Air Navigation and Weather Services

Abstract A severe afternoon thunderstorm (ATS) system developed within the Taipei basin on 14 June 2015, which produced intense rainfall (with a rainfall rate of 131 mm h−1) and urban-scale flooding. A control simulation (CNTL) using the Weather Research and Forecasting (WRF) Model with the horizontal grid size nested down to 500 m was performed to capture reasonably well the onset of the sea breeze, the merger of convective cells, and the evolution of the afternoon thunderstorm system. Four numerical sensitivity experiments with the increase or decrease of midlevel (700–500 hPa) relative humidity (RH) of 10% and 20% were conducted, and simulation results were compared with those from the CNTL. Although the response of convection to midlevel RH was somewhat nonlinear, sensitivity experiments showed that a dry layer at middle levels would result in stronger cold pool, more intense convection, stronger updraft, more graupel particles, stronger net latent heating above the melting level, and a much larger area of the potential flooding region [>40 mm (30 min)−1]. The estimation of bulk entrainment rate provided evidence that the entrainment rate could be reduced by stronger cold pool and the widening of moist convection area. Three terrain-removal sensitivity experiments indicated that Taipei basin modulated the response of convection intensity to midlevel RH. The basin terrain confined the outflow associated with ATS and forced it to converge with the moist sea breeze continuously, providing a favorable dynamic and thermodynamic environment for subsequent convection development. This “basin confinement effect” may be crucial for short-duration rainfall extremes over complex terrain. Significance Statement This study has examined the impact of midlevel moisture on the structure, evolution, and precipitation of an afternoon thunderstorm system that produced intense rainfall at Taipei using eight numerical experiments based on high-resolution model outputs. Our findings explain how a drier layer at middle levels would produce a more intense thunderstorm system, although the response of convection intensity to midlevel moisture is somewhat nonlinear. In addition, it is found that terrain could modulate the response of convection to midlevel moisture, which is rarely discussed in previous studies.

Abstract Montane birds are vulnerable to climate change. However, the mechanisms by which weather drives demographic processes in montane birds have seldom been investigated. We conducted a long-term study (2009–2019) on the Green-backed Tit (Parus monticolus), an insectivorous passerine, in the montane cloud forest of subtropical Taiwan. We explored the effects of weather variability on the productivity and survival of adult Green-backed Tits. Nest survival was negatively associated with seasonal rainfall during the breeding season (April–July) and was lower in early clutches than in late clutches. Higher typhoon-induced precipitation during the postbreeding period (July–September) was related to reduced adult survival, but neither summer temperature nor winter weather conditions were found to be related to adult bird survival. We developed a stochastic simulation model for Green-backed Tit population dynamics based on empirical data. We compared the simulated time-series and observed population growth rates (λ) and found that 80% (8/10 yr) of the observed λ fell within the 5th and 95th percentiles of the simulated data over the 10-yr period. Moreover, the simulated average (± standard deviation) of the geometric mean of λ over 10 yr (1.05 ± 0.07) was close to that observed from 2009 to 2019 (0.99), which provided confidence that the model effectively simulated the population growth rate of the Green-backed Tit. We conducted a sensitivity analysis for λ and found that juvenile and adult survival influenced by typhoon-induced rainfall were the greatest contributors to the variance in the growth rate of the Green-backed Tit population. With the onset of intensified seasonal precipitation associated with global warming, the population growth and density of Green-backed Tits will decline substantially. Our results suggest that under scenarios of high emissions of greenhouse gas, this local population of Green-backed Tits will not persist in the near future.

ABSTRACT Biological processes at low latitudes are often associated with seasonal rainfall, while those at high altitudes are also influenced by low temperature and high humidity. Thus, the breeding phenology of montane species at low latitudes requires clarification. We monitored the timing of nesting for two species of secondary cavity-nesting passerine, the Green-backed Tit (Parus monticolus) and the Rufous-faced Warbler (Abroscopus albogularis), in a montane cloud forest in subtropical Taiwan between 2010 and 2014. To determine the effects of weather and extreme events on nesting success, we used program MARK to model daily nest survival rate as a function of weather variables. The results showed that March temperatures, but not spring (February–March) rainfall, affected the mean laying date of Green-backed Tits. Females shifted their laying dates to lay earlier in response to earlier warm spring weather. The Rufous-faced Warbler exhibited a similar but weaker relationship between March temperature an...