Wintertime land surface albedo of forested environments

Forest canopies are complex 3-D structures at the interface between the atmosphere and the land surface which greatly affect radiative processes, especially across seasonally snow-covered domains. The increase in process complexity associated with these areas continues to be a source of uncertainty in land surface modelling. Model development has been hampered by a limited amount of in-situ radiation measurements, together with contrasting spatial scales of measurements and model resolution. Here, a bespoke cable car system was used to measure incoming and outgoing shortwave and longwave radiation below an evergreen forest stand, while an uncrewed aerial vehicle (UAV) system, equipped with up- and down-looking shortwave radiation sensors, was used to measure land surface albedo (LSA) above alpine, sub alpine and boreal forest stands. These in-situ measurements were combined with point-scale simulations of the Community Land Model Version 5.0 (CLM5), enabling process level assessment of algorithms used within global climate modelling frameworks. Analysis of diurnal radiation patterns, obtained via the cable car and UAV systems, revealed canopy structural shading of the snow surface as a main control on both the sub-canopy shortwave radiation budget and overall LSA. Furthermore, diurnal patterns of measured LSA revealed a strong dependency on both solar azimuth and zenith angles. Corresponding CLM5 simulations did not adequately represent the measured spatial and temporal variability in LSA and sub-canopy incoming shortwave radiation. In sparse forested environments, CLM5 performed especially poorly, as LSA was overestimated by up to 66%. The use of effective Plant Area Index (PAI) values as a simple first-order correction for this discrepancy between measured and simulated LSA substantially improved model results (64-76% RMSE reduction). That being said, such large biases suggest the need for a more robust solution, especially as the use of effective PAI values did not improve the ability of CLM5 to replicate diurnal variability in LSA and sub-canopy shortwave radiation. Hence, a time-varying transmissivity for direct shortwave radiation was integrated into CLM5, meaning that directionality of solar irradiance could be taken into account. Results with this modified version of CLM5 showed measured variability of sub-canopy incoming shortwave radiation was replicated more accurately, suggesting this approach may help to decrease uncertainty in LSA simulations across seasonally snow-covered forested environments. This has far reaching implications for simulations of the snow albedo feedback strength over the entire Northern Hemisphere Extratropics