Abstract : E.3
Orographic precipitation and isotope fractionation in the California Coastal and Sierra Ranges

Ronald Smith, Alison Anders, Anthony Didlake, Brett Tipple, Heather Reeves
ronald.smith@yale.edu
Yale University

We estimated the Drying Ratio (DR) for orographic precipitation in the Mendocino Coastal and Sierra Ranges in California using three independent methods. Balloon soundings show a constant ratio of WV fluxes at upstream and downstream locations, giving a DR~32% for the combined ranges. Raingauge data show a constant ratio of precipitation to upstream WV flux, giving DR~30%. Deuterium and oxygen isotopes from streamwater in June and September show a strong fractionation across the ranges giving DR~25% for the Coastal Range and 30% for both ranges. A tuned linear downscaling model suggests that some precipitation enhancement mechanism, such as “roughness enhancement” probably operates over the Coastal Range. Sapwater isotopes also show a strong eastward fractionation, confirmed the DRs derived from stream water. Compared with the corresponding mountains in South America, the North American case has a smaller DR.