SNOWPACK STRUCTURE AND CLIMATE, MOUNT EGMONT, NEW ZEALAND
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42 W e a t h e r and Climate (1983) 3: 42-51
SNOWPACK STRUCTURE AND CLIMATE,
MOUNT EGMONT, NEW ZEALAND
M. G . Marcus
Geography Department, Arizona State University,
Tempe, Arizona, U.S.A.
R. D . Moore
Geography Department, University o f Canterbury,
Christchurch.
ABSTRACT
Between 29 August and 1 September 1981, four snowpits were dug on
the East Ridge of Mount Egmont at elevations of 1240 m, 1370 m, 1580 m and
1980 m. Snowpack structure at all four sites was characterized by alternating
layers of coarse melt-freeze grains and ice bands, with essentially isothermal
temperature profiles i n all pits. Depth of snow, snowpack water equivalent
and integrated ice band thickness display strong positive relationships with
elevation. These features of snowpack structure and properties reflect strongly
Mount Egmont's r - aritime climate, particularly the effects o f periods o f
above-freezing temperatures throughout the winter and the occurrence o f
rain-on-snow.
INTRODUCTION Although seasonal snowcover i s a n i m -
Seasonal snowcover i n t h e South Island portant component of North Island mountain
mountains has been the object of some research environments, particularly as a recreational
effort. M a n y studies (e.g. Anderton, 1974; resource, measurements a n d interpretations
have been scarce. These are restricted to studies
Chinn, 1969; Fitzharris, 1976a) focus o n
seasonal snow as a water resource component. of the Whakapapanui Glacier, M t Ruapehu,
Other studies, partly as a result of increased neve field i n 1968-9 (Kells and Thompson,
avalanche hazard coinciding with the growth 1970) and Heine's (1962) 4-site sampling o f
of winter mountain sports and tourism, con- the surface of M t Ruapehu's upper snow field.
centrate on snow avalanche phenomena (e.g. Glaciologically oriented studies of Mt Ruapehu
Fitzharris, 1976b; Fitzharris and Owens, 1981; include Heine (1963) and Krenek (1959), but
La Chapelle, 1979; McNulty and Fitzharris, these are not directly concerned with snow
1980). Prowse (1981) made a detailed study of problems.
the influence of physical environment on snow- This paper presents the first effort to meas-
pack characteristics in the Craigieburn range, ure and interpret full profile characteristics of
with attendant consequences f o r avalanche a North Island snowpack. Four profiles were
activity and snowmelt runoff. developed from snow pits dug on the eastern
slopes o f M t Egmont at the end o f the 1981
winter accumulation season. T h e snowpack
characteristics and their altitudinal variation
* Local t i m e i s u s e d throughout t h i s p a p e r. are examined with reference to precipitationSnowpack Structure and Climate, M t . Egmont 43 Fig. 1 : Location m a p o f M t Egmont National Park. Abbreviations indicate weather stations: Cap.! LgnuAll (CE), Dawson Falls ( D F ) , M a n a i a Demonstration F a r m ( M D F ) , Ngarara B l u ff (NB), N e w Plymouth (NP), Plateau (P), Stratford Demonstration Farm (SDF), and Stratford Mountain House (SMH).
44 S n o w p a c k Structure and Climate, M t . Egmont
patterns and the weather sequence during the (Gamier, 1958; Thompson, 1981). Frosts are
1981 accumulation season. uncommon in the lowlands; heatwaves unusual
in summer.
SITE CHARACTERISTICS Prevailing winds at New Plymouth shift be-
tween westerly and southeasterly, with 13%
Mt Egmont (2517 m) lies within Mt Egmont calms (Coulter, 1975). I t follows t h a t t h e
National P a r k (Fig. 1 ) . I t s volcanic cone, western slopes experience strongest orographic
situated on a peninsula on the southwest coast influences from the Tasman Sea, whereas the
of the North Island, is particularly susceptible Stratford area and eastern slopes are open to
to marine influences. On three sides, the ocean Cook Strait a n d southeasterly f l o w. B o t h
is never more than 23 km from the summit.
situations can produce precipitation.
