Urban climate in the 21st century

Recent studies, including the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, have highlighted the need for climate models to account for urban surfaces to more realistically evaluate the impact of climate change on people in the environment where they live. In part, this is because there are significant differences in energy balance and climate (e.g., air temperature and humidity, hydrology) between urban surfaces and the vegetated (i.e., “rural”) surfaces typically represented by the land surface component of climate models. Also, there is evidence to suggest that the response of urban areas to climate change may be different from that of rural areas.

As a first step to investigate these issues, a parameterization for urban areas has been incorporated into the Community Land Model (CLMU) as part of the Community Climate System Model (CCSM). At the coarse spatial resolution of the CCSM (1 degree in latitude and longitude), urbanization has negligible impact on grid cell-averaged climate. However, the urban parameterization allows simulation of the urban environment within a grid cell, and particularly allows scientific study of how climate change affects the urban climate (e.g., near surface air temperature and humidity).

The CLMU urban representation is based on the “urban canyon” concept of Oke (1987) in which the canyon geometry is described by building height and street width. 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., to represent residential lawns, parks) and impervious (e.g., to represent roads, parking lots, sidewalks) fractions. Required global datasets are urban extent, and spatially explicit urban morphology (e.g., roof fraction and building height), and radiative (e.g., albedo and emissivity) and thermal (e.g., heat capacity and thermal conductivity) properties of urban materials. The lower (internal) boundary conditions for roofs and walls are determined by an internal building temperature held between prescribed maximum and minimum temperatures thus providing an estimate of energy required to keep building interiors at comfortable levels (e.g., space heating and air conditioning fluxes). Sources of wasteheat from inefficiencies in the heating and air conditioning systems are incorporated as modifications to the canyon energy budget and thus affect near-surface air temperature. The urban model produces sensible and latent heat and momentum fluxes, emitted longwave, and reflected solar radiation, which are area-averaged with fluxes from non-urban “landunits” (e.g., vegetation, lakes) to supply grid-cell averaged fluxes to the atmospheric model.

Here, we will present results from ensemble members of fully coupled global climate simulations including the urban model for historical and future climate conditions (1850-2100), conducted as part of phase 5 of the Coupled Model Intercomparison Project (CMIP5). Future climate simulations available include a high emissions scenario (RCP8.5; a representative concentration pathway where radiative forcing reaches 8.5 W/m2 near 2100) and a medium mitigation scenario (RCP4.5). We will examine and contrast urban and rural energy balance and climatology under the simulation constraints of no urban growth. In particular, it is of interest to examine how the urban heat island changes under these RCPs. Because human beings are sensitive to the diurnal distribution of climate as well (e.g., heat stress can be quantified by peak daytime temperatures but also by the persistence of warm nighttime temperatures), we will also examine the changes in the diurnal cycle of urban climate in term of maximum and minimum temperatures. Changes in source energy required for heating and air conditioning will be investigated as well.