To understand the possible changes of tropical cyclones in the future, greenhouse-warmed world remains the issue of high priority from scientific and socio-economic viewpoints. As an effort to explore the problem, we have been conducting a series of 10 or 20 years-long climate experiments using an MRI/JMA, 20-km mesh global atmospheric model, including the setup of a present-day climate condition(control case), and greenhouse- warmed climate(CO2 and SST) conditions. The experiments have been performed on the Earth Simulator, as a part of the project RR2002, "Development of Super High Resolution Global and Regional Climate Models" funded by Ministry of Education, Culture, Sports, Science and Technology (MEXT).

In the control case, the model has reasonably reproduced the typical hierarchy of the disturbances constituting the climate system wherein tropical cyclone is observed; the hierarchy ranges from the equatorial intraseasonal oscillation, extra-tropical cyclone, Baiu front, tropical cyclone, the spiral rainband of the tropical cyclone, down to the life cycle of mesoscale convections embedded in the rainband. The use of 20-km mesh model, in this sense, obviously increased the realism of the convections associated with the tropical cyclone, which represents a major advance from the past coarser-mesh GCMs. (The validity of the prognostic Arakawa-Schubert scheme as a cumulus parameterization used in the model should merit investigation in its own right.)

Our preliminary comparison of the tropical cyclone between the control case and greenhouse-warmed condition suggests that the tropical cyclone under the greenhouse-warmed condition has the potential to become stronger, and less frequent than under the present-day climate condition. Most of the tropical cyclones under the greenhouse-warmed condition are characterized by increased supply of latent heat fluxes from the sea, as expected, and higher precipitation rates, compared with the control case. The higher perecipitation rate is likely to be contributed by the increased ratio of precipitation of large-scale condesation origin to that of cumulus-convection-scheme origin, despite the apparently activated convective activities under the greenhouse-warmed condition. The mechanisms behind the simulated results, and the structural change of the tropical cyclone in the greenhouse-warmed condition are now being investigated intensively, which will be discussed in the symposium.