Influence of Upper-Level Trough and Ridge on the Asymmetric Precipitation during Extratropical Transition of Typhoon Usagi
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摘要: 本文基于NCEP再分析资料、TRMM卫星降水资料和RSMC最佳路径集（Best-track Dataset）资料，分析了热带气旋（TC）“天兔”变性过程中降水分布和大尺度环流的演变，并结合片段位涡反演进行修改槽脊的数值试验，研究了环流调整对TC变性过程中非对称降水的影响。结果表明：（1）“天兔”变性前降水集中在路径左侧，伴随着纬向型向经向型调整的背景环流，副热带高压南退的同时减弱西进；（2）TC降水的非对称分布与冷暖锋的相对强弱、水汽输送情况以及高空冷空气下传的落区有直接关系；（3）加强环流调整后，系统斜压性加强，与LOT（降水集中于TC路径左侧）型降水相关的环流指数增大过程随之加强，有利于LOT型降水分布进一步加强。Abstract: Based on the reanalysis data from NCEP/NCAR, rainfall data from TRMM (Tropical Rainfall Measuring Mission), and database from RSMC (Regional Specialized Meteorological Centre) best-track, this paper presents an evaluation of the precipitation distribution and large-scale circulation of tropical cyclone (TC) Usagi during its extratropical transition (ET). Based on this, a numerical experiment was conducted, in which the upper-level trough and ridge were modified using piecewise potential vorticity inversion to consider the influence of the trough and ridge on the asymmetric precipitation of the TC during ET process. The results are as follows: (1) Precipitation from Usagi during ET is concentrated on the left side of the path, accompanied by an adjustment in the meridional-to-zonal circulation, with the subtropical high retreating southward, while weakening and being propelled westward. (2) The asymmetric distribution of TC precipitation is directly related to the relative strength of the cold and warm fronts, the water vapor transport, and the area of falling upper-level cold air. (3) When the circulation adjustment is strengthened, the baroclinity of the system is enhanced. This means that the increased westerly, as indicated by the increased circulation index, promotes an increase in the LOT (left of track) rainfall.
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图 1 台风“天兔”移动路径及2007年8月3日12时至4日12时的累积降水量（彩色阴影，单位：mm d−1）分布。黑色方框为降水分区示意图。台风路径上标注的四位数字前两位表示日期，后两位表示时刻，如2712表示27日12时（协调世界时，下同），下同
Figure 1. Typhoon Usagi movement path and distribution of accumulated precipitation (colorful shadings, units: mm d−1) from 1200 UTC 3 August to 1200 UTC 4 August 2007. Black box shows schematic of the calculation of the precipitation partitions. The four numbers marked on TC (tropical cyclone) movement path, the first two numbers represent date, the last two numbers represent time, for example, 2712 represents 1200 UTC 27 July, the same below
图 2 24 h累积的Brain（左侧纵坐标，蓝色实线，单位：mm d−1）、亚洲环流指数（右侧纵坐标，红色实线，单位：gpm）及TC上下游西风指数（右侧纵坐标，上游：黑色实线，下游：黑色点线，单位：gpm）随时间变化曲线
Figure 2. Evolutions of 24-h accumulated Brain (left y-axis, blue solid line, units: mm d−1), Asian circulation index (right y-axis, red solid line, units: gpm), and west wind index (right y-axis, upstream: black solid line; downstream: black dotted line, units: gpm) of TC. Taking the TC location as the center, the current moving direction as the axis, and within a square with a side length of 10 longitudes, differences of the 24-h accumulated precipitation between the left area and the right area are defined as Brain. The differences of the 500-hPa geopotential height averaged over 60°～150°E between 35°N and 55°N are defined as the Asian circulation index. Taking the TC location as the center and extend 45 longitudes to both sides to calculate the circulation index on the both sides of the TC, which is defined as the western wind index of the upstream and downstream. ETtime is 1200 UTC 4 August 2007
