Potential “downstream blocking” synergistic mechanism for urban warming in Kowloon Peninsula, Hong Kong

Abstract

A notable environmental challenge at present is the occurrence of extreme heat events in many large cities such as Hong Kong. In this study, an idealized two-dimensional model for Kowloon is proposed to investigate the urban thermal environments in various wind conditions. The hourly weather is simulated using the Weather Research and Forecasting (WRF) model to evaluate the synergistic roles of urban heat island circulation, sea breeze and mountain slope wind in urban warming. The results highlight that certain background winds transform mountain slope winds into foehn-like winds that worsen the thermal environment in urban areas. These adverse effects are exacerbated in the presence of urban heat island circulation and sea breezes in Kowloon. This potential warming mechanism would lead to long-lasting high-temperature afternoons in urban areas, during which the region with the highest temperature experiences the lowest wind speed. A new theory termed “downstream blocking” is proposed, which clarifies how air masses descend after passing a mountain ridge when a downwind urban heat island exists. The findings can help explain the mechanisms of extreme heat events in Kowloon and provide novel perspectives for understanding the foehn-like wind effects in urbanized areas adjacent to low-altitude mountains.