RAMS modeled differences between 1970 and 2005 summer daytime temperatures and winds in Coastal California

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Monday, 18 January 2010
B. Lebassi, Santa Clara Univ., Santa Clara, CA; and J. E. González and R. Bornstein

California summertime July to August (JJA) mean monthly air temperatures (1950-2005) were previously analyzed for two California air basins: South Coast (SoCAB) and the San Francisco Bay Area (SFBA), which extended into the Central Valley (CV). Daily Tmin and Tmax values were used to produce average monthly values and spatial distributions of and Tmax values trends for each air basin. Results showed concurrent cooling in coastal areas and warming at further inland areas. This pattern suggests that the regional-warming of inland areas resulted in increased coastal sea breeze activity.

The sea breeze hypothesis was now further investigated by use of RAMS mesoscale simulations, with a horizontal grid resolution of 4 km on an innermost grid over the SoCAB. Simulations of historical effects of global greenhouse gas increases and/or LULC changes on the SoCAB region during typical JJA conditions were also be investigated. Two sets of simulations, i.e., “present” (2002) and “past (1970 vegetation)” land-use and green house gas conditions, have been undertaken for the summertime period. Resulting monthly averaged 0000 UTC (= 5 PM PDST) wind and temperature changes (i.e., current minus past) were analyzed. The comparison of present and past climate conditions reflect significant increase in sea breeze and coastal cooling which supports the hypothesis that coastal cooling is a possible direct result of green house gas warming. These results provide an increased understanding of past and present trends of summer daytime temperatures and of impacts from global and mesoscale (e.g., marine, land use, and topographic) physical processes in coastal environments.

Coastal cooling could impact summer daytime ozone levels, water supply, fossil-fuel use, agricultural production, and human thermal-stress levels. This “reverse-reaction” effect could exist in other coastal regions, while other types of local reverse-reactions may also exist worldwide.