Orographic Precipitation, Drying Ratio and Isotope Fractionation

We derive a new “Upslope Model” of orographic precipitation (OP) proportional to integrated water vapor flux (WVF). This new form of the model can be integrated easily to give a formula for drying ratio DR=1-exp(h/H) where h is the mountain height and H is the saturation scale height. We test this prediction against two types of data: cross mountain balloon sounding pairs and stable isotope fractionation in water from streams and trees. For the Sierras and Cascades ranges, weather balloon data tightly constrains DR=48%±2% . Other mountain ranges such as the Southern Andes and the Southern Alps give similar DR values but with more scatter. An air mass correction is necessary to reduce scatter. The stable isotopes of water can also be used to estimate the drying ratio (DR). Four mid-latitude mountain ranges: Cascades, Sierras, Southern Andes and Southern Alps produce deuterium changes of δD= 50 to 80 per mille , suggesting DR=30 to 50%. For this method, redundant data is available from different isotopes and for parallel and repeat transects. These new formulae are used to judge the sensitivity of OP fractionation to mean cloud temperature and mountain height. We extrapolate from OP in mid-latitudes to OP on the slopes of the high-latitude Greenland and Antarctic Ice sheets to judge the role of local fractionation on Pleistocene ice core climate records.