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Urban Projects 30
Figure 3. Observations at KSS (11/2009-03/2012): (a) quantiles of carbon dioxide flux F CO2 by wind direction in
10° intervals; (b) micro-scale anthropogenic carbon dioxide flux representative of the building scale, by wind
direction and time of day, calculated as the difference between commonly processed turbulent flux (L1) and
results incorporating the IMAS filter (L2). See Kotthaus and Grimmond (2012) for further details: plots are based
on their Figure 8 and Figure 11; eddy covariance flux processing and IMAS filter as outlined by Kotthaus and
Grimmond (in preparation).
periods affected by the close-by anthropogenic sources
based exclusively on the raw time series, i.e. not taking
into account aspects such as wind direction. However,
the filter mainly picks up events under easterly flow conditions
which suggests it to be suitable for its purpose.
The filter is incorporated into the flux processing procedure
so that results can be obtained which are representative
of the respective local scale source area. In order
to investigate the impact of the micro-scale effects, the
fluxes are calculated as usual (L1 processing, see Kotthaus
and Grimmond 2012 for details) and including the
IMAS procedure (L2 processing). The difference between
these results provides a first order estimate of the anthropogenic
(heat or carbon dioxide) flux at the building
scale. The distribution by wind direction of this microscale
turbulent flux estimate (L1-L2, e.g. for carbon dioxide
flux F CO2 , Figure 3b), again emphasizes the directional
dependence of the flux estimates on the locations of the
main emission source at KSS (to the East). Figure 3b further
shows variations of micro-scale fluxes by time of day
(radial axis), with generally larger values recorded during
daytime. This is in agreement with the time of usage of
the building generating the emissions, which is located
in the central business district where the daytime population
far exceeds the one at night. With respect to the
order of magnitude, building scale fluxes of sensible and
latent heat or carbon dioxide observed at KSS show reasonable
agreement with modelling approaches. These
are usually employed to estimates anthropogenic con-
tributions to the surface energy balance (e.g. Iamarino et
al. 2012) and the carbon dioxide budget. The presented
filtering technique depicts the first attempt to directly
measure these fluxes, which are especially important in
high density urban environments.
Flux processing at KCL incorporates an updated version
of the IMAS procedure, so that turbulent surface
exchange can be interpreted based on the local scale
source area (Kotthaus and Grimmond, in preparation).
Given the dense urban setting of the KCL Strand campus
(Figure 1), anthropogenic effects are also clearly evident
at this larger scale (as opposed to the building scale
flux shown in Figure 3b). Median diurnal patterns of energy
fluxes observed at KSS in summer (JJA) and winter
(DJF) reveal how energy exchange is affected in central
London (Figure 4). Naturally, energy input from the net
all-wave radiation Q* is significantly higher in summer,
while its contribution to the surface energy balance is
rather small during times of low sun elevation. Turbulent
sensible heat flux Q H is also weaker during winter, however
its median diurnal maximum of about 100 W m -2 distinctly
exceeds the radiation flux (~65 W m -2 ). And even
more notably, night time sensible heat flux accounts for
strong upward energy transport of over 50 W m -2 with
even higher median values during summer. Significant
storage heat flux is characteristic for dense urban environments
and provides energy for the turbulent fluxes
in times of no radiative input. A few other measurement
studies with sites locates in the city centre (e.g. Basel,
ISSUE NO. 46 DECEMBER 2012 INTERNATIONAL ASSOCIATION FOR URBAN CLIMATE