Park boundaries encompass lower limits of the
bush w i t h the upper l i m i t a t about 1100-
1200 in. Above this level, the mountain gen- PRECIPITATION PAT T E R N S
erally experiences winter snowcover (see Fig. Low level stations w h i c h surround M t
2). The snowpack disappears in most summers.
Egmont (Fig. 1) — Cape Egmont (8 m), New
The climate of Taranaki, because of location Plymouth (55 m), Normanby (122 m), and
and marine effects, i s moist and moderate Manaia Demonstration Farm (98 m) c o n -
Fig. 2 : Photograph o f M t Egmont's eastern aspect.The striking symmetry i s upset o n t h e left-hand
skyline by Fantham's Peak.Snowpack Structure and Climate, M t . Egmont 45
sistently record annual precipitation ranges of THE W E AT H E R SEQUENCE D U R I N G
1000-1500 mm. Stratford Demonstration Farm WINTER 1981
(311 m) shows higher values between 1600 and Figure 4 shows the precipitation and occur-
2500 mm. Winter and summer precipitation are rence of snow as recorded at Stratford Moun-
about equally distributed for all stations. tain House a t 0900 hrs* f o r M a y through
Between the plains and both 846 m high August 1981. I t also shows f o r t h e same
Stratford Mountain House and 1148 m Plateau period the freezing levels as determined from
Station, precipitation increases dramatically. the 1200 hrs upper air soundings at Auckland,
which was the closest station f o r which this
This is illustrated i n Fig. 3, which provides
mean precipitation values f o r the nine year data were available. Although t h e free a i r
period when concurrent records were main- freezing levels in Taranaki would generally be
tained a t New Plymouth, Manaia, Stratford, lower than those a t Auckland because o f
Stratford Mountain House and Plateau. The Taranaki's higher latitude, the relative magni-
two higher stations received three to four times tude and frequency of variation will be similar.
the precipitation of the lower stations on both Figure 4 indicates that during most sequences
a seasonal and an annual basis (because o f of rainy weather, the freezing level varies by
the use o f long-term recorders, winter pre- 1000 to 2000 m. This suggests that through-
cipitation cannot b e separated f o r Plateau out the winter, most of the mountain experi-
Station). ences periods o f above-freezing temperatures
Above 1200 m, precipitation either remains and the occurrence of rain-on-snow. Certainly,
constant o r decreases with elevation, as sug- there were no periods of more than a week or
gested by Kidson (1930) and Thompson (1981). so during which most elevations on the moun-
tain had sustained cold conditions. Further
This is supported by the precipitation record
from a station maintained a t Ngarara Bluff evidence is provided by reports in past issues
of the Australia New Zealand Ski Year Book.
(1560 m ) i n 1971 a n d 1973. I n 1971, the
respective annual precipitation records a t Accounts in the Year Book consistently men-
Stratford M o u n t a i n House, Plateau a n d tion that warm rain storms suppressed skiing
Ngarara Bluff were 6467 mm, 6349 mrn and activity on Mt Egmont, and often caused much
6016 mm; and in 1973, 5218 mm, 5350 mm snowmelt. From the pattern o f variation i n
and 5734 mm. The Ngarara Bluff precipitation the Auckland freezing levels, though, i t i s
is within ten per cent of the other two stations argued that the higher the elevation on M t
for both years. Egmont, the less frequently temperatures rose
above freezing, and the greater the amount of
precipitation which fell as snow.
6000
MEAN P R E C I P I TAT I O N SNOW PROFILES
5000 1 9 6 4 -71 , 1 9 7 3 I LOCATION
7 Snowpits were excavated on the east slopes
4000
of M t Egmont from 29 August t o 1 Sep-
tember 1981, roughly at the end of the winter
3000 accumulation season. Figure 5 locates the four
0.
