Sensitivity of a global climate model to the urban land unit

Given the relative newness of the presence of an urban land type and model in a GCM, there are many more facets of the urban-climate relationship to be investigated. By comparing thirty-year ensembles of CAM4 coupled with CLM4 both with and without the inclusion of the urban land type, the sensitivity of the atmospheric model to urban land cover is assessed. As expected, largest differences tend to be in the Northern Hemisphere due to the location of most of the globe's densest and expansive cities. Significant differences in the basic climate variables of temperature and precipitation are present at annual, seasonal, and monthly scales in some regions. Seasonality to the urban influence also exists with the transition months of Spring and Fall having the largest difference in temperatures. Of the eleven regions defined, three are most influenced by the presence of urban land cover in the model - Europe, Central Asia, and East Asia. While preserving the distinct urban regional characteristics and the geographical distribution of urbanized areas, the model is also run with homogeneous urban types: high density and tall building district. As with the default urban and excluded urban runs there is seasonality to the atmospheric response to homogeneous density types. Overall, the transition and winter months are most sensitive to changes in urban density type.