Investigating the changing spatiotemporal urban heat island and its impact on thunderstorm patterns by Hilbert Huang transform

In the past few decades, the average surface temperature of Earth has been rising specifically in several urbanized cities all over the world. In this context of the phenomenon of Urban heat islands (UHIs), they have a temporal correlation with rainstorms due to the increased heat in urban areas leading to increased atmospheric instability and thunderstorm development. The changing spatial-temporal patterns of urban thunderstorms may lead to short-term heavy rainfall and more severe urban flash flood events. This study aims to understand the UHI impact in the Taipei basin on thunderstorms in order to be a considerable input for future urban development. The first step is to reveal specific details of UHI by understanding the trend of non-linear relation from both time domain and frequency using the Hilbert Huang Transform (HHT). The HHT trend results are used to compared UHI trend between each station. The next step is to extract the main spatial pattern of thunderstorm event in Taipei basin using Empirical Orthogonal Function (EOF). The results show that urbanization in Taipei basin area enhance the UHI intensity for the past 20 years varies between 0.2 and 1.1 degrees C. While according to the EOF analysis, there is increasement of thunderstorm by 42.36% happened in the urban areas of Taipei basin rather than in other area since 1998. Compared to the past, rainstorms are not limited to metropolitan areas recently. These could be asserted as the rising of UHI has a role in the increase of thunderstorm that occurs in the urban area. Proper mitigation is needed in the future as the threat of flash flooding caused by thunderstorm will also rising.