图 3 台风“天兔”的相空间分布：（a）
$ - {{V}}_{{T}}^{{L}}$ 和B；（b）$ - {{V}}_{{T}}^{{L}}$ 和$ - {{V}}_{{T}}^{{U}}$ 。黑色圆点表示TC中心位置Figure 3. CPS (Cyclone Phase Space) distribution of typhoon Usagi: (a)
$ - {{V}}_{{T}}^{{L}}$ (900–600-hPa thermal wind) and B (storm-relative thickness symmetry); (b)$ - {{V}}_{{T}}^{{L}}$ and$ - {{V}}_{{T}}^{{U}}$ (600–300-hPa thermal wind). The black dots indicate the centers of TC
图 4 2007年8月3日18时（a）345 K等熵位涡（阴影，单位：PVU，1 PVU=10−6 K kg−1 m2 s−1）、500 hPa位势高度（黑色实线，单位：gpm）、200 hPa急流（蓝色虚线，>30 m s−1），（b）850 hPa水汽通量（阴影，单位：10−4 g s−1 cm−2 hPa−1）、水平流场（黑色实线）、急流（红色虚线，>12 m s−1）。图a（b）中黑（白）色圆点表示台风中心所在位置
Figure 4. (a) 345-K isentropic potential vorticity (shadings, units: PVU, 1 PVU=10−6 K kg−1 m2 s−1), 500-hPa geopotential height (black solid lines, units: gpm), 200-hPa high-level-jet (blue dashed lines, >30 m s−1), (b) water vapor fluxes (shadings, units: 10−4 g s−1 cm−2 hPa−1), horizontal flow (black solid lines), low-level jet (red dashed lines, >12 m s−1) at 850 hPa at 1800 UTC 3 August 2007. The black (white) dots in Fig. a (Fig. b) indicate the TC centers
图 5 （a）2007年8月1日12时至4日12时CTL试验中Brain（黑色线，单位：mm (6 h)−1）和亚洲环流指数（红色线，单位：gpm）随时间演变；（b）实况（紫色线）和CTL试验（红色线）中TC的移动路径，CTL试验3日12时至4日12时500 hPa平均位势高度场（黑色线，单位：gpm）及24 h累计降水（阴影，单位：mm）；（c）初始场（8月1日06时）的扰动位势高度（黑色实虚线表示正负值，单位：gpm）、扰动位势涡度（阴影，单位：PVU）的反演结果，黑色方框代表修改初始场区域，台风符号代表台风中心位置
Figure 5. (a) Evolutions of Brain (black line, units: mm (6 h)−1) and Asian circulation index (red line, units: gpm) in CTL test from 1200 UTC 1 August to 1200 UTC 4 August 2007; (b) Usagi movement path obtained from observed (purple line) and CTL test (red line), 500-hPa mean geopotential height (black lines, units: gpm) and 24-h accumulated precipitation (shadings, units: mm) obtained from CTL test from 1200 UTC 3 August to 1200 UTC 4 August 2007; (c) disturbed potential height (black solid and dashed lines represent positive and negative values, units: gpm), disturbed geopotential vorticity (shadings, units: PVU) at initial time (0600 UTC 1 August 2007), black box indicates the initial field modification range, the typhoon symbol indicates TC center location
图 6 2007年8月1日12时至4日12时CTL试验（黑色实线）、加强试验（红色实线）和减弱试验（蓝色实线）中（a）亚洲环流指数和（b）Brain随时间演变
Figure 6. Evolutions of (a) Asian circulation index and (b) Brain in CTL test (black lines), Usa_TR+0.2 (Trough enhanced experiment) test (red lines), and Usa_TR-0.2 (Trough weaken experiment) test (blue lines) from 1200 UTC 1 August to 1200 UTC 4 August 2007
图 7 2007年3日12时至4日12时CTL试验与敏感性试验（（a）加强试验、（b）减弱试验）的累计降水量差值（阴影，单位：mm d−1），黑色等值线表示敏感性试验降水量（单位：mm d−1），实线表示降水累计时段内的台风路径
