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.