Wintertime Tree Surface Temperature Dynamics in Boreal and Sub-Alpine Forests Revealed by Thermal Infrared Imaging

Canopy surface temperature is a critical state variable of land surface models. During winter, it plays a key role in modulating energy fluxes between atmosphere, canopy air space, and sub-canopy snowpack. Understanding these surface temperature dynamics spatially and temporally is becoming increasingly important as recent hyper resolution models are now capable of resolving snow-forest interactions at the scale of individual trees and within concrete canopy structure.

Here, we present a novel dataset and analyze spatio-temporal wintertime canopy surface temperaturedynamics derived from ground-based thermal infrared (ThIR) images. Panoramic ThIR images were captured in forest gaps and dense stands at up to hourly intervals throughout diurnal cycles in boreal and sub-alpine forests. Postprocessing enabled documentation of absolute vertical and azimuthal tree surface temperature distributions within the forest under varying meteorological conditions. Our observations revealed the spatiotemporal dynamics of canopy temperatures offsets relative to ambient air temperatures. Positive offsets mainly followed direct insolation patterns within the 3-dimensional canopy structure in case of clear sky conditions. Insolated stems in forest gaps were observed to be up to 20 degrees above the surrounding canopy, while at the same time shaded stems could be up to 3 degrees colder than the canopy. Moreover, combining ThIR observations with RGB imagery further demonstrated evidence of insolation driven unloading of snow intercepted by the canopy, providing valuable data for further development of hyper resolution forest snow models.