Andrew Clifton

Persistence in intra‐annual snow depth distribution: 1. Measurements and topographic control

[1]xa0Terrestrial and airborne laser scanning (TLS and ALS) techniques have only recently developed to the point where they allow wide-area measurements of snow distribution in varying terrain. In this paper we present multiple TLS measurements showing the snow depth development for a series of precipitation events. We observe that the pattern of maximum accumulation is similar for the two years presented here (correlation up to r = 0.97). Storms arriving from the northwest show persistent snow depth distributions and contribute most to the final accumulation pattern. Snow depth patterns of maximum accumulation for the two years are more similar than the distribution created by any two pairs of individual storms. Based on the strong link between accumulation patterns and terrain, we investigated the ability of a model based on terrain and wind direction to predict accumulation patterns. This approach of Winstral et al. (2002), which describes wind exposure and shelter, was able to predict the general accumulation pattern over scales of slopes but failed to match observed variance. Furthermore, a high sensitivity to the local wind direction was demonstrated. We suggest that Winstral et al.s model could form a useful tool for application from hydrology and avalanche risk assessment to glaciology.

Snow saltation threshold measurements in a drifting-snow wind tunnel

Wind tunnel measurements of snowdrift in a turbulent, logarithmic velocity boundary layer have been made in Davos, Switzerland, using natural snow. Regression analysis gives the drift threshold friction velocity (ut), assuming an exponential drift profile and a simple drift to friction velocity relationship. Measurements over 15 snow covers show that ut is influenced more by snow density and particle size than by ambient temperature and humidity, and varies from 0.27 to 0.69 m s -1 . Schmidts threshold algorithm and a modified version used in SNOWPACK (a snow-cover model) agree well with observations if small bond sizes are assumed. Using particle hydraulic diameters, obtained from image processing, Bagnolds threshold parameter is 0.18. Roughness lengths (z0) vary between snow covers but are constant until the start of drift. Threshold roughness lengths are proportional to u 2 t. The influence of macroscopic objects on the roughness length is shown by the lower values measured over the smooth and flat snow surface of the wind tunnel (0.04 � z0 � 0.13 mm), compared to field measurements. Mean drifting-snow grain sizes for mainly new and partly decomposed snow are 100-175 mm, and independent of surface particle size. This paper describes the results of a series of experiments investigating the form of the wind velocity boundary layer over a snow surface, with and without drift. This information is essential to help calculate the energy balance at the snow surface and also to assess mass movement of snow by wind. The process of drift is important both over relatively flat terrain, such as Antarctic regions, where it contributes to mass balance, and also in mountainous terrain, where accumulation on steep slopes can contribute to the danger of avalanches. The velocity boundary layer over a surface is often de- scribed by a log-law, where the wind speed at a particular height is a function of the surface friction velocity, u� , and aerodynamic roughness length, z0 (Stull, 1988). The param- eters uand z0 are then used to model the fluxes of heat and water vapour from the surface, which are important, not only for snowpack development (Marks and Dozier, 1992; Lehn- ing and others, 2002b), but also for mass-balance calculations (Cline, 1997; Liston and Sturm, 1998; Box and others, 2004). Aerodynamically rough, solid surfaces have roughness lengths independent of u� (Schlichting and Gersten, 2003). The same does not always hold over granular surfaces, such as snow or sand; above the threshold friction velocity, ut, particles will be entrained and transported by drift. At low wind speeds, this drift will be predominantly saltation, where particles follow parabolic paths over the surface (Bagnold, 1941, p. 10-37). These drifting particles increase the aerodynamic roughness of the surface. Owen (1964) summarized earlier research relating to the aerodynamic effects of drifting particles, and developed a theoretical framework to explain this effect. Together, these suggest two

On the saltation of fresh snow in a wind tunnel : Profile characterization and single particle statistics

