5-7月大别山地区降水与边界层风场日变化特征关系研究

Research on Diurnal Variations of Precipitation and Wind in Boundary Layer adjacent to the Dabie Mountains During May to July

中文关键词:  地形  降水 低空急流  日变化

英文关键词:Topography, precipitation, low-level jet, diurnal variation

中文摘要:

      [资料和方法]基于2016年至2020年地面自动站、雷达、闪电等多源观测以及ERA5再分析数据，针对5-7月长江中下游关键地区午后和清晨时段降水双峰时空分布特征，[目的]通过分析边界层急流（Boundary Layer Jet, BLJ）频率、对流环境因子日变化特征，[结果]揭示了大别山（Dabie Mountains, DBM）周边区域内边界层风场对降水时空分布的影响机制。[结论]午后在山顶辐射加热影响下，DBM以向山顶爬升的辐合性风场为主，中午至午后时段DBM等山顶地区降水和对流开始明显增强，而在大别山和黄山之间的峡谷地区（Dabie-Huang Mountains Valley, DHV）内以流向两侧山地的辐散性风场为主，平均降水量相对山顶地区较弱。白天时段DHV峡谷BLJ出现频率较低，总体降水量弱于山顶地区。夜间00时前后，长江中下游地区边界层西南风加速度达到最大，BLJ发生频率显著增强，早晨04时至05时左右，来自江汉平原的西风扰动和来自鄱阳湖平原的西南风扰动在DHV峡谷入口区汇合，导致DHV峡谷内西南风日变化扰动分量接近最大，也是BLJ出现频率最高的时段；同时在山顶夜间辐射冷却作用下，来自大别山和黄山山顶地区的下沉山风扰动也接近最大，因在DBM山区与DHV峡谷西南风急流轴之间出现显著的正扰动涡度，并在DBM南部边缘与DHV入口交界处形成水汽通量辐合中心，此处也是夜间对流有效位能高值区顶端，有利于对流触发或进一步东传，导致DHV区域在凌晨时段的强降水日变化峰值。

英文摘要:

      Based on ERA5 reanalysis and observation data from surface auto-weather stations, radars and lighting locators, the characteristics and connection between boundary layer wind and precipitation around Dabie Mountain (DBM) is studied by analyzing the characteristics of dual-peak precipitation in the afternoon and early morning in the middle and lower reaches of the Yangtze River Basin from May to July, and the diurnal variations of Boundary Layer Jet (BLJ) frequency and convective environmental factors. It is confirmed that under the influence of radiative heating in afternoon, DBM area is dominated by convergent wind towards the mountain top, and the precipitation and convection in DBM began to strengthen significantly from noon to afternoon, while the divergent wind flowing to the mountains tops on both sides was dominated in the valley between the Dabie Mountain and the Yellow Mountain (DHV). In the daytime, the southwest jet stream in DHV occurs less frequently, and the precipitation was weaker than that on the top of DBM. After 0000LST at night, BLJ in the middle and lower reaches of the Yangtze River strengthened significantly, and from 0400LST to 0500LST in the morning, accelerated airflow from the Jianghan Plain and Poyang Lake Plain converged in the entrance area of DHV, and the diurnal southwest wind perturbation in DHV was reached to the largest speed in coincidence with the maximum occurrence frequency of the BLJ. At the same time, due to the nocturnal radiative cooling at mountain top, the downhill winds perturbation are also close to the maximal speed in the surrounding areas of Dabie Mountain and Huangshan Mountain, which results in cyclonic wind shear between the downhill wind from the DBM top and the accelerated BLJ in DHV during late night to early morning. Positive vorticity is formed at the adjacent district between the southern edge of DBM and DHV and converges with water vapor flux. This region is also at the tongue top of convection effective potential energy region in night with higher water vapor flux, which is conducive to the triggering and bursting of convection, resulting in diurnal rainfall peak at early morning in the DHV area.