Validation of a numerical model for urban energy-exchange using outdoor scale-model measurements

The objectives of our study are (1) to evaluate the simple urban energy balance model for mesoscale simulation (SUMM) using such data that are free from many real world uncertainties in respect to spatial variability in material, geometry, and land use, and (2) to analyse the sensitivity of land surface parameters (LSP), which are used in the model. The model was evaluated using the data obtained from comprehensive outdoor scale-model (COSMO) experiments during a period, which covers roughly half of a year (winter and spring-early summer) including various wind conditions. SUMM simulated surface layer energy fluxes, surface temperature, and interior temperature fairly well under windy conditions while it underestimated sensible heat flux under calm conditions. On average, simulated sensible heat flux underestimated observed value by 30% (0.73 MJ m^-2 d^-1) in daytime. Errors of net radiation (4%; 0.40 MJ m^-2 d^-1) and heat storage (5%; 0.33 MJ m^-2 d^-1) were smaller than that of sensible heat flux in daytime. This underestimation of sensible heat flux can be attributed to the inadequate parameterization of the surface layer bulk transfer coefficient used in SUMM under calm conditions. On the basis of the sensitivity analyses, parameterization of the surface layer bulk transfer coefficient, using Monin-Obukhov similarity theory (MOST) shows that the model performance is very sensitive to this coefficient, while it is less sensitive to the relative values of the bulk transfer coefficients of local faces.