Based on the NCEP/NCAR reanalysis data and observations collected during the SCSMEX, a mesoscale convective system (MCS) occurring over South China during 23-24 May 1998 has been studied with a numerical simulation using the Fifth Generation Penn-State/NCAR Mesoscale Modeling System (MM5). The successful simulations present us some interesting findings. The simulated MCS was a kind of meso-β scale system with a life cycle of about 11 hours. It generated within a small vortex along a cold front shear line. The MCS was characterized by severe convection. The simulated maximum vertical velocity was greater than 90 cm s-1, and the maximum divergence at about 400 hPa. The rainfall rate of MCS exceeded 20 mm h-1. To the right of the simulated MCS, a mesoscale low-level jet (mLLJ) was found. A strong southwesterly current could also be seen to the right of MCS above the mLLJ. This strong southwesterly current might extend up to 400 hPa. A column of cyclonic vorticity extended through most part of the MCS in the vertical direction. Additionally, the simulated MCS was compared favorably with the observational data in terms of location, precipitation intensity and evolution.

The movement of Typhoon Maggie (9903) in June 1999 isoneof the rare cases ever seen in the history. At 00U on June 6 Maggie was located at about 70km to the southwest of Taiwan. When it arrived at the coastal region of Shanwei City (22.8°N, 116.5°E), it turned suddenly to move south-westward along the southern China coastal line. On June 7 Maggie finally turned to move northward, making landfall to the north of Shangchuan Island. The experimental numerical prediction syst~n on ty-phoon movement that was designed based on MM5 is proved quite successful for the 48h prediction of Maggie' s movement and rainfall. The mean prediction error of typhoon track is 81 km for 0~ 24 h and 74 km for 24～48h. The location of typhoon center in the initial field of the model is approximately 100 km away from the actual observations. In order to modify the location of typhoon center, a bogus typhoon was intro-duced into the model and the prediction of typhoon track was improved in 0～24 h time interval. But the prediction error was enlarged in 24～36 h. We also performed a sensitivity experiment of changing the land of southern China into the ocean. It is found that the orientation of South China coastal line and the topography have no obvious effoct on the movement of Typhoon Maggie.

Now more comprehensive cloud microphysical processes have been included in advanced three-dimensional mesoscale meteorological model such as PSU/NCAR MM5 model, so the model can be used in the prediction of fog. In this paper, MM5 was utilized to simulate an advection fog occurring in Nanling Mountain area. The simulated results were compared with the facts obtained by detailed observation experiment. The results showed that the simulation was successful in the following aspects: (1) the formation and development of the fog; (2) the temporal variation of the maximum liquid water content; (3) the diffusion of the cold air, especially the temporal variation of the ground temperature and (4) the uplift of the air and the formation and development of the low-lever inversion. Besides, we did some sensitivity numerical experiments and discussed the effects of the radiation, the release of condensation latent heat and the change of soil moisture and temperature on the formation and development of fog. The success of numerical simulation experiment of fog has proved that the numerical forecasting of fog is promising.

The influences of heat sources and orography in East Asia on the seasonal change of atmospheric circulation are investigated through numerical experimentation with a two-layer primitive equation model. Simulations show basic agreement with the observed ann.al variation of the average planetary troughs and ridges produced by the orography in East Asia. The simulated results with diabatic heating are more similar to climatology than those without diabatic heating. In addition, experi-mental results reveal that heating over East Asia has an important con-tribution to the southward progression and withdrawal of the westerly jet and the formation of the Tibetan summer anticyclone.

By using the daily-14 year (1983-1996) NCEP/NCAR 2.5