梅雨发生前对流层顶及平流层异常信号的分析

Analysis of the anomalous signals around the tropopause and in the stratosphere before the Meiyu onset.

中文关键词:  梅雨,对流层顶,位涡,平流层,北半球环状模指数

英文关键词:Meiyu, Tropopause, Potential vorticity, Stratosphere, NAM index

中文摘要:

      利用NCEP和欧洲中心的ERA Interim再分析资料结合中国国家气候中心的20 a梅雨观测资料，分析了梅雨发生前江淮地区对流层顶和位涡的异常以及北半球环状模指数的变化。研究表明,江淮地区在梅雨开始前3—5天会出现对流层顶的下降，且高度的经向变化比纬向变化更为明显。对流层顶的降低与北方冷空气的南下入侵、东亚季风的爆发以及急流轴北跳引起的频繁的对流层顶折卷过程有关。对流层顶的下降伴随着来自平流层的高位涡冷空气的入侵。而梅雨发生前江淮地区上空对应正位涡异常，这一异常的建立和维持与贝加尔湖、西西伯利亚、鄂霍次克海附近的正位涡异常有关；而梅雨发生前江淮地区平流层温度达到极大值，梅雨爆发后开始下降，纬向风则处于西风到东风的转换期。梅雨期的总降水量与对流层上部平流层下部的北半球环状模指数存在一致的正相关关系，而在梅雨发生前15—30天，这种相关性尤为显著。这一结果说明，在对流层顶附近的北半球环状模指数对梅雨期降水量的预测有较好的指示意义。

英文摘要:

      Using the NCEP/NCAR reanalysis data and the ERA Interim reanalysis data together with the Meiyu data from the National Climate Center of China during 1989-2008, the tropopause anomalies and the NAM index over the Yangtze River and the Huaihe River valleys before Meiyu are analyzed. It is found that the tropopause height over the Yangtze River and the Huaihe River valleys decreases 3-5 days before the Meiyu onset， and the meridional change of the tropopause height is more significant than that in the zonal direction. It is proposed to that the lowering of the tropopause before the Meiyu onset is caused by frequent tropopause fold events associated with frequent cold surges from the north, the onset of the eastern Asia monsoon as well as the northward movement of the westerly jet. The decrease of the tropopause height before the Meiyu onset is accompanied by the descent of the cold air of high potential vorticity (PV) from the stratosphere. Positive PV anomalies can be noted over the Yangtze River and the Huaihe River valleys before the Meiyu onset， and the establishment and the maintenance of those PV anomalies are associated with PV anomalies over Lake Baikal, Western Siberia and near Sea of Okhotsk. The stratospheric temperatures over the Meiyu area reach a peak value before the Meiyu onset while the zonal winds in the stratosphere show a transition from westerly to easterly. After the onset of Meiyu, the stratospheric temperatures over the Meiyu area are decreased. The total precipitation during the Meiyu season has a consistent positive correlation with the NAM index in the upper troposphere and the lower stratosphere, which is most significant 15-30 days before the Meiyu onset. The results suggest that the NAM index near the tropopause region is one of good factors for forecasting the precipitation in the Meiyu season.