Analysis on the causes of abnormal activities related to landfall tropical cyclones in China during 2008

Based on the NCEP/NCAR reanalysis data and the Outgoing Longwave Radiation(OLR) data from the NOAA,the authors analyze the landfall tropical cyclones in China during 2008.The characteristics of the 2008 tropical cyclone season include the first tropical cyclone coming in much earlier,the autumn tropical cyclones appearing close together,and high proportion of landfall tropical cyclones.The results show that,with the development of the cold high in the southern hemisphere,the equatorial low-level jet was intensified,which strengthened the equatorial westerly wind and spread it eastward,and the convection in the Inter-Tropical Convergent Zone(ITCZ) was more active.These circu1ation conditions resulted in the formation of tropical cyclones.With infrequent cold air in the mid-latitude areas,including the developing southwest monsoon and northwestward advance of the subtropical high,the frequency of landfall tropical cyclones was greater than norma1.By applying the atmospheric heat sources and the complete form of the vertical vorticity tendency equation,it is found that with the ITCZ strengthened and spread northward,the tropical cyclones originated frequently,strong precipitation lead to the abnormal non-uniform heating that played an important role in the northwestward advance of the subtropical high.As the tropical cyclones occurred frequently for some time,the subtropical high could keep such abnormal state,which leads to the tropical cyclones moving westward and landfall frequently in China.