MODELING URBAN HEAT ISLANDS IN CALIFORNIA CENTRAL VALLEY

Temperature maps of the difference between long-term mean monthly temperature and recent monthly means of summertime temperature trends in California Central Valley (CCV) Cites of San Jose, Modesto and Sacramento showed a statistically significant increase of 0.65, 0.46, and 0.49 oF per decade, respectively. The historical 60 year temperature trend for CCV is compared to the local temperature increase, population growth, and urban expansion finding a correlation between urban development and temperature increase. To demonstrate the hypothesis of a regional climate change due to land use for urbanization, a detailed mesoscale modeling effort of the CCV atmospheric dynamics has been conducted, with an emphasis on the heat island and temperature diurnal cycles. The Regional Atmospheric Modeling System (RAMS) is used as the simulation tool and was configured using four grids (0.9 km fine grid). The focus of the analysis is the spatial and temporal variations of the urban heat island under the land use scenarios centered in both Sacramento and Modesto: 60 year ago when the cities were not developed and the present urban landscape. The pre urbanization land use scenario was configured assuming a potential vegetation for the area. Since Sacramento is exposed to the Carquinez strait, it is important to consider the effect of the sea breeze on the heat island as part of the research. Three dimensional-simulations with realistic topography, coastlines and time-varying synoptic forcing were generated. The differences in spatial temperature and sea breeze clearly indicate that the historical land use modification is a driver for temperature increases, and the sea breeze is unaffected by the low pressure generated by the local warming. Consequently, the heat from the cities is in general advected to the eastern Sierra Nevada Hills.