Flexible Treatment of Radiative Transfer in Complex Urban Canopies for Use in Weather and Climate Models

Abstract

We describe a new approach for modelling the interaction of solar and thermal-infrared radiation with complex multi-layer urban canopies. It uses the discrete-ordinate method for describing the behaviour of the radiation field in terms of a set of coupled ordinary differential equations that are solved exactly. The rate at which radiation intercepts building walls and is exchanged laterally between clear-air and vegetated parts of the urban canopy is described statistically. Key features include the ability to represent realistic urban geometry (both horizontal and vertical), atmospheric effects (absorption, emission, and scattering), and spectral coupling to an atmospheric radiation scheme. In the simple case of a single urban layer in a vacuum, the new scheme matches the established matrix-inversion method very closely when eight or more streams are used, but with the four-stream configuration being of adequate accuracy in an operational context. Explicitly representing gaseous absorption and emission in the urban canopy is found to have a significant effect on net fluxes in the thermal infrared. Indeed, we calculate that for the mid-latitude summer standard atmosphere at mean sea level, 37% of thermal-infrared energy is associated with a mean free path of less than 50\xa0m, which is the typical mean line-of-sight distance between walls in an urban area. The interaction of solar radiation with trees has been validated by comparison to Monte Carlo benchmark calculations for an open forest canopy over both bare soil and snow.