Tropical Intra-seasonal Variability in a 20-km mesh MRI/JMA AGCM incorporating a New Convective Scheme

Two versions of 20-km AGCMs are used in the Japanese KAKUSHIN-3 project for the future projection of tropical depressions and extreme weathers. One is the MRI/JMA AGCM with the Arakawa-Schubert-type cumulus scheme. The other is the AGCM developed recently in this project, incorporating a new convective scheme developed by Yoshimura (2010, in preparation), which is based on the Tiedtke-type scheme, but with the cloud ensemble approach.

In the latter AGCM simulation for the present climate specified with the observed monthly global sea surface temperature and sea ice concentration in 1979-2003, the Asian summer precipitation climatology is much improved especially over the northwestern Pacific and East Asia compared with the former one. Tropical depressions' distribution and intensity are also improved.

The tropical intra-seasonal variability (ISV) in the simulations is investigated and compared with the observation. As have reported, the ISV is less realistic in the former model simulation. The new AGCM improves the magnitude of convective activity on the intra-seasonal time scale and their space-time power spectra in low frequency band compared to the former model. Interestingly, a northward ISV propagation signal is clearly detected over the South Asia as observed. However, the spectral power in the MJO band is still weaker than the observation, and the simulated power is distributed more on the time periods longer than 90 days. Further analysis and model improvements are necessary for improving these points.