Idealized Numerical Experiments on Cyclone Development in the Tropical, Subtropical, and Extratropical Environments

The development of cyclones, particularly in the Southern Hemisphere summer, is active in the tropics and extratropics but is inactive in the subtropics. To elucidate the influence of environmental fields on the cyclone development in the tropics, subtropics, and extratropics, idealized numerical experiments are conducted using a nonhydrostatic channel model. The experiments examine the development of a weak initial vortex within a zonally uniform environmental field that consists of five factors: the Coriolis parameter, zonal wind, potential temperature, relative humidity, and surface temperature difference between the ocean and atmosphere. The idealized experiments successfully reproduce the significant cyclone development in the tropical and extratropical environment as well as no cyclone development in the subtropical environment. This result confirms the dominant role of the environmental field in controlling the cyclone development. To clarify which environmental factor is responsible for the suppression of cyclone development in the subtropics, a series of sensitivity experiments is performed. A tropical cyclone cannot develop in the subtropics because of low temperature, strong stratification, and strong vertical shear compared to the tropics. On the other hand, an extratropical cyclone cannot develop in the subtropics because of the small Coriolis parameter and weak vertical shear compared to the extratropics. The relative humidity and surface temperature difference play only secondary roles. These results provide useful insights into the climatological distribution of various types of synoptic-scale cyclones.