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Thermal comfort © G.Scudo, 2002 Gianni Scudo Built Environment Sciences & Technology (BEST), Politecnico di Milano, Via Durando 10, 20158 Milano Text of paper to the COST C 11 "Green structures and urban planning" - Milan Oct 2002 FIRST DRAFT 3. Green urban structures as climate mitigating technology 4. Evaluation and design tools.
Appendixes (to be added) a. "Green design" parameters b. Urban green spaces categories Review of programmes to evaluate outside comfort conditions 
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3. Small scale green urban structures as "thermal comfort giving" technologies The variety of urban grids and buildings generates a wide range of different streets, squares, courts, open spaces which modify local climate into urban microclimates. The urban microclimate can be described as: "The ideal urban climate is an atmospheric situation within the Urban Canopy Layer with an high variation in time and space to develop inhomogeneous thermal conditions for man within a distance of 150 m. It should be free from air pollution and thermal stress by means of more shadow, ventilation and wind protection" (Katzshner 2000). Vegetation contributes to the reduction of thermal stress and pollution as well as to save energy in built areas. Deciduous trees cool by shading urban spaces and buildings during overheated periods whilst allowing radiation transmission in underheated seasons; evergreen trees protect from cold winds and snow; loss of water by vegetation to the atmosphere (evapotranspiration) reduces urban air temperature. Trees also have other important ecological qualities (sound absorption, blocking of rainfall erosion, filtering of pollutants, reduction of ozone, etc) which interact positively with the microclimatic qualities mentioned giving urban trees a very high environmental-economic value. According to Canadian evaluation an old mature tree produces an yearly economic value of about 300 euro for its air conditioning, soil protection, pollution control and wildlife habitat. (Akbari and Taha).
Fig. 6: Ecological qualities of trees (Bernartzsky)
The cooling effect of large parks and greenbelts in terms of modifying urban heat islands has been measured and evaluated by many authors. It is generally agreed that there is an air temperature difference of about 2-3 to 6°C between the inside of large green areas (for example larger than 50 ha) and the surrounding built up areas. Field research and evaluation/simulation work has also recently been carried out on the cooling effect of smaller green urban structures (courtyards, streets, small squares) mainly in North America, Japan and German speaking and Scandinavian countries where knowledge regarding urban climate and successful urban forestry and town "greening" programmes have been developed over the last 20 years. We now have extensive scientific documentation arising from different scientific areas as mentioned above, but little work has been done on simplified methods appropriate to "green" design for comfortable urban spaces applied to Mediterranean climate and cultures. Note: The cooling effect of green areas on urban microclimate is a field of study and interest of many applied sciences: Climatology and meteorology, Forestry and Arboricolture, Bioclimatic Design and Building Physics, Landscape Planning, to name just a few.(See : Yoshino, Oke., Mayer h. et Matzarakis, A.Bernatsky, Santamouris, Carpenter and Walker)
Briefly summing up, the energy budget of a "generic" tree can be simply described as an energy radiation input ( 100%) which is partially reflected (25%), partially transmitted (5-5% in summer for deciduous species), partially absorbed by photosynthetic process (5%) and finally partially emitted as sensible heat (10-15%) and latent heat through evapotranspiration (40%). Trees therefore reduce air temperature through evapotranspiration, reduce the surface temperatures of public spaces, pavements and building facades - and therefore radiant temperature - through shading, control air velocity with the roughness of the crown.