6
pit sites, which were chosen where drift effects
2000 were minimal. The pits were situated on the
Stratford Mountain Club ski field north o f
E D W I N T E R ( M A Y - SEPT )
1000 ANNUAL
Manganui Lodge (1240 m), just above the
upper terminus o f the T-bar tow (1370 m),
( ) S T A T I O N E L E VAT I O N ( r n )
adjacent to the rope tow (1580 m) and on the
East Ridge snowfields above the "Policeman"
Fig. 3 : M e a n w i n t e r a n d a n n u a l precipitation (1980 m). The sites were on slopes of 10', 24'.
(1964-71, 1973) a t selected Taranaki stations. 23' and 32', respectively.
2. METHOD
* Local time is used throughout this paper. Snow density and temperature were measured46 Snowpack Structure and Climate, M t . Egmont
at a l l f o u r sites. A t the three lower pits, termined f o r each layer b y monocular lens
measurements included snowpack stratigraphy, examination o f crystal samples o n a plate
grain type and size, relative hardness and shear marked with 1, 2 and 3 mm grids. Relative
stregh. Procedures are those described b y hardness i n the horizontal direction ( K ) was
Perla and Martinelli (1976). Densities were estimated by the standard hand test. The ease
measured using a 100 m l box cutter snow of shearing between snow layers, which i s
sampler, sampling continuously through t h e important f o r assessing snow stability and
pack in 29 mm increments. Temperatures were avalanche potential, was roughly established
taken by dial stem thermometer at the snow by the shovel test. The shovel test involves
surface, 50 and 100 mm below the surface, and isolating a column of snow from the pit's up-
every 100 rnm thereafter. hill wall, inserting a shovel vertically into the
snow i n the uphill side o f the column, and
gradually increasing the applied leverage until
General stratigraphie characteristics were the snow column shears. The ease with which
established b y inspection o f a snowpit face, the snow shears and the smoothness o f the
with layers identified by brushing the pit face. shearing layer indicate potential for snowpack
Grain type ( F ) and diameter ( D ) were de- failure.
4000
AJ
0 3000
C.,
1
1
LA
LA
z 2000
Lu
LA
CC
.c
1
0
tsd
1000
Snow Recorded ot
S rot ford Mtn House
1
•••••
11 -1H1 11 r
9 Cr,
1 31
10 2 0 3 10 2 0 30 10 20 o 2 0
MAY JUNE JULY AUGUST
Fig. 4 : 0900 hrs observations o f precipitation a n d snow occurrence at Stratford Mountain House and 1200 hrs
freezing levels a t Auckland f o r M a y through August 1981.Snowpack Structure and Climate, M t . Egmont 4 7
DAWSON
7 / 1 / FALLS
2 • Snow Pit
Major Av a l a n c h e
September 5 , 1981
All h e i g h t s a n d c o n t o u r s a r e i n f e e t .
Mt E g m o n t
8260'
Shores To o t h
8220'
Line o f s l a b f r a c t u r e
Fantharrs Peak
6436' prominent 7 0 0 0 - 7 5 0 0 '
Ron91,0 F l o ,
-.t,thuranoi Lodge
- ftettion'tti ,
rh* _
,,•-• •
ROUND W 2 Y . / o c i t . ; " ; . ' ' ,
7/0'' P a , * • . 1 , , • • •
Fig. 5 : Contour m a p (above) a n d eastern perspective ( b e l o w ) o f M t . Egmont. T h e 516 September 1981
avalanche zone i s shaded o n b o t h maps. Snowpit locations are shown o n the contour map.48 S n o w p a c k Structure and Climate, M t . Egmont
Tc'C
3 0
600 2 0 0
/4 ( k g r11-3)
0°C
Iso hermoi
K = Hardness
S Soft
M Medium
H Hard
ELEV 1 9 8 0 r o ✓ Very
—
E • = Groin Diameter ( a i m ;
• S n o w Groin Ty p e
i t E a r l y E . T. p a r t i a l l y s e t t l e d
• L a t e E . T. ; r o u n d e d
• M e l t - Freeze ; rounded
ww• I c e
O.0
Isotherm& • C Conglomerated Crystals
ELEV, 1370rn
600 2 0 0
i /r2 ( kg ro-3 ) •
600 2 0 0
( kg )
Fig. 6 : M t E g m o n t snowpit data, 2 9 A u g u s t - 1 September 1981.
