| The UHI effect can aggravate summertime heat waves and strongly influence human comfort
and health, leading to greater mortality in metropolitan areas. Many geo-engineering
technological strategies have been proposed to mitigate climate warming, and for the
UHI, increasing the albedo of artificial urban surfaces (rooftops or pavements)
has been considered a lucrative and effective way to cool cities. The objective of
this work is to quantify the contribution of urbanization to recent extreme heat
events of the early 21st century in the Beijing-Tianjin-Hebei metropolitan area, using
the mesoscale WRF model coupled with a single urban canopy model and actual
urban land cover datasets. This work also investigates a simulation of the regional
effects of white roof technology by increasing the albedo of urban areas in the
urban canopy model to mitigate the urban heat island, especially in extreme heat
waves.
The results show that urban land use characteristics that have evolved over the past ~20
years in the Beijing-Tianjin-Hebei metropolitan area have had a significant impact on the
extreme temperatures occurring during extreme heat events. Simulations show that new urban
development has caused an intensification and expansion of the areas experiencing extreme
heat waves with an average increase in temperature of approximately 0.60°C. This change is
most obvious at night with an increase up to 0.95°C, for which the total contribution of
anthropogenic heat is 34%. We also simulate the effects of geo-engineering strategies
increasing the albedo of urban roofs. White roofs reflect a large fraction of incoming sunlight
in the daytime, which reduced the net radiation so that the roof surface keep at a lower
temperature than regular solar-absorptive roofs. Urban net radiation decreases by
approximately 200 W m-2 at local noon because of high solar reflectance of white
roofs, which cools the daytime urban temperature afer sunrise, with the largest
decrease of almost -0.80°C at local noon. Moreover, the nighttime temperature also
shows slightly cooler, approximately 0.2°C, because there is still considerable heat
which is stored in the daytime released from urban surfaces at night. The results
also suggest that increasing the albedo of urban roofs can reduce the urban mean
temperature by approximately 0.51°C during summer extreme heat events. In urban areas,
white roofs can counter 80% of the heat wave results from urban sprawl during
the last 20 years. These results suggest that increasing the albedo of roofs in the
Beijing-Tianjin-Hebei metropolitan area is an effective way of countering some hazards of
heat waves.
Using a regional climate model, we proposed that white roofs may be an effective strategy
to complement urban heat wave mitigation efforts as a way of further slowing the rate of
global temperature increase in response to continued greenhouse gas emissions. |