A simple single-layer urban canopy model for atmospheric models: Comparison with multi-layer and slab models

We developed a simple, single-layer urban canopy model, and compared it to both multi-layer and slab models. Our single-layer model has the following features: (a) It is a column model of energy and momentum exchange between an urban surface and the atmosphere, (b) it includes the influence of street canyons, which are parameterized to represent the urban geometry, (c) it includes shadowing from buildings and reflection of radiation, and (d) it estimates both the surface temperatures of, and heat fluxes from, three surface types: roof, wall, and road. In the simulation of the single-layer model, the roof was hottest during the daytime, but coolest from midnight to early morning. This is consistent with output from the multi-layer model and field observations at a residential area on a clear, summer day. The diurnal variation of the energy budget from the single-layer model agrees well with that from the multi-layer model. Our single-layer model's performance is nearly that of a multi-layer model for studying mesoscale heat islands. Nevertheless, it is simply parameterized, and thus easily included in larger-scale atmospheric models. The slab model has the largest nighttime cooling rate of the three models. To overcome this, it needs more adjustments than for the canopy models.