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梅雨期高位涡源区及其传播过程
Sources and Transfer of High Isentropic Potential Vorticity During Meiyu Period
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摘要: 分析了气候平均意义下梅雨前期及期间东亚地区等熵位涡（isentropic potential vorticity，简称IPV）的源区和演变过程。结果表明:梅雨发生前，东亚地区对流层高层经向位涡梯度减弱，而后这里的IPV开始向南延伸出高值带，形成“舌区”。同期对流层低层，经向位涡梯度出现反向，与南边位涡梯度大值带形成经向偶极子型并伴随梅雨发生发展。梅雨期40°N，120°E附近对流层顶折叠处有明显的位涡输送和质量交换。用10~90 d带通滤波和超前相关追踪IPV异常源区和传播路径发现，345 K的IPV异常场和梅雨期前后降水异常的相关系数最大值出现在前者超前后者10 d左右，位置在贝加尔湖东侧，这里是影响梅雨期降水的位涡源区。其向南输送高位涡空气主要在梅雨发生前的6月10日左右，高位涡异常空气沿2 PVU等位涡面以东北—西南路径向南输送，在2 PVU面最陡峭处堆积，然后穿越物质面快速下沉侵入40°N以南，并在对流层呈扇状铺开。因而，贝加尔湖东侧可能是影响梅雨的主要冷空气源区，是梅雨降水中期预报的一个关键区。Abstract: The climatological mean sources and evolutions of isentropic potential vorticity(IPV)during Meiyu period are studied. Compared to the factors in the isobaric coordinate system during the Meiyu period, potential vorticity on the isentropic surface shows clearer "trough" and "ridge", and it is found that there are two high IPV "tongue" areas near the east of Lake Baikal and the southeast of Karafuto in the lower troposphere during the Meiyu period. And pentad IPV evolutions show that the regions where the meridional IPV gradient(MIPVG)weakens in the upper troposphere could be the entrance of high IPV air invading. Before the Meiyu rainy season the MIPVG obviously weakens in upper troposphere over East Asia, then high IPV contours begin to extend equatorward forming "tongue" area. Simultaneously, in lower troposphere MIPVG appears to reverse, shaping local north south dipole between low and high MIPVG. Evident potential vorticity transports and mass exchanges exist at tropopause near 40°N, 120°E during Meiyu period where the tropopause easily folds. By using 10—90 day bandpass and lead/lag correlation analysis, the sources and paths of high IPV anomalies are further investigated and traced. The results show that high IPV anomalies originate from the lower stratosphere and upper troposphere of the high latitude, and the maximum correlation coefficients between IPV anomalies and rainfall anomaly on 345 K isentropic surface before, during and after Meiyu period appear when the former leads the latter by about 10 days, near 55°N, 130°E in the east of Lake Baikal which is an important source of high IPV influencing the rainfall during Meiyu period. On June 10 before Meiyu, the high IPV air is transferred primarily southward along the NE—SW direction at 2 PVU surface from the high IPV source and accumulates near the steepest areas of this surface, developing upward, then crosses downward to the tropopause, and partly invades the south of 40°N when the IPV anomalies fields are typical longitudinal mode. However, in the troposphere it becomes a distribution of latitudinal mode spreading out like a fan. On 315 K isentropic surface, high IPV anomalies invade later than those in the upper troposphere, and are also transferred southward along the NE—SW direction from the high IPV source region near 45°N in the lower troposphere to the Meiyu areas since about June 18. As a result, the region on the east of Lake Baikal is possibly a main source area of cold air influencing Meiyu and key region to make the medium term forecast of Meiyu precipitation.
