Rooftop Greening and Local Climate


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Rooftop Greening and Local Climate

  1. 1. ROOFTOP GREENING AND LOCAL CLIMATE: A CASE STUDY IN MELBOURNE Michael Bruse1 and Carol J. Skinner2 1) University of Bochum, Institute for Geography, Universitaetsstrasse 150, D-44780 Bochum, Germany 2) Bureau of Meteorology, GPO Box 1289K, Melbourne 3001,AustraliaABSTRACTA high-resolution numerical model is used to estimate the microclimatic effect of introducing vegetation at streetand/or rooftop level. Results are presented to show the effect on a hot afternoon at a location in the inner suburbs ofMelbourne, Australia. The study shows that the addition of vegetation reduces temperatures and wind speeds, therebyimproving climatic amenity for pedestrians. Other environmental benefits of rooftop gardens are discussed.INTRODUCTIONWhen trees and soil give way to buildings and paved surfaces, the energy balance near the earth’s surface changes. Lessincoming solar radiation is dissipated as latent heat and more goes into sensible heat. The resulting rise in surfacetemperatures is one contributing factor to the urban heat island effect – an effect which is particularly unwelcome inAustralia’s rather warm climates.As the urban building density increases, the radiatively active surfaces moves upward towards the rooftops. In denselybuilt-up urban environments, roofs are the surface where most of the absorption, reflection and emission of radiationtakes place. It seems likely that changes in roof treatments can alter the local climate.A considerable amount of work has been done on the impact of vegetation on urban climates (Akbari et al, 1997,Finnigan, 1994). Urban vegetation is particularly beneficial in improving thermal comfort in hot dry climates (Givoni,1989). Large-scale planting has been shown to reduce urban heat islands (Akbari, Rosenfeld and Taha, 1990, Goward,1981). Studies of the micro-climatic effect of rooftop gardens have been largely descriptive (Spiller, 1993).The City of Port Phillip, where this case study is located, is an inner-city neighbourhood lying on the shores of PortPhillip Bay in southeastern Australia (see figure 1). The study estimates the change in local climate achievable at aspecified site through:1) adding rooftop gardens to all buildings (the green roofs case); and2) combining rooftop gardens with street-level vegetation (the all-greened case).
  2. 2. Figure 1 Location of the case-study area (the City of Port Phillip, Melbourne)The South Melbourne Market occupies most of a city block. The southern section of the Market building is 6m highand the northern section is 4m. Surrounding buildings vary in height from 3 to 11 m. There is very little vegetation inthe study area. In the “all-greened” case, the asphalt parking lots to the W and SW of the Market were planted withgrass and trees; the roads around the market were narrowed to allow the addition of a 9m wide strip of grass, plantedwith a central row of trees (see figure 2).Figure 2 The Market (the large central building) and proposed tree plantings; strip of grass on roads not shown.METHODSA three-dimensional non-hydrostatic urban climate model, ENVI-met, (Bruse and Fleer, 1998) was used to simulate themicroclimate of the South Melbourne Market and its immediate surroundings. The ENVI-met model is able tosimulate microscale interactions between urban surfaces, vegetation and the atmosphere. The model input parametersused are shown in Table 1.Table 1 Basic input parameters to the modelLocation 37.82 deg. S, 145.00 deg. E (South Melbourne)
  3. 3. Date, time of simulation 23rd January; 1400 hoursInitial wind 3m/s at 10m from 350 deg. (slightly west of north)Boundary conditions Temperature (at 2500m) = 293K Specific humidity (at 2500m) = 3g/KgGrid size 73 x 74 x 20; X-Y grid spacing, 3m; Z grid spacing, 2mPlants Trees in parking lots: 20m high, dense foliage, deciduous Street trees: 10m high, distinct crown, dense foliage, deciduous Grass trip around Market, 9m wide. Rooftop plants: 5-cm grass covering all building roofs. The Market building roof has a 3m wide border of 2m high acacia bushes.Surfaces/soil profiles Asphalt road profile: asphalt to 60 cm, then loam down to 2m. Soil in parking lot: loamSoil initial conditions Temperature (-2m) = 290K.A hot day was chosen for analysis, because small increases in comfort deliver greatest health benefits in thermallystressful conditions.RESULTSReduced temperatures and wind speeds were observed due to the introduction of vegetation, with the greatestreductions in the all greened case.Selections from the voluminous model output are shown in figures 3, 4 and 5. Temperature and wind speed values aredifferences from the ungreened (status quo) case for the same level and time of day. Green areas indicate vegetation atthis height; black areas are buildings. The horizontal views are cross-sections just above the roof level of the Marketbuilding. The cross-sections cut through the grassed roof of the southern part of the Market building and are 2m abovethe grassed roof of the lower northern part (and therefore not shown green).Influence on wind speedAs figure 3 shows, the maximum wind speed reduction just above market roof level is around 0.90 ms-1 in the green-roofs case whereas in the case with maximum greening a reduction up to 1.30 ms-1 can be found. The zones of reducedwind speed are mainly limited to the areas where the plants are placed. Due to the sheltering row of street trees aroundthe market, the wind speed in the case with maximum greening is a little lower across the whole market platform.Inside the stand of tall trees on the western (left) side of the model domain, the trees reduce wind speed inside the leaflayer but between the plants the wind is canalised and the reduction effect is considerably less.
