Statistical analysis of quasi-linear convective systems over the Beijing-Tianjin-Hebei region during the warm season

Abstract

The quasi-linear convective systems (QLCSs) in the warm season (May–September) during the 10-year period 2011–2020 over the Beijing-Tianjin-Hebei region are investigated. The synoptic patterns are divided into four types according to the geopotential height field at 500 hPa. The environment is unstable near the locations of the formation of the QLCSs, where the temperature and humidity are high and there are weak updraft and low-level vertical wind shear. These features are beneficial for the formation and rapid development of strong convection. Furthermore, the spatio-temporal distribution characteristics of the shape, evolution, and propagation of the QLCSs are quantified using a skeleton-based automatic identification technology (SBAIT). The QLCSs form in both the mountains and plains, with the highest frequency near the foothills at approximately 200 m, because of the topographic forcing that causes the southerly warm and moist flow to rise near the foothills. There are more QLCSs in the plains and in the daytime than in the mountains and at night, owing to the peak solar heating of the mountains during the day. Further, the environment in the plains is generally characterized as more moist and unstable with vertical wind shear compared with those in the mountains. Most QLCSs are less than 600 km long and more than 4 km and less than 60 km wide. Furthermore, they have an aspect ratio of less than 45 and an area of less than 20,000 km2. The terrain and formation time strongly influence the shape features and intensity of the QLCSs. Following formation, most QLCSs dissipate within 300 km away from the formation locations. Those initiate in the daytime dissipate mostly in the early evening, whereas those initiate at night dissipate mostly in the early morning. Most QLCSs in the mountains propagate to the southeast during the day, while those in the plains propagate to the northeast and southeast at night, and the least number of QLCSs propagate to the northwest. Slow-moving QLCSs with long lives exhibit a slightly reduced area, whereas the area of the fast-moving ones with long lives shows a slight increase. Most QLCSs are in the westsouthwest-eastnortheast (WSW-ENE), southwest-northeast (SW-NE), and west-east (W-E) directions, and a few of them are in the westnorthwest-eastsoutheast (WNW-ESE) direction as well. Most fast-moving QLCSs are downslope systems whose directions are nearly vertical to their propagation directions. If they continue to develop in the plains, then they become long-duration systems. However, the slow-moving QLCSs may exhibit the train effect, i.e., the transmission direction of the convective cells is consistent with the direction of the convective system, which remarkably enhances their sustainability. Moreover, the synoptic patterns exude a certain impact on the diurnal variations of the shape features, evolution, and propagation of the QLCSs, because the locations of QLCSs are distinct in different synoptic patterns.