The ambient field characteristics for quasi-straight long path and multi-turning path of eastward moving Tibetan Plateau vortex
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摘要: 利用1998—2018年NCEP/NCAR 全球最终分析数据、大气观测资料、青藏高原低涡切变线年鉴,采用合成方法分析了准平直长路径和多折向路径东移高原低涡的环境场特征,探讨了低涡折向的主导因素。结果表明: 准平直长路径低涡、多折向路径低涡长时间活动的共同环境场特征是有明显影响低涡活动的天气系统, 副热带高压(简称副高)位于高原低涡东南方,高原低涡以北上空伴有东、西段急流;低涡有正涡度平流输入,高原低涡上空为辐散区,高空高位涡下传到低涡。同时,二者环境场特征存在明显差异,多折向路径低涡伴有较强的热带低压活动,是在副高、西风带天气系统、热带低压相互作用的环流背景下,高原涡东移受阻而折向; 准平直长路径低涡是在西风带天气系统为主导的环流背景下向东移动;准平直长路径低涡受冷空气、西南气流与高空锋区的影响比多折向路径低涡强,造成了准平直长路径低涡的正涡度平流、位涡、斜压性、高空辐散比多折向路径低涡强。多折向路径低涡折向的主导因素是环境场条件使低涡在减弱、东移受阻的情况下高空高位涡中心在低涡西部上空,高位涡下传使低涡加强的强正位涡异常区出现在低涡西部,低涡移向低涡加强的区域。Abstract: Composite methods are applied to analyze atmospheric observations and the NCEP/NCAR Final Operational Global Analysis data as well as the Tibetan Plateau vortex (TPV) and shear line yearbooks from 1998 to 2018 to reveal the ambient field characteristics for the groups of the Tibetan Plateau vortices that move eastward following quasi-straight long-path (QSLTPVs) and multi-turning path (MTTPVs), respectively. The leading factors that lead to the TPVs's turning are also discussed. The results show that the ambient field characteristics common for long-lasting QSLTPVs and MTTPVs activities are that there are obvious weather systems affecting TPVs. And the subtropical high is located to the southeast of the TPVs, while the east and west segments of jet stream exist in the upper levels to the north of the TPV. These systems promote positive vorticity advection into the TPVs and there is positive divergence region above the TPVs. Potential vorticity in the upper levels are transported downward to TPVs. The difference in ambient field conditions between the QSLTPVs and the MTTPVs is obvious, too. The MTTPVs are accompanied by strong tropical low-pressure activities. They are blocked and forced to turn under the influences of the sub-tropical high, the westerly wind belt, the topical low-pressure systems and their interaction. The QSLTPVs move eastward in the ambient background field dominated by westerly synoptic systems. And the QSLTPVs are more affected by cold air, southwesterly flow and upper-level front belt than the MTTPVs. These systems lead to stronger positive vorticity advection, larger potential vorticity, stronger baroclinicity and positive divergence in upper levels. The main factors causing the turning of MTTPVs are that the TPVs are weakened and blocked by the ambient field conditions, and the downward transport of high-level potential vorticity to the TPV results in strong positive vorticity in the west of the TPV. Thereby, the TPV moves to the area where it becomes stronger, which explains why the TPV makes turn.
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图 1 1998—2018年5—9月东移路径 (a) 准平直长路径涡,(b) 多折向涡 (数字为表1中高原涡序号;实心圆为08时高原涡位置,空心圆为20时高原涡位置)
Figure 1. The eastward paths of (a) QSLTPVs and (b) MTTPVs from May to September during 1998—2018 (the numbers denote the sequence of TPVs as listed in Table 1;the solid circle and hollow circle are the positions of TPV at 08:00 and 20:00 BT,respectively)
图 3 合成的500 hPa位势高度 (黑色实线,单位:gpm)、温度 (红色虚线,单位:℃) 分布 (a. 组1涡移出时,b. 组1涡加强时,c. 组1涡持续时,d. 组2涡移出时,e. 组2涡折向时,f. 组2涡加强时,g. 组2涡持续时;红色原点为高原涡中心,棕色粗实线表示槽线或切变线,蓝色粗实线表示副热带高压脊线,绿色框线表示锋区位置;x、y轴分别是以低涡中心为中心的纬线、经线方向的相对位置)
