A STATISTICAL ANALYSIS ON THE DIFFERENT FEATURES OF TC EXTRATROPICAL TRANSITION OVER LAND AND OCEAN

The extratropical transitions(ETs)of tropical cyclones(TCs)over China and the ocean east to 150°E are investigated by the use of best-track data and JRA-25 reanalysis spanning 1979-2008.The ET events occurring north of 25°N and in the warm season(from May to October)are extracted from the reanalysis to emphasize the interaction between TC and midlatitude circulation.Statistical analysis shows that 18.5%of the warm-season TCs go through land ETs north of 25°N in the western North Pacific.And 20.5%of the ET events occur over the ocean east of 150°E.Most(62.2%)ET TCs over China gradually die out after ET,but more(70.7%)ocean ET cases have post-ET reintensification.The evolutions in cyclone phase space and the composite fields for land and ocean ETs,as well as the ET cases with and without post-ET reintensification,are further analyzed.It is found that most TCs with ET over China and those without post-ET reintensification evolve along the typical ET phase path as follows:emergence of thermal asymmetry→losing upper-level warm core→losing lower-level cold core→evolving as extratropical cyclone.The TCs undergoing ETs over ocean and those with post-ET reintensification form a high-level cold core before the ET onset.The TCs with land ET have long distance between the landing TC and a high-level trough.That makes the TC maintain more tropical features and isolates the TC flow from the upstream and downstream jets of the midlatitude trough.The structure of circulation leads to weak development of baroclinicity in land ET.On the contrary,shorter distance between ocean TC and high-level trough makes the high-level trough absorb the TC absolutely.Under that baroclinicity-favorable environment,strong cold advection makes the TC lose its high-level warm core before ET onset.The composite fields confirm that the TC with ocean ET has stronger baroclinic features.Generally,the TC at land ET onset is located to the south of the ridge of the subtropical high,which tends to prevent the TCs from interacting with midlatitude circulation.But for the ocean ET,the situation is just the opposite.Similar analyses are also carried out for the TCs with and without post-ET reintensification over both land and ocean east of 150°E.The results further prove that the TC with stronger baroclinic characteristics,especially in the circumstance favorable to its interaction with high-level midlatitude systems,has more opportunity to reintensify as an extratropical cyclone after ET.

The extratropical transitions （ETs） of tropical cyclones （TCs） over China and the ocean east to 150~E are investigated by the use of best-track data and JRA-25 reanalysis spanning 1979-2008. The ET events occurring north of 25°N and in the warm season （from May to October） are extracted from the reanalysis to emphasize the interaction between TC and midlatitude circulation. Statistical analysis shows that 18.5% of the warm-season TCs go through land ETs north of 25°N in the western North Pacific. And 20.5% of the ET events occur over the ocean east of 150°E. Most （62.2%） ET TCs over China gradually die out after ET, but more （70.7%） ocean ET cases have post-ET reintensifieation. The evolutions in cyclone phase space and the composite fields for land and ocean ETs, as well as the ET cases with and without post-ET reintensification, are further analyzed. It is found that most TCs with ET over China and those without post-ET reintensification evolve along the typical ET phase path as follows： emergence of thermal asymmetry--losing upper-level warm core--losing lower-level cold core - evolving as extratropical cyclone. The TCs undergoing ETs over ocean and those with post-ET reintensification form a high-level cold core before the ET onset. The TCs with land ET have long distance between the landing TC and a high-level trough. That makes the TC maintain more tropical features and isolates the TC flow from the upstream and downstream jets of the midlatitude trough. The structure of circulation leads to weak development ofbaroclinicity in land ET. On the contrary, shorter distance between ocean TC and high-level trough makes the high-level trough absorb the TC absolutely. Under that baroclinicity-favorable environment, strong cold advection makes the TC lose its high-level warm core before ET onset. The composite fields confirm that the TC with ocean ET has stronger baroclinic features. Generally, the TC at land ET onset is located to the south of the ridge of the subtropical high, which tends to prevent the TCs from interacting with midlatitude circulation. But for the ocean ET, the situation is just the opposite. Similar analyses are also carried out for the TCs with and without post-ET reintensification over both land and ocean east of 150°E. The results further prove that the TC with stronger baroclinic characteristics, especially in the circumstance favorable to its interaction with high-level midlatitude systems, has more opportunity to reintensify as an extratropical cyclone after ET.