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John W. Pomeroy
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Warren Helgason and John Pomeroy

Abstract

Application of the energy balance approach to estimate snowmelt inherently presumes that the external energy fluxes can be measured or modeled with sufficient accuracy to reliably estimate the internal energy changes and melt rate. However, owing to difficulties in directly measuring the internal energy content of the snow during melt periods, the ability to close the energy balance is rarely quantified. To address this, all of the external energy balance terms (sensible and latent heat fluxes, shortwave and longwave radiation fluxes, and the ground heat flux) were directly measured and compared to changes of the energy content within an extensive, homogeneous, snowpack of a level field near Saskatoon, Saskatchewan, Canada. The snow was observed to lose significant amounts of energy because of a persistent longwave radiation imbalance caused by low incoming fluxes during cold, clear-sky periods, while solar heating of the snow surface caused an increase in the outgoing fluxes. The sum of the measured turbulent heat fluxes, ground heat flux, and solar radiation fluxes were insufficient to offset these losses, however the snowpack temperatures were not observed to cool. It was concluded that an unmeasured exchange of sensible heat was occurring from the atmosphere to the snowpack. The exchange mechanism for this is not known but would appear to be consistent with the concept of a windless exchange as employed to close the energy balance in various snow models. The results suggest that caution should be exercised when using the energy balance method to determine changes in internal energy in cold snowpacks.

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Richard Essery and John Pomeroy

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A finescale model of blowing snow is used to simulate the characteristics of snow cover in a low-Arctic catchment with moderate topography and partial shrub cover. The influence of changing shrub characteristics is investigated by performing a sequence of simulations with varying shrub heights and coverage. Increasing shrub height gives an increase in snow depth within the shrub-covered areas, up to a limit determined by the supply of falling and blowing snow, but increasing shrub coverage gives a decrease in snow depths within shrubs as the supply of blowing snow imported from open areas is reduced. A simulation of snow redistribution over the existing topography without any shrub cover gives much greater accumulations of snow on slopes in the lee of the prevailing wind than on windward slopes; in contrast, shrubs are able to trap snow on both lee and windward slopes. A spatially aggregated, or tiled, model is developed in which snow is relocated by wind transport from sparsely vegetated tiles to more densely vegetated tiles. The vegetation distribution is not specified, but the simulation is parameterized using average fetch lengths along the major transport axis. The aggregated model is found to be capable of matching the average snow accumulation in shrub and open areas predicted by the distributed model reasonably well but with much less computational cost.

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Warren Helgason and John W. Pomeroy

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Within mountainous regions, estimating the exchange of sensible heat and water vapor between the surface and the atmosphere is an important but inexact endeavor. Measurements of the turbulence characteristics of the near-surface boundary layer in complex mountain terrain are relatively scarce, leading to considerable uncertainty in the application of flux-gradient techniques for estimating the surface turbulent heat and mass fluxes. An investigation of the near-surface boundary layer within a 7-ha snow-covered forest clearing was conducted in the Kananaskis River valley, located within the Canadian Rocky Mountains. The homogeneous measurement site was characterized as being relatively calm and sheltered; the wind exhibited considerable unsteadiness, however. Frequent wind gusts were observed to transport turbulent energy into the clearing, affecting the rate of energy transfer at the snow surface. The resulting boundary layer within the clearing exhibited perturbations introduced by the surrounding topography and land surface discontinuities. The measured momentum flux did not scale with the local aerodynamic roughness and mean wind speed profile, but rather was reflective of the larger-scale topographical disturbances. The intermittent nature of the flux-generating processes was evident in the turbulence spectra and cospectra where the peak energy was shifted to lower frequencies as compared with those observed in more homogeneous flat terrain. The contribution of intermittent events was studied using quadrant analysis, which revealed that 50% of the sensible and latent heat fluxes was contributed from motions that occupied less than 6% of the time. These results highlight the need for caution while estimating the turbulent heat and mass fluxes in mountain regions.

