Influence of urban morphology and sea breeze on hot humid microclimate: the case of Colombo, Sri Lanka

Abstract

Urbanisation leads to increased thermal stress in hot-humid climates due to increased surface and air temperatures and reduced wind speed. We examined the influence of urban morphology and sea breeze on the microclimate of Colombo, Sri Lanka. Air and surface temperatures, humidity and wind speed were measured at 1 rural and 5 urban sites during the warmest season. The urban sites differed in their height to width (H/W) ratio, ground cover and distance to the sea. Intra-urban air temperature differences were greatest during the daytime. A maximum intra-urban difference of 7 K was recorded on clear days. Maximum temperatures tended to decrease with increasing H/W ratio and proximity to the sea. All urban sites experienced a nocturnal urban heat island (UHI) when the sky was clear or partly cloudy. The temperature differences between sunlit and shaded urban surfaces reached 20 K, which shows the importance of shade in urban canyons (reducing long-wave radiation from surfaces). Within the urban areas, the vapour pressure was high (> 30 hPa) and showed little diurnal variation. Wind speeds were low (< 2 m s -1) and tended to decrease with increasing H/W ratio. Shading is proposed as the main strategy for lowering air and radiant temperatures; this can be achieved by deeper canyons, covered walkways and shade trees. It is also suggested to open up wind corridors perpendicular to the sea to facilitate deeper sea breeze penetration.KEY WORDS: Urban geometry · Tropical climate · Climate-sensitive urban design · Urban heat island · Coastal city structure Resale or republication not permitted without written consent of the publisher

Review of urban climate research in (sub)tropical regions

“…The available work is primarily concerned with the influence of the built form (geometry, height-to-width of street canyons) to promote ventilation, building materials and colour as means to reduce the absorption of solar radiation and storage of heat as well as the role of vegetation for shading and evaporative cooling to moderate the thermal and moisture climate. Studies were conducted in the following climate types and cities: (1) equatorial wet Colombo (Emmanuel, 2003(Emmanuel, , 2005aEmmanuel and Johansson, 2006), Sao Paolo (Ribeiro, 2005) and Singapore (Wong et al, 2003;Wong and Yu, 2005;Chen and Wong, 2006), (2) tropical wet/dry Belo Horizonte (Sad de Assis and Frota, 1999), (3) tropical highland Mexico City and (4) subtropical dry San Juan, Argentina (Papparelli et al, 1996), Gaborone (Jonsson, 2004), Fez, Marocco (Johansson, 2006), various cities in Algeria (Ali-Toudert et al, 2005;Ali-Toudert and Mayer, 2006), Dimona, Israel (Pearlmutter et al, 2007a) and various settlements in the Negev desert of Israel which are reviewed by Pearlmutter et al (2007b) with special attention given to the relationship between urban geometry and thermal stress.…”

Screening of Tree Species for Improving Outdoor Human Thermal Comfort in a Taiwanese City

“…The sky view factor (SVF) describes the impact of canopy density on air circulation [8] and the irradiance reduction of plant canopies [9,10,21]. Because SVF can be measured conveniently, some studies have used SVF to discuss variations in shading levels and air temperature [22][23][24][25]. Areas with relatively dense broadleaf trees tend to have low SVF values [26].…”

Analysis of Sea-breeze Frontogenesis over the Coastal Urban Area Using Urbanized MM5

“…특히 종관적으로 하안풍(offshore wind)이 존재할 경우, 해풍의 내륙침투는 제한되나 해안역에 서 뚜렷하게 발달한 해풍전선과 함께 강한 해풍을 발 생시킬 수 있다 (Reible et al, 1993). Lee et al (2009) (Emmanuel and Johansson, 2006;Nichol, 1996), (Zhang et al, 1994;Taniike, 1992;Bailey and Kwok, 1985). 한편 Yang and Li (2009) (Hwang et al, 2010).…”