Simulation of the Asian Summer Monsoon and its Dependence on Model Horizontal Resolution

[摘要] The 1986-1989 Asian summer monsoons have been simulated using a state-of-the-art atmospheric General Circulation Model (GCM), spectrally truncated at 63, 42, 31 and 21 total wavenumbers, corresponding to horizontal resolutions ranging from 200-600km. The mean June-September Asian monsoons have been analyzed and compared to the observed mean monsoon behaviour. Large-scale features such as the lower tropospheric westerly jet, the upper tropospheric tropical easterlies, the Tibetan anticyclone, and copious rainfall over continental Asia are captured by the model at all resolutions. As the resolution is increased, the core of the low-level westerly jet moves towards Somalia and becomes more realistic. The model, however, produces excessive precipitation over the equatorial Indian Ocean and over the southern slopes of the Tibetan plateau, and these errors become accentuated at higher resolution. Furthermore, the monsoon is displaced southward at higher resolutions as is clearly evidenced by a shift in the position of the Tibetan anticyclone. A budget analysis of the upper-level vorticity suggests that this may be related to the excessive ascent over the southern slopes of the Tibetan plateau. A smooth orography test has been made at T63 truncation using T21 truncated orography, in order to assess the contribution due to orographic changes. Smooth orography alleviates the excessive precipitation over the southern slopes of the Tibetan plateau and has a strong and generally beneficial impact on the monsoon over land. It also gives large-scale dynamical monsoon indices in better agreement with observations, yet does not alleviate the excessive precipitation over the equatorial Indian Ocean. The excessive oceanic precipitation is partly due to a systematic intensification of the equatorial convergence zones with increased resolution, yet this appears to be only weakly coupled to the dynamics of the monsoon circulation.

[学科分类] 大气科学