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,Australia

ABSTRACT

A high-resolution numerical model is used to estimate the microclimatic effect of introducing vegetation at street

and/or rooftop level. Results are presented to show the effect on a hot afternoon at a location in the inner suburbs of

Melbourne, Australia. The study shows that the addition of vegetation reduces temperatures and wind speeds, thereby

improving climatic amenity for pedestrians. Other environmental benefits of rooftop gardens are discussed.

INTRODUCTION

When trees and soil give way to buildings and paved surfaces, the energy balance near the earth’s surface changes. Less

incoming solar radiation is dissipated as latent heat and more goes into sensible heat. The resulting rise in surface

temperatures is one contributing factor to the urban heat island effect – an effect which is particularly unwelcome in

Australia’s rather warm climates.

As the urban building density increases, the radiatively active surfaces moves upward towards the rooftops. In densely

built-up urban environments, roofs are the surface where most of the absorption, reflection and emission of radiation

takes 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 Port

Phillip Bay in southeastern Australia (see figure 1). The study estimates the change in local climate achievable at a

specified site through:

1) adding rooftop gardens to all buildings (the green roofs case); and

2) combining rooftop gardens with street-level vegetation (the all-greened case).

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 high

and the northern section is 4m. Surrounding buildings vary in height from 3 to 11 m. There is very little vegetation in

the study area. In the “all-greened” case, the asphalt parking lots to the W and SW of the Market were planted with

grass and trees; the roads around the market were narrowed to allow the addition of a 9m wide strip of grass, planted

with 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.

METHODS

A three-dimensional non-hydrostatic urban climate model, ENVI-met, (Bruse and Fleer, 1998) was used to simulate the

microclimate of the South Melbourne Market and its immediate surroundings. The ENVI-met model is able to

simulate microscale interactions between urban surfaces, vegetation and the atmosphere. The model input parameters

used are shown in Table 1.

Table 1 Basic input parameters to the modelLocation 37.82 deg. S, 145.00 deg. E (South Melbourne)

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, deciduousGrass trip around Market, 9m wide.Rooftop plants: 5-cm grass covering all building roofs. The Market buildingroof 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: loam

Soil initial conditions Temperature (-2m) = 290K.A hot day was chosen for analysis, because small increases in comfort deliver greatest health benefits in thermally

stressful conditions.

RESULTS

Reduced temperatures and wind speeds were observed due to the introduction of vegetation, with the greatest

reductions in the all greened case.

Selections from the voluminous model output are shown in figures 3, 4 and 5. Temperature and wind speed values are

differences from the ungreened (status quo) case for the same level and time of day. Green areas indicate vegetation at

this height; black areas are buildings. The horizontal views are cross-sections just above the roof level of the Market

building. The cross-sections cut through the grassed roof of the southern part of the Market building and are 2m above

the grassed roof of the lower northern part (and therefore not shown green).

Influence on wind speed

As 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 reduced

wind speed are mainly limited to the areas where the plants are placed. Due to the sheltering row of street trees around

the 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 leaf

layer but between the plants the wind is canalised and the reduction effect is considerably less.

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 temperature

In general, a maximum temperature reduction of 1.4 K above market ground level in the green-roofs case and of 2.4 K

in the all-green case can be observed (figure 4). The areas of local air temperature reduction are largely restricted to the

green roof locations and are advected with the main northerly airflow toward the south of the area. Although this

advection increases the area affected by the rooftop cooling effect, there is a clear area of influence for each greened

roof.

By contrast, in the all-greened case, the reduction of air temperature is more uniform. There is a maximum temperature

reduction of 2.4 K, south of the tall trees on the western side of the Market. Here, the cooling effects of the rooftops and

of the street-level vegetation combine to act as a single system.

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 temperature

extends upward to around 42m above ground level. Little difference in vertical extent can be found between the two

greening scenarios. But in intensity, the all-green case shows an effective reduction in air temperature up to twice the

reduction 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.

DISCUSSION

In 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

open parking lots, offer less wind shelter to pedestrians. As bricks and concrete have replaced vegetation, the cooling

effect of plants, through shading and transpiration, has been lost. This case study has shown how addition of vegetation

can 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” the

city. 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). Urban

runoff is greater than in rural areas, because of the city’s impervious surfaces (Oke, 1987) and its quality is poor

compared with that from forested areas (Duncan, 1999). Although not studied here, it appears likely that rooftop

gardens can reduce the amount of urban stormwater runoff by adding more permeable surfaces to the city and can

improve the quality of runoff, by filtering runoff water through garden soils. Because of its positive impact on urban

climate, on energy use, on urban runoff and on air and stormwater quality, it appears likely that, if introduced on a

wider 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 the

advantages of “gardens in the sky”.

ACKNOWLEDGEMENTS

The authors wish to thank Geoff Connellan of Burnley Horticultural College, University of Melbourne for providing

information about Australian native plants.

REFERENCES

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