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# Casson_ISSW poster-Revised2.ppt - ftp - University of Washington by xiaopangnv

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Background                                                                                                                                                                                                                                                                                                                                                                                           Equations for Determining Snow Ratio
In modeling natural releases in storm snow cycles
Evaluating the importance of                                                                                                                                                                                                                                                                       Snow Ratio Equations applied to all observations
from Stevens Pass (2007-8) with snow depths greater
(i.e., direct action avalanches) it is important to
accurately parameterize some of the physical
properties of new snow. In particular how quickly
crystal type on new snow instability                                                                                                                                                                                                                                                                  than 2.5 cm.

new snow gains strength, whether it is through
mechanical compression or metamorphic                                                                                                                                                                      Jerry Casson, Mark Stoelinga and                                                                                                                               American
strengthening, is of critical significance to simple
strength vs. stress avalanche models such as SNOSS
John Locatelli                                                                                                                                        Avalanche
Avg. values from observations:
Association
(Conway, Wilbour et al., 1999, Hayes et al., 2004).                                                                                                                                                             University of Washington                                                                                                                                                            Graupel-like Snow: density = 91.7 kg/m3
Graupel: density = 151.5 kg/m3
Poster 8046
Frozen Drops: density = 200 kg/m3
SNOSS Snow Slope Stability Model                                                                                                                                                                                                                                                                                                                                                                    Equations from linear fit to observations:
Dendritic: density = 0.27*(% riming) + 23.0 kg/m3
This model is based on a strength vs. stress approach                                                                                        Observing Crystal Type and New Snow Density                                                                                                                                                                                                            Non-dendritic: density = 1.43*(% riming) + 70.2 kg/m3
to slope stability. When the stability index (or ratio
of slope strength to slope stress) is less than one,
Measuring Densification Rate
slope failure is predicted to occur.                                                                                                                                                                                                                                               Measurements of new snow height and mass                                                                           •Settlement tubes for measuring the compactive viscosity of snow samples
calculations every 15 min during storms provided                                                                   •Results below show the comparison of observations to the current SNOSS parameterization
for 7 different test samples. Values above (below) the SNOSS line indicate a lower (higher)
STABILITY INDEX = Stress on layer due to                                                                                                                                                                                                                                           detailed density measurements coincident with
overburden of new snow( xz) divided by shear
observed densfication rate than predicted by SNOSS.
crystal observations using a high-powered                                                                                                                              Densification Rate (Compactive Viscosity) and Crystal Type

strength of snow layer ( fz).                                                                                                                                                                                                                                                     stereoscopic microscope and camera.
Note:  xz is a function of slope angle and
cumulative precipitation.  fz is a function of
density only.
GS
Sample Data from Crystal Observations
SNOSS parameterization of density change:
      m   zz             zz = overburden stress = wt. of snow above layer (= piston pressure in
                            settlement tube experiment)
t         zz                                                                                                                                                               5%    6%

 = density          zz = compactive viscosity (see equation below)                                                                            1%   6%    8%
90
L

 m = metamorphic stress (increase in density due to destructive metamorphism)                                                                 1%   6%    8%    2%
80
1%   6%    8%    2%

L                                        few                  70
 E 
 zz  ( A)    exp 
4
       A = compactive viscosity constant = .616X10-13 (see graph at far right)
1%

2%
1%

2%
6%

6%
8%

8%
4%

5%
1%

1%
1%

2%   2%   1%
2%

2%                                   60                                                                                                           Determining Shear Strength

Snow Ratio
 RTs                                                                                                                          3%                   3%   6%    8%    5%   2%        3%   2%        2%   2%   2%   1%        2%        5%    1%    2%   1%   1%   50
R   = gas constant      E   = activation energy=67.3   KJ-1mol-1                                                  5%            1%     3%   6%    8%    5%   3%   1%   3%   2%   1%   3%   5%   4%   2%   3%   3%   2%   15%   4%    3%   3%   2%
40

Ts = temp. of snow layer (= air temp from sensor near settlement tubes)                                             7%            4%     4%   6%    8%    6%   5%   1%   3%   3%   2%   3%   5%   5%   3%   5%   4%   5%   15%   5%    5%   3%   3%
30
7%            5%     4%   6%    8%    7% 10%    5%   3%   5%   5%   12% 20%   7%   5%   15% 10% 15%          10% 15% 5%      5%

20
Surface Air Temperature ( C)

10% 5%               5%   6%    8%    8% 15% 10%     4%   10% 17% 15% 20% 20% 10% 17% 15%         18% 15% 15% 15% 15% 19%

Precipitation Rate (mm/hr)
o

30% 20%            10% 15%      8%    16% 25% 13% 15% 35% 25% 25% 20%              15% 30% 20%    30% 25% 25% 25% 17% 30%         10

Key Questions Regarding Shear                                                                                                                 -8
35% 65%            65% 20%      8%    35% 40% 70% 65% 43% 50% 40% 35% 60% 47% 30% 42% 30% 25% 40% 35% 55% 40%
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Strength, Density and Crystal
-10                                                                                                                                        4
-12                                                                                                                                        2
0
1100                            1000              0900            0800                     0700                 0600
Pacific Standard Time

Type:                                                                                                                                              Snow Ratio= water density(1000 kg m-3)/snow density.

