Stephen J. Déry

Characteristics and Trends of River Discharge into Hudson, James, and Ungava Bays, 1964–2000

Abstract The characteristics and trends of observed river discharge into the Hudson, James, and Ungava Bays (HJUBs) for the period 1964–2000 are investigated. Forty-two rivers with outlets into these bays contribute on average 714 km3 yr−1 [= 0.023 Sv (1 Sv ≡ 106 m3 s−1)] of freshwater to high-latitude oceans. For the system as a whole, discharge attains an annual peak of 4.2 km3 day−1 on average in mid-June, whereas the minimum of 0.68 km3 day−1 occurs on average during the last week of March. The Nelson River contributes as much as 34% of the daily discharge for the entire system during winter but diminishes in relative importance during spring and summer. Runoff rates per contributing area are highest (lowest) on the eastern (western) shores of the Hudson and James Bays. Linear trend analyses reveal decreasing discharge over the 37-yr period in 36 out of the 42 rivers. By 2000, the total annual freshwater discharge into HJUBs diminished by 96 km3 (−13%) from its value in 1964, equivalent to a reduction...

Modeling drifting snow in Antarctica with a regional climate model: 1. Methods and model evaluation

To simulate the impact of drifting snow on the lower atmosphere, surface characteristics and surface mass balance (SMB) of the Antarctic ice sheet regional atmospheric climate model (RACMO2.1/ANT) with horizontal resolution of 27 km is coupled to a drifting snow routine and forced by ERA-Interim fields at its lateral boundaries (1989–2009). This paper evaluates the near-surface and drifting snow climate of RACMO2.1/ANT. Modeled near-surface wind speed (squared correlation coefficient R2 = 0.64) and temperature (R2 = 0.93) agree well with observations. Wind speed is underestimated in topographically complex areas, where observed wind speeds are locally very high (>20 m s!1). Temperature is underestimated in winter in coastal areas due to an underestimation of downward longwave radiation. Near-surface temperature and wind speed are not significantly affected by the inclusion of drifting snow in the model. In contrast, relative humidity with respect to ice increases in regions with strong drifting snow and becomes more consistent with the observations. Drifting snow frequency is the only observable parameter to directly validate drifting snow results; therefore, we derived an empirical relation for fresh snow density, as a function of wind speed and temperature, which determines the threshold wind speed for drifting snow. Modeled drifting snow frequencies agree well with in situ measurements and novel estimates from remote sensing. Finally, we show that including drifting snow is essential to obtaining a realistic extent and spatial distribution of ablation (SMB < 0) areas.

Simulation of blowing snow in the Canadian Arctic using a double-moment model

We describe in this paper the development of a double-moment modelof blowing snow and its application to the Canadian Arctic. Wefirst outline the formulation of the numerical model, whichsolves a prognostic equation for both the blowing snow mixingratio and total particle numbers, both moments of particles thatare gamma-distributed. Under idealized simulations, the modelyields realistic evolutions of the blowing snow particledistributions, transport and sublimation rates as well as the thermodynamic fields at low computational costs. A parametrizationof the blowing snow sublimation rate is subsequently derived. The model and parametrization are then applied to a Canadian Arctictundra site prone to frequent blowing snow events. Over a period of210 days during the winter of 1996/1997, the near-surfacerelative humidity consistently approaches saturationwith respect to ice. These conditions limit snowpack erosion byblowing snow sublimation to ≈3 mm snow water equivalent (swe)with surface sublimation removing an additional 7 mm swe.We find that our results are highly sensitiveto the proper assimilation of the humidity measurements and the evolving thermodynamic fields in the atmospheric boundary layer during blowingsnow. These factors may explain the lower values of blowing snow sublimationreported in this paper than previously published for the region.

A BULK BLOWING SNOW MODEL

We present in this paper a simple and computationally efficient numerical model that depicts a column of sublimating, blowing snow. This bulk model predicts the mixing ratio of suspended snow by solving an equation that considers the diffusion, settling and sublimation of blowing snow in a time-dependent mode. The bulk model results compare very well with those of a previous spectral version of the model, while increasing its computational efficiency by a factor of about one hundred. This will allow the use of the model to estimate the effects of blowing snow upon the atmospheric boundary layer and to the mass balance of such regions as the Mackenzie River Basin of Canada.

An intercomparison among four models of blowing snow

Four one-dimensional, time-dependent blowing snow models areintercompared. These include three spectral models, PIEKTUK-T,WINDBLAST, SNOWSTORM, and the bulk version of PIEKTUK-T,PIEKTUK-B. Although the four models are based on common physicalconcepts, they have been developed by different research groups. Thestructure of the models, numerical methods, meteorological field treatmentand the parameterization schemes may be different. Under an agreed standardcondition, the four models generally give similar results for the thermodynamic effects of blowing snow sublimation on the atmospheric boundary layer, including an increase of relative humidity and a decrease of the ambient temperature due to blowing snow sublimation. Relative humidity predicted by SNOWSTORM is lower than the predictions of the other models, which leads to a larger sublimation rate in SNOWSTORM. All four models demonstrate that sublimation rates in a column of blowing snow have a single maximum in time, illustrating self-limitation of the sublimation process of blowing snow. However, estimation of the eddy diffusioncoefficient for momentum (Km), and thereby the diffusion coefficients for moisture (Kw) and for heat (Kh), has a significant influence on the process. Sensitivitytests with PIEKTUK-T show that the sublimation rate can be approximately constant with time after an initial phase, if Km is a linear function with height. In order to match the model results with blowing snow observations, some parameters in the standard run, such as settling velocity of blowing snow particles in these models, may need to be changed to more practical values.

