The Australian seasonal snowpack can be classified as a marginal maritime snowpack with a temperature near 0 °C throughout the snow season. Subtle changes in atmosphere – snow energetics therefore result in rapid change in snowpack properties, which occur against a background of a warming climate. This has been attributed to a 40% decline in spring snow depths in the past 40 yrs. and geologic records suggest the seasonal snowpack is now near a 2000 yr. minimum. Modelled future snow cover predicts further decline by 57 % to 78 % of current maximum snow depth by the 2040s. Such research primarily attributes this decline in snow cover to global warming. However, the past decline in Australian snow cover can also be attributed to change in synoptic wintertime precipitation patterns that have resulted in a dramatic increase in proportional winter and spring precipitation of tropical origin since the 1950s. Tropical moisture is predominantly transported into southeast Australia during negative phases of the Indian Ocean Dipole (IOD) by northwest cloud bands – visible expressions of atmospheric rivers coupling tropical moisture sources northwest of Australia to the Australian Alps. Here we present a case study of one such event that occurred from the 21 to 23 July 2016 when 118 mm of rain-on-snow over a 12 hr period led to near complete ablation of the snowpack. While predictions of future variability of the IOD due to global warming remain uncertain, we suggest that warming atmospheric temperatures increase the risk of such extreme rain–on-snow events during negative IOD events. Combined with reduced snow cover in response to warmer ambient wintertime temperatures, such rain-on-snow events may further accelerate the reduction in seasonal snow cover in the Australian Alps, possibly on occasions after which the snowpack does not recover before spring. These conditions would present significant challenges to the Australian snow sports industry which is worth $2 billion annually and lead to change in snow dependent ecosystems and alpine hydrology.

Research on modification to snowpacks as a result of forest disturbance has typically focused on spatiotemporal patterns of snow depth and snow water equivalent, snowpack energy fluxes, and melt/ablation characteristics. However, little work has been conducted on relationships between tree trunks and snowpack dynamics. Insight into drivers of internal snowpack thermodynamics around trees and their response to forest disturbance is crucial to understanding hydrological processes in forested regions of the cryosphere, especially as forest disturbance through climate change continues. This work investigates relationships between energy fluxes and thermodynamic patterns surrounding tree trunks and within the greater snowpacks of forest stands in the Snowy Mountains of the Australian Alps. Measurements of vertical and horizontal snowpack temperature profiles and sub-canopy energy fluxes were collected during the 2018 winter season in non-disturbed and fire-disturbed Eucalyptus pauciflora (Snow Gum) stands. Primary heat sources were identified for each measurement location in the snowpack through employing the Random Forest machine learning regression method. Preliminary results indicate that soil heat flux is the dominant control on snowpack temperature at all locations in the un-disturbed forest stand. However, outgoing longwave radiation is shown to be the prevalent driver at numerous locations within the fire-disturbed stand that are close to the snowpack surface and tree well. This work aims to develop the physical basis for a 3-dimensional thermodynamic model of snowpacks contained in forests that could be used in conjunction with existing 1-dimensional snowpack models to determine melt and variability.