AN ANALYSIS OF THE STRUCTURAL EVOLUTION OF EXTRATROPICAL TRANSITION OF TYPHOON FAXAI (1403) IN LOW LATITUDE
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摘要: 利用气旋相空间法(cyclone phase space，CPS)对1403号台风Faxai变性前后的环境场及结构演变特征进行分析。结果表明：相空间法能够很好地指示低纬变性台风Faxai的变性起止时间。此次过程是由减弱的台风环流与TC西北侧的短波槽结合发展产生，分析台风Faxai的结构演变特征可知，变性阶段TC低层厚度场由均匀对称分布转为非均匀分布，增大了环境斜压性，变性后B值最大达30 m，为弱的斜压非对称结构。Faxai东侧的偏南风暖湿气流与偏北风气流相交汇，使得经向位温梯度增加从而在TC东北象限形成一带状锋区，锋区正好位于南北两大风圈之间的位置。整个变性阶段Faxai西侧几乎无明显冷锋锋生，只在环流东北侧有一定程度的暖锋锋生，这与典型的锋面气旋的发展过程有所不同。变性前，TC呈现对称分布的暖核结构；变性阶段，冷空气从热带低压西侧对流层中低层下沉入侵，TC呈现左侧冷、右侧暖的非对称斜压结构，中层增温可能与槽后强的下沉气流有关。对锋生函数各分量分析发现，散度场主导了气旋周围的标量锋生，倾斜项的贡献次之，涡度场是引起旋转锋生的主要因素，其余两项可忽略不计。Abstract: The cyclone phase space (CPS) method is used to analyze the environmental field and structural evolution characteristics of typhoon Faxai(1403) during extratropcial transition (ET). The results show that the CPS method can indicate the beginning and end time of the ET of typhoon Faxai at low latitudes well. The process is developed by the combination of weakened typhoon circulation and short-wave trough on the northwest side of TC. With the structural evolution characteristics of Faxai being analyzed, it can be seen that the TC low-layer thickness field in the ET stage changes from a uniform symmetrical distribution to a non-uniform distribution, which increases the environmental baroclinicity, and the B value after ET is up to 30, which is a weak baroclinic asymmetric structure. The southerly warm and humid air flow on the east side of Faxai converge with the northerly air flow, which increases the meridional potential temperature gradient and forms a banding front zone in the northeast quadrant of TC, which is exactly located between the north and south wind speed circle. There is almost no obvious cold frontogenesis in the west side of Faxai during the whole ET stage, but only a warm frontogenesis in the northeast side of the circulation, which is different from the development process of typical frontal cyclones. Before ET, TC presents a symmetric structure of warm core; during the ET stage, cold air sinks and invades from the middle and lower troposphere in the western tropical depression, TC presents an asymmetric baroclinic structure with cold left and warm right, and the temperature increase in the middle layer may be related to the strong downdraft behind the trough. The analysis of the components of the frontogenesis function shows that the divergence field dominates the scalar frontogenesis around the cyclone, and the tilting term has a secondary contribution. The vorticity field is the main factor causing the rotation frontogenesis, and the other two are negligible.
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图 1 1979—2017年西北太平洋累年生成TC和变性TC的月际变化及变性TC所占比例(黑线) (a)；逐月TC变性完成时纬度分布特征(b，冷季(蓝色)和暖季(红色))
箱线图的中间线段表示平均纬度。
图 3 台风Faxai 2014年2月27日12时—3月6日18时的CPS路径图
a.-VTL与B; b.-VTL与-VTU。字母“A”表示可用分析中绘制的气旋生命史的开始，“Z”表示结束，“ET_B”表示变性开始，“ET_E”表示变性结束。每间隔6 h绘制一个圆点，圆点的颜色代表该时刻气旋的强度，与图 2所示一致。
图 4 台风Faxai 500 hPa位势高度场(黑线，单位：10 gpm)，风场(单位：m/s)，涡度场(阴影区，单位：10-5 s-1)及500~200 hPa厚度场(红线，单位：10 gpm) (a~b)；850 hPa风场(单位：m/s)，温度平流(阴影区，单位：10-4 K/s) (c~d)
ET_B，ET_E及下图的ET_B-24，ET_E+12分别表示变性开始和变性完成及变性前24 h和变性后12 h，台风符号表示TC中心(下同)。
图 5 气旋中心附近900~600 hPa厚度场
单位：m，圆圈表示以气旋中心为圆心，500 km为半径的圆，圆箭头所示方向为气旋移动方向，箭头两侧的数字为运动方向两侧半圆内900~600 hPa的平均厚度，右上角的B参数为两半圆间的差值。
图 6 925 hPa相当位温(阴影区，单位：K)和辐散风场(单位：m/s) (a~d)；mslp(黑线，单位：hPa)，925 hPa全风速(阴影区，单位：m/s)和-Fn(蓝线，≥2×10-9 K/(m·s)，间隔：4×10-9 K/(m·s)) (e~h)
图 9 a~d分别为ET_E时刻925 hPa标量锋生函数总项(-Fn)(≥ 2 × 10-9K/(m·s), 间隔：4 × 10-9 K/(m·s))，散度项(-Fn - div），变形项（-Fn - def）及倾斜项（-Fn - tilt）
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