Abstract:A series of process-based algorithms has been developed to describe the accumulation, unloading and sublimation of intercepted snow in forest canopies. These algorithms are unique in that they scale up the physics of interception and sublimation from small scales, where they are well understood, to forest stand-scale calculations of intercepted snow sublimation. Evaluation of results from the set of algorithms against measured interception and sublimation, in a southern boreal forest jack pine stand during late winter, found that the coupled model provides reasonable approximations of both interception and sublimation losses on half-hourly, daily and event bases. Cumulative errors in the estimate of intercepted snow load over 23 days of test were 0 . 06 mm SWE, with a standard deviation of 0 . 46 mm SWE. Sublimation losses during the evaluation were high, approximately two-thirds of snowfall within this period. Seasonal intercepted snow sublimation as a portion of annual snowfall at the model test site was lower than sublimation during the tests, ranging from 13% for a mixed spruce±aspen, 31% for the mature pine and 40% for a mature spruce stand. The results indicate that sublimation can be a signi®cant abstraction of water from mature evergreen stands in northern forests and that the losses can be calculated by application of process-based algorithms. #
Improved representations of snow interception by coniferous forest canopies and sublimation of intercepted snow are implemented in a land-surface model. Driven with meteorological observations from forested sites in Canada, the USA and Sweden, the modified model is found to give reduced sublimation, better simulations of snow loads on and below canopies, and improved predictions of snowmelt runoff. When coupled to an atmospheric model in a GCM, however, drying and warming of the air because of the reduced sublimation provides a feedback which limits the impact of the new canopy snow model on the predicted sublimation. There is little impact on the average annual snowmelt runoff in the GCM, but runoff is delayed and peak runoff increased by the introduction of the canopy snow model.
Abstract:In the calculation of the melting of a patchy snow cover, the energy advected from the adjacent bare soil to the snow surface is an important consideration. The quantity or rate of energy advected depends on the fetches and sizes of snow and bare ground patches. Any successful method to estimate advection will necessarily require the incorporation of relationships describing the same. Complex boundary-layer methods require detailed spatial knowledge of the patch sizes, wind direction and fetch distances, and are computationally intensive. A physically based approach that can be spatially applied using distributions of snow patch geometry is required. This paper presents a new approach, in which boundary-layer integration is used to provide a means of calculating the amount of energy removed by the snow patch surface as warmer air moves over it. The method is reduced to a simple parametric form, and the relationships describing the coefficients required for its application are developed.The applicability of this new approach is discussed in light of the fractal nature of snow patches, the relationship between the individual patch length and area, and the distribution of patch sizes as they develop and disappear on the landscape.
Physically based equations describing snow interception and sublimation processes were applied to canopyintercepted snow using a fractal scaling technique to provide a snow-covered forest boundary condition for a onedimensional land surface scheme. Modi®cation of the land surface scheme's calculation of turbulent transfer and within-canopy ambient humidity was required to accommodate this nested control volume approach. Tests in late winter in a southern boreal forest mature jack pine stand against measured sublimation found that the coupled model provides good approximations of sublimation losses on half-hourly and event bases. Daily sublimation averaged 0Á5 kg m À2 daily, with minimum and maximum daily losses of 0Á16 and 0Á72 kg m À2 . Cumulative errors in estimating canopy temperature, humidity and intercepted snow load over 7 days of simulation were À 0Á7 K, À 4Á15% of the average observed vapour pressure, and 0Á103 kg m À2 , respectively. At a nearby regenerating jack pine site, measured peak latent heat ranged from À 14Á6 W m À2 to -40Á9 W m À2 . Testing of the model at this site yielded reasonable estimates of latent and sensible heat ¯uxes during an overnight event, but did not estimate latent heat ¯ux as well during events involving larger snow loads and incoming solar radiation, possibly as a result of errors introduced by solving for within-canopy humidity and neglect of subcanopy snow energetics. Further work to improve heat storage terms, and the inclusion of subcanopy snow energetics could help improve the coupled model performance.
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