shrink the TC size; for example, the radius of the storm-force wind area (over 25 m s^-1) in the cold-rain simulation is two-thirds of that in the warm-rain one. The TC evolution depends greatly on strong cooling due to melting and sublimation of snow and graupel around the melting layer outside of the TC eyewall. The cooling reduces the pressure gradient below the melting level, and weakens the inflow toward the TC center. It suppresses the inward transport of high absolute angular momentum (AAM), and decreases the energy conversion rate from the available potential energy to kinetic energy of axisymmetric flows. As a result, the reduction of AAM around the eyewall shrinks the TC eyewall size, and the reduction of energy conversion rate delays the TC organization. The influence of the terminal fall velocities of snow and graupel is also examined by performing sensitivity experiments, with the horizontal resolution of 5 km. The results show that the increased terminal fall velocity lowers the actual melting altitude, and enhances the TC development through the change in the vertical profile of diabatic heating.