Observational and modeling study of global warming and urbanization impacts on coastal California climate

Analyses were carried out of observational climatological (1970--2004) California summer temperatures from 253 NWS Coop sites in two air basins: South Coast Air Basin (SoCAB) and San Francisco Bay Area (SFBA). Results for all the California 253 COOP sites together showed increased (-0.23°C decade-1) Tave-values), asymmetric warming, as Tmin-values increase faster than Tmax-values (0.27 vs. 0.04°C decade-1), and thus decreased daily temperature range (DTR) values (0.15°C decade-1). While summer nighttime SoCAB and SFBA min temperatures showed expected greenhouse-gas induced warming, their spatial distributions of observed Tmax-values exhibited a complex pattern, with cooling (-0.30°C decade-1) in low-elevation coastal-areas open to marine air penetration and warming (0.32°C decade-1) at inland areas, which suggests that the inland warming resulted in increased coastal sea-breeze activity.;Further investigations by use of mesoscale model simulations with the Regional Atmospheric Modeling System (RAMS) meso-met model with a horizontal grid resolution of 4 km on an inner grid over SoCAB were undertaken to investigate the effects of long-term changes due to green house gas (GHG) warming and land-use land-cover changes on coastal flows. Comparison of simulated present and past climate conditions showed significant increases in sea breeze activity and thus coastal cooling, which supports the observational analysis results that coastal cooling is an indirect "reverse reaction" of GHG warming. Urbanization effects on coastal environment are twofold: increased urban mechanical surface roughness retards sea breeze flows, while urban heat islands (UHIs) enhance them.;Significant beneficial societal impacts will result from this observed reverse-reaction to global-warming, especially during UHI-growth periods, include decreased maximum: agricultural production, O3 levels, per-capita energy requirements for cooling, and human thermal-stress levels. Similar "reverse-reaction" effects should be found in other mid-latitude western coastal-regions.