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Exploring historical and future urban climate in the Earth System Modeling framework: 2. Impact of urban land use over the Continental United States

Using a newly developed urban canopy model (UCM) coupled to the Geophysical Fluid Dynamics Laboratory (GFDL) land model LM3 (LM3‐UCM), this study examines the urban land use impacts over the Continental United States (CONUS) under the present‐day climate and two future scenarios. Using natural (undisturbed) vegetation systems as references where no land use has occurred, the LM3‐UCM simulations show that the spatial pattern of summer (June, July, and August) temperature differences between urban and natural vegetation systems is primarily controlled by the spatial pattern of differences in evapotranspiration, which further depends on the spatial distribution of precipitation. The magnitude of temperature differences generally increases as the summer precipitation amount increases and then levels off when the total summer precipitation amount exceeds 400 mm, which is broadly consistent with previous studies but with significant variability. In winter (December, January, February), the magnitude of temperature differences is more controlled by the building heating than the precipitation amount. At high latitudes where snow is an important factor in radiative balance, the magnitude is also affected by a larger net shortwave radiation input for urban areas due to the lower albedo of cities. Although both urban and natural vegetation temperatures increase as the climate warms, their increasing rates are different and hence their differences change with time. It is found that the multidecadal trend of summer temperature difference is negligible. However, the winter temperature difference shows a strong negative trend, which is caused by reduced building heating requirements under a warming climate.