When we design the above mentioned small urban green structures, the effect of evapotranspiration - which is the cooling of the air around the crown &endash; can be large (a large tree thirty metres tall evapotranspires 300-400 litres of water per day which corresponds to a cooling power of approximately 10 kW) but it is quickly mixed and "dissipated" through the air unless elements of vegetation are not completely closed in by walls. It is difficult therefore to evaluate the real cooling effect of evapotranspiration at a small scale (for example the scale of an urban block), because it is difficult (and expensive) to evaluate wind behaviour in the specific geometric and surfaces conditions of urban spaces. (Dimoudi et al. 2000, SAGA Cités 2002). Green elements in the landscape affect wind in a variety of ways: size, location, orientation, porosity, and proximity. Unlike the effect on radiation, however, the effect of vegetation on wind cannot be determined with certainty. We can make educated guesses, based on theory and observation, to suggest how wind can be modified in a landscape. The denser (less porous) a windbreak, the greater the effect on wind speed, but the smaller the area of affected air. Conversely, the looser (more porous) a wind break the lower the effect on wind speed, but the larger the area affected. Furthermore wind velocity and direction is a hard problem to quantify at design level, because, as mentioned before, wind behaviour in a real urban grid is difficult and expensive to evaluate. Wind tunnel techniques and fluid dynamics simulations are unaffordable for small scale urban design; we have a lot of simplified models giving interesting information (wind around the building, shadow, pressure etc), but when geometry changes their reliability is very low. Furthermore wind velocity at 1.5 m above ground level in dense urban tissue is not usually very high, as pointed out in many microclimatic analysis. Much information on wind barriers is useful for landscape architecture but it cannot be used in urban structures unless the urban grid is not open like a park. Wind barriers could be useful in "pilotis" building typologies to mitigate the negative effect of increasing wind velocity on thermal discomfort The main effect we can really control in Mediterranean urban spaces is the field of radiation: solar radiation (direct, diffused and reflected) and long wave radiation or "terrestrial radiation" (from the sky, the ground and objects above ground). The evaluation of solar and terrestrial radiation fields in urban structures is a very complex problem which is usually approached by urban microclimatologists through complex simulation models and/or infrared teledetecting at a scale (1:10.000) which is not useful for small urban block design purposes (from 1:500 to 1:200). Plants have the following characteristics which affect solar radiation: - Individual leaves that allow some radiation to be transmitted though them (normally about 20%), absorb some radiation (normally about 55%), and reflect some radiation (normally about 25%); - The dates when individual species come into leaf in spring and dates when they lose their leaves in autumn; - The shape and maximum height of the plant; - The transmissivity of the canopy in different seasons (a combination of the characteristic of the leaves, twigs, branches, dates and size).
Fig. 7: Solar and terrestrial radian fluxes in a urban green structure
If we compare a street canyon with trees and without trees the difference of air temperature is very low, let's say about 1 °C, but the difference of mean radiant temperature (MRT), due to the shaded and unshed surface temperature differences, is very high: during summer in N-S oriented streets the difference can be up to 30 °C. Furthermore a large contribution to lowering the radiant temperature is given also by green surfaces (leaves and grass) which usually have a temperature a little lower than the air temperature. Figures 8 and 9 show the differences of surface temperature, global temperature, and PET index in streets with and without trees. Fig. 8: Surfaces temperatures in shaded and unshaded areas in a green street in summer (Rambla Catalunya, July) (Ochoa de la Torre, Serra) Fig.9: Microclimatic differences in green and grey spaces: air temperature (up), globe temperature (centre), and index PET (below) in Munchen (Mayer H., and Matzarakis )
4. Evaluation and design tools When an architect begins to address the design of an urban space (street, square and courtyard) he needs some basic information on small scale urban microclimate which usually is either very general (for example: urban canyons) or very specific (a lot of detailed radiant field measures) in a geographic context which he does not know. For example we have extended literature on methods and case studies for contexts in northern Europe and North America, mainly oriented towards moderating cold climates. These contributions often are the fallout of the agroforestal ecologic studies (wind breaks …) with landscape, urban and architectural design. In relation to the Mediterranean context only in the last 15 years the transfer of knowledge from meteorological, climatological and bio-meteorological fields of studies to urban architectural design has taken place and new urban bioclimatic approaches begun. Inside this recent development many methods were developed (some of them contained in appendix c) but still a large gap exists between this knowledge and what designers need in terms of guidelines and simplified methods. Methods available are still too complex . The basic information and simplified method we are now elaborating attempts to fill this gap and to spread the knowledge of radiation control in design culture and practise. Design tool can be simply classified into three categories: guidelines, simplified methods and simulation programme more or less integrated to urban GIS or similar urban thematic maps.
