UPWARD LONGWAVE RADIATION FROM A NON-BLACK URBAN CANOPY

The longwave upward radiation was calculated for an urban canopy by using a Monte Carlo model. The effects of the urban geometry were examined in terms of the fractional roof area, the height of the buildings and the emissivity. The urban canopy consists of identically sized buildings and the ground surfaces. The model allows for the temperature differences between the buildings and the ground surface and for multiple reflections in the canyon. The Monte Carlo results show that neglect of the geometric effects causes significant errors in calculated upward radiation: calculations with area-weighting of the radiation emitted from flat homogeneous surfaces are not appropriate. The upward flux is a nonlinear function of the fractional roof area, which may be approximated by a function of the square or cube of the fractional roof area. Neglect of the reflections by non-black surfaces (emissivity < 1) underestimates the upward flux by a few percent for a canopy of emissivity = 0.9. Radiation effects due to multiple reflections in the canyon are parameterized by use of the view factor and the fractional roof area. The parameterization scheme yields accurate results.