London

  This time it is all about the city I currently live in: London. R.L. Wilby at his time at King's College London published a paper in 2003 on London's urban heat island which will be reviewed in this post. 

  The UK Climate Impacts Programme (UKCIP) predicts an increase in solar radiation and a decrease in cloud cover over southern parts of the UK. More global radiation (see post Urban Heat Island) will warm the land surface. The largest urban area inside the EU, London is located within this area. As I discussed in the last post, urban regions disproportionally warm a lot faster during heatwaves. Will hot days become an increasing problem in the UK capital famous for its fog and rain? 

  Over the summer of 1999 Wilby (2003) measured an average peak temperature difference between the British Museum and rural reference station Bracknell (30km to the west) of 3°C. London's Urban Heat Island (UHI) is strongest at night and it weakens with increasing wind speed. Figures 1 & 2 confirm a high night time (nocturnal) UHI which is strongest in inner city areas. Measurements representing Westminster (see Figure 1) were taken in St. James Park and therefore do not represent evening temperatures in built up areas which would be significantly higher as shown in Figure 2.  

Figure 1: Hourly variation in London's Urban Heat Island with increasing distance from the city centre (British Museum) to a rural site 30km to the west (Bracknell): Westminster (~2km), Hammersmith (~6km), Kew (~13km) and Heathrow (~26km), during six days in July and August 1999 & 2000, source: Wilby, R.L. (2003)

Figure 2: London Heat Island on 07/08/2003 at 9:30 pm, Inner city areas are significantly warmer than outer regions, source: Website 1

  London's UHI is strongest during summer where nocturnal averages reached 2.2°C in August of 1999. Whereas daytime UHI values were highest in December, however, their impact with 0.4°C on average is much less and can be attributed to anthropogenic heat production. From 1960 to 2000 the number of days with an UHI of 4°C or more (=intense UHI) has gone up by 4.4 days per decade. In the same time period the average nocturnal UHI during spring and summmer has gone up by 0.12°C per decade.

  How will London's UHI develop in the future? Wilby (2003) created a model. [For all of you who are keen to understand the model, here are the variables included: near-surface wind strength, vorticity, relative humidity and 850 mb geopotential height. This combination gave a correlation coefficient of R=+0.6 for observed vesus modelled values.] He then took the medium-high greenhouse gas emissions scenario by the IPCC for future atmospheric composition. Under this scenario, Wilby expects an increase in intense UHI days by 15 days per year in the 2080s compared to the 1961-2000 average. He also modelled an intensification of the UHI by +0.26°C on average. All of this is on top of an expected warming of 3.5-4°C in southern parts of the UK (UKCIP). Considering that the summer UHI is much stronger than in the winter, London's city centre could become a very hot place by the 2080s. If Wilby (2003)'s model is correct, Inner London will experience less night-time relief during heatwaves and reduced cooling of the already at times unbearably warm underground system.

And just imagine this lot...

...down here!

  On this note we should start thinking about techniques that can be incorporated into the built environment of a city to reduce the UHI. The next post will explore how cities start to take measures against further warming. An interview with this blog's compagnion Professor Brian Stone Jr. on Public National Radio will feature as well.

References

Wilby, R.L. (2003) Past and Projected Trends in London's Urban Heat Island, Weather, Vol. 58, pp.251-260

Website 1: http://climatelondon.org.uk/climate-change/heatwaves/ (accessed 14/11/2012, 17:05)