Meteorological assessment of a mountainous convective precipitation event triggering debris flows in the subtropical Andes.

Convective precipitation in mountainous zones often induces downstream effects, such as increased river turbidity,  debris flows, and flooding, which can result in infrastructure damage, casualties, and even fatalities. Observing these events in the Andes is particularly challenging because of the lack of surface observations in the highlands, and sometimes, they are not even captured by meteorological stations. Nevertheless, debris flows are observed by the small communities of mountain inhabitants. In this research, we analyze a convective precipitation event induced by a cut-off low that triggered several debris flows in the upper Huasco River basin at the subtropical Andes (~28°S) during April 2024 (austral fall). We follow the typical hydrologic approach, extrapolating low elevation measurements from the Chilean water and meteorological agencies, comparing the estimation of freezing level with in-situ low-cost temperature sensors, and MODIS images sensing for snow detection. In addition, we perform a convection-permitting simulation through the Weather Research and Forecasting model (WRF), to reproduce rainfall in locations where debris flows occur. Our results show that the available pluviometers did not observe significant precipitation, except in some areas with intensities of up to 7 mm/hr and total precipitation of 64 mm at 1370 m a.s.l., as well as intensities of up to 2 mm/hour and total precipitation of 10 mm at 467 m a.s.l. during two to three days of the event. The temperature sensors indicate a high freezing level decreasing from 4000 m a.s.l, to 3000 m a.s.l. within the first day. The WRF simulations revealed, in the 4 km resolution domain, that total precipitation exceeds 100 mm, surpassing the highest observed records. Our findings demonstrate the importance of having observational data in mountain zones and the key role that may play in convection-permitting simulations in complex and ungauged terrain.