Statistical Characteristics Of Nocturnal Heavy Rainfall And A Case Study Of Mechanism In North China

Existing research results presented mesoscale and diurnal variation characteristics of convective systems causing heavy precipitation at night.The nocturnal peak of heavy rainfall is evident in North China,but its mechanism needs further study.This study uses hourly precipitation data from 1980 to 2018 to select nocturnal heavy precipitation events from June to August in North China,and comprehensively uses conventional observation data,geostationary satellite data,radar data,and CFSR reanalysis data to analyze the synoptic situations,environmental conditions,distribution and diurnal variation of precipitation,statistical characteristics of physical variables,and the convective systems that caused the intense precipitation at night.A nocturnal heavy precipitation event that occurred at the junction of Hebei and Shandong on August 2nd,2015 is selected,focusing on analyzing the development and evolution of the convective system triggered at night and the causes of local heavy rainfall.The main conclusions are as follows:?1?Based on the criterion that the number of national stations in North China with a precipitation accumulation more than 100 mm in 12 hours at night reached 5,31 cases of heavy nighttime precipitation are selected and classified four categories as tropical cyclones,the vortex and shear,the edge of subtropical high under a weak synoptic forcing,and the other type.The second category accounts for 51.61%.The overall distribution of nighttime precipitation has a center in the east of Beijing-Tianjin-Hebei.The diurnal variation characteristic of these cases is that the average precipitation amount and frequency peaks at night,which stands for convective generation or development at night.?2?Analyze the environment by interpolating CFSR reanalysis data to stations with rainfall intensity greater than 20 mm/h at night.Cases,especially tropical cyclones and low vortex shears categories,have better moist environment compare to short-time heavy rainfall researches,but the thermal and energy conditions are weaker.The vertical wind shear and environmental uplift conditions are consistent with former studys.The overall statistical results show that the south Bohai Bay and the eastern part of Shandong Peninsula is prone to NLLJs.The core of LLJs gradually strengthen and decline in height at night.?3?Using geostationary satellite data since 1996 to capture the general features of convective systems in 23 cases.The active area of nighttime convective matches spatial distribution pattern of precipitation with extra area above the west Bohai Bay.41.4%of heavy precipitation occurs in the area with TBB gradient larger than 0.3 K/km.5 of 7 cases with reliable composite reflectivity data show convection triggered in afternoon,and triggered or developing at night.The nocturnal convective systems are mostly M_?ECSs,and have high frequency of generation in the 12-18 BT and maturity in 18-00 BT.43.75%of convections are triggered in the eastern part of Taihang Mountain.93.75%of systems gradually moves eastward during the development process,and the distance from generation to maturity is 300-400 km in average.?4?A case of nocturnal heavy precipitation in the night of August 2^nd,2015,during which 6stations accumulated more than 100 mm precipitation is carefully studied.This process was closely related to a trough passing by in the south of a strong cold vortex.The M_?ECS triggered at night which caused heavy rainfall in the second half of the night had a very complex evolution process--convection triggered in Wutai Mountain and later combined with multiple newborn convective lines on the east side,and became a squall line at last.In the stage of merging,two cold pools merged into one with a cyclonic circulation center.Due to a better water vapor condition and the centroid of the convection system declining in altitude,the precipitation intensity became greater.Both the large-scale uplifting condition cooperating with the mesoscale convergence and the local train effect in the southern section of the squall line caused the system's long-time maintainance.This case was accompanied by OLLJ and SLLJ.NLLJ strengthened after midnight and its core declined which led to the stronger transportation of energy and humidity,and offered a favorable convergence conditions on its left front.?5?Focusing on the intense rainfall events during the nighttime,the main finding of this paper are concluded.First,the active area of nighttime convective,the east of Beijing-Tianjin-Hebei,conform to the high incidence area of nocturnal precipitation,except for the western part of Bohai Sea,and is located in the left front of the NLLJs frequent area in the east Shandong Province.Second,the overall statistical results of physical quantities show that the abundant water vapor is rather vital to the occurrence and development of night convective.The meso-?-scale convective systems are dominant in the nocturnal heavy rainfall events.Last,there are two development modes of convective system:generation in the afternoon and generation or enhancement at night.