大气环境条件对夜间飑线影响的敏感性试验

利用CM1数值模式，以2017年8月7日发生在长江三角洲地区的一次夜间飑线过程为例，开展弱切变背景下中层相对湿度、低层风切变和对流有效位能的敏感性试验。结果表明:中层相对湿度升高，有利于夜间飑线雷达回波面积、回波强度和地面降温幅度增大。湿度降低，虽导致夜间飑线的雷达回波宽度变窄，但有利于夜间飑线结构和强度的维持。中层相对湿度的改变对夜间飑线成熟阶段的地面最大风速的影响并不十分明显，但是中层相对湿度的降低会增大地面最大风速的波动；低层风切变的增大使夜间飑线雷达回波强度增强、面积增大、移速变慢，也使飑线冷池强度增强，而对成熟飑线的冷池厚度和地面最大风速影响不大，但是更弱的环境风垂直切变更容易出现脉冲风暴地面强风。低层风切变的减小不利于夜间飑线的发展以及成熟夜间飑线结构和强度的维持；对流有效位能越大，越有利于夜间飑线雷达回波强度和回波面积以及冷池强度和厚度的增大，也有利于夜间飑线地面降温幅度和地面最大风速的增大。中等大小的对流有效位能更有利于成熟夜间飑线强度和结构的维持。低对流有效位能不利于夜间飑线发展，但在中层湿环境条件下依然能发展成为成熟的夜间飑线。该研究揭示了中层相对湿度、低层风切变和对流有效位能等大气环境条件对夜间飑线发生、发展的影响机制，为夜间飑线的预报提供了参考依据。 

Sensitivity of a nocturnal squall line to atmospheric conditions

The influence of atmospheric conditions in the form of changing midlevel relative humidity, low-level wind shear and convective effective potential energy (CAPE) on the 7 August 2017 nocturnal squall line in the Yangtze River Delta is investigated using CM1 numerical model. The idealized simulation results show that the increase in midlevel relative humidity is conducive to increases in radar echo area, echo intensity and surface temperature deficit of the nocturnal squall line. The humidity decrease will narrow the radar echo of the nocturnal squall line, but it is favorable for the squall line to maintain its structure and intensity. The results also show that the influence of midlevel relative humidity on maximum surface wind speed is not very obvious while the decrease of the relative humidity increases the fluctuation of maximum surface wind speed. The increase in low-level wind shear results in radar echo enhancement and area increase of the nocturnal squall line, slows its propagation, and also strengthens the cold pool intensity while has little effect on maximum surface wind speed and cold pool depth, but the weak low-level wind shear is prone to produce strong surface winds caused by pulse storms. The decrease in low-level wind shear is not favorable for the development of nocturnal squall line and maintenance of the structure and intensity of the mature nocturnal squall line. A larger CAPE value is more conducive to increases in radar echo intensity and area as well as cold pool intensity and depth of the nocturnal squall line. It can also lead to larger surface temperature deficit and maximum wind speed. A moderate CAPE value is more beneficial to the maintenance of the intensity and structure of the mature nocturnal squall line, while a low CAPE value is not conducive to the development of the nocturnal squall line, but initial convective storm with the low CAPE value can still develop into a nocturnal mature squall line in midlevel moisture condition. This study reveals the influence mechanism of atmospheric environmental conditions such as midlevel relative humidity, low-level wind shear and CAPE value on the occurrence and development of nocturnal squall lines, which provides a scientific basis for the forecasting of nocturnal squall lines.