Orography as well as land-sea distribution have a profound impact on regulating various characteristics of precipitation. Here we use a very high resolution AGCM (20-km mesh TL959L60 MRI/JMA AGCM) to investigate modulation of precipitation distribution on orography.

First, we investigate tropical rainfall diurnal variations on model's horizontal grid size. In view of continental and basin–wide scale precipitation, no phase difference is found between 20km AGCM and 120km AGCM. Spatial distribution of peak time and amplitude of rainfall diurnal variation are well represented in both AGCMs, although a peak time in the model occurs a few hours earlier than the observation. More realistic orographic rainfall is captured in 20km AGCM, due to better simulations of land/sea breeze–like and/or mountain–valley wind–like local circulations.

Second, impact of horizontal resolution of mountain heights on rainfall distribution and its diurnal cycle is investigated by conducting numerical experiments with 20km AGCM with lower resolution mountains. A control case (20km AGCM simulation with 20km-scale mountain) reveals terrain-induced rainfall. It is found that the 20km AGCM simulation with 60km-scale mountain produced very similar features to those obtained in the control case, while the 20km AGCM simulation with 180km-scale mountain resulted in qualitatively different precipitation fields due to a very smooth mountain. These differences appear when and where cumulus type precipitation dominates. Modulation of diurnal cycle will also be explored.