Review of urban climate research in (sub)tropical regions

Changes in land use and land cover alter the local energy balance and contribute to distinct urban climates. This paper presents a local-scale above-canopy study of intra-urban land cover mixes in two cities to analyse the relative effects of surface morphology and local climate on the surface energy balance (SEB). The study is conducted for urban areas in Phoenix, Arizona, and Portland, Oregon, cities with distinct climates but similarly warm and dry summers. A Local-Scale Urban Meteorological Parameterization Scheme (LUMPS) is used to analyse the relative contributions of local weather extremes and land cover variations on the urban energy balance. The partitioning of net all-wave radiation into turbulent sensible and latent heat fluxes as well as heat storage is investigated for a typical dry summer month and two extreme weather scenarios in the two cities. Results of sensitivity analyses show that incoming solar radiation is an important driver of the SEB in LUMPS and should be considered in the generation of climate scenarios. The relationship between individual land cover fractions and SEB fluxes is not clear because of interrelated effects of surface characteristics in the land cover mix. Daytime Bowen ratios vary inversely with vegetation fraction between and within cities for all weather scenarios. Impervious surface cover is positively correlated to the available energy that is partitioned into sensible heat. Cumulative evapotranspiration (ET) is similar for average weather conditions across medium wet sites in Phoenix and Portland but varies more in Portland than in Phoenix under extreme weather conditions. Results suggest that land cover manipulation could offset influences of weather extremes on ET in Portland to a certain degree but not in Phoenix. These findings highlight the importance of spatial and climatic context in the urban design process to mitigate the effects of urbanization.

Section: Analysis Of Energy Partitioningsupporting

confidence: 87%

Section: Introductionmentioning

confidence: 99%

Land cover, climate, and the summer surface energy balance in Phoenix, AZ, and Portland, OR

Middel

Brazel

Gober

et al. 2011

“…Many (sub)tropical cities show a clear seasonal difference in NUHI intensities, with 0.5-3 K lower intensity during the wet season compared to the dry season (Okoola, 1990;Jauregui et al, 1992;Jauregui, 1997;Robaa, 2003;Jonsson, 2004;Chow and Roth, 2006;Roth, 2007;Balogun et al, 2009). The lower NUHI intensities during the wet season are explained by the higher amount of cloud cover, but also by thermal admittance due to increased soil-water (Jauregui, 1997;Jonsson, 2004;Jonsson and Lindqvist, 2005;Roth, 2007).…”

Section: Introductionmentioning

confidence: 99%

“…The lower NUHI intensities during the wet season are explained by the higher amount of cloud cover, but also by thermal admittance due to increased soil-water (Jauregui, 1997;Jonsson, 2004;Jonsson and Lindqvist, 2005;Roth, 2007). The importance of soil wetness to cooling was emphasized by Jonsson and Lindqvist (2005) who compared cloud-free nights during the dry and the wet season, respectively.…”

Section: Introductionmentioning

confidence: 99%

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

Holmer

Thorsson

Lindén

2012

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.

“…The examination of the UHI effect and associated aspects is still relevant and important (McCarthy et al, 2010;Oleson et al, 2010), especially because an increasing percentage of the world population -particularly in the tropics -live in (mega)cities (Roth, 2007;Grimm et al, 2008) or are regular visitors to urban areas for inter alia employment or social/recreational reasons.…”

Section: Introductionmentioning

confidence: 99%

Using phenology to assess urban heat islands in tropical and temperate regions

Jochner

Alves‐Eigenheer

Menzel

et al. 2013

ABSTRACT:The study of urban phenology is relevant to assess the effects of heat islands and the potential impacts of climate change on plants. Temperature is the main factor regulating plant development in moist/temperate regions. However, plants in tropical cities may respond to additional environmental cues, such as air humidity. We examined whether differences in air temperature and humidity along an urban-rural gradient influenced phenological onset dates of trees in a temperate mid-latitude city (Munich, Germany) and a city in the neotropics (Campinas, Brazil). (Dis)similarities were identified incorporating equivalent study design based on identical methods for recording phenology and site-specific meteorological data. We found that phenological onset dates of silver birch (Betula pendula) were able to describe local temperature variations in Munich. Correlation coefficients between mean temperature and onset dates of Betula were significant and varied between r = −0.48 and r = −0.72. However, onset dates for yellow trumpet tree (Tabebuia chrysotricha), false Brazilwood (Caesalpinia pluviosa) and rosewood (Tipuana tipu) in Campinas were rather variable, and the magnitude and significance of correlation coefficients with temperature varied greatly among species/phenophases. Nevertheless, we detected significant correlations, especially between Tipuana phenophases and temperature and between leaf unfolding of Tabebuia and humidity. Our findings revealed that the phenology of Tipuana was sufficiently sensitive to detect urban heat island effects in Campinas and might be a useful indicator of temperature variations and, in turn, of global warming. Because Tipuana is widely used for urban arborization in South America, we foresee many applications for monitoring heat islands in the neotropics. Furthermore, the fact that most phenophases of Tabebuia were not responsive to temperature stressed the importance of humidity variables. Additional species and phenophases should be explored to identify the further potential of phenology for monitoring heat islands in tropical cities.