FROM RAINDROPS TO MOUNTAINTOPS: TOPOGRAPHIC CONTROLS ON THE ISOTOPIC COMPOSITION OF PRECIPITATION

The stable isotopic composition of precipitation has the potential to provide insight into a range of atmospheric, hydrologic, and even geologic processes. Correlations between the stable isotopic composition of precipitation, temperature, and elevation have been used to support interpretations of paleo-isotopes as proxies for paleoclimate and paleotopography. While such models are motivated by the fundamental physics controlling fractionation during the condensation of water vapor in the atmosphere they do not consider the full range of microphysical processes operating on hydrometeors throughout their lifecycle. However, stable hydrogen and oxygen isotopes of water can provide a powerful tool for investigating atmospheric microphysics and are increasingly used for discerning precipitation processes. A process-based understanding of the isotopic composition of precipitation will allow for assessment of the use of water isotopes in paleotopographic reconstructions. I present maps of the spatial variability in the isotopic composition of precipitation in western North American on timescales from minutes to decades and probe the extent to which topography forces these patterns. I also suggest a conceptual model in which isotope fractionation is controlled by atmospheric drying rather than parcel temperature, which more easily allows for representation of a range of processes contributing to precipitation production. Paleo-isotopes potentially represent a rich archive of information about past topography and climate and mechanistic understanding of precipitation processes provides insight into this archive.