Tropical Cyclone Formations in 30-day Simulation Using Cloud-System-Resolving Global Non-hydrostatic Model (NICAM)

We have been developing an icosahedral-grid non-hydrostatic AGCM, which can explicitly represent cumulus or meso-scale convection over the entire globe. We named the model NICAM (Nonhydrostatic ICosahedral Atmospheric Model). In 2005, we performed a first simulations with the horizontal grid intervals of 14, 7 and 3.5 km using realistic topography and sea surface temperature in April 2004 (Miura et al., 2007; GRL). It simulated a typhoon Sudal that actually developed over the western North Pacific in 2004.

In the present study, the NICAM model with the horizontal grid interval of 14 km was used for perpetual July experiment with 30-day equilibrium integration. In this simulation, several tropical cyclones formed over the western and eastern North Pacific, although the formation over the western North Pacific occurred a little further north to the actually observed region. The mature tropical cyclones with intense wind speed had a realistic structure of a cloud-free eye, eye wall and a warm core.

We have found that the environmental parameters related to the tropical cyclone genesis explain well the characteristics of the simulated region of the cyclone genesis. The low-level flow such as monsoon was well simulated over the eastern North Pacific, while it was a little different from the actually observed one over the western North Pacific. Other environmental characteristics such as Maximum Potential Intensity (MPI) and vertical shear were also well reproduced over the eastern North Pacific. The vertical shear over the North Pacific in the simulation was stronger than that in the actual atmosphere, which appears to inhibit the formation over the North Atlantic.

Over the North Atlantic and eastern North Pacific, westward-moving disturbances like easterly waves were simulated, which was related to the cyclone formation over the eastern North Pacific. On the other hand, the simulated tropical cyclones over the western North Pacific appears to form through different factors as has been suggested by the previous studies based on observation.

The structure change of each tropical cyclone during its life cycle was analyzed. The radius of maximum wind (RMW) reduced during the development stage of some tropical cyclones, while the convective cloud organized at the radius of eye-wall at the beginning of this stage.

Although the model is still under continuous improvement, we can demonstrate what dynamics can be represented using a global high-resolution model.