R. Perla

The Ottawa Hospital, Ottawa, Ontario, Canada

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Publications (12)14.13 Total impact

  • Jeff Dozier · R.E. Davis · R. Perla
    Snow Property Measurement WorkshopSnow Property Measurement Workshop; 01/1987
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    Jeff Dozier · R.E. Davis · R. Perla
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    ABSTRACT: It is a long-standing problem to find an appropriate small set of parameters that best describes snow micro structure as observed on sec­ tions or thin slices, for the purposes of relating physical properties of snow to its electromagnetic, thermal, or mechanical properties. The stereological parameters point density and intercept length (uniquely related to surface area per unit volume) are likely members of the minimal set of descriptors, but a measure of their distribution, such as the variance, is also needed. In addition a topological classifier of ice- or pore-connectivity, such as the first Betti number, is possibly useful to describe thermal or mechanical proper­ ties, but that parameter is only retrieved after tedious preparation and analysis of serial sections. For models of microwave emission or metamor- phism at weak temperature gradients a geometric description from orthog­ onal sections provides useful parameters.
    01/1987: pages 49-59; International Association of Hydrological Sciences Publication No. 162.
  • R.E. Davis · Jeff Dozier · R. Perla
    01/1987: pages 53-74; D. Reidel.
  • Snow Property Measurement WorkshopSnow Property Measurement Workshop; 01/1987
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    ABSTRACT: Field trials of the dilution technique for measuring snow liquid water content show that the refined procedure is rapid and simple. Measurements of the liquid water mass fraction with an absolute error of ˜1.5% can be obtained by one operator at a rate of 10-15 samples per hour, but if the water content is low (0-2%), the relative error can be high. Electrolytic conductivity is the preferred method for measuring concentrations, using a stock solution of 0.01 N HCl. The recommended amount of stock solution to add is 0.5-0.8 times the mass of the snow sample. Extraction of the resulting mixture of stock solution and snow liquid water is best done with a screened pipette, instead of by decanting.
    Water Resources Research 09/1985; 21(9-9):1415-1420. DOI:10.1029/WR021i009p01415 · 3.55 Impact Factor
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    ABSTRACT: Photomicrographs of section-planes from snow samples exposed to very strong or very weak temperature gradients were converted to video images and digitized. Brightness levels were set to discriminate ice from pore profiles. Section-plane density, mean and maximum intercept lengths for the ice profiles, and surface area per unit mass were measured and compared. Although results were somewhat obscured by density variations, the mean and maximum intercept lengths increased and the surface area per unit mass decreased under the influence of strong or weak gradients. The increase in maximum intercept length was more pronounced in the strong gradient tests.
    Cold Regions Science and Technology 09/1985; 11(2):181-186. DOI:10.1016/0165-232X(85)90016-3 · 1.37 Impact Factor
  • R PERLA · C.S.L. Ommanney
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    ABSTRACT: Snow samples were exposed to very strong or very weak temperature gradients and studied as disaggregated grains and by section. Very strong gradients (≈1600°C/m) caused obvious morphological change in short periods (≈100 h). Low density snow metamorphosed toward a weak assembly of skeletal and faceted grains. High density snow metamorphosed toward a strong, crusty texture, which appeared fibrous or diffuse in section. These results are contrasted with observations of snow stored under weak gradients (< 1°C/m) for periods up to 6 months at −35°C, −5°C, and −1°C. Under weak gradients there was much less morphological change. There was a tendency to form facets and crusts at −1°C, but the possibility of melting and refreezing due to temperature fluctiations in storage cannot be excluded. Some facets survived or developed in long term storage at −35°C and −5°C.
    Cold Regions Science and Technology 07/1985; 11(1):23-35. DOI:10.1016/0165-232X(85)90004-7 · 1.37 Impact Factor
  • R. Perla · C. S. L. Ommanney
  • R. Perla · Jeff Dozier
    Proceedings, Sixth International Snow Science WorkshopProceedings, Sixth International Snow Science Workshop; 01/1984
  • R. Perla · T. M. H. Beck
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    ABSTRACT: The shear frame is a simple in situ device for indexing the shear strength of thin weak layers. The index is sensitive to shear-frame geometry, rate-of-pull, and shear-frame mass. It is time-consuming to carefully align the device on the Gleitschicht (shear failure plane) in a slab avalanche zone. Despite these limitations, the shear frame is a useful tool for gathering statistical data on strength distributions and anisotropies of the Gleitschicht until a more fundamental technique is developed.-from Authors
    Journal of Glaciology 01/1983; 29(103):485-491. · 3.24 Impact Factor
  • R PERLA · T.M.H. Beck · T.T. Cheng
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    ABSTRACT: A shear frame (area 0.025 m2) was used to measure an index of the shear strength of alpine snow. Shear indices were stratified according to snow crystal morphology, and then were correlated with measurements of snow density, temperature, and crystal size. The correlations of shear index with density were significant for all crystal morphologies except for the melt-refreeze morphology. Shear indices correlated erratically with snow temperatures and crystal size.Shear strength was also measured with a large rotary vane (0.5 m diameter). A comparison of rotary vane indices, shear frame indices, and slab avalanche measurements suggests that shear indices decrease with increasing sample size.
    Cold Regions Science and Technology 08/1982; 6(1):11-20. DOI:10.1016/0165-232X(82)90040-4 · 1.37 Impact Factor
  • R.I. Perla · T. T. Cheng · D. M. McClung
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    ABSTRACT: Voellmy's method for computing the run-out distance of a snow-avalanche includes an unsatisfactory feature: the a priori selection of a midslope reference where the avalanche is assumed to begin decelerating from a computed steady velocity. As an alternaative, a differential equation is derived in this paper on the premise that the only logical reference is the starting position of the avalanche. The equation is solved numerically for paths of complex geometry. -from Author
    Journal of Glaciology 01/1980; 26(94):197-207. · 3.24 Impact Factor