Influence of upper-tropospheric cold anomalies on heavy rainfall along the Eastern Foothills of Taihang Mountains

Using hourly rain records and reanalysis data, the climatic composite analysis and two case studies of heavy rainfall along the eastern foothills of Taihang Mountains are used to examine the influence of the upper troposphere cold anomalies (UTCA) on heavy rainfall. Three UTCAs, i.e., the cold anomaly located upstream of North China (upNC), the cold anomaly dominating the Japan Sea (JS), and the cold anomaly near the East China Sea (ES) are revealed to play positive roles in the heavy rainfall. The upNC cold anomaly and JS cold anomaly both stretch southeastward from the upper to the lower troposphere. The upNC cold anomaly can trigger a negative geopotential height anomaly accompanied by cyclonic circulation anomaly in the upper troposphere to strengthen upper-layer divergence, and a negative geopotential height anomaly in the middle and lower troposphere to enhance the low-level extratropical cyclone (ETC), resulting in heavy rainfall. JS cold anomaly can enhance the downstream trough to prevent the eastward propagation of the upstream convective systems, and further, promote significant southeasterly flows in the northeast of ETC in the middle and lower troposphere with more moist air transported to the eastern foothills of Taihang Mountains. The two heavy rainfall events indicate that ES cold anomaly stretches northwestward to the lower troposphere with a smaller horizontal scale. ES cold anomaly can increase the precipitation by enhancing the easterly anomaly to its north, which supplies a large amount of water vapor from the Northwest Pacific and amplifies the uplift of topography. The evolutions of upNC, JS, and ES cold anomalies 12-h in advance are found to be closely connected to hourly precipitation, which could be precursor signals to provide helpful information in forecasting heavy rainfall.