We present experimental results on the snow drift in a turbulent boundary layer over a flat fresh snow-covered surface. Vertical profiles of mass flux and of the distribution of particle diameters were obtained by means of a pair of Snow Particle Counters parallel with measurements of the stream-wise velocity profile. The aim of the paper is to discuss current parameterizations of the vertical mass flux profile for fresh snow and to investigate the range of timescales involved in a developing saltation layer occurring in a turbulent boundary layer. The novelty of the work consists of using an intact fresh snow cover as an erodible surface able to provide realistic snow crystals as drifting particles. Results show that (1) the parameters scaling the vertical mass flux profiles of fresh snow can significantly differ from those given in the literature for ice or compacted snow particles; (2) though drifting snow covers an extremely wide range of temporal scales, the mean time interval between saltating particles 〈Δt 〉 is the key timescale of the saltation process; (3)〈Δt〉 allows for the optimal reconstruction of the mass flux as a continuous signal and for neglecting the effects related to the heterogeneous distribution of particle size on the mass flux. Implications on the modeling of snow drift and on the processing of field observations are discussed.

Blowing Snow Fluxes in the Cariboo Mountains of British Columbia, Canada

Abstract The Cariboo Mountains form the northern extension of the Columbia Mountains, spanning a distance of about 300 km in central British Columbia (BC), Canada. Cool air temperatures, abundant snowfall, and strong winds (especially above treeline and along exposed ridges) would suggest frequent and intense blowing snow events. The occurrence of intense blowing snow episodes is confirmed by automated wind and snow depth measurements at several sites in the area. Simulations conducted with a numerical model forced by meteorological observations recorded from 2006 to 2009 reveal a high frequency of blowing snow episodes at three high-elevation sites in the Cariboo Mountains. This process is especially prominent on the exposed ridge of Browntop Mountain (elevation of 2031 m a.s.l.) where snow transport by wind is calculated to occur as much as two-thirds of the time during some winter months. Simulated blowing snow fluxes remain high at this site with monthly transport and sublimation rates reaching 5301 Mg m−1 and 31 mm snow water equivalent (SWE), respectively. Blowing snow is also shown to be a dominant process in snow accumulation at the upper Castle Creek Glacier site (elevation of 2105 m a.s.l.), with strong winds generating sharp declines in snow depth and the erosion of more than 200 cm of snow depth during two successive winters. The results presented in this study suggest that blowing snow contributes significantly to snow accumulation and the mass balance of glaciers in BCs Cariboo Mountains.

Verification of moisture budgets during drifting snow conditions in a cold wind tunnel

[1]xa0Experiments in a cold wind tunnel were used to verify drifting snow sublimation models. A layer of drifting snow particles was formed over a sintered snow surface. Sublimation and drifting snow flux were estimated from two vertically resolved profile measurements separated along the flow path and were compared to a simple, one-dimensional diffusion model of drift and drifting snow sublimation. The experiments show an increase in water vapor content of the air from drifting snow sublimation. The measured drifting snow sublimation appeared to be consistent with albeit somewhat larger than theoretical values found in the model study. Under wind tunnel conditions, particle number density appears to be the most important controlling factor on the sublimation rate. For experiments with external solar radiative forcing, the increase of the sublimation rate was also larger than theoretical predictions. The experiments suggest that irregular snow crystals and solar radiation might increase sublimation rates more than described by many drifting snow models.

Temporal Coherence: A Model for Non-stationarity in Natural and Simulated Wind Records

We present a novel methodology for characterizing and simulating non-stationary stochastic wind records. In this new method, non-stationarity is characterized and modelled via temporal coherence, which is quantified in the discrete frequency domain by probability distributions of the differences in phase between adjacent Fourier components. Temporal coherence can also be used to quantify non-stationary characteristics in wind data. Three case studies are presented that analyze the non-stationarity of turbulent wind data obtained at the National Wind Technology Center near Boulder, Colorado, USA. The first study compares the temporal and spectral characteristics of a stationary wind record and a non-stationary wind record in order to highlight their differences in temporal coherence. The second study examines the distribution of one of the proposed temporal coherence parameters and uses it to quantify the prevalence of nonstationarity in the dataset. The third study examines how temporal coherence varies with a range of atmospheric parameters to determine what conditions produce more non-stationarity.