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Long Li and John W. Pomeroy

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The threshold wind speed for snow transport is related to properties of the surface snowpack: snow particle bonding, cohesion, and kinetic friction. These properties are controlled by meteorological factors. A method is proposed that relates the threshold wind speed for the initiation of snow transport to standard surface meteorological observations. A complete dataset on the hourly threshold condition for snow transport as determined from visual observation was developed for 16 stations on the prairies of western Canada over six winters. The threshold wind speeds for wet snow transport are significantly different from those for dry snow transport. The majority of recorded threshold 10-m wind speeds ranged from 7 to 14 m s−1 with an average of 9.9 m s−1 for wet snow transport, and from 4 to 11 m s−1 with an average of 7.7 m s−1 for dry snow transport. The observations display a nonlinear but generally positive correlation between threshold wind speed and air temperature. An empirical model between threshold wind speed and air temperature was developed for dry snow conditions. The model, on average, provides a good estimate of the threshold wind speed.

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Sebastian A. Krogh and John W. Pomeroy

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The rapidly warming Arctic is experiencing permafrost degradation and shrub expansion. Future climate projections show a clear increase in mean annual temperature and increasing precipitation in the Arctic; however, the impact of these changes on hydrological cycling in Arctic headwater basins is poorly understood. This study investigates the impact of climate change, as represented by simulations using a high-resolution atmospheric model under a pseudo-global-warming configuration, and projected changes in vegetation, using a spatially distributed and physically based Arctic hydrological model, on a small headwater basin at the tundra–taiga transition in northwestern Canada. Climate projections under the RCP8.5 emission scenario show a 6.1°C warming, a 38% increase in annual precipitation, and a 19 W m−2 increase in all-wave annual irradiance over the twenty-first century. Hydrological modeling results suggest a shift in hydrological processes with maximum peak snow accumulation increasing by 70%, snow-cover duration shortening by 26 days, active layer deepening by 0.25 m, evapotranspiration increasing by 18%, and sublimation decreasing by 9%. This results in an intensification of the hydrological regime by doubling discharge volume, a 130% increase in spring runoff, and earlier and larger peak streamflow. Most hydrological changes were found to be driven by climate change; however, increasing vegetation cover and density reduced blowing snow redistribution and sublimation, and increased evaporation from intercepted rainfall. This study provides the first detailed investigation of projected changes in climate and vegetation on the hydrology of an Arctic headwater basin, and so it is expected to help inform larger-scale climate impact studies in the Arctic.

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Sebastian A. Krogh, John W. Pomeroy, and James McPhee

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A physically based hydrological model for the upper Baker River basin (UBRB) in Patagonia was developed using the modular Cold Regions Hydrological Model (CRHM) in order to better understand the processes that drive the hydrological response of one of the largest rivers in this region. The model includes a full suite of blowing snow, intercepted snow, and energy balance snowmelt modules that can be used to describe the hydrology of this cold region. Within this watershed, snowfall, wind speed, and radiation are not measured; there are no high-elevation weather stations; and existing weather stations are sparsely distributed. The impact of atmospheric data from ECMWF interim reanalysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR) on improving model performance by enhancing the representation of forcing variables was evaluated. CRHM parameters were assigned for local physiographic and vegetation characteristics based on satellite land cover classification, a digital elevation model, and parameter transfer from cold region environments in western Canada. It was found that observed precipitation has almost no predictive power [Nash–Sutcliffe coefficient (NS) < 0.3] when used to force the hydrologic model, whereas model performance using any of the reanalysis products—after bias correction—was acceptable with very little calibration (NS > 0.7). The modeled water balance shows that snowfall amounts to about 28% of the total precipitation and that 26% of total river flow stems from snowmelt. Evapotranspiration losses account for 7.2% of total precipitation, whereas sublimation and canopy interception losses represent about 1%. The soil component is the dominant modulator of runoff, with infiltration contributing as much as 73.7% to total basin outflow.

