A study of summertime wind and air temperature in Kowloon

Abstract

The increasingly frequent urban extreme heat events are adversely affecting human health and the economy worldwide. As one of the most densely constructed and populated cities in the world, Kowloon, Hong Kong has been experiencing increasingly intense heatwaves in recent years and witnessed a rapid warming rate of 0.31°C/decade in the last three decades. An in-depth study of the wind and temperature environment is needed to explore the urban warming mechanisms in Kowloon. The relationship between background wind and daily maximum temperature in Kowloon is investigated using observed weather data for clear summer (June to August) days from 2000 to 2020, and how this relationship is affected by geographical location, land cover, and topography is also found. The results demonstrate the distribution of near-surface wind fields under calm weather conditions, which provides a basis for analysing the relationship between background wind and maximum temperature in urban areas under windy conditions. The research shows that the maximum surface air temperature in coastal regions is significantly influenced by the background wind direction. For weather stations located in the Kowloon Peninsula, a larger background wind speed leads to a faster increase in daily maximum temperature when the background temperature rises. The results also suggest that the mountain warming effect is influential in areas at the foot of a mountain, even though the maximum terrain height is only approximately 500 m. An idealised two-dimensional model for Kowloon is proposed to investigate the urban thermal environments in various wind conditions. The hourly weather is simulated using a 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. 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. Using semi-idealised WRF simulations, the studies investigate the wind flow fields over the Kowloon area under various background wind conditions in real scenarios. Weather photos of Kowloon are examined to identify cloud locations and evaluate the predicted urban plumes. The research also explores the association between background wind and spatial air temperature distribution in Kowloon. Northeasterly wind conditions can lead to high temperatures at the foothills of mountains situated northeast of Kowloon; a streamline analysis shows that heat advection accounts for this phenomenon during the day, while a possible foehn-like wind plays an important role at night. These findings provide a possible way to predict extreme high-temperature patterns in different regions of Hong Kong and explain the mechanisms of extreme heat events in Kowloon. And the understanding of the wind descending mechanism in urbanized areas adjacent to low-altitude mountains may be applied to other cities around the world.