Changes in the landscape by urbanization have lead to drastic climate modifications, which ranks among the most significant human impacts on the environment. Most remarkable is that cities are exposed to higher air temperatures than those in the natural surroundings. This phenomenon - known as the urban heat island - causes serious health risks for many people in the city. Especially in large cities, mortality rates are higher during heat waves, such as that of the European summer of 2003. It is expected from global climate change that cities are more often exposed to extreme weather, including the increased number of strong heat waves. Sensitivity experiments with the regional climate model CCLM coupled to a new efficient urban land-surface parametrization TERRA-URB over Belgium have led to an overall better understanding of the climatic drivers of the urban heat island and their seasonal dependency at the regional scales. It turns out that both urban structure and energy waste into the atmosphere by human activity, but also their interaction determine the seasonal variability of the urban heat island intensity. Remarkably, the averaged contribution of urban structure to the nocturnal urban heat island for the cities in and around Belgium (+0.41 K for Brussels) is smaller than that from the energy waste (+1.24 K) during winter. Conversely, the contribution of urban structure (+1.97 K) dominates that of the energy waste (+0.68 K) during summer. The respective contributions mostly counteract each other during summer (-0.21 K), whereas they enhance each other during winter (+0.25 K). The above-mentioned findings allow to take wiser decisions for keeping the climate in cities comfortable within the context of climate change and urban expansion. Hereby, it is recommended to account for both the impact of urban structure, energy waste, their interactions and their seasonal dependency on urban climate. Hereby, the respective influences on the urban thermal comfort both in the positive as in the negative sense needs to be considered. It is expected that putting in place policies in terms of urban modifications – that are needed to temper climate change on the global scale - will influence the climate in cities on the local scale as well.

1. 16/07/2014
The seasonal dependency of the
urban heat island of cities in
Belgium with CCLM/TU
Hendrik Wouters, Matthias Demuzere, Koen De Ridder, Nicole van Lipzig
Meteoclim 2014

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Urban expansion!

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Urban expansion!
figure from Poelmans and Van Rompaey., 2009

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Urban expansion!
figure from Poelmans and Van Rompaey., 2009

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Large modifications in the Earth’s landscape
» Drastic local climate changes…. Higher local temperatures!
-> Urban heat island
» Two processes:

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Large modifications in the Earth’s landscape
» Drastic local climate changes…. Higher local temperatures!
-> Urban heat island
» Two processes:
1 2

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²
Trapped infrared radiation
sensible heat
stored
efficiently
by building
materials
atmosphere
sensible heat
atmosphere
Stored in ground
Very hot surface!!
Trapped solar radiation
Dry surface: no evaporation
Efficiënt conversion

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Excess
release
during the
night
Large heat
Storage
In buildings
atmosphere
Stored in ground
Hot Surface is cooling
sensible heat
atmosphere 1

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2. Human activity

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2. Human activity -> heat emission
Energy converted to heat -> emitted to the atmosphere! 2

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Urban heat island… Big deal?
» Prolongued growing season
» More agreable temperature
temperaturen
» Infrastructure

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» energieconsumption (air-co)
» infrastructure
» (Sports) Events
» Air quality
» More casualties in cities
versus natural surroundings
during heat waves
Urban heat island… Big deal?

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+ Global climate change!

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» 1. Which of the processes and contribute the
most to the urban heat-island of cities in and around
Belgium?
» 2. Is there a seasonal dependency?
Research questions
21

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Why do we care?
» To know which policies are important to keep cities livable, sustainable,
and resilient as much as possible!
livable
smart
cities

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How?
» We adapt the regional climate model CCLM to take account for
» by including the processes and
» Applicable for both summer as for winter
» as efficiënt as possible (cfr. Computational cost, needed input
parameters)
21

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Regional climate model? 2 components.
Landoppervlakmodel
Atmospheric model
Sensible heat,
Evapo(transpi)ration,
Upward radiation
Drag on the wind
Temperature,
Precipitation,
Wind,
Downward radiation

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Regional climate model? 2 components.
Land-surface model
Atmospheric model
Wouters et al., (2012). Comprehensive parametrization of surface-layer transfer
coefficients for use in atmospheric numeric models. Boundary-layer Meteorology
Wouters et al., The impact of impervious water-storage parametrization
on urban climate modelling. Urban Climate (under revision)
21
Sensible heat,
Evapo(transpi)ration,
Upward radiation
Drag on the wind
Temperature,
Precipitation,
Wind,
Downward radiation

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Model configuration
Global weather forecasts from ECMWF
resolution:12.5 km
Regional climate model CCLM
resolution: 2.8 km

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Measurements urban heat island
» Antwerp
Antwerp
“Aren’t we a
bunch of
modelers?”

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Urban heat island of Antwerp

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Urban heat island of Antwerp
» Mid-summer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013
Uur van de dag

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Urban heat island of Antwerp
Uur van de dag
21
» Mid-summer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013

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Urban heat island of Antwerp
» Mid-summer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013
Uur van de dag
21

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Urban heat island of Antwerp
» Mid-summer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013
Uur van de dag
21

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Effect of heat emission in the city?
» Mid-summer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013
Uur van de dag
21

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Effect of heat emission in the city?
» Mid-zomer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013
Uur van de dag

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Effect heat emission in the city?
» Mid-zomer
21/07/2012 -> 21/08/2012
» Mid-winter
21/01/2013 -> 21/02/2013
Uur van de dag

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geen stedelijke opbouw
anthropogene warmte
mid-summer
midnight
(averaged)
21 21
21

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mid-summer
midnight
(averaged)
21 21
2121

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mid-summer
midnight
(averaged)
21 21
2121
21
21

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mid-summer
midnight
(averaged)
21 21
2121
21
21

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mid-summer
midnight
(averaged)
21 21
2121
21
21 21

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stedelijke opbouw energieverspilling interactie
mid-summer
midnight
21 21 21

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+ +
=
totale impact
mid-summer
midnight
21 21 21
21
urban structure interactionenergy waste

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+ +
=
totale impact
Values for Brussels
1.97°C 0.68°C -0.21°C
2.44°C
mid-summer
midnight
21 21 21
21
urban structure interactionenergy waste

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+ +
=
total impact
Values for Brussels
+0.41°C +1.20°C +0.25°C
+1.97°C
mid-winter
midnight
21 21 21
21
urban structure interactionenergy waste

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Conclusions
» Both , as well as their determine the urban heat-
island intensity of cities in Belgium, and their respective contributions
strongly depend on the season
» Both positive as negative influences on the urban thermal comfort needs
to be accounted for with regard to:
» Tempering urban heat during heat waves, versus cold waves
» Climate change (e.g. , …)
» (urban) land-use change
21 21

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Thank you for your attention!
livable
smart
cities