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

Malle, Johanna, Rutter, Nick, Webster, Clare, Mazzotti, Giulia, Wake, Leanne and Jonas, Tobias (2021) Effect of forest canopy structure on wintertime Land Surface Albedo: Evaluating CLM5 simulations with in-situ measurements. Journal of Geophysical Research: Atmospheres, 126 (9). e2020JD034118. ISSN 2169-897X

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Official URL: https://doi.org/10.1029/2020JD034118


Land Surface Albedo (LSA) of forested environments continues to be a source of uncertainty in land surface modeling, 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 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 Plant Area Index (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, for example, by introducing a canopy metric that represents canopy gap fraction rather than assuming a spatially homogeneous canopy.

Item Type: Article
Additional Information: Funding information: The authors thank many students and interns of the SLF snow hydrology group (Elena Stautzenbach, Luca Iacolettig, Sarah Barr, Mira Ehrler, Benedikt Friedrich, Robin Maedel, Kalliopi Koutantou, Eliane Brändle) for assistance in the field. J. Malle received funding from Northumbria University, and G. Mazzotti was supported by the Swiss National Science Foundation, Project 169213. Our field campaign in Sodankylä has received funding from INTERACT under the European Union H2020 Grant Agreement No.871120, project IME4Rad. The authors thank Steven Hancock, who was supported by Natural Environment Research Council Grant GEF/loan 1108, for acquiring TLS data at Sodankylä and Richard Essery for help in the field during our campaign there. The authors are also grateful to Anna Kontu and Leena Leppänen (Finnish Meteorological Institute) for the hospitality at FMI and for providing AWS and snow survey data from FMI. The authors further acknowledge the support of the SLF electronics and mechanics workshop in assembling and maintaining the deployed instrumentation. Finally, we thank Dr. Eric Sproles and one anonymous reviewer for their insightful comments and suggestions which helped improve the quality of the manuscript.
Uncontrolled Keywords: Land Surface Albedo, Canopy Shading, UAV measurements
Subjects: F800 Physical and Terrestrial Geographical and Environmental Sciences
Department: Faculties > Engineering and Environment > Geography and Environmental Sciences
Depositing User: Elena Carlaw
Date Deposited: 19 Apr 2021 12:34
Last Modified: 10 Dec 2021 19:19
URI: http://nrl.northumbria.ac.uk/id/eprint/45945

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