Urban energy fluxes in built-up downtown areas and variations across the urban area, for use in dispersion models

Surface energy fluxes are needed as inputs to most state-of-the-art dispersion models. The sensible heat flux is of major priority, since it is combined with the momentum flux to estimate the Monin-Obukhov length, which is a key stability indicator. Observations of urban heat flux components from 11 locations in suburban and built-up downtown areas in Oklahoma City during the Joint Urban 2003 (JU2003) field experiment are analyzed. At street level in the downtown area, the ground heat flux and the sensible heat flux are relatively large and the latent heat flux is relatively small, when compared with concurrent fluxes observed in the upwind suburban areas. The latent heat flux measured at heights less than 5 m at the downtown sites is found to be strongly influenced by the local (within 50 m) surface conditions, especially the presence of irrigated lawns. The sensible heat flux in the downtown area is observed to be slightly positive at night, indicating nearly neutral or slightly unstable conditions. As found elsewhere, a delay of a few hours in the peak of the sensible heat and ground heat fluxes with respect to the net radiation flux is found at both suburban and downtown sites. Some simple parameterizations for the heat fluxes as a function of urban surface type are suggested.