Urban effects on current and future diurnal cycle of atmospheric variables in global climate simulations

Urban areas are responsible for about 70% of global greenhouse gas emissions, contributing to global climate change. At the same time, cities and, in particular, urban populations are severely affected by climate change. For example, the projected increase in heatwaves has negative impacts on human health, and the increase in heavy precipitation events can cause urban flooding. In addition, temperature, precipitation and other atmospheric variables such as wind and humidity are known to be influenced by urban areas. These urban impacts can vary greatly from city to city, and for local time of day, and may be influenced by climate change. However, a global analysis of these effects is still rare.

Using simulations at 50 km resolution with the global Conformal Cubic Atmospheric Model (CCAM) coupled to a state-of-the-art urban parameterization, we analyze the impact of urban areas on the diurnal cycle of atmospheric variables in the current climate and in a future climate based on the RCP8.5 scenario. First, we generated time zone-corrected three-hourly Local Time Period (LTP) data to compute mean diurnal cycles for grid cells with more than 10% urban fraction for the two periods 1985-2010 and 2070-2099. Second, we grouped the data into latitudinal bands: Northern Extratropics (NET), Southern Extratropics (SET) and Tropics. For the current climate, we found statistically significant urban influences on temperature at night, consistent with many previous studies on urban heat islands and evidence of urban cooling during daytime. For NET, two-thirds of the urban grid cells show significantly increased precipitation, while for SET and Tropics show more decreases than increases. The urban impact on the precipitation signal in the future time period is small, while urban areas alter the signal of temperature and specific humidity in NET with a tendency towards reducing the increases in both variables.