Development of the Surface Urban Energy and Water balance Scheme (SUEWS) for cold climate cities

Abstract

Abstract. The Surface Urban Energy and Water balance Scheme (SUEWS) is developed to include snow. The processes addressed include accumulation of snow on the different urban surface types; snow albedo and density aging; snow melting and re-freezing of melt water. Individual model parameters are assessed and independently evaluated using long-term observations in two cold climate cities, Helsinki and Montreal. Eddy covariance sensible and latent heat fluxes and snow depth observations are available for two sites in Montreal and one in Helsinki. Surface runoff from two catchments (24 and 45 ha) in Helsinki and snow properties (albedo and density) from two sites in Montreal are also analysed. As multiple observation sites with different land-cover characteristics are available in both cities, model development is conducted independently of evaluation. The developed model simulates snowmelt related runoff well (within 10% and 6% for the two catchments in Helsinki when there is snow on the ground), with the springtime peak estimated correctly. However, the observed runoff peaks tend to be smoother than the simulated ones, likely due to the water holding capacity of the catchments and the missing time lag between the catchment and the observation point in the model. At all three sites the model simulates snow accumulation and melt events well, but underestimates snow depth by 18–20% in Helsinki and 29–33% in Montreal. The model is able to reproduce the diurnal pattern of net radiation and turbulent fluxes of sensible and latent heat during cold snow, melting snow and snow free periods. Largest model uncertainties are related to the melting period. The results show that the enhanced model can correctly simulate the exchange of energy and water in cold climate cities, and is appropriate to be nested in a larger scale atmospheric model or used independently for urban planning.