Nocturnal Cool Island in the Sahelian city of Ouagadougou, Burkina Faso

Abstract

ABSTRACT:Relatively little is known about the urban climate processes of the fastest growing cities in the world today. Described in this paper are urban-rural and intra-urban variations in air temperature and humidity in one of these cities; Ouagadougou, Burkina Faso. Measurements were collected from car traverses and fixed site measurements during two field studies. The aim of the study was to determine the influence of land use on the urban climate in this hot-dry region, with a focus on the role of vegetation and of a large centrally located reservoir.Analyses of results show that vegetation is the most important factor in the nocturnal urban climate in Ouagadougou, while effects of built up and paved areas are limited. Average urban-rural temperature differences demonstrate that while the evening urban heat island (UHI) reached only 1.9°C, the cool island in a densely vegetated area was 5.0°C cooler than the dry rural reference. Temperature differences of the same magnitude persisted throughout the night between vegetated and non-vegetated areas, regardless of urban or rural setting. Cooling from the open water of the reservoir was the most important parameter during daytime, while during evenings and nights, direct effects of the open water of the reservoir are very limited, and the availability of water, allowing enhanced vegetation cover, is of much greater importance compared to the water itself.The results presented here show that the urban land cover, together with climatic region, can create significant differences in the physical properties driving the urban climate in a dry-tropical city compared to those commonly found in temperate cities.

Evening evapotranspirative cooling in relation to vegetation and urban geometry in the city of Ouagadougou, Burkina Faso

“…Site 5 DMN is used as a reference station in our study and it was also used as reference by Offerle et al (2005), Eliasson et al (2009) andLindén (2010). Details of the instruments are given in Table 2.…”

“…However, the LCZs may also be considered as source areas. In Ouagadougou, Lindén (2010) obtained the best relationship between intra-urban temperatures and source areas with a radius of 400 m. This is smaller than the circle with 500 m radius used by Chow and Roth (2006) but a larger area than the 300 m grid used by Hart and Sailor (2009) and Yokobori and Ohta (2009) or 180 m sector in the wind direction used by Murphy et al (2010). Li and Roth (2007) used both 100 and 500 m radius and found best correlation between vegetation cover and UHI intensity for the smaller circle.…”

Transpiration of urban trees and its cooling effect in a high latitude city

“…According to a review by Caird et al (2007), night-time transpiration rates of various plants, including trees, usually amount to 5-15 % of the daytime values. While not directly measured, evening evapotranspiration has also been suggested by Lindén (2011) and Holmer et al (2013) as a reason behind intensive nocturnal cooling of densely vegetated areas in the tropical city of Ouagadougou, Burkina Faso. The rapid cooling of vegetated areas in Ouagadougou around sunset, in the so-called Phase 1 of nocturnal cooling (Holmer et al 2007), was accompanied by an increase of specific humidity not observed at non-vegetated sites, which cooled more slowly.…”

Temporal variations in microclimate cooling induced by urban trees in Mainz, Germany

“…Although numerous studies of urban climate have shown that cooler places within the city are closely connected to an increased vegetation cover (Alavipanah et al 2015;Fan et al 2015;Harlan et al 2006;Lindén 2011;Middel et al 2012;Norton et al 2015), the diurnal variations as well as influence of changing weather conditions on the transpiration-induced cooling are still unclear. For example, cooling was strongest in the afternoon in irrigated parks and parks in humid climates (Jonsson 2004;Potchter et al 2006;Spronken-Smith and Oke 1998).…”