SNOW CHARACTERISTICS grains, o n the other hand, are older grains
Figure 6 and Table 1 summarize snowpack which have experienced repeated freeze-thaw
characteristics and their variation with ele- cycles. The melting and refreezing produces
vation. As shown in Fig. 6, snowpack structure bonded, coarse polygranular units which are
shows little variation with elevation. A t a l l relatively h a r d w h e n f r o z e n (Perla a n d
four sites, the snowpack is characterised by Martinelli, 1976). Evidence of another process,
alternating layers o f melt-freeze ( M F ) i c e temperature gradient (TG) metamorphism, was
grains and ice bands, with the upper layers not present. Temperature gradient m e t a -
containing some relatively fresh snow crystals morphism involves the vertical migration o f
which have undergone to varying degrees equi- water molecules through the snow pack i n
temperature (ET) metamorphism. response to temperature and vapour pressure
gradients, and produces poorly-bonded cup-
Equi-temperature metamorphism is the pro- shaped crystals often called depth hoar o r
cess b y which fresh snow crystals lose their sugar snow.
original complex form and settle into rounded
grains. The process results from the migration The ice bands observed i n the pack can
of water molecules from areas o f the snow form from two complementary processes. The
crystal having high free energy, such as the first is the freezing o f condensation onto the
convex points on a classic stellar crystal, t o snow surface, producing a sheath of clear ice
areas of low free energy, such as the concave over the mountain. This process is reported in
zones between the points. Melt-freeze ( M F ) the 1935 Ski Year Book. What is required isSnowpack Structure and Climate, M t . Egmont 4 9
a cold, clear night during which the snow sur- The temperature profiles a t a l l sites are
face cools radiatively, followed b y the ad- essentially isothermal at 0°C, the upper limit
vection of a warm, humid air mass. Moisture for ice. Again, this reflects the influence of a
from the air mass condenses and freezes onto maritime climate, i n which t h e snowpack
the snow surface. The second process involves rarely undergoes prolonged periods of net heat
rain o r meltwater which percolates d o w n loss. The sub-zero temperatures i n the upper
through the pack until i t reaches a relatively layers of the two higher pits result from radi-
impermeable layer, stops, and freezes into the ative and conductive cooling during the clear
ice matrix. The impermetable layer could be weather preceding pit excavation.
an old buried surface ice crust formed by melt
and refreeze, a buried ice sheath, or the bound- As seen i n Table 1, integrated density (p)
ary between two different snow layers. The shows a fairly strong positive linear relation-
ice bands effectively isolate intervening, layers ship w i t h elevation ( E ) (though t h e small
from liquid and vapour transfers until the sample number limits the use of the relation-
bands decay during spring thaw. ships i n Table 1 f o r other than description).
However, this is not due to increasing snow
density w i t h either elevation o r snowpack
Density profiles are also similar for all four depth; rather, it reflects an altitudinal increase
sites. They consist of the layers of M F grains in the mass fraction of the snowpack which is
having densities f r o m 350-450 kgm-3 sand- composed of high density ice bands. This fact
wiched between the ice bands o f about 800
is apparent from the strong relationship be-
kgm-3 density. The E T grains i n the upper tween integrated ice band thickness (IT) (the
layers have densities o f 200-300 kgm-3, de- sum of the thicknesses of the ice bands in each
pending on the degree of ET metamorphism. profile) and elevation.
The density o f the layers o f M F grains is
fairly constant with depth. This is because, in Snowpack depth (HS) and water equivalent
Mt Egmont's maritime climate, freshly fallen (WE) exhibit strong relationships w i t h ele-
snow i s subjected t o E T a n d M F meta- vation. Fitzharris (1978) investigated the form
morphism, causing a rapid increase in density. of relationship between WE and elevation for
Subsequent snowfall and ice band formation two North American and two New Zealand
prevent vertical mass transfers which would South Island sites for several years. He found
result in density changes. The hardness of the that the form of the best-fit relationship var-
MF grains helps resist further density increase ied both ultra- and inter-seasonally, depending
caused by the overriding snowpack pressure. on three factors: variation o f precipitation
TA B L E I : A LT I T I I D I N A L VA R I AT I O N O F SELECTED SNOWPACK PROPERTIES.
