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

Abstract

Intra-urban cooling in the city of Ouagadougou, capital of Burkina Faso in the Sahel zone of West Africa, was studied during the dry seasons in 2003, 2004 and 2007. The aim was to see how vegetation, built structure and position within the built-up area influenced the nocturnal cooling. Cooling was divided into two phases. In Phase 1 (16 : 00-20 : 00 hours LST = CET), cooling was very different between the sites while in Phase 2 (20 : 00-06 : 00 hours LST), cooling rates differed insignificantly and the whole area cooled almost at the same rate. Thus the temperature differences between the sites developed during these few hours in Phase 1 were preserved during the rest of the night. In Phase 1, Evening Evapotranspirative Cooling was intensive at vegetated sites that cooled almost twice as fast as sparsely vegetated. This was indicated by a humidity rate (increase of specific humidity per hour) that was high at a vegetated site, but considerably lower at a sparsely vegetated. In Phase 2 the humidity rate was slightly negative with little difference between the sites. After a division in vegetated and sparsely vegetated sites built structure (sky view factor) were shown to influence cooling, but no influence of the position within the built-up area was traced. Thus, the site-specific properties dominated cooling, giving large intra-urban temperature differences. The study also showed the importance of considering a large enough source area to account for micro-scale advection.

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

“…Transpiration induced cooling is most pronounced during the nighttime when transpiration is generally considered to be insignificant as stomata are thought to be close (Berninger et al 1996;Cowan and Farquhar 1977;Hari et al 1986). However, a stronger nocturnal cooling in vegetated sites has also been found in other studies (Bowler et al 2011;Holmer et al 2013;Lindén 2011;Qiu et al 2013;Spronken-Smith and Oke 1998 Barbour et al 2005;Bucci et al 2004;Cavender-Bares and Bazzaz 2000;Dawson et al 2007;Grulke et al 2004;Matyssek et al 1995;Scholz et al 2007;Snyder et al 2003). Nocturnal SF and transpiration has been documented in other studies (Daley and Phillips 2006;Dawson et al 2007;Fisher et al 2007;Marks and Lechowicz 2007;Moore et al 2008;Rosado et al 2012;Scholz et al 2007;Snyder et al 2003;Zeppel et al 2010) and quantified to reach 5-40% of the daytime levels (Caird et al 2007).…”

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

“…However, the contribution of transpiration to the evening cooling was indicated by the relationship between E L of studied trees and the cooling rate of air in the first part of the night (Phase 1 of nocturnal cooling), as well as a less intensive cooling observed at a non-vegetated reference site with a similar SVF. While transpiration was also observed later in the night, in Phase 2, it was no longer correlated with the cooling rate, possibly due to the development of a capping inversion leading to a spatially uniform cooling (Holmer et al 2013). These results are in line with studies by Upmanis et al (1998), Holmer et al (2007 and Holmer et al (2013) where a stronger cooling at a vegetated site was only observed in the first part of the night.…”

“…While transpiration was also observed later in the night, in Phase 2, it was no longer correlated with the cooling rate, possibly due to the development of a capping inversion leading to a spatially uniform cooling (Holmer et al 2013). These results are in line with studies by Upmanis et al (1998), Holmer et al (2007 and Holmer et al (2013) where a stronger cooling at a vegetated site was only observed in the first part of the night. It should be noted that due to spatially uniform cooling in Phase 2, T a differences developed around sunset, partly caused by evening tree transpiration, are preserved throughout the night.…”

“…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.…”

Urban heat island and indoor comfort effects in social housing dwellings

“…As pointed out by Holmer, Thorsson, and Lindén (2012), such phenomenon (termed 'two-phase cooling') seems to be ubiquitous and apparently independent from local climate, as it has been observed in Gothenburg, Sweden, in Adelaide, Australia, in Athens, Greece, in Singapore and in Ouagadougou, Burkina Faso. The fastest cooling takes place during the first phasea more detailed analysis of the outdoor temperature measurements at the Technological Village of Curitiba (location of the monitored low-cost houses) exhibited a similar pattern: a rapid cooling phase during the first hours of the night with diminishing intensity closer to sunrise hours (Fig.…”