Abstract:After a programme of integrated field and modelling research, hydrological processes of considerable uncertainty such as snow redistribution by wind, snow interception, sublimation, snowmelt, infiltration into frozen soils, hillslope water movement over permafrost, actual evaporation, and radiation exchange to complex surfaces have been described using physically based algorithms. The cold regions hydrological model (CRHM) platform, a flexible object-oriented modelling system was devised to incorporate these algorithms and others and to connect them for purposes of simulating the cold regions hydrological cycle over small to medium sized basins. Landscape elements in CRHM can be linked episodically in process-specific cascades via blowing snow transport, overland flow, organic layer subsurface flow, mineral interflow, groundwater flow, and streamflow. CRHM has a simple user interface but no provision for calibration; parameters and model structure are selected based on the understanding of the hydrological system; as such the model can be used both for prediction and for diagnosis of the adequacy of hydrological understanding. The model is described and demonstrated in basins from the semi-arid prairie to boreal forest, mountain and muskeg regions of Canada where traditional hydrological models have great difficulty in describing hydrological phenomena. Some success is shown in simulating various elements of the hydrological cycle without calibration; this is encouraging for predicting hydrology in ungauged basins.

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Saltation of snow, the transport of snow in periodic contact with and directly above the snow surface, is governed by the atmospheric shear forces applied to the erodible snow surface, the nonerodible surface, and the moving snow particles. Empirical data measured over a snow-covered plain suggest functions for parameters important to the apportionment of atmospheric shear forces; the aerodynamic roughness height during saltation, the mean horizontal velocity of saltating particles, and the efficiency of the saltation process. The resulting mass transport expression shows an approximately linear increase in snow saltation transport rate with friction velocity, in agreement with the measurements presented. The expression is sensitive to the cohesion of the snow surface, as indexed by the threshold wind speed, that wind speed at which transport ceases; for wind speeds well above the threshold condition, higher threshold wind speeds correspond to higher transport rates. An adaptation of the expression allows calculation of the mass concentration of saltating snow from measured data and the transport rate of saltating snow from the mean wind speed at 10 m height. Application of the transport rate expression using measured wind speeds, directions, and weather observations demonstrates that the directional component of annual saltating snow transport does not always correspond with wind direction frequency. INTRODUCTION Redistribution of surface snow by wind transport signifi-cantly affects the winter microclimate, snow cover accumulation, and hence snowmelt runoff patterns in cold, windswept regions. Several authors suggest that a large component of the mass flux of wind-transported snow travels in saltation, which is the horizontal movement of particles in curved trajectories, which are near to and periodically impact the surface [Dyunin, 1967; Male, 1980; Schmidt, 1986]. Saltating snow particles are ice particles changing in form from both sublimation and abrasion. $chmidt's [1981] observations show particles approximating spheroids with mean diameters of 200 /am, and a range of diameters from approximately 10/xm to several hundred micrometers. They are assumed to have the density of ice. Saltating snow differs significantly from saltation of other materials in that the "bed" is not a layer of rounded particles but is a cohesive matrix of bonded crystals which are metamorphosed by the impact of saltating particles and the vapor transfer during ventilation. Saltating snow particles are derived from shattered surface snow crystals, and Schmidt [1980] has shown that the intercrystal bond strength, "cohesion," is a much more important parameter than crystal size in calculating the force required to eject a crystal. Particle impact is expected to result in rebound, with a shattering and reestablishment of bonds in the matrix and a "splashing" of crystals from the matrix. Schmidt, 1986] link greater cohesion to higher threshold wind speeds for transport. The threshold wind speed indexes the atmospheric shear stress not av...

Pomeroy, J. W., Gray, D. M., Hedstrom, N. R., Janowicz, J. R. (2002). Prediction of seasonal snow accumulation in cold climate forests. Hydrological Processes, 16(18), 3543-3558. 59th Eastern Snow Conference (ESC), Stowe, Vermont, 5-7 June 2002.Accumulation of snow under forest canopies is known to decline with increasing canopy density and leaf area because of snow interception and sublimation in the canopy. Seasonal snow accumulation measurements, collected over a decade from various forest stands in western Canada, were used to test and develop methods to relate forest snow accumulation to stand properties and observations of either small-clearing seasonal snow accumulation or seasonal snowfall. At sub-stand scales, the variability of seasonal snow accumulation was not well related to stand leaf area, seasonal interception or small-clearing seasonal snow accumulation. At the stand scale, physically based snow interception equations predicted seasonal snow accumulation from the stand leaf area and the seasonal snow accumulation or snowfall in adjacent clearings. A simple parametric form of these equations showed the sensitivity of seasonal snow accumulation to leaf area at the forest stand scale and suggested a relationship to extrapolate snow accumulation or snowfall measurements from clearings to forests. These relationships, developed from Canadian boreal forest observations, are consistent with Kuz'min's (1960. Formirovanie Snezhnogo Pokrova i Metody Opredeleniya Snegozapasov. Gidrometeoizdat: Leningrad) relationship between accumulation and canopy density derived from Russian observations, suggesting a good degree of transferability. Copyright 2002 Crown in the right of Canada. Published by John Wiley Sons, LtdNon peer reviewe

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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. #

Abstract:This paper discusses the development and testing of snow algorithms with speci®c reference to their use and application in land surface models. New algorithms, developed by the authors, for estimating snow interception in forest canopies, blowing snow transport and sublimation, snow cover depletion and open environment snowmelt are compared with ®eld measurements. Existing algorithms are discussed and compared with ®eld observations. Recommendations are made with respect to: (a) density of new and aged snow in open and forest environments; (b) interception of snow by evergreen canopies; (c) redistribution and sublimation of snow water equivalent by blowing snow; (d) depletion in snow-covered area during snowmelt; (e) albedo decay during snowmelt; ( f) turbulent transfer during snowmelt; and (g) soil heat¯ux during meltwater in®ltration into frozen soils.Preliminary evidence is presented, suggesting that one relatively advanced land surface model, CLASS, signi®cantly underestimates the timing of snowmelt and snowmelt rates in open environments despite overestimating radiation and turbulent contributions to melt. The cause(s) may be due to overestimation of ground heat loss and other factors. It is recommended that further studies of snow energetics and soil heat transfer in frozen soils be undertaken to provide improvements for land surface models such as CLASS, with particular attention paid to establishing the reliability of the models in invoking closure of the energy equation.

Circular dichroism (CD) spectroscopy is widely used to characterize the structures of DNA G-quadruplexes. CD bands at 200-300 nm have been empirically related to G-quadruplexes having parallel or antiparallel sugar-phosphate backbones. We propose that a more fundamental interpretation of the origin of the CD bands is in the stacking interactions of neighboring G-quartets, which can have the same or opposing polarities of hydrogen bond acceptors and donors. From an empirical summation of CD spectra of the d(G)5 G-quadruplex and of the thrombin binding aptamer that have neighboring G-quartets with the same and opposite polarities, respectively, the spectra of aptamers selected by the Ff gene 5 protein (g5p) appear to arise from a combination of the two types of polarities of neighboring G-quartets. The aptamer CD spectra resemble the spectrum of d(G3T4G3), in which two adjacent quartets have the same and two have opposite polarities. Quantum-chemical spectral calculations were performed using a matrix method, based on guanine chromophores oriented as in d(G3T4G3). The calculations show that the two types of G-quartet stacks have CD spectra with features resembling experimental spectra of the corresponding types of G-quadruplexes.