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Phillip Harder, Warren D. Helgason, and John W. Pomeroy

Abstract

On the Canadian Prairies, agricultural practices result in millions of hectares of standing crop stubble that gradually emerges during snowmelt. The importance of stubble in trapping wind-blown snow and retaining winter snowfall has been well demonstrated. However, stubble is not explicitly accounted for in hydrological or energy balance snowmelt models. This paper relates measurable stubble parameters (height, width, areal density, and albedo) to the snowpack energy balance and snowmelt with the new, physically based Stubble–Snow–Atmosphere Model (SSAM). Novel process representations of SSAM quantify the attenuation of shortwave radiation by exposed stubble, the sky and vegetation view factors needed to solve longwave radiation terms, and a resistance scheme for stubble–snow–atmosphere fluxes to solve for surface temperatures and turbulent fluxes. SSAM results were compared to observations of radiometric snow-surface temperature, stubble temperature, snow-surface solar irradiance, areal-average turbulent fluxes, and snow water equivalent from two intensive field campaigns during snowmelt in 2015 and 2016 over wheat and canola stubble in Saskatchewan, Canada. Uncalibrated SSAM simulations compared well with these observations, providing confidence in the model structure and parameterization. A sensitivity analysis conducted using SSAM revealed compensatory relationships in energy balance terms that result in a small increase in net snowpack energy as stubble exposure increases.

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Kabir Rasouli, John W. Pomeroy, and Paul H. Whitfield

Abstract

How mountain hydrology at different elevations will respond to climate change is a challenging question of great importance to assessing changing water resources. Here, three North American Cordilleran snow-dominated basins—Wolf Creek, Yukon; Marmot Creek, Alberta; and Reynolds Mountain East, Idaho—each with good meteorological and hydrological records, were modeled using the physically based, spatially distributed Cold Regions Hydrological Model. Model performance was verified using field observations and found adequate for diagnostic analysis. To diagnose the effects of future climate, the monthly temperature and precipitation changes projected for the future by 11 regional climate models for the mid-twenty-first century were added to the observed meteorological time series. The modeled future was warmer and wetter, increasing the rainfall fraction of precipitation and shifting all three basins toward rainfall–runoff hydrology. This shift was largest at lower elevations and in the relatively warmer Reynolds Mountain East. In the warmer future, there was decreased blowing snow transport, snow interception and sublimation, peak snow accumulation, and melt rates, and increased evapotranspiration and the duration of the snow-free season. Annual runoff in these basins did not change despite precipitation increases, warming, and an increased prominence of rainfall over snowfall. Reduced snow sublimation offset reduced snowfall amounts, and increased evapotranspiration offset increased rainfall amounts. The hydrological uncertainty due to variation among climate models was greater than the predicted hydrological changes. While the results of this study can be used to assess the vulnerability and resiliency of water resources that are dependent on mountain snow, stakeholders and water managers must make decisions under considerable uncertainty, which this paper illustrates.

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Siqiong Luo, Jingyuan Wang, John W. Pomeroy, and Shihua Lyu

Abstract

The freeze–thaw changes of seasonally frozen ground (SFG) are an important indicator of climate change. Based on observed daily freeze depth of SFG from meteorological stations on the Tibetan Plateau (TP) from 1960 to 2014, the spatial–temporal characteristics and trends in SFG were analyzed, and the relationships between them and climatic and geographical factors were explored. Freeze–thaw changes of SFG on a regional scale were assessed by multiple regression functions. Results showed multiyear mean maximum freeze depth, freeze–thaw duration, freeze start date, and thaw end date that demonstrate obvious distribution characteristics of climatic zones. A decreasing trend in maximum freeze depth and freeze–thaw duration occurred on the TP from 1960 to 2014. The freeze start date has been later, and the thaw end date has been significantly earlier. The freeze–thaw changes of SFG significantly affected by soil hydrothermal conditions on the TP could be assessed by elevation and latitude or by air temperature and precipitation, due to their high correlations. The regional average of maximum freeze depth and freeze–thaw duration caused by climatic and geographical factors were larger than those averaged using meteorological station data because most stations are located at lower altitudes. Maximum freeze depth and freeze–thaw duration have decreased sharply since 2000 on the entire TP. Warming and wetting conditions of the soil resulted in a significant decrease in maximum freeze depth and freeze–thaw duration in the most area of the TP, while drying soil results in a slight increase of them in the southeast of the TP.

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