Abstract ID: 3.9481 | Accepted as Talk | Talk/Oral | TBA | TBA

Rain-on-snow (ROS) events are critical hydrometeorological phenomena influencing snowpack dynamics, river discharge, and flood risks in alpine regions. In the maritime Southern Alps of New Zealand, ROS events significantly modulate seasonal snowmelt, triggering avalanches and extreme runoff, particularly during winter and spring. Maritime snow-dominated regions are especially vulnerable in a warming climate, where even small temperature increases can drive substantial changes in snow accumulation and melt processes. This study examines observed ROS events using high-elevation meteorological and snowpack data, focusing on their seasonal distribution, atmospheric drivers, and impacts on catchment hydrology. We utilized the Snow and Ice Network (SIN) dataset, alongside data from the Pisa Range, Brewster Glacier, and Mt Belle (Milford Road), to identify and classify ROS events based on their occurrence during snow accumulation and snowmelt periods. While spring ROS events are more frequent, our findings confirm that winter ROS events also exhibit significant hydrological impacts, highlighting the need to account for year-round ROS dynamics in hydrological modelling and flood risk assessments. Additionally, we investigated the relationship between atmospheric rivers (ARs) and ROS events over seasonal snowpacks in the Southern Alps. Case study analyses revealed that AR-related ROS events are associated with an increase in vertical integral of heat flux, leading to anomalously warm mid- and lower-tropospheric temperatures and a freezing level rise to ~725-650 hPa. Air temperature increases of up to 10°C were recorded near the Main Divide, contributing to rapid snowmelt rates (up to 200 mm day⁻¹). These events were further intensified by turbulent latent heat flux and rain-heat flux, exacerbating snowmelt-driven floods. Streamflow data from alpine rivers confirm that high melt rates combined with AR-driven precipitation can lead to major flooding events in upper terrains of the Southern Alps during both winter and spring. The findings of this study contribute to the development of a national-scale snowmelt forecasting system, improving flood prediction accuracy and hazard assessments across New Zealand. This work underscores the importance of integrating ROS processes into hydrological models, particularly in the context of a warming climate and increasing extreme weather events.