Figure 3. Distributions of composite 500 hPa geopotential height (black solid line,unit:gpm) and temperature (red dashed line,unit:℃)(a. Group 1 moving out,b. Group 1 during strengthening,c. Group 1 persistence,d. Group 2 moving out,e. Group 2 turning direction,f. Group 2 during strengthening,g. Group 2 persistence;the red solid circle is the centre of TPV,the brown thick solid line denotes the trough line or shear line,the blue thick solid line denotes the ridge line of the western Pacific Subtropical High,and the green frame line represents the position of the front area;the coordinates in the x-axis and y-axis are the relative coordinates from the center of vortices in zonal and meridional directions)
图 4 合成的500 hPa相对湿度 (色阶,单位:%)、温度 (红色虚线,单位:℃) 和风 (矢量,单位:m/s) 分布 (a. 组1涡移出时,b. 组1涡加强时,c. 组1涡持续时,d. 组2涡移出时,e. 组2涡折向时,f. 组2涡加强时,g. 组2涡持续时;红色原点为高原涡中心)
Figure 4. Distributions of composite 500 hPa relative humidity (shaded,unit:%),temperature (red dashed line,unit:℃) and wind (vector,unit:m/s)(a. Group 1 moving out,b. Group 1 during strengthening,c. Group 1 persistence,d. Group 2 moving out,e. Group 2 turning direction,f. Group 2 during strengthening,g. Group 2 persistence;the red solid circle is the centre of TPV)
图 5 合成的200 hPa风 (矢量,色阶为大风速区,单位:m/s)、位势高度 (黑色实线,单位:gpm) 分布 (a. 组1涡移出时,b. 组1涡加强时,c. 组1涡持续时,d. 组2涡移出时,e. 组2涡折向时,f. 组2涡加强时,g. 组2涡持续时;红色原点为高原涡中心)
Figure 5. Distributions of composite 200 hPa wind (vector,color-shaded areas denote large wind velocity areas,unit:m/s) and geopotential height (black solid line,unit:gpm)(a. Group 1 moving out,b. Group 1 during strengthening,c. Group 1 persistence,d. Group 2 moving out,e. Group 2 turning direction,f. Group 2 during strengthening,g. Group 2 persistence;the red solid circle is the centre of TPV)
图 6 过高原涡中心 (红色圆点) 的合成位涡(等值线,单位:PVU) 纬向垂直剖面 (a. 组1涡移出时,b. 组1涡加强时,c. 组1涡持续时,d. 组2涡移出时,e. 组2涡折向时,f. 组2涡加强时,g. 组2涡持续时)
Figure 6. Height-zonal cross-sections of composite potential vorticity (contour,unit:PVU) passing through the center of the plateau vortex (red dot)(a. Group 1 moving out,b. Group 1 during strengthening,c. Group 1 persistence,d. Group 2 moving out,e. Group 2 turning direction,f. Group 2 during strengthening,g. Group 2 persistence)
表 1 1998—2018年5—9月东移路径的准平直长路径涡、多折向涡过程
Table 1. List of processes of quasi-straight long-path plateau vortices (QSLTPVs) and multi-turning path plateau vortices (MTTPVs) from May to September during 1998 to 2018
路径
类别序号 高原涡
编号过程时间 移出高原
时间中心降水量
(mm)暴雨分布最广的日期\省份 准平直 1 C0012 7月1日08时—3日20时 1日20时 246.6 7月1日\川、鄂、渝 2 C0115 6月1日08时—5日20时 2日20时 235.4 6月4日\桂、粤、闽、赣 3 C1323 6月4日20时—10日08时 5日20时 287.9 6月7日\苏、浙、皖、鄂、湘、贵 4 C1619 5月17日20时—22日20时 19日08时 111.1 5月19日\渝、湘、贵、赣 多折向 1 C0010 6月17日08时—23日08时 17日20时 413.5 6月22日\鄂、浙、皖、湘、赣、闽、贵、桂 2 C0319 7月12日20时—14日20时 13日08时 174.7 7月12日\陕、浙 3 C0726 6月6日20时—13日20时 8日08时 189.7 6月11日\浙(大暴雨) 4 C0819 6月5日20时—10日20时 7日08时 303.2 6月10日\鄂、湘、浙、皖、沪 5 C0831 7月20日08时—23日08时 21日20时 345.7 7月22日\川、渝、贵、鄂、陕、豫、皖、苏 6 C1141 7月30日20时—8月3日08时 31日20时 221.4 8月1日\川、渝、陕、鄂、豫 注:高原涡编号是以“C”字母开头,由年份的后2位数与当年低涡顺序2位数组成。 表 2 与高原涡相伴合成的200 hPa最大散度
Table 2. Composite 200 hPa maximum divergence accompanied with the TPVs
准平直长路径涡 多折向涡 主要时次 最大散度值
(×10−5 s−1)主要时次 最大散度值
(×10−5 s−1)形成 4.324 形成 3.497 移出 4.308 移出 5.178 加强 3.997 折向 2.949 持续 3.828 加强 1.959 将消失 0.921 持续 1.775 将消失 1.570 表 3 高原涡合成的涡中心区(PV)1平均值
Table 3. Composite mean values of (PV)1 in vortex central area for the TPVs
准平直长路径涡 多折向涡 主要时次 (PV)1平均值 (PVU) 主要时次 (PV)1平均值(PVU) 形成 0.6832544 形成 0.8333738 移出 1.0195254 移出 0.8571036 加强 1.0785650 折向 1.0334851 持续 1.0899944 加强 1.0574530 将消失 1.0156387 持续 1.0790279 将消失 1.1936706 表 4 高原涡合成的涡中心区(PV)2 平均值
Table 4. Composite mean values of (PV)2 in vortex central area for the TPVs
准平直长路径涡 多折向涡 主要时次 (PV)2平均值 (PVU) 主要时次 (PV)2平均值 (PVU) 形成 − 0.0255057 形成 − 0.0583614 移出 − 0.0338160 移出 − 0.0371240 加强 − 0.0356576 折向 − 0.0248671 持续 − 0.0099520 加强 − 0.0261846 将消失 − 0.0142794 持续 − 0.0264430 将消失 − 0.0254278 -
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