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Abstract

Vegetation surface temperature is a critical state variable of forest snow and 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 model developments have become more centred around resolving snow-vegetation interactions at the sub-tree scale. Here, we present an analysis of wintertime tree surface temperatures which were derived from ground-based thermal infrared (ThIR) images. Images were acquired at up to hourly intervals over full diurnal cycles in boreal and sub-alpine forests to document vertical and azimuth surface temperature distributions within the forest under varying meteorological conditions.Our observations show that tree temperatures could be both warmer and cooler than air temperature providing detailed insight into the spatial distribution, timing, and magnitude of these variations. Direct insolation conditions quickly led to differential patterns of increased surface temperatures. Sun-exposed stems warmed up most and remained the warmest forest components throughout most of the night. However, in other places we observed nighttime forest surface temperatures well below air temperatures, particularly during clear-sky conditions and in the vicinity of snow (intercepted or on the ground). Under overcast conditions, forest surface temperatures were rather uniform and approximated air temperatures. These observations will be valuable for informing and verifying forest snow model developments targeted at multi-layer canopy structure representations.