Figure 7. Differences of accumulated precipitation (shadings, units: mm d−1) between CTL test and sensitivity tests [(a) Usa_TR+0.2 test, (b) Usa_TR-0.2 test] from 1200 UTC 3 August to 1200 UTC 4 August 2007. The black contours represent precipitation (units: mm d−1) obtained from sensitivity tests, solid curves represent the Usagi movement path during the precipitation accumulated period
图 8 2007年8月4日06时（a）CTL试验、（b）加强试验、（c）减弱试验500 hPa位势高度场（阴影，单位：gpm）和4日00时至06时累积降水量（红色实线，单位：mm）以及TC移动路径（白色实线）
Figure 8. 500-hPa geopotential height (shadings, units: gpm) at 0600 UTC 4 August 2007, accumulated precipitation (red lines, units: mm) from 0000 UTC to 0600 UTC 4 August 2007, and TC movement path (white lines) in (a) CTL test, (b) Usa_TR+0.2 test, and (c) Usa_TR-0.2 test
图 9 2007年8月4日06时（a）CTL试验、（b）加强试验和（c）减弱试验850 hPa假相当位温（阴影，单位：K）、水汽通量（黑色实线，单位：g s−1 cm−1 hPa−1）和4日00时至06时累积降水量（黑色粗实线，单位：mm）以及TC移动路径（白色实线）
Figure 9. 850-hPa pseudo-equivalent potential temperature (shadings, units: K), water vapor fluxes (black lines, units: g s−1 cm−1 hPa−1) at 0600 UTC 4 August 2007, accumulated precipitation (black thick lines, units: mm) from 0000 UTC to 0600 UTC on 4 August 2007, and TC movement path (white lines) in (a) CTL test, (b) Usa_TR+0.2 test, and (c) Usa_TR-0.2 test
图 10 2007年8月4日06时（a）CTL试验、（b）加强试验和（c）减弱试验950 hPa锋生函数（阴影，单位：K）、水平风场（单位：m s−1）和4日00时至06时累积降水量（黑色实线，单位：mm）以及TC移动路径（红色实线）
Figure 10. 950-hPa function of frontogenesis (shadings, units: K), horizontal wind (units: m s−1) at 0600 UTC 4 August 2007, accumulated precipitation (black lines, units: mm) from 0000 UTC to 0600 UTC 4 August 2007, and TC movement path (red line) in (a) CTL test, (b) Usa_TR+0.2 test, and (c) Usa_TR-0.2 test
图 11 （a）2007年8月4日00时至06时CTL试验累积降水量（黑色实线，单位：mm）分布，蓝色实线为图b、c的剖线，红色实线为台风路径，台风符号为4日06时台风中心位置；（b）2007年8月4日06时湿位涡[MPV1（阴影）、MPV2（红色虚线），单位：PVU]和水汽通量（黑色实线，单位：g s−1 cm−1 hPa−1）的剖面分布；（c）2007年8月4日06时位势高度距平（阴影，单位：gpm）、位涡（黑色实线，单位：PVU）、垂直速度（箭头，单位：m s−1）的剖面分布以及每个剖点的6 h降水量累积值（白色实线，单位：mm）
Figure 11. (a) Accumulated precipitation (black lines, units: mm) from 0000 UTC 4 August to 0600 UTC 4 August 2007, blue line indicates cross section line of Figs. b and c, red line is TC track, and the typhoon symbol indicates the TC center at 0600 UTC 4 August 2007; (b) vertical distribution of humidity potential vorticity [MPV1 (shadings), MPV2 (red dashed lines), units: PVU], and water fluxes (black lines, units: g s−1 cm−1 hPa−1) at 0600 UTC 4 August 2007; (c) vertical distribution of geopotential height anomalies (shadings, units: gpm), potential vorticity (black lines, units: PVU), vertical velocity (arrows, units: m s−1), and 6-h accumulated precipitation (white lines, units: mm) at cross section points obtained from CTL test at 0600 UTC 4 August 2007
表 1 WRF模式运行的参数化方案
Table 1. Parameterization schemes used in WRF model
参数化方案 积分区域中心位置 35.0°N，130.0°E 水平分辨率 30 km×30 km 格点数目 368×257×35 初始时刻 2007年8月1日06时 积分步长/时间 Δt=60 s/78 h 微物理方案 Ferrier (new Eta)方案 长波辐射方案 Rrtm方案 短波辐射方案 Dudhia方案 近地面层方案 Monin-Obukhov方案 陆面过程方案 Noah方案 边界层方案 Mellor方案 积云参数化方案 Grell-Devenyi方案 
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