The sensitivity of the model to the albedo values of the urban surfaces was investigated. The albedo of a surface is defined as its hemispherically and wavelength integrated reflectivity (Taha 1997).
The difference in albedo between rural and urban surfaces is one of the factors which can affect the surface energy balance in urban areas. The albedo for the urban area is typically lower than that of the rural surroundings, and therefore less radiation is reflected back from the urban area, which can lead to a higher heat content, thus contributing to the UHI phenomenon (Oke 1987; Taha 1997; Atkinson 2003). A decrease in the UHI intensity is
expected if the difference between the urban and rural albedo is removed by increasing the albedo of the urban surface (Atkinson 2003).
A summary of albedo values for typical urban and rural surfaces is shown in Table 4.3. For most surfaces a range is presented, as precise albedo values are not certain.
Table 4.3: Typical albedo values for rural and urban surfaces, taken from Oke (1987) and Taha (1997)
Surface type
Albedo
Grass 0.16 - 0.28 Crops 0.15 - 0.24 Forests 0.10 - 0.18 Gravel 0.09 Tile roof 0.10 - 0.35 Asphalt 0.05 - 0.20 Concrete walls 0.10 - 0.35 Brick 0.20 - 0.40 Stone 0.20 - 0.35
Average urban areas (Oke 1987) 0.15 European and US cities (Taha 1997) 0.20
White paint 0.50 - 0.90
Simple modifications of the albedo in urban areas, for example by using high reflective building materials, white surface coatings for roofs and walls, and lighter street surfaces, could be part of a mitigation strategy in attempting to counter the effects of rising urban temperatures, reduce cooling energy use and improve air quality (Oke 1987; Sailor 1995; Bretz et al. 1998; LCCP 2002). Increasing the albedo will affect the surface energy balance by increasing the percentage of incoming solar radiation which is reflected back to space. The use of solar reflective, or high albedo, materials maintains low surface temperatures in sunlight, and thus reduces the convective heat transfer from the surface to the ambient air, and helps create cooler communities (Taha 1997; Bretz et al. 1998). For example Taha et al. (1992) found a difference of 25 K between the temperature of a white surface (albedo of
0.61) and that of conventional gravel (with an albedo of 0.09), compared to the ambient temperature. For this reason in warm climates buildings are often painted white.
A number of modelling studies have shown reductions in temperature over urban areas by decreasing the surface albedo. For example Taha et al. (1988) showed, using one- dimensional meteorological simulations, that for a typical mid latitude city in summer it is possible to reduce local air afternoon temperatures by as much as 4 K by increasing the surface albedo from 0.25 to 0.40. Seaman et al. (1989) showed that by increasing the urban-rural albedo difference from 0.025 to 0.05 the UHI is weakened by 0.3 K. Sailor (1995) used a 3-D meteorological simulation of Los Angeles, USA to show decreased peak summertime temperatures of up to 1.5 K by increasing the surface albedo by 0.14 downtown, and 0.08 over the entire basin. Atkinson (2003) found a reduction in temperature over the urban area of 0.3 K by increasing the urban albedo from 0.15 to that of the rural surroundings (0.18).
Lower ambient air temperatures, achievable through an increase in albedo, also have implications on air quality (Sailor 1995; Taha 1997) and can effect substantial energy savings in areas where air conditioning is prevalent (Rosenfeld et al. 1998) since the direct heat transfer from the building surface into the building is also reduced.
In the control simulation of the present study the albedo for all urban surfaces, i.e. roofs, walls and roads, was set to 0.20 as suggested in the original BEP scheme code. This is a typical urban albedo value (Taha (1997) suggested a range of 0.15 to 0.20 for US and European cities). However, this value of 0.20 is equal to the albedo of the rural surface, which is also set to 0.20 for the METRAS ‘Meadows’ urban class. For this reason a
simulation was carried out in which the urban albedo is either reduced to 0.15, a value at the lower end of the range suggested by Taha (1997), or increased to 0.30. It is expected that the decrease in the albedo to a value 0.5 lower than the rural surroundings would enhance the temperature over the urban area, and vice versa.
The two simulations were therefore carried out with the following parameters: • alb1: Albedo for all three surface types increased to 0.30
• alb2: Albedo reduced to 0.15, the urban value used by Sailor (1998) and Atkinson (2003)
In Figure 4.17 it is apparent that if the albedo of the urban surfaces is reduced to 0.15 then temperatures over the city increase by up 0.2 K. On the other hand increasing the albedo of all three urban surface types (roofs, roads and walls) determines a net reduction in the temperature near to the urban surface which is observed during the daytime, whereas night time temperatures are very similar. Around 06:00 the results, especially for the high albedo situation, appear to show an uncertain behaviour; however this is likely to be due to the fact this time corresponds to sunrise and the fact these are point measurements, not averaged over the urban surface. The maximum reduction observed is 0.4 K, and it peaks around midday when the solar radiation is highest. These results compare well with those in the literature discussed above.
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 00:00 04:00 08:00 12:00 16:00 20:00 00:00 Time Dif fere n ce i n po tent ial te mp eratu re (K ) alb1-a01 alb2-a01
Figure 4.17: Diurnal variation of the difference in potential temperature (K) between the simulations alb1 and the control simulation a01 (blue line) and between the simulations alb2 and a01 (pink line) at x=0, y=0, z=10 m for the second day of simulation. The alb1 simulation represents an urban albedo of 0.30 and the alb2 simulation represents an urban albedo of 0.15.
It is clear that changing the albedo could be a relatively economical and achievable way of influencing the urban daytime temperatures, and this could be investigated further at the city scale for London, UK. The potential for increasing urban albedo has been investigated for some others cities, for example Bretz et al. (1998) estimated the potential to modify the albedo of Sacramento, California by 18%.