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A High Resolution Atmospheric Model (HiRAM) at 20-km resolution is adopted to simulate tropical storm (TS) activity over the western North Pacific (WNP) and Taiwan/East Coast of China (TWCN) at the present time (1979 -2003) and future climate (2075 -2099) under the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report (AR5) representative concentration pathway (RCP) 8.5 scenarios. The results show that in contrast to TS simulation activities in most of the low-resolution climate models, TS activities except intensity over the WNP and TWCN region are well simulated by HiRAM at 20-km resolution. The linkage between large-scale environments and TS genesis simulated by HiRAM are dramatically superior to those in low-resolution fifth Coupled Model Intercomparison Project (CMIP5) models. During 2075 -2099, both TS genesis numbers and TS frequency over the WNP and TWCN are projected to decrease consistent with the IPCC AR5 report. However, the rate of decrease (49%) is much greater than that projected in IPCC AR5. The decrease in TC genesis numbers under global warming is primarily attributed to the reduction in mid-level relative humidity and large-scale ascending motion, despite the warmer sea surface temperature (SST) providing more favorable conditions for TS formation. TS intensity and the maximum precipitation rate are projected to increase under global warming. At the end of the 21 st century, the mean precipitation rate within 200 km of TS storm center over the TWCN region is projected to increase by 54%.

Summer rainfall in Taiwan is composed of four types of rain events: tropical cyclone (TC), frontal convection (FC), diurnal convection (DC), and other southerly convection (SC) that propagates from the nearby ocean. In this study, we accessed the present‐day simulation (1979–2003) and future projection (2075–2099, the Representative Concentration Pathway 8.5 scenario) of rainfall in Taiwan by using the regional Weather Research and Forecasting model driven by the global High Resolution Atmospheric Model. The results indicated that the dynamical downscaling process adds value to the present‐day simulation of summer rainfall in Taiwan and the contribution of different types of rain events. It was found that summer rainfall in Taiwan will increase in a warmer future and that this change was mainly due to an increase in SC rainfall (i.e., light rainfall event). The following trends in Taiwan rainfall were also predicted in a warmer future: (1) SC rainy days will increase because the intensified monsoonal flow facilitates the propagation of more SC toward Taiwan, (2) TC rainy days will decrease as the Western North Pacific subtropical high extends southwestward and prevents TC systems from passing over Taiwan, (3) DC rainy days will decrease in response to the increased local thermal stability, and (4) FC rainy days will show no significant changes. Moreover, all types of rainfall are projected to become more intense in the future due to the increased moisture supply in a warmer climate. These findings highlight how the rainfall characteristics in East Asia may change in response to climate change.

This paper is the first of a two-part study that investigates summer convective afternoon rainfall (CAR) activity over the East Asian continent through simulation and projection. We focus on the CAR activity over Taiwan in Part I. In Part II, the changes in CAR activity over South China and Luzon are examined. Using the WRF (Weather Research and Forecasting) model driven by two super-high resolution global climate models: HiRAM (High Resolution Atmospheric Model) and MRI (Meteorological Research Institute Atmospheric General Circulation Model), this study evaluates the performance of models (HiRAM, MRI, and WRF) used to simulate the summer CAR activity over Taiwan. The evaluations focus on the spatialtemporal variations in CAR activity during two time periods: the present-day (1979 -2003, historical run) and the future (2075 -2099, RCP8.5 scenario). For the present-day simulations, analyses indicate that both HiRAM and MRI cannot accurately simulate the temporal evolution of diurnal rainfall. This timing shift problem can be fixed after dynamical downscaling using WRF. The WRF dynamical downscaling approach also helps generate a more realistic CAR amount simulation over western Taiwan, where most residents live. In future projections all models predict that a significant decrease in CAR amount will occur over southwest Taiwan. This decrease in CAR amount is suggested due to the decrease in CAR frequency, not a change in the CAR rate. All models suggest that the cause of decrease in CAR frequency over southwest Taiwan is weaker local afternoon surface wind convergence and thermal instability to suppress CAR genesis.