  4. 4. Figure 3 Difference in wind speed; horizontal section at 6 m above ground level. Left: Green roofs only. Right:Maximum greening. Isoline interval 0.2 ms-1.Influence on Air temperatureIn general, a maximum temperature reduction of 1.4 K above market ground level in the green-roofs case and of 2.4 Kin the all-green case can be observed (figure 4). The areas of local air temperature reduction are largely restricted to thegreen roof locations and are advected with the main northerly airflow toward the south of the area. Although thisadvection increases the area affected by the rooftop cooling effect, there is a clear area of influence for each greenedroof.By contrast, in the all-greened case, the reduction of air temperature is more uniform. There is a maximum temperaturereduction of 2.4 K, south of the tall trees on the western side of the Market. Here, the cooling effects of the rooftops andof the street-level vegetation combine to act as a single system.
  5. 5. Figure 4 Difference in air temperature; horizontal section at 6 m above ground level. Left: Green roofs only. Right:Maximum greening. Isoline interval 0.2 K.Figure 5 shows the vertical extension of the vegetation cooling effect. In both cases, the area of reduced air temperatureextends upward to around 42m above ground level. Little difference in vertical extent can be found between the twogreening scenarios. But in intensity, the all-green case shows an effective reduction in air temperature up to twice thereduction of the “roofs only” scenario.Figure 5 Difference in air temperature. Vertical section running north south, 72 m from the western boundary. Top:Green roofs only. Bottom: Maximum greening. Isoline interval 0.2 K.DISCUSSIONIn centuries past, town dwellers were sheltered from the wind by closely spaced buildings of roughly uniform height.Streets were narrow, as movement was mostly on foot. Our cities, with their wider roads, varied building heights and
  6. 6. open parking lots, offer less wind shelter to pedestrians. As bricks and concrete have replaced vegetation, the coolingeffect of plants, through shading and transpiration, has been lost. This case study has shown how addition of vegetationcan improve climatic amenity for pedestrians, in an area which depends on visitors for its financial survival.In densely built-up areas where land is at a premium, rooftop gardens may be the only practicable way to “green” thecity. It has been shown that urban vegetation reduces the energy needed for indoor climate control (Akbari et al, 1997).Vegetation is known to remove particulate pollutants from the air, by impaction on leaves (Givoni, 1989). Urbanrunoff is greater than in rural areas, because of the city’s impervious surfaces (Oke, 1987) and its quality is poorcompared with that from forested areas (Duncan, 1999). Although not studied here, it appears likely that rooftopgardens can reduce the amount of urban stormwater runoff by adding more permeable surfaces to the city and canimprove the quality of runoff, by filtering runoff water through garden soils. Because of its positive impact on urbanclimate, on energy use, on urban runoff and on air and stormwater quality, it appears likely that, if introduced on awider scale, rooftop greening could reduce the ecological footprint of our cities. Some German cities already have by-laws requiring new industrial buildings to incorporate rooftop gardens: planners in other cities may like to consider theadvantages of “gardens in the sky”.ACKNOWLEDGEMENTSThe authors wish to thank Geoff Connellan of Burnley Horticultural College, University of Melbourne for providinginformation about Australian native plants.REFERENCES1. Akbari, H ., Kurn, D., Taha, T., Bretz, S., and Hanford, J. 1997. Peak power and cooling energy savings of shadetrees. Energy and Buildings. 25(2): 139-148.2. Finnigan, J.J., 1994. Improving the physical urban environment with trees. In: Baird, I A (ed.). Proceedings of the2nd National Urban Tree Seminar on Urban Trees: the challenge for Australian cities and towns.3. Givoni, B., 1989. Urban design in different climates, World Meteorological Organization4. Akbari, H., Rosenfeld, A., and. Taha, T. 1990. Summer heat islands, urban trees, and white surfaces. ASHRAEProceedings, Atlanta, GA, (February). Also Lawrence Berkeley National Laboratory Report LBL-28308, Berkeley,CA.5. Goward, S.N., 1981: Thermal behavior of urban landscapes and the urban heat island. Phys. Geog. 2(1), 19-336. Spiller, M , 1993. Roof gardens and green facades for the improvement of urban environments. Thesis B. L. Arch,University of NSW7. Bruse, M., and Fleer, H. 1998. Simulating surface-plant-air interactions inside urban environments with a three-dimensional numerical model. Environmental Software and Modelling. Vol.13: 373-384.8. Oke, T. R., 1987. Boundary Layer Climates, Methuen.9. Duncan, H. P., 1999. Urban stormwater quality: a statistical overview, Cooperative Research Centre forCatchment Hydrology.