Development and evaluation of a vegetated urban canopy model for use in mesoscale models

A vegetated urban canopy model (VUCM) for use in mesoscale meteorological and environmental models is developed on the basis of a single-layer model. The urban configuration elements in the VUCM are homogeneous buildings characterized by canyon aspect ratio and mean building height, paved road, and natural surfaces with vegetation and soil. The VUCM considers five energy exchanging surfaces (roof, wall, road, vegetation, and soil), and energy conservation relation is applied to each component. The temperature and specific humidity of the canopy air are prognostically calculated without resorting to a thermal equilibrium. For radiative transfer in the canyon, multiple reflection for shortwave radiation and one reflection for longwave radiation are taken into consideration to include the effect of radiation trapping, while shadowing and the absorption of radiation by vegetation are calculated using the transmissivity function. The VUCM is evaluated against the field measurement data of ESCOMPTE (French acronym for a field experiment to constrain models of atmospheric pollution and transport of emissions)-the urban boundary layer (UBL) campaign in Marseille, France. The VUCM quite well simulates observed surface temperatures as well as surface energy balance such as net radiation, sensible and latent heat fluxes, and storage heat flux. Observed features such as nocturnal positive sensible heat flux, large heat storage by urban fabric, and latent heat flux from natural surfaces are also well simulated. Sensitivity tests show that vegetation has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In addition, surface energy balance can be affected by soil moisture content and LAI as well as the fraction of vegetation. These sensitivity-test results suggest that vegetation effects should be included in urban canopy parameterization schemes. The VUCM model is implemented in a mesoscale model RAMS (Regional Atmospheric Modeling System) and its impacts on meteorological fields over an urban area (Seoul metropolitan area) are examined.