The energy
balance of central
T. R.
Oke, R. A. Spronken-Smith, E. Jáuregui and C. S. B. Grimmond
Atmospheric Environment, 33, 3919-3930
The first measurements of the energy balance
fluxes of a dry, densely built-up, central city site are presented. Direct
observation of the net radiation, sensible and latent heat flux densities above
roof-top in the old city district of Mexico City allow the heat storage flux
density to be found by residual. The most important finding is that during
daytime, when evaporation is very small (<4% of net radiation), and
therefore sensible heat uses dominate (Bowen ratio >8), the uptake of heat
by the buildings and substrate is so large (58%) that convective heating of the
atmosphere is reduced to a smaller role than expected (38%). The nocturnal
release of heat from storage is equal to or larger than the net radiation and
sufficient to maintain an upward convective heat flux throughout most nights.
It is important to see if this pattern is repeated at other central city, or
dry urban sites, or whether it is only found in districts dominated by massive
stone structures. These findings have implications for the height of the urban
mixing layer and the magnitude of the urban heat island.