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
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.