1) How well does crystal type predict new snow                                                                                                       Note: Poor correlation between temperature and snow density as indicated by large fluctuations in snow ratio (as
denoted by shaded blue area) but little change in surface air temperature (as denoted by dotted red line). Primarily
low density snow of dendritic type form early in storm followed by denser, cold-type crystals in latter half of storm.
parameters, such as air temperature?                                                                                                                                                                                                                                                                                                                                                             Performing shear tests on tables, instead of directly on the snow pack, ensured that only new
layers were tested and allowed for an easy “reset” between measurements by brushing snow off
of the tables. A new light-weight, large-area shear frame allowed measurements to be made in
2) Does the rate of densification depend entirely on                                                                                                  Plot of All Data with Consistent Crystal Types During 15-Minute Observation Period.                                                                                                                                                           low density layers too weak to support a standard-size metal frame.
snow temperature, overburden, type and rate of                                                                                              Snoqualmie Pass data (larger grey circles) and Stevens Pass data (smaller dark dots)
metamorphism or is crystal type an important factor.                                                                                                                                                                                                                                                                                                                                                                           Results from Shear Tests
Data ordered by crystal type and degree of riming (dendrites to graupel, light riming to dense riming).

3) How well does density determine the initial shear                                                                                                                                                                                                                                                                                                                                                                                                      SNOSS best fit power law relation of density
Snow Density & Crystal Type
strength for new snow? Is the change in shear                                                                                                                                                                                     Stevens Pass 2007-2008 & Snoqualmie Pass 2006-2007
to shear strength:
2
strength over time well correlated to changes in
60.0
  s  where A1 = 1.95X104 Pa
 fz  A1  
density? Is there a crystal-type dependence?                                                                                                                                                                                                                                                                                                                                                                                                                             i 
50.0

Jamieson & Johnston best fit for precipitation
particles:
Field Observations                                                                                                                                                  40.0

 s 
1.35
Snow Ratio

 fz  A2  
 i 
30.0                                                                                                                                                                                                                                                                                                                 where A2 = 5320 Pa
Locations in the Washington Cascades at Snoqualmie
Pass (elev. 921 m) for winter 2006-7 and Stevens Pass                                                                                                               20.0

(elev. 1238 m) for winter 2007-8.
10.0
Summary
0.0                                                                                                                                                                                •One of the most promising results from this study is the potential for parameterizing a link between crystal
0
GLS       Graupel   type and new snow density. Surface air temperature alone proved to be a very poor predictor of density.
Lump-

GLS
Graupel
Performing regression analysis on the crystal-type data from this study is a first step to a direct prediction of
Graupel                       new snow density solely based on crystal type. This could potentially improve the accuracy of the
initialization of avalanche forecast models which rely on density as a predictor of snow stability.
Crystal Type and Degree of Riming

•The SNOSS model’s equations for snow densification seem reasonably accurate but could be improved
with better predictions for initial density. Also, corrections for dendritic crystal types may need to be added
to the densification equations.

Relationship between Crystal Type and Density:                                                                                                                                                     •A new initial shear strength parameterization may further improve the performance of the model.
Snow Lab Research Trailer at Stevens Pass site.
Weather instrumentation above trailer allowed                                                                                                                  INCREASING DENSITY                                                                                                                                                                       References:
collection of real-time meteorological data. Shear                                                                                                                                                                                                                                                                                                      [1] Conway, H., C. Wilbour, 1999. Evolution of snow slope stability during storms. Cold Regions Science and Technology. 30, 67-77.
needles/sheaths,                                                                      [2] Hayes, P., C. Wilbour, R. Gibson, H.P. Marshall, H. Conway, 2004. A simple model of snow slope stability during storms. Proceedings of
tables right of trailer and settlement tube shelter                                                                                                Dendritic forms                                                                                                                                                                                      the 2004 International Snow Science Workshop, Jackson Hole, WY, 165-171.
columns, plates, sideplanes,
attached to front enabled densification and shear                                                                                                                                                                                                                                                                                                       [3] Jamieson, J.B. and C.D. Johnston, 2001. Evaluation of the shear frame test for weak snowpack layers. Annals of Glaciology. 32, 59-69.
bullets, etc.
strength measurements.                                                                                                                                                                                                                                                                                                                                  Acknowledgements:
Unrimed                                                                                    Densely rimed                                                                Graupel                      Thanks to the National Science Foundation and the American Avalanche Association for funding support. Thanks especially to the Stevens
Pass Ski Area for allowing us to locate our research trailer in their area.

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