Effect of Forest Canopy Structure on Wintertime Land Surface Albedo: Evaluating CLM5 Simulations With In‐Situ Measurements

Abstract

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.

Canopy Structure and Air Temperature Inversions Impact Simulation of Sub‐Canopy Longwave Radiation in Snow‐Covered Boreal Forests

“…This challenge was identified as a poorly constrained source of uncertainty in the SnowMIP2 intercomparison of forest snow process models (Rutter et al., 2009), and remains persistent. However, the development of effective bulk canopy parameterizations for radiative transmission and emittance (e.g., Malle et al., 2021) is now starting to provide first‐order empirical solutions in lieu of more explicit solutions using canopy gap fraction.…”

Impact of measured and simulated tundra snowpack properties on heat transfer

“…The ground is underlain by continuous permafrost to a depth of 350-500 m (Wilcox et al, 2019), with a maximum active-layer depth of up to 1 m at the end of the summer (Grünberg et al, 2020). Snow cover at TVC has a typical duration of 8 months (Pomeroy et al, 1993), with typical depths of 0.2-0.5 m, though drifts exceeding 1-2 m occur surrounding tall shrubs and in proximity to steep slopes (Marsh and Pomeroy, 1999).…”

An Operational Methodology for Validating Satellite-Based Snow Albedo Measurements Using a UAV

“…The most physically consistent approach for relating pyranometer measurements to surface properties is to compare each pyranometer measurement to a cosine-weighted average of the pixels within a defined PFOV (Jäkel et al, 2019;Malle et al, 2019;Malle et al, 2021). The proposed correction calculates a cosine-weighted mean of σ s and c s for every pixel (n) that lies within the specified PFOV of the downward-facing pyranometer, which is also dependent on altitude (Figure 7).…”

“…Therefore, topographic data from these areas should not be included in the topographic corrections of pyranometer measurements over snow, and the radiance signal from these portions of the scene should not be corrected. Moreover, since the albedo of tree-shaded areas is mainly a function of the magnitude of tree shading as opposed to snow physical properties (Malle et al, 2021), interpretations of the physical controls on measured albedo depend on the ability to partition measurements based on fractions composed of trees and shading. This partitioning was out of the scope of this research, and the implications are discussed in Limitations and Future Work.…”