An anisotropic parameterization scheme for longwave irradiance and its impact on radiant load in urban outdoor settings

A robust representation of the radiative properties in complex urban settings is important for accurate estimations of radiant load. Here, we present a new parameterization scheme in the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model that partitions the upper hemisphere into 153 patches. Partitioning of the upper hemisphere enables determination if longwave irradiance originates from the sky, vegetation, sunlit building surfaces, or shaded building surfaces from each patch. Furthermore, a model for anisotropic sky longwave irradiance where emissivity increases with zenith angle is included. Comparisons between observations and simulations show high correlation, with R(2) and RMSE for T(mrt) of 0.94 and 4.6 °C, respectively, and R(2) and RMSE for longwave radiation of 0.89 and 14.1 Wm(−2), respectively. Simulations show that mean radiant temperature (T(mrt)) can be up to 1.5 °C higher with an anisotropic sky compared to a uniform sky as an effect of higher radiant load on the vertical of a human when sky longwave irradiance increases with zenith angle. In comparisons of simulated T(mrt) with the new parameterization and old parameterization schemes, previously overestimated T(mrt) under trees (high sky obstruction, sky view factor (SVF) < 0.3) can be decreased by up to 3 °C from more realistic estimations using the patches. Moreover, T(mrt) close to sunlit walls (SVF ~ 0.5) is increased by up to 2–3 °C from increased exposure to sunlit surfaces. Concluding, anisotropic sky longwave radiation and directionality of longwave radiation from different sources are important in estimations of T(mrt) of humans in outdoor settings. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00484-023-02441-3.