Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 227

materials and morphology of the urban surface, is the surface energy balance
(SEB), defined for an urban areas as:
Q þ Q F ¼ Q H þ Q E þ Q S þ Q A ½ units : Wm 2
where Q* is the net all-wave radiation (the net balance of the incoming and
outgoing radiative fluxes), Q F is the anthropogenic heat flux (the energy
released by human activities), Q H is the turbulent sensible heat flux (the
energy that heats the air), Q E is the latent heat flux (the energy taken up/
released with the phase change of water, i.e. with evaporation and condensa-
tion), Q S is the net storage heat flux (the energy that heats and is stored in
the urban fabric and volume), and Q A is the net horizontal heat advection
(the lateral movement of energy into or out of an area).
Given advances in instrumentation, measurements of surface energy bal-
ance fluxes increasingly are being made in urban areas to complement
measurements of more standard meteorological variables, for example, tem-
perature, wind speed, humidity, to gain greater insight into the processes that
underlie the generation of urban climates. In these studies, careful attention
has to be directed to the siting and operation of the instruments to ensure
reliable and representative data are collected. Recent work has demonstrated
that turbulent sensible, latent, and storage heat fluxes all are important terms
in the SEB of most cities. Each of the heat fluxes varies both spatially and
temporally. Under low-wind conditions, storage heat flux is most important at
downtown and light industrial sites (at least 50% of daytime Q*), and the
sensible heat flux is most important at residential sites (40 to 60% of daytime
Q*) (Grimmond and Oke 2002 ). At all sites there is distinct hysteresis in the
diurnal course of the storage heat flux; much more of the net radiation is used
to heat the urban fabric in the morning. In addition, the sensible heat flux
remains positive after the net all-wave radiation turns negative at night. This
has important implications for stability (mixing) of the urban atmosphere and
therefore for urban air quality. At residential sites, latent heat flux, if sus-
tained by garden irrigation and/or frequent rainfall, is also significant (20 to
40% of daytime Q*). Cities are not just impervious building materials (con-
crete and asphalt sidewalks, roads and parking lots, rooftops, etc.); in many
urban areas, trees and other vegetated surfaces cover a significant area
(commonly up to 40% of the plan area of a city in North America) and
surface detention ponds are common. Surface cover, notably the fraction of
the surface vegetated and irrigated, has been shown to exert an important
control on Q E and the relative heat partitioning between sensible (Q H ) and
latent (Q E ) heat.
Here select data derived from Oke and Grimmond ( 2002 ) are presented to
illustrate the importance of both urban and rural surface properties in SEB
comparisons. Whilst most cities have an increased sensible heat flux