4.1 Guidelines Three are simple or complex guidelines which give different level of design proposal about the use of vegetation coming from the Bioclimatic and Landscape approaches to small scale urban design ( Robinette, Carpenter and Walker ecc..) Some interesting guidelines are going to be elaborated inside the Green Projects financed by UE inside the key action City of Tomorrow ( RUROS, BUGS, GREENSPACE) and will be completed at the end of the next year. The existing guidelines usually give simple information on microclimate, vegetation form and physiology and a lot of examples of what to do ( prescriptive logic). We are elaborating a simplified design guideline which is based on performance approach to green urban structures: street, squares and courts coupled with different vegetation elements (linear, extended, surface and pergola).
Having in mind the vegetation parameters which affect solar radiation ( and wind ) it is easy to set out the first requirements in the preliminary design phase (by trial and error) in relation with the control strategies ( Fig. 10) and the different green elements ( Fig. 11). Let us consider a simple example. Looking at a very specific requirement: to shade a café area in the centre of a small rectangular South-North oriented square in a mid latitude city during lunch time in summer and winter. In summer I need to choose the control strategy: shading and lowering surface temperature (strategies 1, 3 Fig. 11) which means protecting people from direct and reflected solar radiation and shading a nearby area to lower terrestrial radiation. Shaded areas tend to have a surface temperature close to the air temperature, while wet surfaces have an even lower temperature because of the cooling effect of evaporation.
better efficiency but over a long time (see appendix b) we have to choose trees of branching shape, continuous disposition, low transmission (dense crown), in order to get an efficient green umbrella over the café area.
Fig. 11: Microclimatic control strategies through vegetation characteristics (Scudo, Ochoa de la Torre In winter I need to choose the strategies of increasing surface temperature and ground temperature (strategies 2 and 4) to get more radiation. The choice of trees is therefore critical because the seasonal changing in radiation transmission has to be balanced (fig1a, appendix a): so I choose a deciduous trees with high % of solar transmission in winter and low transmission in summer. Once the preliminary approach is completed the designer can evaluate the effect of the choices made on comfort using one of the simplified or simulation methods below and in appendix c briefly described.
Fig. 12: Microclimatic control strategies through the main different green structures: linear, extended, superficial and pergola (Scudo, Ochoa de la Torre)
4.2 Simplified methods The simplified design tool is partially based on Landscape design tool (ComFa elaborated by Brown and Gillespie 1995) and bioclimatic urban design tools ( Dessì 2001, Ochoa 1999) . The main focus is on radiant field. All landscape elements modify terrestrial radiation (longwave) in a microclimate as a function of their ability to absorb and hold solar radiation and their ability to emit terrestrial radiation. Mineralised elements (paved areas and building surfaces) reflect solar radiation, convert radiation into heat and emit radiant heat depending on albedo, emission, conductance and thermal capacity (or thermal admittance). Vegetation also behaves in a similar way, but with some substantial differences due to the specific energy-water balance of its metabolism. While mineralized or hard elements ( concrete or asphalt paved area) can easily reach surface temperatures in the rage of 50-70 °C , leaves temperature are around air temperature with variations in the range on -1° to + °4 °C. Original ComfFA method was not evaluating radiant contribution of hard vertical elements which can now be done through the radiaspace method presented in Appendix c. The results of the simplified method is a ComFA index , which gives the energy balance of a person in outside space correlated to comfort sensation (Tab.3).
Tab. 3: ComFa index: interpretation of energy balance of a person with the sensation of comfort (Brown and Gillepsie,)
4.3 Urban evaluation and simulation programmes. Many programmes have been and are going to be implemented also inside the EU research programmes. Some of these programmes are specifically "green oriented" ( i.e. Greenspace above mentioned), other ones could be adapted to evaluate green environmental effect on urban structures at different scales. Appendix c is a short review of four programmes. A simple classification and short description of some available programmes will be done in the second draft of the paper (end of dicember).
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updated 25 oct 2002