| Author: | Osei-poku, Louis |
| Title: | Investigating the ionosphere response to thunderstorm/lightning in Hong Kong using GNSS and local lightning data |
| Advisors: | Chen, Wu (LSGI) Chen, Mingli (BEEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Rainstorms -- China -- Hong Kong Lightning -- China -- Hong Kong Ionosphere Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Land Surveying and Geo-Informatics |
| Pages: | xix, 163 pages : color illustrations |
| Language: | English |
| Abstract: | Ionosphere disturbances caused by the processes of space, meteorological and man-made events like thunderstorms adversely affect Global Navigation Satellite Systems (GNSS) operations. However, the wide spatial-temporal capabilities of GNSS is useful to investigate the ionosphere response to these events. By deep convective process, thunderstorms alter electron density in the atmosphere which affect GNSS signals. This study utilizes GNSS and lightning data to study spatial-temporal response of ionosphere to thunderstorm/lightning in Hong Kong (HK). To obtain the ionosphere disturbances caused by thunderstorms/lightning, the low frequency components of ionospheric TEC changes are filtered out known as detrending. However, with the numerous detrending methods, there lacks a systematic means to select an appropriate detrending technique for a particular circumstance. To solve this, the detection and distinguishing condition (2DC) approach is proposed. Firstly, the ionosphere disturbances of days and non-days of an event at a given criteria are compared after applying the detrending method. Detrending methods meeting the criteria are chosen. Then, through statistical means, the method with the least root mean square error is finally chosen as the most suitable. 2DC was used in evaluating detrending methods during thunderstorm/lightning activities in HK. Savitzky-Golay method (order 6, time window 120 minutes) performed better than the commonly used polynomial methods for the detection of ionosphere disturbances. 2DC provides a systematic means to evaluate and select the most suitable detrending method rather than choosing randomly. By analysing long term GNSS and lightning data in HK, the correlation between thunderstorm/lightning events, ionospheric gravity waves and disturbances was shown. The thunderstorm/lightning induced gravity waves exhibited a nighttime peak enhancement which was not observed in other geographical regions. On the diurnal ionosphere response to thunderstorm/lightning, the ionosphere disturbances associated with dominant daytime thunderstorm/lightning activity were seen after about three to five hours while that of dominant evening time were seen almost immediately. The underlying mechanisms for these observations are mainly the processes of gravitational waves and Trimpi effect for the daytime and evening time thunderstorm/lightning activities respectively. These observations from HK region were different from those in the mid latitudes of Southern Africa and US Plains. The low latitude region is a complex dynamism which requires mechanisms to explain. This study has demonstrated this phenomenon and provides insight into some of the mechanisms contributing to this complex nature. The ionosphere response to simultaneous and consecutive occurrence of geomagnetic storm and thunderstorm/lightning was studied as usually only that of one event is studied. Continuous wavelet transform (CWT) was applied to the detrended total electron content to obtain the CWT parameters of magnitude, frequency and period. The highest magnitude of CWT (CWT_mag) was averagely between 0 – 0.1, 0.1 – 0.3 and 0.3 – 0.4 when there was no ionosphere disturbance, and ionosphere disturbance from geomagnetic storm only and thunderstorm/lightning only respectively. CWT_mag was the same regardless of the simultaneous and consecutive occurrence of these events. Although no significant changes were recorded, the CWT_mag values address the limitation of identifying which event occurred by observing the ionosphere disturbances as other parameters like increment in total electron content (TEC) values and rate of TEC index (ROTI) could not do so. To better understand atmospheric gravity wave (AGW) propagation characteristics, the atmospheric ozone enhancement, ionospheric perturbations, and AGW during the passage of Tropical cyclone (TC) Nida accompanied by thunderstorm/lightning in HK region in 2016 were studied. The results showed that the relative position of the GPS stations on the left side of the trajectory had a decrement in perturbations compared to those on the right. This demonstrates that the relative position of an observing GPS station along the TC trajectory could affect the perturbation information retrieved and hence the propagation characteristics of AGW. In general, the findings from this research are beneficial in developing the requisite tools to mitigate the thunderstorm/lightning-generated ionosphere disturbances on GNSS operations in HK. |
| Rights: | All rights reserved |
| Access: | open access |
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