Impact of Urban Density Type in a Global Climate Model

The recent inclusion of an urban land model within a large-scale global climate model has allowed for a more realistic representation of the Earth’s surface, aiding in a better understanding of land cover and climate relationships. This research uses the NCAR Community Climate System Model version 4.0 (CCSM4.0) with the land component, the Community Land Model version 4.0 (CLM4.0), coupled with the atmospheric component, the Community Atmosphere Model version 4.0 (CAM4). Although the urban land type is a sub-grid phenomena that rarely occupies more than half of a grid cell at the 1° by 1° scale, significant changes in basic climate variables are present in some regions. These changes are primarily seen where a denser network of grid cells exist with an urban presence. Seasonality to the urban influence also exists with the transition months of Spring and Fall having the largest difference in temperatures.

Additional relationships beyond the present day urban land cover and climate are investigated. By setting all urban land cover to only one urban density type, the importance of city composition on climate, even within the same city, is highlighted. While preserving the distinct urban regional characteristics and the geographical distribution of urbanized areas, the model is run with homogeneous urban types: high density and tall building district. Although it is unrealistic to assume any of these regions will ever be completely covered with either high density or tall building district density types, it is reasonable to say that the ratio of high density and tall building district areas may increase as population continues to increase.

As with the default urban and excluded urban runs, there is strong seasonality to the differences between the all high density, the all tall building district, and default urban simulations. Overall, the transition and winter months are most sensitive to changes in urban density type. The dramatic increases in waste heat, impervious surface fraction, and height to width ratio in the tall building district simulation resulted in significantly increased temperatures. A large significant increase in surface albedo with the tall building district density type also exists although the warming influence of these other variables outweighs the extra reflection from the surface.