| Numerical experiments are conducted on typhoon Rananim (0414) by use of PSU/NCAR nonhydrostatic mesoscale model MM5.Verified against observations,the track, the speed,intensity change,detailed mesoscale precipitation distributions are well reproduced.The experiments also show that those are sensitive to the topography and the initial field.Based on the successful simulation of the typhoon process, the characteristics of typhoon before and after landfall stage are analyzed in cirulation and dynamical and thermodynamic structures. It is found that the asymmetric distribution of moisture transporting on east and west sides dues to the chage of underlying surface after landfall,moisture transportation affects the thermodynamic structure of typhoon to a great extent. In vertical direction, extension of physical elements is not only low in height but also small in amount.The causes are analyzed In the light of three key problems-westwards turning track abruptly, sustaining over land long and heavy rain caused by Rananim. Results demonstrate that the enhance of subtropical high is the main cause for its westward motion, physical elements such as 3h ground surface allobaric, 500hPa vorticity variability and change of u-component of wind are useful in short-range forecast of track .It is also found that lower layer moisture channel connect with typhoon circulation, upper level strong divergence field, move towards and favorable underlying suface associated with the sustaining of typhoon over land after it made landfall.Based on the main influene system, the heavy rain process is divided into two phases.It was shown that the first strong precipitation phase was affected by typhoon itself and for the second precipitation process, the major casue was the result of interaction between the typhoon circulatin and minlatitude systems. Then focusing on the characteristic of asymmetrical distribution of rainfall at the both sides of landfall site, the study was carried out and indicated that this phenomenon associated with asymmetrical structure of typhoon itself, different moisture transportation and Mesoscale terrain. At the same time, the MPV was applied to diagonse space-time distribution of rainfall, the results suggested that the corresponding relation between the negative center of MPV and heavy rain area was better, and its absolute value had positive correlation to the intensitiy of one hour precipitation.lt provided more valuable information for short range weather forecasting, the typhoon torrential rain especially. |
