HOT TREES AND MELTING SNOW IN THE ROCKIES
Research Cluster: Ecohydrology, Meteorology, and Watersheds
Snowmelt is the major contributor to streamflow generation from the Canadian Rockies and much of the seasonal snowcover lies under the evergreen blanket of sub-alpine and montane coniferous forests that cover valley bottoms and slopes of the region. The warming and melting of this sub-canopy snow is a crucial event in the annual life cycle of the forest, and governs the water supply and climate interaction of the forest. For instance, snowmelt controls the peak and duration of high spring streamflows, initiates soil thawing and transpiration, and governs the decay of surface albedo (Pomeroy et al., 1998). Forest cover moderates snowmelt rates compared to open areas; however, the melt rates in areas of sparse forests is enhanced from that of homogeneous, continuous stands (Pomeroy and Granger, 1997). Forest cover reduction can result in earlier and faster snowmelt and can change the pattern and delivery of water flow through a mountain basin.
Radiation, rather than air temperature, is usually the primary source of energy for snowmelt under forest canopies (Link and Marks, 1999) because turbulent transfer of atmospheric heat is strongly attenuated by the canopy structure (Harding and Pomeroy, 1996). The relative importance of shortwave (visible to near infrared) and longwave (thermal infrared) radiation to snowmelt energy depends on albedo, cloudiness and canopy density, and both forms of radiation are important to snowmelt (Sicart et al., 2004). Whilst shortwave radiation under forest canopies has received much attention from both observations and model development (Ellis and Pomeroy, in press), less attention has been paid to longwave radiation effects on snowmelt. It has generally been assumed that longwave radiation flux to snow can be partitioned between that deriving from a ‘sky view’ component and that deriving from a canopy component which has an effective temperature equal to the local air temperature (Marks et al., 1999).
At Marmot Creek Research Basin in the Kananaskis Valley, arrays of 12 pyrgeometers were placed in a uniform lodgepole pine stand at a level site and a slightly discontinuous lodgepole pine stand on a south facing slope during a snowmelt period. Infrared thermometers (IRTCs) were placed to measure canopy and trunk temperatures from north and south sides at the south and north facing slopes and the level site. A reference site was established in a large clearing near these stands for measuring incoming longwave and shortwave radiation from open sky. Thermal infrared images were recorded with a scanning digital radiometer (IR camera). Skyview was measured by analyzing upward-looking digital photographs taken using a hemispherical lens.
Needle and air temperatures matched each other well at night and on cloudy days (Figure 1). However, trunk temperatures remained warmer than needle and air temperatures at night, with strongest effects (4.5 to 7.0 oC) on clear nights. During the day the trunks became substantially warmer than air with the largest warming on clear days (Figure 2). The trunks and needles were 5-15 oC warmer than air temperature when sunlit.
The longwave radiation exitance (emission and reflection) from various sources to the sub-canopy snow was measured using IRTC’s and above canopy pyrgeometers (sky) and converted to exitance in W m-2 using the Stefan-Boltzmann equation and an emissivity of 0.98. The major difference is that between exitances from the sky and that from canopy elements (Figure 3). This difference between sky and canopy exitances is greatest in clear periods where it is on the order of 100 W m-2. On some cloudy nights the sky and canopy exitances were extremely well matched. Needle and air temperature-based exitances were in excellent agreement at night and on cloudy days but needle exitances exceeded that from air temperature during sunlit periods with the largest differences of 20-34 W m-2 occurring during clear days. Substantive differences were found between trunk and air temperature-derived exitances with the largest differences due to day-time heating of trunks in direct sunlight ranging from 76 to 226 W m-2 and a secondary difference, ranging from 20 to 30 W m-2, due to trunks remaining warm on cold clear nights when air temperature dropped rapidly. As a result, mean differences in exitance between trunk and air temperature-derived values ranged from 10 to 24 W m-2 over the range of conditions — a substantial potential source of snowmelt energy.
The failure of the assumption of canopy and air temperature equality can lead to substantial enhancement of snowmelt rates under forest canopies in sunlit periods compared to cloudy or night-time conditions in which the canopy temperature is more likely to be equal to air temperature. The observed warm canopy effect was caused by sunlit trunks that were substantially warmer than air temperature and sunlit needles that were sometimes warmer than air temperature. The largest warm canopy effects occurred for situations with strong sunlight to discontinuous canopies where small gaps permitted shortwave radiation to be extinguished well down within the canopy. Smaller effects occurred at night and during cloudy periods, and for homogeneous canopies. Changes in forest structure due to pine beetle infestation, fires, or harvesting may cause enhancement of snowmelt rates as forest stands become more open and remaining trunks become more exposed to direct sunlight.
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