Modeling the Impacts of Urbanization vs. Global Warming in California Southern Coast Air Basin

A recent study by Lebassi et al. 2009 evaluated long-term (1948-2005) air temperatures in California (CA) during summer (June-August, JJA). The aggregate CA results showed asymmetric warming, as daily minimum temperatures increased faster than daily maximum temperatures. The spatial distributions of daily maximum temperatures in the heavily urbanized South Coast (SoCAB) and San Francisco Bay Area (SFBA) air basins in California, however, exhibited a complex pattern, with cooling at low-elevation coastal-areas and warming at inland areas. Previous studies have suggested that cooling summertime maximum temperatures in CA were due to increased irrigation, coastal upwelling, or cloud cover. The current hypothesis, however, is that this temperature pattern arises from a “reverse-reaction” to greenhouse gas (GHG) induced global-warming. In this hypothesis, the global warming of inland areas resulted in an increased (cooling) sea breeze activity in coastal areas. Further investigation using the Regional Atmospheric Modeling System (RAMS), at a horizontal grid resolution of 4 km over the SoCAB has been undertaken to investigate the combined effect of urbanization and global warming on coastal temperature and flow patterns during the same period as the observational study. Comparison of simulated present and past climate conditions shows significant increases in sea breezes and coastal cooling over time, supporting the hypothesis that this coastal cooling is a ”reverse reaction” to GHG and urban warming. The simulated effect of urbanization on the coastal environment is two fold, as surface mechanical processes deter sea breeze flows, while its thermal aspects enhances sea breezes. Results show that increase in sea breeze induced by GHG is a dominant factor over urbanization. Further analyses of the results quantify impacts on sea breeze change penetration, intensity, and depth in the boundary layer evidencing a regional reverse reaction. Significant societal impacts may result from this reverse-reaction. Possible beneficial effects (especially during periods of UHI growth) include decreased: maximum: O3 levels, per-capita energy requirements for cooling, and human thermal-stress levels.