Effect of forest canopy structure on wintertime Land Surface Albedo: Evaluating CLM5 simulations with in-situ measurements

Johanna Malle*, Nick Rutter, Clare Webster, Giulia Mazzotti, Leanne Wake, Tobias Jonas

*Corresponding author for this work

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Abstract

Land Surface Albedo (LSA) of forested environments continues to be a source of uncertainty in land surface modelling, especially across seasonally snow‐covered domains. Assessment and improvement of global scale model performance has been hampered by the contrasting spatial scales of model resolution and in‐situ LSA measurements. In this study, point‐scale simulations of the Community Land Model 5.0 (CLM5) were evaluated across a large range of forest structures and solar angles at two climatically different locations. LSA measurements, using an uncrewed aerial vehicle (UAV) with up and down‐looking shortwave radiation sensors, showed canopy structural shading of the snow surface exerted a primary control on LSA. Diurnal patterns of measured LSA revealed strong effects of both azimuth and zenith angles, neither of which were adequately represented in simulations. In sparse forest environments, LSA were overestimated by up to 66 %. Further analysis revealed a lack of correlation between PAI, the primary canopy descriptor in CLM5, and measured LSA. Instead, measured LSA showed considerable correlation with the fraction of snow visible in the sensor’s field of view, a correlation which increased further when only considering the sunlit fraction of visible snow. The use of effective PAI values as a simple first‐order correction for the discrepancy between measured and simulated LSA in sparse forest environments substantially improved model results (64‐76% RMSE reduction). However, the large biases suggest the need for a more generic solution, e.g. by introducing a canopy metric that represents canopy gap fraction rather than assuming a spatially homogenous canopy.
Original languageEnglish
Article numbere2020JD034118
Number of pages15
JournalJournal of Geophysical Research: Atmospheres
Volume126
Issue number9
Early online date3 May 2021
DOIs
Publication statusPublished - 16 May 2021

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