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ARS Home » Pacific West Area » Boise, Idaho » Northwest Watershed Research Center » Research » Publications at this Location » Publication #174581

Title: Variability of the below canopy thermal structure over snow

item Marks, Daniel

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 9/5/2004
Publication Date: 9/5/2004
Citation: Hardy, J.P., Marks, D., Link, T., and Koenig, G. 2004. Variability of the below canopy thermal structure over snow, EOS Transactions of the American Geophysical Union, Vol 85 (47): F448 (CD-ROM abstract)

Interpretive Summary:

Technical Abstract: Due to the complexity of energy exchange in forested environments, existing canopy models have difficulty capturing the impact of canopy elements on thermal signatures. Prior research suggests that below canopy solar and thermal energy and forest snow pack variability is controlled by canopy type and structure. Considerable research has been conducted on the interactions of solar radiation (visible through the near infrared) with canopy elements. This degree of research has not been conducted for the interaction of longwave infrared radiation with canopy elements and the underlying surface. The thermal contribution from the different tree elements varies spatially and temporally due to differential effects of point sources (i.e. the sun) and extended sources (terrain, sky, canopy, and canopy gaps). The objectives of this work are 1. to document the complexity and variability of the thermal signature within a forest canopy, and 2. to make simple calculations to asses the thermal impact of the canopy elements on the energy balance incident on the snow surface. Measurements of the thermal environment beneath forest canopies were made in April 2004 at the Reynolds Creek Experimental Watershed in Idaho as part of a larger effort to characterize the sub-canopy energetics at the snow surface in a sub-watershed complicated by forest cover and variable terrain. We made measurements of the thermal environment using a Flir System ThermaCAM S60 infrared camera (spectral range, 7.5 ' 13'm; thermal sensitivity, 0.08ºC). Our measurements captured the spatial and temporal variability of thermal radiation in both conifer and deciduous stands. The infrared camera obtained images of the trees from all cardinal orientations over a 24-hour period as well as images of forest litter on the snow surface. Surface temperatures of aspen tree trunks ranged from 2 to 25ºC depending on orientation and time of day. Conifer stem temperatures were measured at 20ºC higher than the surrounding air temperature. The temperature of a single Douglas fir cone on the snow surface was measured at 13ºC; while small, thin fir needles on the snow surface were measured at 2ºC. A dead tree limb on the snow surface reached an extreme temperature of 37ºC. Simple calculations show the contribution of these tree elements and forest litter to the energy balance at the snow surface.