Surface elevation change and optical retrieval products to benchmark the next generation of surface mass balance models, Place Glacier, Canada

Mass balance models are typically evaluated using a sparse collection of point measurements on one or more glaciers. Discrete measurements of mass or elevation loss often reflect surface elevation/mass change along centreline ablation stakes. More recent approaches use geodetic data to help constrain these mass balance models, but the infrequent nature of these distributed datasets often precludes an evaluation of how well these models capture important physical properties such as albedo evolution, short-term events such as impurity deposition following wildfire or dust loading, or the importance of heat waves. Here we describe the unique observational dataset for Place Glacier, a small (2 km2) benchmark glacier monitored since 1965. Our dataset includes 62 airborne laser altimetic and 30 hyperspectral surveys from which we derive monthly surface elevation change and optical retrievals (e.g. grain size, albedo and radiative forcing caused by light-absorbing particles-LAPs) during the ablation seasons (2020-2025). These 2-m data are currently being used to assess the performance of distributed surface mass balance models such as COSIPY, GEMB, CROCUS. The dense observational record allows us to perform suitable calibration and validation exercises for each model but also for spaceborne-derived datasets of surface elevation and albedo change. Surface mass balance model performance during the validation period is typically excellent, but significantly degrades when attempting to simulate mass change prior to 2020. Factors which account for this poor pre-2020 performance includes major changes in the extent of firn, late-lying snow and impurity deposition. In addition to elevated flux of LAPs from dry and wet deposition, thinning of firn has elevated surface concentration of LAPs and surface debris thereby darkening the glacier and accelerating mass loss. Work is underway to physically model these important physical processes leading to albedo reduction and attendant glacier mass loss.