Orographic precipitation and isotope fractionation

Over the last four years, several orographic precipitation projects have been carried out along the “American Cordillera”, ranging from 50 North to 50 South. Some of these projects included stable isotope estimates of Drying Ratio (DR). In this paper we summarize this work and try to identify common aspects of the results from different regions.

The “baseline” site for these projects is the recent project in northern California. The Drying Ratio (DR) for orographic precipitation in the Mendocino Coastal and Sierra Ranges in California was estimated 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 40% for both ranges. Sapwater isotopes also show a strong eastward fractionation, confirmed the DRs derived from stream water. Estimates of cloud delay times are about 2000 seconds. Compared with the corresponding mountains in South America, the North California case has a smaller DR. Preliminary results from the tropics will also be presented.