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Key words:
- isentropic potential vorticity;
- Meiyu;
- lead correlation;
- IPV source
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图 1 多年平均梅雨期6月17日-7月18日315 K等熵面位涡(实线: IPV, 单位: PVU, 1 PVU=10-6m2·K·s-1·kg-1, 下同) (a), 700 hPa位势高度和位温(实线:位势高度, 单位: gpm; 点线:位温, 单位: K)(b), 图 1a中点划线范围内区域平均的I PV逐候演变(c), 沿110°~120°E平均的等压位涡和位温经向-气压剖面(实线:位涡, 单位: PVU; 粗虚线:位温, 单位: K)(d)和1998年7月16-31日平均的315 K IP V(等值线)、风场(矢量箭头)及雨强(阴影)(e) (a, b和e中虚线表示3000 m地形高度, d中阴影亦表示地形)
Fig. 1 The climatological mean IPV distributions on the 315 K isentropic surface(solid lines:IPV, unit:PVU, 1 PVU=10-6m2·K·s-1·kg-1, the same hereinafter)(a), height and temperature fields at 700 hPa(solid lines:height, unit:gpm; dot lines:temperature, unit:K)during Meiyu period(b), evolution of area-averaged IPV over 35°-60°N, 110°E-180°from pentad 32 to 42, meridion-pressure cross-section of PV(solid lines)and potentia l temperature(thicked dashed lines, unit:K)averaged over 110°-120°E during Meiyu period(d), and the 315 K IPV(contour), wind vector and rainfall intensity(shaded)ave raged during Jul 16-Jul 31, 1998(e)(dashed lines denote the Qinghai-Tibet Plateau with the altitude≥3000 min Fig.a, Fig.b, Fig.e, and in Fig.d shadow part denotes terrain too)
图 2 梅雨期平均的经向位涡梯度(a中阴影, 单位: 9×10 -6PVU/ m)、高空西风急流(b中阴影为风速≥24 m/s)和经向位涡输送(a和b中等值线, 单位: 9×10 -6 PVU/s)(a)315 K,(b)345 K
Fig. 2 Mean meridional IPV gradients(shaded areas in Fig. a, unit : 9×10 -6PVU/m), upper-level westwind jet(shaded areas in Fig. b, ≥24 m/s)and meridional IPV transports(contours, unit: 9×10-6 PVU/ s)during Meiyu(a)315 K,(b)345 K
图 3 第32, 34和36候的IPV(实线, 2 PVU线被加粗)和经向位涡梯度GPθy (阴影, 浅色为负, 深色代表高值区, 单位: 9×10 -6 PVU)
Fig. 3 IPV(solid lines, 2 PVU contour is thickened)and meridional IPV gradients GPθy (shaded areas, unit: 9×10 -6PVU)at pentad 32, 34 and 36
图 4 第30-38候10~90 d尺度IPV异常(阴影为其正值, 虚线为负值, 从外到里分别为-0.02, -0.05和-0.2 PVU线, 单位: PVU)、未滤波的经向风(箭头表示, 单位: m/s)、等高线(细实线, 单位: 103 gpm)和对流层顶(未滤波的加粗2 PVU线表示)经向-等熵面逐候演变(100°~130°E平均)
Fig. 4 The meridion-isentropic surface section of IPV anomalies in the 10-90-day band(shaded areas:positive, dashed lines:negative, respectively being-0.02, -0.05 and-0.2 PVU contours from interior to exterior, unit:PVU), unfiltered meridional winds(arrow heads, unit :m/s), potential height(thin lines, unit : 103 g pm)and the tropopause(unfiltered thickened 2 PVU lines)averaged over 100°-130°E from pentad 30 to 38
图 5 345 K等熵面10~90 d尺度IPV异常场和降水量异常超前/滞后相关系数(实线)与5月24日-7月23日IP V异常场(阴影)经向-时间剖面图(100°~150°E平均)(a)和纬向-时间剖面图(40°~70°N平均)(b)(0.27和0.6等值线分别代表通过95%信度检验的范围和相关系数的高值区)
Fig. 5 The meridion-time(averaged over 100°-150°E)(a)/ zone-time(averaged over 40°-70°N)(b)section of the cross correlations(thin solid curves, 0.27 solid curves denote the range of 95% confidence level, 0.60 solid curves denote high correlation areas)between 345 K IPV and area-aver aged rainfall anomalies in the 10-90-day band and IPV anomalies(shaded areas)in the same band at 345 K from May 24 to July 23
图 6 345 K等熵面10~90 d尺度IPV异常场超前/滞后降水量异常的相关系数(实线, 从外到里分别为0.35, 0.60, 其中0.35是99%信度检验线)与5月24日-7月18日异常IPV和异常风场(阴影)
Fig. 6 The cross correlations(thin solid lines, respectively being 0.35 and 0.60 from interior to exterior, herein 0.35 standing for the range of 99% confidence level)between 345 K IPV and area-averaged rainfall anomalies in the 10-90-day band and IPV anomalies(shaded areas)in the same band at 345 K from
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