沧州辐合线热动力特征及其对强降水高频区的影响

Observational study of a warm-season high-frequency heavy-precipitation region triggered by surface convergence lines over Cangzhou

摘要:

基于2016—2021年5—9月(暖季)逐小时地面自动站观测数据、雷达资料和ERA5再分析资料的统计分析发现,渤海湾以西、河北沧州附近常存在一条准东西向中尺度辐合线(沧州辐合线),其位置与沧州附近短时强降水中心有较好的对应关系,且无天气尺度系统强迫类时,沧州辐合线附近发生短时强降水的比例远高于辐合线以外的区域。辐合线过程日合成场与平均场对比分析显示,沧州辐合线形成于西风带低槽底部与副热带高压西侧的过渡区,天气尺度强迫弱,低层位于弱南风风速辐合的湿舌中。热动力结构合成分析表明,沧州辐合线及其南北两侧20~30 km范围为高温高湿的辐合区,伴随降水类辐合线的辐合更强且更暖湿,地面温度和露点均较无降水类高2~3 ℃。沧州辐合线本质上是弱的地面中尺度系统,但在高温高湿的环境背景下,可成为主导降水显著增强的中尺度系统。沧州辐合线可在无天气尺度系统强迫下独立触发、组织强降水,亦可在存在天气尺度系统强迫情形下使得对流风暴增强,进而引发强降水。

Abstract:

Statistical analyses of hourly surface observations,radar data,and ERA5 reanalysis during the warm seasons (from May-September) from 2016 to 2021 reveal the presence of a quasi-east-west-oriented mesoscale surface convergence line over the North China Plain west of the Bohai Gulf,hereinafter referred to as the Cangzhou convergence line.This convergence line is predominantly confined to a region approximately 75 km in the north-south direction and 300 km in the east-west direction between the Bohai Gulf and the Taihang Mountains,with nearly 60% of occurrences near 38.5°N.The convergence line typically persists for 3—5 h and exhibits pronounced diurnal variability,forming most frequently in the early morning and evening.Its occurrence peaks in May and June,whereas the proportion of events accompanied by precipitation is highest in July and August,reaching approximately 50%.Overall,precipitation is associated with 37% of all convergence-line occurrences.The spatial distribution of the Cangzhou convergence line closely corresponds to the high-frequency center of hourly heavy rainfall,with the proportion of heavy rainfall events near the convergence line significantly exceeding than in surrounding regions.Composite analyses relative to climatological mean fields indicate that the convergence line generally forms within a transition zone between the base of a mid-tropospheric westerly trough and the western flank of the western Pacific subtropical high.This environment is characterized by weak synoptic-scale forcing,enhanced low-level moisture,and weak southerly wind convergence.Under such weak synoptic conditions,the frequency of hourly heavy rainfall near the convergence line is markedly higher than that in areas more than 30 km to the north or south,highlighting its critical role in producing the local-heavy rainfall maximum near Cangzhou.Thermodynamic and dynamic composite analyses further show that a 20—30 km zone on either side of the convergence line is characterized by pronounced low-level convergence,elevated temperature,and high humidity.The convergence line is typically embedded within a surface warm center (~29 ℃) and a moist region with dew-point temperatures near 17 ℃.During precipitation events,surface temperature and dew point near the convergence line are approximately 2—3 ℃ higher than during non-precipitation cases,providing more favorable conditions for convective initiation and intensification.Although the Cangzhou convergence line is a relatively weak mesoscale feature,it can become the dominant system governing precipitation enhancement under warm and moist environmental conditions.Analyses of representative cases indicate that environments near the convergence line are characterized by high convective available potential energy (CAPE) and low convective inhibition (CIN).Convective storms producing heavy rainfall are either initiated directly along the convergence line or intensify substantially upon as they approach it.In the absence of strong synoptic forcing,the convergence line alone can trigger and organize heavy rainfall.When synoptic-scale systems are present,interactions with mesoscale features such as gust fronts further enhance convective development.Consequently,a pronounced heavy-rainfall center forms near the Cangzhou convergence line,with precipitation intensity significantly exceeding that of the surrounding regions.