A climatology of adverse winter-type weather events

Using the European Centre for Medium-Range Weather Forecasts Re-Analysis gridded data, a global climatology of blowing snow, blizzard, and high-windchill events is conducted for the period 1979–1993. The results show that these phenomena occur primarily over flat, open surfaces with long seasonal or perennial snow covers such as the Greenland and Antarctic ice fields as well as the Arctic tundra. On a regional scale, emphasis is given to the Mackenzie River Basin (MRB) of Canada, where fewer events take place within the boreal forest as opposed to the Arctic tundra. Interannual and monthly variabilities in the number of events are also evident and are due primarily to 10-m wind speed anomalies at high latitudes for blowing snow and blizzard events, while high-windchill events are more sensitive to air temperatures near the surface. We also find that high-windchill episodes are the more frequent events, since they occur at 9.3% of all possible grid points and times on a yearly basis, while blowing snow at 6.5% and blizzards at 1.4% are less common events. Compositing of principal meteorological fields show that anticyclones and lee cyclones are prominent features associated with blowing snow events in some sections of the MRB.

A century of hydrological variability and trends in the Fraser River Basin

This study examines the 1911‐2010 variability and trends in annual streamflow at 139 sites across the Fraser River Basin (FRB) of British Columbia (BC), Canada. The Fraser River is the largest Canadian waterway flowing to the Pacific Ocean and is one of the world’s greatest salmon rivers. Our analyses reveal high runoff rates and low interannual variability in alpine and coastal rivers, and low runoff rates and high interannual variability in most streams in BC’s interior. The interannual variability in streamflow is also low in rivers such as the Adams, Chilko, Quesnel and Stuart where the principal salmon runs of the Fraser River occur. A trend analysis shows a spatially coherent signal with increasing interannual variability in streamflow across the FRB in recent decades, most notably in spring and summer. The upward trend in the coefficient of variation in annual runoff coincides with a period of near-normal annual runoff for the Fraser River at Hope. The interannual variability in streamflow is greater in regulated rather than natural systems; however, it is unclear whether it is predominantly flow regulation that leads to these observed differences. Environmental changes such as rising air temperatures, more frequent polarity changes in large-scale climate teleconnections such as El

Modeling Snow-Cover Heterogeneity over Complex Arctic Terrain for Regional and Global Climate Models*

The small-scale (10 to 100 m) and local-scale (100 m to 10 km) effects of topography (elevation, slope, and aspect) and snow redistribution by wind on the evolution of the snowmelt are investigated. The chosen study area is the 142 km2 Upper Kuparuk River basin located on the North Slope of Alaska. Two land surface models (LSMs) designed for regional and global climate studies apply different techniques to resolve these additional processes and features and their effects on snowmelt. One model uses a distributed approach to simulate explicitly the effects of topography on snowmelt at a 131-m resolution across the entire Upper Kuparuk watershed. By contrast, the other LSM employs a simple parameterization to implicitly resolve the effects of wind-blown snow on the hydrology of the Upper Kuparuk basin. In both cases, the incorporation of these local- and small-scale features within the LSMs leads to significant heterogeneity in the 1997 end-of-winter spatial distribution of snow cover in the Upper Kuparuk watershed. It is shown that the consideration of subgrid-scale snow-cover heterogeneity over complex Arctic terrain provides a better representation of the end-of-winter snow water equivalent, an improved simulation of the timing and amount of water discharge of the Upper Kuparuk River, and an alteration of other surface energy and water budget components.

SOME ASPECTS OF THE INTERACTION OF BLOWING SNOW WITH THE ATMOSPHERIC BOUNDARY LAYER

Several possible effects of blowing snow on the atmospheric boundary layer are investigated, mostly within the general framework of the Prairie Blowing Snow Model (PBSM). The processes of snow saltation and suspension are first described. Variations to the drift density profile are tested and the effects of stratification and density variation calculations are evaluated. Despite high density gradients of blowing snow, stratification effects on turbulence and the velocity profiles can generally be neglected. However, with saltating or suspended snow in a constant shear stress layer, part of the shear stress is carried by the particles. A highly simplified, single-phase approach, based on the density variation of the air-snow mixture coupled to a simple turbulent stress-strain relationship, is used to illustrate this. Sublimation rates in a column of blowing snow are calculated using the PBSM and results are compared with those obtained with a modified formulation which incorporates a spectrum of sublimating particles of varying sizes at each height in a steady-state surface boundary layer and different specifications of the ventilation velocity.

Modeling the Effects of Wind Redistribution on the Snow Mass Budget of Polar Sea Ice

Abstract A two-dimensional numerical model of blowing snow specifically designed for sea ice environments is presented. This new model is used to quantify the snow mass lost because of blowing snow into leads, blowing snow sublimation, and the effects of snow redistribution in the presence of surface irregularities (e.g., pressure ridges and snowdrifts) and on the conductive heat flux through the ice. Results show that the percentage of blowing snow lost into open waters (i.e., the lead trap efficiency) ranges between 60% and 100%. The lead trap efficiency increases with fetch over open waters, decreases as the upwind fetch over sea ice expands, and diminishes as wind speeds and friction velocities are enhanced. Its dependence on air temperature and relative humidity, however, is relatively small. Results from the time evolution of a snowdrift show that considerable snow cover heterogeneity arises because of interactions between winds and the surface; however, the corresponding increase in the conductive ...