华北地区持续性极端暴雨过程的分类特征

Classification characteristics of continuous extreme rainfall events in North China

中文关键词:  华北地区  持续性极端暴雨  暴雨的环流分型  环流特征

英文关键词:North China  Continuous extreme rainfall events  Classification  Atmospheric circulation characteristics

中文摘要:

      利用1960—2015年日降水资料，筛选出华北地区56次持续性极端暴雨过程。基于距平相关系数的客观聚类分析方法和天气学检验，将它们进行分类，并使用NCEP（2.5°×2.5°）再分析资料进行分类合成，对比分析不同环流背景下华北地区持续性极端暴雨过程的基本特征。结果表明，这些持续性极端暴雨事件按照环流背景可分为经向型、纬向型、减弱的登陆热带气旋型和初夏型4类。它们一般都与不同天气系统配置结构下的锋面动力学过程有关，由于锋面结构特征、环境大气层结状态以及与低空急流有关的暖湿气流输送通道和强度不同，造成不同环流形势背景下，暴雨日的高频站点与过程平均累计降水量在空间分布上存在差异。（1）纬向型对应的锋区强度明显强于经向型，但是其对应的层结稳定度与整个夏季状态相当，而经向型存在弱的层结不稳定异常，这表明，纬向型的对流活动一般不如经向型强，持续性锋面降水特征更清晰，造成站点上日降水量超过50 mm的最大频率明显低于经向型，但是过程累计平均最大降雨量却比经向型大。（2）从水汽输送通道来看，源于西太平洋副热带高压南侧的水汽通道只在纬向型环流主导下的华北区域持续性极端暴雨过程中起主导作用。初夏型以及减弱的登陆热带气旋与西风带系统相互作用造成的极端暴雨过程中，活跃的印度季风造成25°N以南异常强盛的纬向低空西南气流携带充沛的水汽，穿过中南半岛后以西南低空急流或者通过减弱的登陆热带气旋“中转”，是这两类暴雨区的主要水汽供应方式；经向型环流背景下的水汽输送也与这支源于青藏高原南侧的西风气流异常有关。这可能是华北地区夏季降水与印度季风降水的相关显著强于中国东部其他地区的主要原因。（3）减弱的登陆热带气旋与西风带系统相互作用造成的极端暴雨事件同样由经向型环流主导，但是，更充沛的水汽输送、更强的上升运动和更深厚的大气不稳定层结状态是它比一般的经向型强度更大的直接原因；此外，中高纬度弱冷空气侵入对减弱的登陆热带气旋顶部形成持续性极端暴雨过程非常重要。

英文摘要:

      Based on daily precipitation data provided by China Meteorological Administration for the period 1960—2015, 56 continuous extreme rainfall events in North China were filtered. Using the objective cluster analysis method based on the anomaly correlation coefficient and synoptic appraisal, these events were classified. Furthermore, the NCEP reanalysis data (2.5°×2.5°) was used to classify and synthesize the synoptic characteristics. Results show that the patterns of continuous extreme rainfall events in North China include warp-wise type, across-warp type, weakened landfalling tropical cyclone type and early summer type. The four types are generally related to the frontal dynamics process under different weather configurations. However, due to differences in the frontal structural features, the environmental atmospheric stratification state and the warm and humid transport channels associated with low-level jets, the spatial distributions of meteorological stations with high-frequency of daily precipitation ≥50 mm and the average cumulative precipitation show different patterns. (1) The intensity of the frontal zone corresponding to the continuous extreme rainfall event of across-warp type is significantly stronger than that corresponding to the event of warp-wise type. However, the stratification stability for the across-warp type is similar to the climate state of summer, while there is a weak-instability anomaly in the warp-wise type. As a result, the convective activity in the continuous extreme rainfall event of across-warp type is generally less violent than that of warp-wise type, and it exhibits a clearer frontal precipitation characteristics with continuous rainfall, that is, the frequency of daily precipitation ≥50 mm in the across-warp type is significantly lower than that in the warp-wise, while the across-warp type has more cumulative precipitation. (2) The water vapor transport channel, which was derived from the south side of the Western Pacific Subtropical High, only plays a leading role in the across-warp type. In the early summer type and weakened landfalling tropical cyclone type, the active Indian monsoon causes a strong low-level zonal southwesterly airflow anomaly to the south of 25°N, which becomes the main source of water vapor after passing through the Indo-China Peninsula or is "transferred" by tropical cyclone. In addition, the water vapor transport of the warp-wise type is also related to the westerly airflow anomaly originating from the south side of the Qinghai-Tibet Plateau. This factor might be the main reason why the summer precipitation in North China has a higher correlation with the Indian monsoon precipitation than that in other parts of eastern China. (3) Similar to the warp-wise type, the weakened landfalling tropical cyclone type is also dominated by meridional atmospheric circulation. However, as a result of more water vapor transport, greater vertical velocity and deeper unstable atmospheric state, the precipitation intensity of the tropical cyclone type is more concentrated. In addition, for the extreme rainfall events located at the northern ridge of weakened landfalling tropical cyclones, the invasion of weak cold air from higher altitudes is significantly effective.