**1. Introduction**

The influence of urban environment on the lightning activity remains a debatable topic, beginning with first publication of Westcott [1], followed by several studies in US, Europe, Asia, Brazil and Australia [2–7]. The change of LULC in urban areas leads to a change of surface characteristics. Such change affects not only the dynamic and physical processes in the atmospheric boundary layer, but also affects the surface heat budget to form an urban heat island (UHI). The UHI effect and its associated impact on environment and regional climate have been receiving wide attention in recent years. It has already been established that the intensity of the UHI effect is strongly associated with the urban size, urban surface characteristics, anthropogenic heat release, topography, and meteorological conditions [8–13]. It has been hypothesized that convection is enhanced by a UHI-induced mesoscale circulation and as a result of such enhanced convection; increased thunderstorm and lightning activity accompanied with enhanced precipitation are observed

over or downwind of major urban areas. During the Metropolitan Meteorological Experiment [14,15], this hypothesis was extensively investigated and substantiated. Modern urbanization, which caused dramatic change in LULC, is basically the demand of increased population and human activity. Contributions of manmade aerosols are also enhanced because of urbanization having significant impacts on local thunderstorm formation leading to an enhanced CG lightning activity particularly over urban area [16–18]. However, the physical mechanisms responsible for these effects are not fully understood due to the complex correlations.

The climate of coastal cities like Taipei is influenced by the development of cumulus convection caused by boundaries that occur when moist air is transported by the sea breeze from ocean to land. Taipei City is surrounded by mountains in all directions, except the two river valleys. The two river valleys, Tanshui and Keelung, respectively, in the northwest and northeast of Taipei City carry surface airflow to the open sea. Sea breezes funnel toward the southern part of the Taipei city through two river valleys. These low-level flows are expected to interact not only with the mountains situated south of Taipei City, but also with the UHI of Taipei City. Blending of UHI effect and sea breezes with the local synoptic and orographic structure causes the summer thunderstorm and lightning activity within the New Taipei City and Taipei City to develop a unique climate system. Exploring the impact of such triple interaction on the local atmospheric circulation Kitada et al. [19] indicated that urbanization has nominal effect on the diurnal airflow if urbanization is built up 100–200 km away from mountains. In contrast, a shift of the highest temperature zone from the center of the city to inland suburbs is evident in the case of extensive urbanized coastal areas. The development of Taipei City was started since 1960s, and with time it has become one of the largest trade centers in Eastern and Southeast Asia. Due to such rapid development in the past several decades, Taipei City has been well urbanized. The urban population has increased by a factor of 3.5. The land use for construction has also increased by a factor of three in the last few decades [20].

The effect of UHI leads to many serious environmental problems, such as regional climate change, air pollution, and visibility deterioration. For example, the diurnal temperature change has been decreasing since the 1970s in Taiwan [21]. The occurrence of fog in hours has decreased significantly over the last 40 years [21]. Temperature has been increasing at a rate of approximately 1.1–1.6 ◦C/century as recorded at eight lowland meteorology stations in Taiwan since 1900 to 2009. This rate of increase is consistent with the warming conditions of other East Asia countries, but significantly higher than the global rate of 0.74 ◦C/century [22,23]. During the last 30 years, the warming rate in Taiwan has almost doubled and reached 0.23–0.40 ◦C/decade. LULC caused by rapid urbanization has been attributed as one of the leading cause of such increasing trend of warming by researchers [24,25].

Taiwan, a mountainous island located at western Pacific coast region, is bisected by the Tropic of Cancer and has 300-km Central Mountain Range (CMR) stretching along north to south with more than 100 peaks above 3,000 m. The hot and humid weather condition provides plentiful lightning strikes over this island during premonsoon and monsoon seasons. Considerable research on thunderstorms and lightning has been conducted over tropical areas and in the United States. Compared to those studies the number of studies relating to Taiwan is limited [5,26,27]. The purpose of this study is to investigate the impact of LULC change caused by rapid urbanization on the formation of local lightning during the warm seasons (May–October) from 1998 to 2012 over Taipei, the capital of Taiwan. Possible influence of urban particulate matter on the enhancement of CG lightning activity is another focus of this study. More precisely we have tried to investigate in this study whether the local climate of Taipei is affected by this urbanization through an increase in lightning activity particularly during summer.
