Limor Shashua-Bar

Vegetation as a climatic component in the design of an urban street: An empirical model for predicting the cooling effect of urban green areas with trees

Abstract The cooling effect of small urban green wooded sites of various geometric configurations in summer is the object of this study. It was studied experimentally at 11 different wooded sites in the Tel-Aviv urban complex during the period July–August 1996. An empirical model is developed in this study for predicting the cooling effect inside the wooded sites. The model is based on the statistical analysis carried out on 714 experimental observations gathered each hour from the 11 sites on calm days, when urban climate is expressed. Two factors were found to explain over 70% of the air temperature variance inside the studied green site, namely, the partial shaded area under the tree canopy and the air temperature of the non-wooded surroundings adjoining the site. The specific cooling effect of the site due to its geometry and tree characteristics, besides the shading, was found to be relatively small, about 0.5 K, out of an average cooling of about 3 K at noon. The cooling effect of the green wooded areas on their immediate surroundings at noon was also analyzed. The findings corroborate earlier studies that the range is noticeable. At small green sites, the cooling effect estimated in this study is perceivable up to about 100 m in the streets branching out from the site. The empirical findings in this study permit development of tools for incorporating the climatic effects of green areas in the urban design. Some policy measures are proposed accordingly, for alleviating the “heat island” effect in the urban environment.

Geometry and orientation aspects in passive cooling of canyon streets with trees

As streets usually cover more than a quarter of the urban area, canyon street morphology plays an important role in creating the urban climate. It directly influences the air temperature, moisture and wind flow within the streets as well as the urban surrounding area and has been the topic in several urban climatology studies. Recently, studies based on the street cluster thermal time constant (CTTC) model have been carried out by the authors with a view to assessing the thermal effects of alternative architectural designs of the flanking buildings and inner courtyards. The effect of green spaces, especially that of shade trees which plays a significant role in solar radiation penetration, has not yet been considered. In the CTTC model, passive cooling of the street by solar heating attenuation is governed mainly by the street orientation and its geometry as measured by the aspect ratio of flanking buildings height to street width. The tree shading coverage largely offsets the contribution of these two factors. Moreover, significant thermal effects are provided by the tree canopy, in addition to the direct solar radiation. Accordingly, adjustments are called for in the currently used canyon street models. The present paper discusses the geometry and orientation aspects of the canyon street climate and how these aspects are affected and can be reconciled in the presence of shade trees. Some consequences of environmental design of urban spaces and their effects on outdoor thermal comfort are also considered.

The Green CTTC model for predicting the air temperature in small urban wooded sites

Abstract An analytical model, the Green CTTC (cluster thermal time constant) model, for predicting diurnal air temperature inside an urban wooded site, is the object of this study. The proposed model is based on the same principles as the CTTC model, developed earlier by M.E. Hoffman and colleagues, with the addition of vegetation effects. It is shown that the tree thermal effect can be evaluated either as the shade effect partly offset by the convection component of the tree radiation balance or, equivalently, as the combined effect of evapotranspiration and the change in the plant heat storage. In this paper, the former approach is adopted. Simulations for testing the validity of the Green CTTC model were carried out on summer data of 11 small urban wooded sites in the Tel-Aviv metropolitan area near the Mediterranean sea coast. Results show a satisfactory fit, with average root-mean-square-error K for all studied sites and time intervals at 09:00, 15:00, and 18 : 00 h (summer time). The CTTC values and the convection parameters were estimated from the empirical data, using a novel procedure. The proposed model, which can be enlarged to encompass the cases of groves and lawns, is an appropriate tool for assessment of the climatic impact of trees and other greeneries on urban design alternatives.

Quantitative evaluation of the effects of built-up geometry and trees on diurnal air temperature in canyon-type courtyards

Publisher Summary This chapter emphasizes on the role of urban courtyards in passive cooling. The passive design control elements that are considered—courtyards geometry and the density of shade trees. The chapter presents the case of attached canyon-type courtyards in residential buildings, using summer data in the Tel-Aviv metropolitan area near the Mediterranean seacoast, characterized by hot and humid climate during the summer season. Measurements of air temperature were taken hourly at 9:00, 15:00, and 18:00h in two courtyards, situated in the same locality in Tel-Aviv. Both courtyards consist of a 20–25 meter open space enclosed between two stretches of buildings of about 12 m high. Simulations were conducted for each courtyard, with and without trees, using an analytical model, the “Green CTTC model”. The simulations provide quantitative evaluation of a courtyards diurnal air temperature pattern over the course of 24hours. The effects of changes in the courtyards geometry and the density of shade trees were determined by simulations. The simulated values were compared with measurements in situ. The combined cooling effect of the two courtyards due to trees and deepening was found to reach 4.5K at noon out of 9K rise of air temperature from sunrise to noon in August.