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登陆热带气旋入黄渤海强度变化的环境场特征
Characteristics of Environment Flow Related to Intensity Change of Landing Tropical Cyclones Towards the Yellow Sea and the Bohai Sea
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摘要: 为了研究登陆热带气旋进入黄渤海域（YBTC）强度变化的规律和环境场特征，利用1949—2007年台风资料和NCEP/NCAR逐6 h的客观再分析资料，首先对YBTC强度变化进行统计分析，发现其入海加强比率达49%，9月加强的频数最多、加强幅度最大，登陆福建的YBTC加强比率高，中心最低气压Pmin和中心最大平均风速Vmax变化不完全同步，Vmax加强幅度比减弱幅度大，Pmin加强幅度与减弱幅度相当。选择入海加强和入海减弱的YBTC各5例，合成诊断、对比分析两类YBTC的大尺度环境条件表明：前者YBTC西北部有深槽移近、YBTC与其锋区入海时逐渐耦合，后者在YBTC西部有浅槽、北部为弱脊。前者副热带高压经向度大利于YBTC与中纬槽系统相互作用加强。前者有强的高空急流快速靠近、Y BTC行进到高空急流入口区右侧，而后者位于较弱的高空急流入口区外、没有快速靠近过程。两者都有低空偏南风急流输送水汽，但前者西侧有较强北风带来的干冷空气。前者具有较强的湿斜压性和θse陡立区适宜倾斜涡度发展。Abstract: 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 Vmax, and the reinforcement is equivalent to the attenuation according to Pmin, but they are not synchronous. The reinforcement maintains till it enters sea or 6 hours later at most, but Vmax 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 θse, 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.
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图 1 入海加强､减弱､不变的YBTC各月频数分布
Fig. 1 Distribution of frequencyper month of reinforced, weakened and changeless YBTC
图 2 入海加强的YBTC (a) 和减弱的YBTC (b) 选例路径图
Fig. 2 Path of reinforced (a) and weakened (b) YBTC after entering sea
图 3 两类YBTC中心涡度 (单位:10-6s-1)､散度 (单位:10-6s-1)､垂直速度 (单位:10-2m·s-1)､与环境温度 (单位:K) 偏差在入海前后的垂直分布
Fig. 3 Sections of compositive vorticity (unit:10-6s-1), convergence (unit:10-6s-1), vertical speed (unit:10-2m·s-1) and temperature deviation (unit:K) around the center of YBTC
图 4 两类YBTC入海前后500 hPa高度场 (单位:gpm)(·为TC中心, 横坐标为相对于YBTC中心的经度, 纵坐标为相对于Y BTC中心的纬度)
Fig. 4 Compositive 500 hPa height (unit:gpm) of two type YBTCs (·is TC center, abscissca is longitude relative to the TC center, y-coordinate is latituder elative to the TC center)
图 5 两类YBTC入海前后涡度平流 (单位:10-10m·s-2) 和温度平流 (单位:10-4K·s-1) 演变
(横坐标为相对于入海时刻的逐6h观测时刻, 0为入海时刻)
Fig. 5 Compositive vorticity advection (unit:10-10m·s-2) and temperature advection (unit:10-4K·s-1) of two type YBTCs
(time is 6hours before and after sea-enterring, 0 is the time of entering)
图 6 入海前后两类YBTC合成的200 hPa风场 (横坐标为相对于YBTC中心的经度, 纵坐标为相对于YBTC中心的纬度; 阴影区为风速大于30 m/s, 阴影深度间隔10 m/s)
Fig. 6 Compositive 200 hPa wind of two type YBTCs (abscissca is longitude relative to the TCcenter, y-coordinate is latitude relative to the TC center; shade for wind speed bigger than 30 m/s, shade skip is 10 m/s)
图 7 入海前后两类YBTC合成的θse垂直剖面图 (单位:K; 横坐标为相对于YBTC中心的经度)
Fig. 7 Compositive θse sections of two type YBTCs (unit:K; abscisscais longitude relative to the TC center)
表 1 YBTC入海强度变化统计
Table 1 The statistic of YBTC intensity change
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