Shading by Trees and Fractional Snow Cover Control the Subcanopy Radiation Budget

Abstract

Radiative processes are substantially altered by the presence of forest canopies, further affecting snow energetics during wintertime. In situ measurements of subcanopy radiation can help improve process‐scale understanding of these complex interactions, which are needed to further constrain and improve land surface models. In this study, a custom‐made cable car was used to measure incoming and outgoing, shortwave and longwave radiation below an evergreen forest stand. Hemispherical photographs taken concurrently from the cable car measured view fractions of shaded snow, sunlit snow, and bare ground. With this setup it was possible to quantify diurnal and seasonal radiation patterns together with their potential drivers at high spatiotemporal resolution. Measurements were performed between January and May 2018, along a 48‐m transect in a discontinuous needleleaf forest in the Swiss Alps. Analysis of diurnal radiation patterns revealed a strong linear relationship (R = 0.94) between outgoing shortwave radiation and sunlit snow‐view fraction, highlighting shading as the main control on the subcanopy shortwave radiation budget. Measurements of outgoing longwave radiation were strongly controlled by the snow cover extent, with locations of diminished snow cover showing an increase in outgoing longwave radiation of up to 60 W/m2. The subcanopy radiation budget was shown to be dominated by shortwave radiation when surrounding canopy structure and the position of the sun allowed for direct insolation of the forest floor, but longwave radiation was the dominating component in the absence of direct insolation.

How Surface Radiation on Forested Snowpack Changes across a Latitudinal Gradient

“…By considering a range of forest densities across a latitudinal gradient, our experiments and results highlight the changes in the relative dominance of shortwave and longwave radiation components vis-à-vis forest density, topographic aspects and slope, and site-specific climatology. These results provide additional perspective to the previous observational and modeling studies (e.g., [13][14][15]21,26,[47][48][49][50][51][52][53]), which reported radiation components on forested snowpack at selected vegetation densities and sites.…”

“…The estimation of the NSRF depends on the accurate evaluation of shortwave radiation, S Net , (direct, diffuse and reflected from snow and canopy) and longwave radiation, L Net , (from tree crown, trunk, sky and snow) components [10][11][12][13][14][15]. Both S Net and L Net radiation components are dependent on vegetation density.…”

Process‐Level Evaluation of a Hyper‐Resolution Forest Snow Model Using Distributed Multisensor Observations

“…The areas span varying climatic conditions and forest types, with a focus on conifer species. In Switzerland, the field sites in the area near Davos Laret have hosted numerous forest snow research projects, as documented in Malle et al (2019), Mazzotti, Malle, et al (2019), Moeser et al (2014), Moeser et al (2015), Webster and Jonas (2018), and Webster et al (2016b). The forest in Laret primarily consists of Norway spruce, including both new and old growth (trees up to 45 m tall).…”

Resolving Small‐Scale Forest Snow Patterns Using an Energy Balance Snow Model With a One‐Layer Canopy

“…However, forest snow dynamics are shaped by complex interacting processes that are controlled by the structure of the overhead canopy and thus display large spatial and temporal variation. Snow interception by the canopy Moeser, Stähli, et al, 2015;Roth & Nolin, 2019) and subsequent sublimation and unloading to the ground (MacKay & Pomeroy et al, 1998), shading of shortwave radiation (Hardy et al, 2004;Malle et al, 2019;Musselman, Molotch, Margulis, Kirchner, et al, 2012), and emission of longwave radiation by the vegetation (Essery, Pomeroy, et al, 2008;Pomeroy et al, 2009;Webster et al, 2016) all vary with canopy structure in specific ways and thus contribute to heterogeneous snow depth distribution patterns, which are difficult to replicate with models (Clark, Hendrikx, et al, 2011). The forest snow model intercomparison project SNOWMIP2 Rutter et al, 2009) evaluated 33 forest snow models differing in both process complexity and canopy implementation approaches.…”