Impacts of land use change on California's climate

California has seen a huge change in land use since mid-1800s with a sharp increase in population. The Regional Spectral Model of ECPC at Scripps Institution of Oceanography is used to investigate the impact of land use change on California's climate. The model is run at 25km resolution for 1995-1996 with NCEP/DOE Reanalysis 2 as boundary conditions. The experiment compares two runs with different land cover datasets: 1) pre-1890 natural vegetation land surface, and 2) modern land surface with irrigation and urbanization. In addition, ensemble runs are performed to see how initial model variability adds uncertainty to the interpretation of differences between modern and pre-industrial runs. Urban areas are investigated at 10km resolution. One notable difference is a decrease of the mean temperature in the irrigated land from presettlement to modern run in summer. Daily maximum temperature decrased more than minimum temperature, resulting in a reduced diurnal temperature range. We discuss the role that past land use/cover changes and irrigation may have had on California's climate through the analysis of model response to the changes.