Convective Modes Associated with Precipitation Exceeding Postfire Debris Flow Thresholds in the Southwestern United States

Postfire debris flows can occur when surface-water runoff dislodges and carries a slurry of loose sediment and vegetation downslope, usually in steep, recently burned terrain. Short-duration, high-intensity rainfall embedded within short- or long-duration rainstorms (e.g., thunderstorms versus atmospheric rivers) is most commonly associated with this mode of debris-flow initiation. The resolutions of global numerical weather prediction and climate models are too coarse to resolve these convective precipitation features on sub-daily timescales, which poses an obstacle to short-term forecasting and investigating future trends in the frequency, magnitude, and characteristics of debris-flow generating rainfall. We used 15-minute precipitation and simulated reflectivity from a set of dynamically downscaled, convective-permitting simulations (~3.75 km) over three time periods—historical (1990–2005), mid-century (2040–2055), and late-century (2085­–2100)—to assess the convective characteristics associated with postfire debris-flow triggering across the southwestern United States. In the historical simulation, our results show regional and seasonal differences in reflectivity signatures with some areas and seasons (e.g., Northern California during the cool season) experiencing more linear modes and others (e.g., Northern California during the warm season) dominated by more cellular modes. In the mid- and late-century simulations, we find a substantial increase in postfire debris-flow rainfall threshold exceedances and we are currently evaluating whether these threshold exceedances are occurring in a more convective mode. Understanding the differing characteristics of rainfall that triggers postfire debris flows is crucial to the development of mitigation strategies to protect communities vulnerable to the effects of this compounding hazard.