Characteristics of Environment Flow Related to Intensity Change of Landing Tropical Cyclones Towards the Yellow Sea and the Bohai Sea

To study the intensity changing patterns and the large scale environmental flow characteristics of landing tropical cyclones moving out to Yellow Sea and Bohai Sea (ab. YBTC), based on 1949—2006 typhoon data and NCEP object reanalysis data every 6 hours, the intensity change of YBTCs is statistically analyzed. The annual YBTC number is 0.9, and the reinforced ratio is 49% when moving into the sea. The maximum value and the largest variation of reinforcement occur in September. The reinforcement ratio of YBTC which land at Fujian Province is higher. The number of reinforced YBTC in 1970s and in 1990s is smaller, and then it increases. The reinforcement is stronger than the attenuation according to V_max, and the reinforcement is equivalent to the attenuation according to P_min, but they are not synchronous. The reinforcement maintains till it enters sea or 6 hours later at most, but V_max doesn't weaken much. 5 cases of reinforced YBTCs and another 5 cases of weakened YBTCs are analyzed and compared using large scale circulation composed diagnostic method. It's found that middle latitude trough occurs in both type of cases, but the intensity and collocation are different. The trough of reinforced YBTC cases is stronger and coupled with YBTC when entering the sea. Strong high level trough brings strong vortices advection and warm advection, and then YBTC gets baraclinic energy thereby develops. In the other type of cases, trough is from west to YBTC with no coupled frontal zone, and north to YBTC is weak ridge. The subtropical high of the former type is massive with the guide flow mainly from southerly wind, facilitating interaction with middle latitude trough. While for the later type, subtropical high is strong and extend westward, blocking the interaction with middle latitude trough. YBTC of the former type moves rapidly to right side of high level jet when entering the sea. But for weakened YBTCs the high level jet is weaker. On low levels, there are southwest yet, but for the reinforced YBTCs, there is also stronger northerly flow which accelerates baraclinic developing in the west of TC. In the first type of cases, there is much stronger moist baraclinity and steep θ_se, which are favorable for vortices to develop. The impairing function to the intensity of YBTC of the strong speed vertical sheer is not remarkable, but the speed vertical sheer of the former type is stronger than the latter type of YBTCs.