高级检索

华南前汛期粤西海岸低空急流特征及其对地形降水的影响

Characteristics of Low-Level jets over the West Coast of Guangdong Province during the first rainy season in South China and its influence on Orographic Precipitation

  • 摘要: 本文基于2014~2016年华南前汛期(4~6月)广东省(简称粤)阳江市海陵岛风廓线雷达观测资料、地面自动观测站降水资料和ERA5再分析数据集,分析了粤西海岸低空急流的结构特征、日变化特征及其形成机制,并探讨了不同强度边界层低空急流对广东三个关键区域的地形降水时空分布影响。研究表明:(1)基于本文提出的低空急流四个等级判定标准,低空急流累计发生概率为21.2%,其中以1-3级低空急流为主,4级低空急流较为罕见。大部分(77.1%)低空急流的风速不超过14 m?s-1,84.7%的低空急流风向为西南风,低空急流中心最大风速下方的风速垂直切变大多介于(5~25)×10-3s-1。低空急流最大风速出现的高度呈现出双峰结构,大部分低空急流出现在1 km以内的边界层。(2)天气尺度系统相关的低空急流日内发生频数表现为夜间单峰结构,而边界层急流发生频数为昼夜双峰结构。边界层急流夜间主峰值出现在上半夜至早晨,与局地海陆风触发的惯性振荡机制有关,白天次峰值主要出现在下午。近地面附近的低纬亚洲大陆低压与西北太平洋洋面高压两个高低值系统间的压力差对不同强度边界层急流形成起着关键的作用,白天大陆低压发展是强边界层急流午后峰值形成的主要原因。(3)边界层急流对广东地形降水分布和强度的影响机制复杂。粤中北部内陆和粤东沿海区域均以大尺度山脉迎风坡地形降水为主,边界层急流越强,地形降水越强;粤东沿海强地形降水落区稳定,而粤中北部内陆地形降水中心随边界层急流增强而西北移。粤西海岸带中小尺度地形的迎(背)风坡及尾流辐合区均可产生明显地形降水,强地形降水需在合适的低空入流风速背景下发生。(4)在地形降水日变化方面,粤中北部内陆区域降水在不同强度边界层急流影响下均出现了下午和早晨双峰结构,下午峰强度约为早晨峰的两倍,双峰强度随急流加强而增大;粤东沿海区域降水随着急流强度增强,降水由日内双峰结构演变为三峰结构;粤西海岸区域降水在较弱急流影响下为双峰,在4级强急流影响下为三峰结构,而在3级中等偏强急流影响下表现为中午单峰结构。

     

    Abstract: In this study, based on the Hailing Island wind profile radar observation data, precipitation data of Automatic Weather Station and ERA5 reanalysis data during the first rainy season of 2014-2016 in South China, we investigate the structural characteristics, diurnal variation characteristics and formation mechanism of low-level jets (LLJs) over the west coast of Guangdong Province. The effects of low-level boundary layer jets with different intensities on the temporal and spatial distribution of orographic precipitation in three key regions of Guangdong are also discussed. Results show that: (1) Based on the classification criteria of LLJs proposed in this study, the cumulative occurrence probability of LLJs is 21.2%. Most LLJs fall into the class 1-3 category, and class 4 is less common. 77.1% of the speed of LLJs are less than 14 m?s-1, 84.7% of the direction of LLJs are southwest. Most of the vertical shear of wind speed below the maximum wind speed of LLJs are between (5~25) ×10-3s-1. The height of the maximum wind speed of LLJs show a bimodal structure, and most of LLJs appear in the boundary layer within 1 km. (2) The synoptic-system-related LLJs (SLLJ) show a nocturnal peak, while the boundary layer jets (BLJ) show a double peak at midnight to the early morning and afternoon. The inertial oscillations triggered by the local-scale land-sea breeze circulation is responsible for nocturnal peak of BLJ. The pressure difference between the low latitude Asian continental low pressure and the Northwest Pacific Ocean high pressure near the surface, plays a key role in the formation of BLJs with different intensities. The development of continental low pressure during daytime is the main reason for the formation of afternoon peak of strong BLJ. (3) The influencing mechanism of BLJs on the distribution and intensity of orographic precipitation in Guangdong Province is complex. The orographic precipitation on the windward slope of large scale mountains dominates the inland of northern central Guangdong and coastal areas of eastern Guangdong. The stronger the BLJs, the stronger the orographic precipitation. The locations of the heavy orographic precipitation along the east coast of Guangdong are stable, while the inland orographic precipitation center in northern central Guangdong moves northwest with the intensification of BLJ. In the coastal area of west Guangdong, the windward slopes and the downwind slopes of the the medium and small-scale mountains, the lee-side wake convergence area of the terrain can produce obvious orographic precipitation. The heavy orographic precipitation must occurs under the background of suitable low-level inflow wind speed. (4) Under the influence of different intensity BLJ, the orographic precipitation in the inland regions of central and northern Guangdong shows a bimodal structure in the afternoon and in the morning. While the precipitation intensity of the afternoon peak is about twice that of the morning peak, the precipitation intensity of the bimodal peak increased with the strengthening of BLJ. As the intensity of BLJ increases, the diurnal bimodal structure of precipitation evolves into trimodal structure in the eastern coastal area of Guangdong Province. Under the influence of weak or strong jet stream, the precipitation in the west coast of Guangdong shows bimodal or trimodal structure, while under the influence of class 3 BLJ, it shows a single-peak structure at noon.

     

/

返回文章
返回