Computing Atmospheric Back Trajectories to Determine Moisture Source Regions and Their Role in the Variability of Oxygen Isotope Ratios in Precipitation

Variations in oxygen isotopes in water samples collected from precipitation depends on the temperature of water at the time of condensation as well as the source region of the water and its path prior to condensation. The evaporation of heavier oxygen isotopes in water increases linearly as temperature increases, while heavier oxygen are more likely to condense with the first precipitation produced by a rain event. In the past, studies have often exclusively used condensation temperature variations to understand the makeup a given rainwater sample. However, given a sample set of precipitation, the water isotope makeup will not only depend on the temperature at which the water was condensed, but also the source region of the water, the temperature at which it was evaporated, and any phase changes along its path before final condensation and rainout. The purpose of this project is to further study the effect that source region coupled with temperature of condensation has on the variation in oxygen isotope for a given location through the use of a climatology of spring and fall precipitation events in North Platte, Nebraska.

The project will use oxygen isotope data from North Platte, NE collected and published by F.E. Harvey and J.M. Welker. The project will also use data from the North American Regional Reanalysis (NARR) project in order to run and analyze back trajectories created using a program called HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) which was developed by the Air Resources Laboratory, a subset of the National Oceanic and Atmospheric Administration. Back trajectories will be run from fixed heights (geopotential meters) in the atmosphere and from a fixed spatial grid. They will start from the hour before precipitation began according to historical METAR observations and end when the trajectory reenters the boundary layer. From this point, the precipitation events can be stratified by source region, temperature of condensation, and temperature of evaporation. These results should give a better understanding of the role that moisture source region plays in the variation of oxygen isotope makeup in precipitation samples.