Evolutional Study Of The Convective Storms Moving From Taihang Mountains To The North China Plain

Convective storms that move from the Taihang Mountains often affect Beijing and Hebei regions.In order to improve the ability of short-term thunderstorm forecast in the North China Plain,it is necessary to increase the understanding of climatological laws of downhill convective storms in the Taihang Mountains.This article is based on the national composite radar reflectivity factor data mosaics,ERA5 reanalysis data,sounding data,conventional surface and upper air observations,single-station Doppler weather radar data and high-resolution visible cloud images.We selected 645 strong convective storms at the eastern foothills of the Taihang Mountains during the warm season(May-September)from 2011 to 2020,and studied the climatic characteristics and key environmental influential factors of the downhill evolution.The main conclusions are as follows:(1)Downhill convective storms often intensify significantly in the near-mountain plain north of Shijiazhuang and western Cangzhou.There are four high-frequency centers with strong echo of 45 d Bz and above in plain area: the near-mountain plain north of Shijiazhuang,western Cangzhou,southern Beijing,and central and southern Baoding.The probability of successfully reaching the plains for downhill convective storms with reflectivity factor intensity of 45 d Bz or above is 86%,and the probability of successfully reaching the plains for downhill convective storms with horizontal scale greater than 100 km and high degree of organization is greater than 90%.The strong echo area of enhanced storm rapidly expands during the downhill process,and the average area in the plain area can reach 4.6 times the initial area.Most of the enhanced convective storms take a short time to go down the hill and affect the plains for a long time,up to 4-8 hours.From 18:00 to 02:00 the next day,the main period during which mountainous convective storms affect plain areas.The closer to the west the direction of downhill convective storm comes from,the bigger the enhancement ratio and moving speed of the convective storm.The number of convective storms going downhill from due west is the largest.(2)The environmental characteristics of the successful downhill event of the mountainous convective storm at the eastern foothills of the Taihang Mountains are similar to those of thunderstorm and gale weather in North China,indicating that the cold high and outflow generated by mountainous storm activity are key factors for the convective storms moving downhill.The stronger the synoptic scale forcing,the higher the probability of mountainous convective storms reaching the plains.The common characteristics of environmental factors for successful downhill events under different forcing conditions are favorable for the formation of strong downdrafts and strong cold pools.The difference is that dynamic factors such as strong wind vertical shear dominate under strong forcing conditions,while thermal factors such as high temperature,high humidity,and large convective effective potential energy dominate under weak forcing conditions.There is a significant difference in environmental factors between successful and unsuccessful downhill events under strong force.The vertical shear of ambient wind of unsuccessful downhill event is weak,the wet layer is shallow,the middle dry layer is not significant,the contrast between upper dry and lower wet is not obvious,and the DCAPE value is relatively small.The difference in environmental factors under weak and moderate forcing is far less significant than that under strong forcing.The common feature is that the 850 h Pa and 500 h Pa false equivalent temperature difference values for successful downhill events of convective storms are large,and the difference is that an environment with thicker wet layer,slightly higher convective available potential energy,and moderate wind under weak forcing is more conducive to the storms successfully going downhill,while an environment with less convective inhibition under moderate forcing is more conducive to the storms successfully going downhill.(3)A considerable proportion of successful downhill events occur under relatively weak synoptic scale system forcing,and research results show that mesoscale systems in the boundary layer(925 h Pa)are very important for downhill events: the presence of wind shear and wind convergence in the 925 h Pa wind field will increase the probability of convective storms successfully moving downhill by about 17% and 11% respectively.Compared with southerly winds,easterly winds are more conducive to successful downhill of convective storms;Both warm advection and temperature advection differences are conducive to the successfully moving downhill of convective storms.The stronger the weather scale forcing,the greater the impact of 925 h Pa significant airflow and warm advection on the convective storms moving downhill.The analysis of typical cases further shows that the synoptic-scale forcing and mesoscale system in the boundary layer plays an important role in the development and maintenance of convective storms.