Spatially Continuous Characterization of Forest Canopy Structure and Subcanopy Irradiance Derived from Handheld Radiometer Surveys

Abstract

Small-scale variations in radiative transfer through forest canopies are strongly linked to canopy structural heterogeneity. To date, upscaling of radiative transfer parameterizations developed at the point scale is hampered by (i) poor representation of canopy structure variability and (ii) limited spatially explicit subcanopy irradiance data to assess the performance of radiative transfer schemes at typical resolutions of land surface models. This study presents a novel approach for efficient in situ characterization of canopy structure and subcanopy irradiance over large spatial extents. The method involves a handheld radiometer assembly mounted on a motorized gimbal developed for nonstationary continuous measurements of shortwave and longwave radiation along forest transects. In combination with radiation and temperature data from a stationary reference station, spatially resolved estimates of sky-view fraction, canopy transmissivity, and longwave enhancement could be obtained. Under favorable meteorological conditions, validation against sky-view fraction data from hemispherical photographs yielded an RMSE of 0.03 (i.e., 3%). Irradiance measurements under heterogeneous canopy cover revealed strong spatial coherence between longwave radiation enhancement, shortwave radiation attenuation, and sky-view fraction on overcast days. Under clear-sky conditions, however, sun flecks caused highly variable shortwave radiation transmissivity patterns. This study demonstrates the potential of handheld radiometer surveys to deliver valuable spatially distributed datasets of collocated canopy structure and subcanopy irradiance which can be used (i) as reference data for alternative approaches to derive canopy structure parameters, (ii) to improve modeling of subcanopy radiation across a wide range of canopy distributions, and (iii) to support respective model upscaling efforts.

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

“…Improved observational systems and resulting data sets constitute the key novelty of our model evaluation approach. Malle et al (2019) and Mazzotti, Malle, et al (2019) already demonstrated the utility of these mobile systems for generating spatially and temporally resolved radiation data to complement earlier studies (Lawler & Link, 2011; Sicart et al, 2004). In the context of this work, additional benefits arise from combining observations of irradiance with the acquisition of air and snow surface temperature data.…”

“…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).…”

“…The portable meteorological stations ( Figure 1) correspond to the stationary setup used by Mazzotti, Malle, et al (2019), with the addition of an ultrasonic wind sensor. Besides providing open-site reference data of all meteorological variables, two additional stations deployed at within-stand locations served to obtain within-canopy wind speed data at 2 m above the surface, which is unfeasible with moving platforms.…”

“…Exact georeferencing of the transects was facilitated by UAV orthoimagery acquired at the sites, with clearly visible trajectories of both the cable car (i.e., the wire) and the gridded plots (i.e., our tracks). Continuous meteorological data from the mobile platforms were converted to point information at fixed locations following Mazzotti, Malle, et al (2019) and Malle et al (2019). As the instruments move along the transects, data acquired within a specific time interval correspond to a transect section, depending on the instruments' velocity.…”

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

“…They include: Leaf area index (LAI): the dimensionless ratio of one‐sided needle leaf area per unit ground area (e.g., Chen et al, ). Note that in some literature vegetation area index (VAI) is used instead, which may also include other vegetation elements such as stems and branches. Sky view fraction (V F ): the visible portion of sky in the hemispherical field of view seen from a specific point, weighted by the sine of elevation angle (e.g., Essery, Pomeroy, et al, ). Canopy cover fraction (CC): the ratio of area covered by the vertical projection of the canopy relative to ground area in a two‐dimensional bounding box (e.g., Mazzotti, Malle, et al, ). Mean canopy height (mCH): the average height of the canopy elements in a two‐dimensional bounding box (e.g., Varhola & Coops, ). …”