Integrated modeling of pedestrian energy exchange and thermal comfort in urban street canyons

412

171

ABSTRACT:The effects of vegetation on human thermal stress in a hot-arid region were tested in two semi-enclosed urban spaces with various combinations of mature trees, grass, overhead shading mesh and paving. The index of thermal stress was calculated hourly from measured meteorological data in the studied sites to evaluate thermal comfort in the different spaces based on radiative and convective pedestrian-environment energy exchanges and sweat efficiency, and expressed on a thermal sensation scale ranging from 'comfortable' to 'very hot'. The efficiency of water use in providing improved comfort was gauged for each of the vegetative landscaping treatments by comparing the total evapotranspiration with the reduction in thermal stress, both expressed in terms of their values in equivalent energy. While conditions in a paved, unshaded courtyard were found to be uncomfortable throughout the daytime hours (with half of these hours defined by severe discomfort), each of the landscape treatments made a clear contribution to improved thermal comfort. With shading, either by trees or mesh, discomfort was reduced in duration by over half and limited in maximum severity when the shading was placed above paving. When combined with grass, both shading mechanisms yielded comfortable conditions at all hours. In both cases, the effect of trees was more pronounced than that of the mesh, but by a small margin. With unshaded grass, 'hot' conditions in the courtyard were restricted to a short period in mid-afternoon, a considerable improvement over unshaded paving, attributable mainly to the lower radiant surface temperatures.

Section: Computation Of Thermal Stressmentioning

confidence: 99%

Section: Computation Of Thermal Stressmentioning

confidence: 99%

Section: Computation Of Thermal Stressmentioning

confidence: 99%

Section: Computation Of Thermal Stressmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

The influence of trees and grass on outdoor thermal comfort in a hot‐arid environment

Shashua‐Bar

Pearlmutter

Erell

2011

412

171

Urban climatology in arid regions: current research in the Negev desert

Pearlmutter

Berliner

Shaviv

2007

Self Cite

Abstract:Desert cities provide climatologists with circumstances that can be qualitatively different from those encountered in more temperate environs. These distinctions are expressed in the balance of heat and moisture between a dry atmosphere and an urbanized terrain, and they are experienced by city dwellers in urban outdoor spaces. But, while such places are characterized by harsh thermal extremes, they also present unique opportunities for microclimatic enhancement.This potential has motivated a series of studies in the arid Negev region of Israel, beginning with a measurement campaign showing that a dense urban fabric can provide significant thermal benefits due to protection from solar radiation, which is dominant within the overall energy exchange. To generalize these findings, an innovative modelling approach -using an open-air, scaled urban array -was used to study variations in urban geometry under realistic radiative loading and boundary-layer turbulence. The surface energy budget above this small-scale urban canopy was found to be similar to observed energy exchange patterns of actual cities.Measurements within the scaled urban canyons served as input to a pedestrian-centred energy exchange model, and a semi-empirical model was also developed for predicting pedestrian energy exchange and thermal discomfort as a function of urban attributes and meteorological data for a given land-use area.This integrated modelling approach revealed that increased urban density, while serving to increase radiative trapping and storage of heat within the urban fabric, also reduces pedestrian thermal stress during the critical daytime hours. This 'cool island' effect is enabled by the high thermal inertia of the built-up area, and the sharp diurnal fluctuations that are peculiar to a hot-arid climate. Its impact is contingent, however, upon the orientation of the street canyon in question -increasing as street-axis orientation approaches north-south and becoming negligible in the east-west direction.

Review of urban climate research in (sub)tropical regions

Roth

2007

346

210

Abstract:Over the last 50 years the developing world, much of which is located in (sub)tropical regions, has seen a dramatic growth of its urban population associated with serious degradation of environmental quality. The total number of (sub)tropical urban climate studies, however; is still small (<20% of all urban climate studies). The available work is further biased towards descriptive studies rather than process work that seeks to indicate the physical climatology of (sub)tropical cities. The available results allow for a preliminary comparison with data from temperate latitudes. Urban heat island (UHI) intensities are generally lower compared to those of temperate cities with comparable population and show a seasonal variation with lower (higher) intensities during the wet (dry) season. (Sub)tropical population-based relations may exist but insufficient appropriate data is available to confirm a logarithmic relationship or systematic differences between different climate types. The (sub)tropical energy balance studies are biased towards dry, clear sky conditions. The amount of net radiation dissipated by sensible heat during daytime is about 40% which is similar to values observed in (sub)urban areas of cities located in temperate climates. Energy partitioning is modulated by water availability and higher percentage of vegetation promotes latent heat flux at the expense of surface heat storage. The apparent strong influence of vegetation and water availability on the energy partitioning irrespective of the climate type, suggests vegetation to be an effective means to reduce heat storage uptake during daytime and hence has the potential to effectively mitigate the nocturnal heat island. It is important to ensure that the rapidly expanding cities of the developing world incorporate climatological concerns in their design to provide a better living and working environment for a large segment of the world's inhabitants. Copyright  2007 Royal Meteorological Society KEY WORDS tropical urban climatology; tropical urban heat island; tropical urban energy balance