Intra-Event Trends in Stable Isotopes: Exploring Summer Monsoon Convective Precipitation in Arizona

The evaluation of the stable isotope ratios of δ2H and δ18O in precipitation can provide insight into the origin of the source water vapor, the type of precipitation mechanisms present, and the compositional changes in and below the cloud. Long-term collection of these measurements has occurred globally, but few studies have examined the variability of the data at the within-event level. Research at the USDA-Agricultural Research Service (ARS) Long-term Agroecosystem Research (LTAR) network site, the Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona has shown δ2H values in shallow storage lower than daily integrated rainfall samples from the source rainfall. This suggests that portions of the rain within an event may contain isotopically different composition. To further investigate this hypothesis, this study was conducted to examine within- and whole-event isotopic variability in summer, convective-dominated precipitation onset by the North American Monsoon in the southwestern United States. A low-cost auto-sampler was deployed at WGEW from 2017-2019 along with temporally high-resolution instrumentation including a weighing-type rain gauge, a meteorological station, disdrometers, and two vertical profile radars. Additionally, the NOAA HYSPLIT model was used to source air parcel trajectories for each event. Within-event concentrations of δ2H and δ18O ranged from -75 to 31 and -12 to 28 per mille respectively. Whole-event deuterium-excess (d = δ2H -8*δ18O) values showed significant positive relationships with rainfall amount, rainfall intensity, drop size, and drop number. Low intensity events showed the greatest deviation from the local meteoric water lines, implying significant sub-cloud evaporation and/or exchange with subcloud vapor. Convective storms sourced from the same tropical maritime air parcel showed differences in whole-storm δ2H concentrations of up to 53 per mille, reinforcing the idea that microphysical interactions can significantly alter the isotopic composition of the water during rainfall in this precipitation regime. Understanding isotopic variability within rainfall events not only informs our understanding of hydro-meteorologic processes, but also provides valuable insight to controls in terrestrial monitoring using isotopes.