On The Characteristics And Genesis Of Wintertime Tropical Cyclones That Landed China

Using the 6-hourly and daily data from NCEP/NCAR reanalysis, the characteristics of the circulation anomalies of two typhoons, 0428 and 7427 that landed China the most late in wintertime has been analyzed in the present paper. The results are summarized as follows:(l)The two typhoons that happened in wintertime have the similar characteristics as those happened in summer. The typhoon center is warm and wet. The convergence is observed in lower troposphere but divergent in higher troposphere. The upward motion is observed in wintertime typhoons, which is similar as in the summertime typhoons. However, the cyclonic circulation with round iso-lines is only found at levels not higher than 400-3 OOhPa, which is quite different from wintertime typhoons. No southwest low-level jet stream in the convergent part of the typhoon circulation is found in winter, which is unique as compare with those typhoons occurred in summer season. The equatorial west wind or the southwest cross-equatorial flow is the essential component for both the summertime and wintertime typhoons.(2)The EOF analysis has been performed for SLP of tropical cyclone (TC) 0428. More than 90% of total variance of the temporal changes in SLP for TC 0428 can be explained by two leading EOF modes, i.e., EOF1 and EOF2. The EOF1 indicates the structure and intensity variations of the TC 0428 while the EOF2 shows the changes in SLP distributions as an environmental pattern that influences the moving directions of this tropical cyclone.(3) The cold air blow plays an important role in the genesis and development of typhoons in wintertime. When the tropical cyclone moves in low latitudes, the cold air from higher latitudes can modulate the intensity of the tropical cyclones. If the cold air mass is not so strong, its invasion into the center part of a wintertime tropical cyclone strengthens the tropical cyclones. However, a typhoon will die if the strong cold air invades the typhoon center.(4)The TC occurs in the tropical vapor convergence zone. Its development makes the convergence of water vapor stronger. However, the water vapor transports are weaker in a growing TC in winter as compared with that in summer. No lower-level jet stream that transports the water vapor in found in wintertime typhoons.(5) TC 0428 moves along the eastlies in the southern flank of the westernPacific subtropical high. The airflow steering direction of TC immigration is observed at both high and low levels in troposphere. However, the steering flow at high levels for TC 7427 is observed different from that at low levels. The direction in which TC 7427 moves westward is probably controlled by the airflow in lower troposphere. The southwest jet-stream of southbranch trough in westerlies makes both TCs to change their move directions, causing to TC land China before their moving away into the northwest Pacific.(6) Some factors facilitate the genesis and strengthening of a wintertime tropical cyclone, which are the anomalous intensification of western Pacific subtropical high and monsoon trough, the anomalous accumulation of water vapor, the anomalously high SST, as well as the anomalously weak wind shear in vertical in the areas where a tropical cyclone occurs. The tropical convection that related to the genesis of a TC is triggered by the northeast monsoon flow a pentad before the tropical cyclone occurs.