OPI: A FULL PROCESS-BASED MODEL FOR STUDYING PRECIPITATION ISOTOPES AS A FUNCTION OF OROGRAPHIC LIFTING AND CLIMATE

Smith and Barstad (2004) developed a fast method for simulating the precipitation field created by steady atmospheric flow of fully saturated air over an arbitrary mountain topography at a spatial resolution of 1 km or better. I report here on OPI (Orographic Precipitation Isotopes), which is an isotope-enabled version of their model. OPI fully accounts for fractionation associated with condensation, isotopic exchange along the fall path, and recycling of evaporated water vapor back into the atmosphere. The precipitation climatology is defined by 6 parameters: wind speed, wind direction, surface air temperature, Brunt-Väisälä frequency, eddy diffusion, and time delays for formation and fallout of hydrometeors. Evaporation is defined by 3 parameters: surface temperature, humidity, and the fraction of precipitation lost by evaporation. I use a non-linear search algorithm to fit 185 measurements of modern δ2H and δ18O for meteoric water samples collected in a 500 km wide region straddling the Patagonian Andes from 40 S to 48 S. The fit is excellent and matches well with a 30-year climate reanalysis for southern South America.

Isotope methods for paleotopography all give highly averaged estimates of the isotopic composition of ancient precipitation, and thus are directly comparable with an OPI estimate of the mean precipitation state and mean isotopic fractionation. The best-fit parameter can be perturbed to see how results are influenced by changes in topography or climate. I find that the fractionation measured in precipitation is linearly dependent on the size of the topography. Climate is also important: a 5 C increase in surface temperature produces an isotopic fractionation equal to a 20% decrease in topography. The other parameters show little influence on isotopic fractionation, including the blocking effect. The quasi-equilibrium model for tropic precipitation indicates that the OPI model should work for both stratiform and convective precipitation as well.