Longwave radiation flux from an urban canopy: Evaluation via measurements of directional radiometric temperature

Longwave radiation flux, an important part of the surface heat budget, is generally represented by εσTr4, where ε is the surface emissivity, σ is the Stefan–Boltzmann constant, and Tr is the measured radiometric temperature. εσTr4 differs from hemispheric emission because the measured radiometric temperature is anisotropic for an uneven surface. This paper analyzes the anisotropy-related error in measurements of longwave radiation flux from a building canopy. The flux difference between εσTr4 and directly measured flux was up to 8% of the directly measured flux, which could be greater than the typical error in measurement of turbulent heat flux over a building canopy. The flux difference increased as the temperature variation within the urban street canyon increased, and also with increasing difference between the incident solar radiation of the building walls and street canyon floors (pavement, roads, ground surface). Theoretical calculations indicate that the flux difference is due to the structure of the building canopy and the temperature difference between the walls and canopy floors. A numerical model of a building canopy heat budget shows that the flux difference increases as the street canyon aspect ratio increases.