The Role of Atmospheric Circulation on Rain-on-Snow Driven Ablation in the Eastern United States

Rapid ablation of a snowpack has many hydrologic, ecological, and societal impacts, which can be further enhanced by rainfall during a “rain on snow” (ROS) event. The severity of rain-enhanced ablation of a snowpack is dependent on several factors including the geography, climatology, and hydrology of an area. Past studies have considered the implications of ROS throughout the world, but few have explicitly examined the eastern United States – an area where rapid ablation events enhanced by rain have caused significant flood damage. Additionally, little work has examined ROS and associated variations in lower- to mid-tropospheric circulation nor its impact on the energy budget of the snowpack, particularly in this region.  Here the Temporal Synoptic Index (TSI) is used to examine air mass characteristics during ROS events. Composites of multiple lower and mid tropospheric fields are examined for each type during ROS events. Meteorological data for case studies from each of multiple synoptic types is used with a coupled mass and energy budget model to assess the relative importance of the heat flux from rainfall compared to the other energy fluxes. Results indicate that melt during ROS is associated with higher sensible and latent heat fluxes, and these fluxes are dependent on the availability of moisture and the low-level winds. Model-derived melt rates indicate some dependence on the initial snow depth and the local geography. Understanding how the variations in lower- to mid-tropospheric circulation influence the surface energy budget and ablation due to ROS is important as synoptic types most conducive to ROS are expected to increase in the future. This work will serve not only to aid in the predictability of severe ROS-induced flooding but also to better understand the impact of ROS in future climate scenarios.