The Relationship Between Narrow Bipolar Events And Thunderstorm Convective Strength

Narrow bipolar events (NBEs) are thirty years ago discovered special intracloud lightningdischarges. NBEs are distinguished from ordinary lightning by the narrow (about10μs) bipolarVLF/LF electric field-change pulse width, high signal-to-noise ratios and the most powerfulVHF/HF radiation observed from lightning, NBEs are thus easily observed from the ground andspace. Generally, NBEs appear isolated or as initiator of the flash. They are prevalent instronger storms and could indicate the storm strength in some degree. On the basis ofobservations recorded by the Chongqing three-dimensional lightning location network, therelationships between NBEs and storm convective strength (inferred by total lightning and radar)are discussed. The conclusions are as follows:(1) In the storms of southwest China, the positive NBEs (PNBEs) and the negative NBEs(NNBEs) respectively account for81.1%and18.9%of total NBEs. NNBEs are less or nooccurrence in most storms, however, there are some especially intensity storms could producemore NNBEs than PNBEs.NBEs have obvious seasonal variation characteristics, they mainly occur from June toAugust, in which the ratios of NNBEs are obviously higher. NNBEs are most active in August,the proportion of which is higher than other months, besides some storms could produceamounts of NNBEs, which is more than PNBEs. Basically all storms which produce moreNNBEs than PNBEs are happened in August.(2) NBEs cluster more closely than ordinary lightning in both space and time, they occurduring the intense period of lightning activity and in the core of densely lightning area,especially the NNBEs.(3) We statistically compare NBEs rates to the Doppler radar-inferred proxies ofconvective strength. It is concluded that although NBEs rates show much weaker dependenceon the volume of40dBZ radar reflectivity (V40) than ordinary lightning, they increaserapidly with the increased V40and largely occur in high V40area, especially for NNBEs.The spatiotemporal relation of positive and negative NBEs to thunderstorm convection isalso investigated. Compared with PNBEs, the NNBEs cluster more closely in time and space,they generally occur in the strongest development stage of the thunderstorm, largely cluster atthe cloud top of the core of deepest convection. It seems that a very vigorous thunderstorm tendto produce more NNBEs. PNBEs are scattered around the convective cores. These results indicated that NNBEs are closely related to the deep convection, which is useful andmeaningful to indicate thunderstorm convective strength.(4) The first appearance of NNBEs in the storm always corresponds to a reflectivity height‘jump’—abrupt increase in30dBZ and40dBZ height, the reflectivity heights stay high duringthe presence of NNBEs. Furthermore, the longer reflectivity height stay high, the more chanceof producing NNBEs in the storm. Therefore, NNBEs could provide us some indication toestimate the thunderstorm top height and the convective strength.The appearance and height of NBEs are closely related to updraft intensity. There’s avery clear coincidence between PNBEs’ height and updraft variation, NNBEs tend to beproduced in the period of strongest updraft. In intense storms, the PNBEs’ heights are1-2kmtaller than less intense storms on average, in addition the NNBEs rate increase massively, evenbeyond PNBEs, and the NNBEs-to-PNBEs ratio per6minutes could be ten times as many asthe less intense storms. The NBEs rate and height increase probably due to stronger chargingdriven by the intense updraft and the elevated negative charge. Therefore, updraft is animportant factor that affects NBEs arising.