Development of an urban parameterization for a global climate model

A parameterization for urban surfaces is being developed for the Community Land Model (CLM) as part of the Community Climate System Model (CCSM) housed at the National Center for Atmospheric Research (NCAR). The model is designed to be simple enough to be compatible with structural and computational constraints of a land surface model coupled to a global climate model, yet complex enough to explore physically-based processes known to be important in determining urban climatology.

The city representation is based upon the “urban canyon” concept of Oke (1987) in which the canyon geometry is described by building height to width ratio. The canyon system consists of roofs, walls, and canyon floor. Walls are further divided into shaded and sunlit components. The canyon floor is divided into pervious (e.g., residential lawns, parks) and impervious (e.g., roads, parking lots, sidewalks) fractions. Trapping of longwave radiation by canyon surfaces is calculated based on multiple re-emissions. Solar radiation absorption and reflection is based on the analytical solution of Masson (2000) for an infinite number of reflections. Momentum fluxes are determined for the entire urban surface using a roughness length and displacement height appropriate for the urban canyon and stability formulations from CLM. Separate energy balances and surface temperatures are determined for each canyon surface. Anthropogenic sources of heat from traffic and industry modify the canyon energy budget. A one-dimensional heat conduction equation is solved numerically for a ten-layer column to determine conduction fluxes into and out of canyon surfaces. The lower (internal) boundary conditions for roofs and walls are determined by an internal building temperature held between prescribed maximum and minimum temperatures thus explicitly resolving domestic heating and cooling fluxes, while the canyon floor has a zero flux lower boundary.

The model is forced either with output from the host atmospheric model or observed forcing (e.g., reanalysis or field observations). Required forcing includes incident longwave and solar radiation and atmospheric level wind, specific humidity and air temperature. The urban model produces sensible, latent heat, and momentum fluxes, emitted longwave, reflected solar radiation, which are area-averaged with fluxes from non-urban surfaces (e.g., vegetation, lakes) to supply grid-cell averaged fluxes to the atmospheric model.

In general, the model reproduces some known features of urban climatology. The model produces higher urban nighttime air temperatures than rural areas (e.g., nocturnal heat island) due to increased daytime storage of heat within canyon surfaces and subsequent nocturnal release of this heat resulting in a positive nighttime sensible heat flux. Daytime maximum temperature is slightly higher in urban areas resulting in a reduced diurnal temperature range compared to rural areas. These characteristics increase with height to width ratio. However, detailed validation of the model awaits the application of suitable observed datasets of urban fluxes and temperatures, atmospheric forcing, and urban surface characteristics. Sensitivity experiments indicate that the model turbulent fluxes and air temperature are sensitive to wind speed, height to width ratio, fraction of pervious canyon floor, anthropogenic fluxes, and thermal and radiative properties of canyon surfaces.