Comparative Studies Of NBEs At Greater Khingan Range And Jianghuai Area Based On Multi-station Observations

Narrow Bipolar Event (NBE) is a kind of distinct intracloud discharge. It had attracted a great deal of attention due to their unusual electromagnetic signatures, such as, extremely strong RF radiation, large amplitude and isolated short-duration very low frequency and low frequency (VLF/LF) bipolar electric field pulses, and weak optical radiation. In this dissertation, by using the GPS based VLF/LF multi-station field measuring networks and VHF receivers operated at Greater Khingen Range (51°N) in northeast China and Jianghuai area (32°N), comparative studies of the phenomenon and meterology context of NBE activity at different latitudes was made. The main conclusions are summarized as follows:(1) The waveform characteristics of VLF/LF bipolar electric field pulses of NBEs were reported. There is no distinction between the VLF/LF waveform characteristics of NBEs occurred at different latitude regions. At Greater Khingen Range, the total pulse duration, initial width and full width at half maximum of the narrow VLF/LF bipolar pulse was27.2μs,7.8us, and4.6μs, respectively. At Jianghuai Area, the positive (nagative) narrow bipolar pulse was characterized with the initial width, rise time, and full width at half maximum of8.3μs (7.6μs),3.1μs (2.9μs),4.4μs (4.0μs), respectively. These results expand the understanding of waveform characteristics of NBEs at different latitude regions.(2) The NBE occurrence appears somewhat different at different geographical location and in different thunderstorms. During the2-year observation periods, only493positive NBEs (negative charge moving up) were recorded at Greater Khingan Range, which accounted for0.034%of all the lightning events, and no negative NBEs were detected. While at Jianghuai Area, a total of21,257NBEs were recorded during8storms, which accounted for1.45%of all the located lightning events. And both positive and negative NBEs were detected in all the thunderstorms, a mean ratio of positives to negatives of9:1was found. NBEs activity in Jianghuai Area is much more virgorous than that in Greater Khingan Range. At Greater Khingan Range, NBEs clustered at altitudes ranging from5km to12km, with a median height of7.9km. While at Jianghuai Area, positive NBEs ranged at7-16km, and negative NBEs clustered at15-18km. The height of positive NBEs occurred at Greater Khingan Range is a littile lower than that in Jianghuai Area and other lower latitude regions. (3) The meterology context of NBEs with the thunderstorm convection and precipitation at different latitudes were analyzed based on the ground-based radar data or satellite-based TRMM PR data. The thunderstorms at Greater Khingan Range had a typical maximum height of30dBZ radar reflectivity (H30dBZ-max) of9-12km (74%of all the317convective cells with lightning having the H30dBZ-max in the range from9km to12km), which showed a relatively weak convection compared to that in lower latitude regions. However, in Jianghuai Area, the maximum height of40dBZ (H40dBZ-max),35dBZ (H35dBZ-max),30dBZ (H30dBZ-max) of convective cells with only positive NBEs was9-11km,11-12km and15km, respectively. And the H40dBZ-max, H35dBZ-max, H30dBZ-max and H18dBZ-max of convective cells with both positive and negative NBEs was14km,16km,17km, and18km, which were little higher than those in convective cells with only positive NBEs. Nevertheless, the height of NBEs at different latitude regions is limited under the top of the thundercloud, especially for those negative NBEs, whose occurrence heights were quite equivalent to the height of rain top (H18dBZ-max). In combination with the differences of NBE height and occurrence of NBEs with different polarities, we suggested that the meterology context of the thunderstorm convection in different latitude regions could be the main reason for the differences of NBEs occurrence. From a certain extent, this also shows that the occurrence of the NBEs, especially the occurrence of negative NBEs, could serve as a proxy of the strongest convection in thunderstorms.(4) NBEs selectively occurred during active period of storm and clustered at the surrounding area of some particular convective cores, which related to strong lightning activity, low cloud top temperature, and high radar reflectivity. Statistical analysis on the temporal context of NBEs with normal lightning activity showed that there is qualitative correlation between NBE and normal lightning activity, but there is a large scatter distribution between them. However, no precise quantitative relationship could be established.(5) The short temporal-spatial context of NBEs with normal lightning was examined by using the multi-station observation and location system in Jianghuai Area. Statistical results showed that the most of NBEs (83.6%) occurred in isolation or ahead of the normal lightning events. The temporal interval between NBE and the following lightning events is less than184.7ms (median value of64.7ms), with the spatial gap of3.1km (median value of1.5km).In the final part of the dissertation, we summarized NBEs activity observed at different latitudes, such as at Greater Khingan Range, Jianghuai Area, as well as in other lower latitude regions. And a comparison was made between these similarities and differences.