Validation of an urban canyon radiation model for nocturnal long-wave radiative fluxes and the effect of surface geometry on cooling in urban canyons

The urban canyon radiation model of Arnfield (1976, 1982) is validated using measurements of long-wave fluxes taken within a scale urban canyon constructed from concrete building blocks. A custom-designed traversing system allowed miniature radiometers to be automatically moved around the perimeter of a canyon cross-section thereby providing for the validation of individual model grid-points. Measured model input consists of surface temperatures obtained using fine wire themocouples, incident long-wave radiation at the canyon top, and emissivity of canyon materials. Tests were conducted to establish the expected accuracy and precision of the input data. Surface temperature data were filtered to remove a noise component. A probable error analysis of all measured model input and validation data is made. Sensitivity tests of the model to variations in input data are presented. Surface temperature is the dominant control under the conditions tested. Model-calculated view-factors are shown to be in error for adjacent corner points and are replaced with view-factors calculated using equations derived from the Nusselt Unit Sphere method (Steyn, pers. comm.) Validation results for a range of canyon height-to-width ratios, meteorological conditions and model parameters are presented. Excellent agreement between modelled and measured fluxes is obtained for points on the canyon floor and top. The agreement for fluxes at points on the canyon walls is generally good but is shown to suffer from errors in sensor orientation relative to the canyon walls. Use of the Unsworth and Monteith (1975) radiance distribution improves model performance statistics for incident and net long-wave radiation. Four different estimates of surface temperature are used as model input in place of the measured values to investigate the differences in the model output. Surface temperature-based estimates are found to be superior to those based upon air temperature. The use of unmodified screen-level air temperatures measured at Vancouver Airport produces the poorest agreement. The temporal and spatial variation of in-canyon temperatures and radiation are presented for three canyon height-to-width ratios. The canyon geometry is shown to significantly reduce the surface cooling on the canyon floor compared to an open site under ideal radiative cooling conditions. Results are compared to previous results from scale models (Oke, 1981) and field studies (Oke and Maxwell, 1975; Hogstrdm et al., 1978). . Atmospheric controls of incident long-wave radiation, wind speed and direction are also shown to affect the observed cooling.Arts, Faculty ofGeography, Department ofGraduat