Elevation Snowpack Water Integrated Integrated Ice
Depth Equivalent Density Band Thickness
(E) (HS) (WE) — (IT)
(p)
ni B kgm-3 C
1240 0.57 223 391 6.3
1370 1.14 472 414 12.5
1580 2.22 1059 477 29.3
1980 3.95 1916 485 .5
62.5
Linear Regression Relationships
HS = —5.11 -1-- 0.00459E r' = 0.999 S.E. = 0.09
WE = —2663 + 2.32E r' = 0.997 S.E. = 74
p = 2 4 3 -I- 0.129E r 2 = 0.804 S.E. = 36
IT = —92.4 4- 0.0771E r ' = 0.979 S.E. = 6.2
(SE. = standard error o f the estimates)50 Snowpack Structure and Climate, M t . Egmont
with elevation; the variation of melt with ele- Knowledge o f snowpack structure and the
vation; a n d elevation o f t h e rain-snow effects o f weather are important for assessing
boundary, which is correlated with the freez- avalanche hazard (Fitzharris e t al., 1983).
ing level (although falling snow can pene- Although the snowpack o n M t Egmont did
trate some distance below the freezing level). not contain any TG crystals, the shovel tests
For the M t Egmont data, a linear relation- indicated that many of the layers of MF grains
ship gives a good fit because only four points were poorly bonded t o t h e underlying ice
were fitted, and our transect did not extend bands, presenting another form o f potential
to the mountain's summit. The scouring o f failure plane. I n fact, during a heavy snow
snow from the tops of mountains by the wind storm in September 1981, an avalanche began
tends to flatten the WE vs elevation curve. on the eastern slopes o f M t Egmont on the
As discussed previously, evidence does not evening o f 5/6 September and ran down the
support the possibility of precipitation increas- Manganui Gorge. The zone affected b y the
ing with elevation on M t Egmont. Therefore, avalanche i s shown i n Fig. 5 . T h e runout
the increase in WE and HS with elevation is length and the depth o f debris i n the gorge
probably caused b y Fitzharris' latter t w o suggest that more than just fresh snow was
factors. That is, the greater the elevation, the involved i n t h e event. A failure probably
greater the amount of precipitation which falls occurred i n the old snow on one o f the ice
as snow, and the lesser the amount of accumu- bands; the release of old snow contributed to
lated snow which melts. This also explains the the magnitude of the event.
increase i n I T with elevation. Since less ice
and snow melt at higher elevations, more o f
ACKNOWLEDGEMENTS
the ice bands are preserved in the pack.
We wish to express our appreciation to Julie
DISCUSSION Gardner f o r field assistance; t o Snow Mace
The M t Egmont snowpack characteristics for sharing Stratford Mountain House weather
reflect strongly the effects of a moist, moder- records and his many years of experience with
ate climate. The predominance of MF and ET Mt Egmont's weather and climate; to Alaric
metamorphism, and the lack o f T G meta- Tomlinson of the Christchurch Weather Office;
morphism, indicates t h a t prolonged periods and to the Taranaki Catchment Commission.
(more than several days) of net heat loss from
the snowpack occur rarely. The thick ice bands REFERENCES
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SPECTACULAR LIGHTNING DISPLAY OVER ASHBURTON
Photograph b y Nigel Yates, Ashburton Guardian.
This spectacular display o f lightning, photographed a t Ashburton about 9.40 p.m. on 21 November 1982, was
associated w i t h o n e o f a group o f thunderstorms which advanced along the Canterbury coast that evening.
These storms, accompanying a southerly change, also brought considerable amounts o f h a i l which damaged
crops, and there were reports o f power failures caused b y lightning strikes.You can also read
