Isotopic Variability in Precipitation: Integrating HYSPLIT Raster Data Into Isoscape Modelling

ABSTRACT Stable isotopes of hydrogen and oxygen in precipitation provide insights into water cycle dynamics; yet, characterising the meteorological processes driving isotopic variability remains challenging. We introduce a method to rasterize air mass trajectory data from HYSPLIT for isotopic analysis and assess the impact of trajectory initiation height on HYSPLIT's representation of local precipitation and temperature. Incorporating HYSPLIT rasters into spatial analysis improved deuterium composition isoscape accuracy for precipitation compared with a model relying solely on surface predictors (i.e., temperature, elevation). The resulting isoscape patterns reflected strong, temperature‐dependent seasonality, aligning well with previous studies. Trajectory cluster analysis further elucidated the links between meteorological patterns and observed seasonality in isotope compositions. We also analysed daily precipitation at two sites ( n = 204, n = 138) over ~2 years using Random Forest and Multiple Linear Regression. Our analysis identified a significant amount effect and evidence for sub‐cloud evaporation. However, additional factors—such as cloud‐top temperature, reflectivity, and convective versus stratiform fractions—are likely needed to explain residual daily‐scale variance. Finally, we determined that a HYSPLIT model initiation height of 1000 m was sufficient for our study domain. Using the local condensation level as the initiation height provided no substantial improvement in correlations between modelled and observed precipitation or temperature.