42 Weather and Climate (1983) 3: 42-51

SNOWPACK STRUCTURE AND CLIMATE, MOUNT EGMONT,

M. G. Marcus Geography Department, Arizona State University, Tempe, Arizona, U.S.A.

R. D. Moore Geography Department, , .

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 in 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 of periods of above-freezing temperatures throughout the winter and the occurrence of rain-on-snow.

INTRODUCTION Although seasonal snowcover is an im- Seasonal snowcover in the portant component of mountain mountains has been the object of some research environments, particularly as a recreational resource, measurements and interpretations effort. Many studies (e.g. Anderton, 1974; have been scarce. These are restricted to studies Chinn, 1969; Fitzharris, 1976a) focus on seasonal snow as a water resource component. of the Whakapapanui Glacier, Mt Ruapehu, Other studies, partly as a result of increased neve field in 1968-9 (Kells and Thompson, avalanche hazard coinciding with the growth 1970) and Heine's (1962) 4-site sampling of of winter mountain sports and tourism, con- the surface of Mt 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 for avalanche a North Island snowpack. Four profiles were activity and snowmelt runoff. developed from snow pits dug on the eastern slopes of Mt Egmont at the end of the 1981 winter accumulation season. The snowpack characteristics and their altitudinal variation * Local time is used throughout this paper. are examined with reference to precipitation Snowpack Structure and Climate, Mt. Egmont 43

Fig. 1: Location map of Mt Egmont National Park. Abbreviations indicate weather stations: Cap.! LgnuAll (CE), Dawson Falls (DF), Manaia Demonstration Farm (MDF), Ngarara Bluff (NB), New Plymouth (NP), Plateau (P), Stratford Demonstration Farm (SDF), and Stratford Mountain House (SMH). 44 Snowpack Structure and Climate, Mt. 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). It follows that the National Park (Fig. 1). Its 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 is never more than 23 km from the summit. Cook Strait and southeasterly flow. Both situations can produce precipitation. Park boundaries encompass lower limits of the bush with the upper limit at about 1100- 1200 in. Above this level, the mountain gen- PRECIPITATION PATTERNS erally experiences winter snowcover (see Fig. Low level stations which surround Mt 2). The snowpack disappears in most summers. Egmont (Fig. 1) — Cape Egmont (8 m), New The climate of , because of location Plymouth (55 m), Normanby (122 m), and and marine effects, is moist and moderate Manaia Demonstration Farm (98 m) con-

Fig. 2: Photograph of Mt Egmont's eastern aspect.The striking symmetry is upset on the left-hand skyline by Fantham's Peak. Snowpack Structure and Climate, Mt. Egmont 45 sistently record annual precipitation ranges of THE WEATHER SEQUENCE DURING 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 at 0900 hrs* for May through Between the plains and both 846 m high August 1981. It also shows for the 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 , which was the closest station for which this This is illustrated in Fig. 3, which provides mean precipitation values for the nine year data were available. Although the free air period when concurrent records were main- freezing levels in Taranaki would generally be tained at New Plymouth, Manaia, Stratford, lower than those at Auckland because of 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 of of rainy weather, the freezing level varies by the use of long-term recorders, winter pre- 1000 to 2000 m. This suggests that through- cipitation cannot be separated for Plateau out the winter, most of the mountain experi- Station). ences periods of above-freezing temperatures Above 1200 m, precipitation either remains and the occurrence of rain-on-snow. Certainly, constant or decreases with elevation, as sug- there were no periods of more than a week or so during which most elevations on the moun- gested by Kidson (1930) and Thompson (1981). tain had sustained cold conditions. Further This is supported by the precipitation record evidence is provided by reports in past issues from a station maintained at Ngarara Bluff of the Australia New Zealand Ski Year Book. (1560 m) in 1971 and 1973. In 1971, the respective annual precipitation records at Accounts in the Year Book consistently men- Stratford Mountain House, Plateau and 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 of variation in and 5734 mm. The Ngarara Bluff precipitation the Auckland freezing levels, though, it is is within ten per cent of the other two stations argued that the higher the elevation on Mt for both years. Egmont, the less frequently temperatures rose above freezing, and the greater the amount of precipitation which fell as snow. 6000

MEAN PRECIPITATION SNOW PROFILES 5000 1964 -71 , 1973 I LOCATION 7 Snowpits were excavated on the east slopes 4000 of Mt Egmont from 29 August to 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 of EDWINTER (MAY- SEPT ) Lodge (1240 m), just above the ANNUAL 1000 upper terminus of the T-bar tow (1370 m), ( ) STATION ELEVATION (rn) adjacent to the rope tow (1580 m) and on the East Ridge snowfields above the "Policeman" Fig. 3: Mean winter and annual precipitation (1980 m). The sites were on slopes of 10', 24'. (1964-71, 1973) at selected Taranaki stations. 23' and 32', respectively. 2. METHOD

* Local time is used throughout this paper. Snow density and temperature were measured 46 Snowpack Structure and Climate, Mt. Egmont at all four sites. At the three lower pits, termined for each layer by monocular lens measurements included snowpack stratigraphy, examination of crystal samples on a plate grain type and size, relative hardness and shear marked with 1, 2 and 3 mm grids. Relative stregh. Procedures are those described by hardness in the horizontal direction (K) was Perla and Martinelli (1976). Densities were estimated by the standard hand test. The ease measured using a 100 ml box cutter snow of shearing between snow layers, which is sampler, sampling continuously through the important for 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 in the uphill side of the column, and gradually increasing the applied leverage until General stratigraphie characteristics were the snow column shears. The ease with which established by inspection of a snowpit face, the snow shears and the smoothness of 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 r Cr, 11 -1H1 9 1 31 10 20 3 10 20 30 10 20 o 20 MAY JUNE JULY AUGUST

Fig. 4: 0900 hrs observations of precipitation and snow occurrence at Stratford Mountain House and 1200 hrs freezing levels at Auckland for May through August 1981. Snowpack Structure and Climate, Mt. Egmont 47

DAWSON 7/1/ FALLS

2 • Snow Pit Major Avalanche September 5, 1981

All heights and contours are in feet. Mt Egmont 8260' Shores Tooth 8220'

Line of slab fracture Fantharrs Peak 6436' prominent 7000 - 7500' Ron91,0 Flo,

-.t,thuranoi Lodge

- ftettion'tti , rh* _

,,•-• • ROUND W2Y./ocit.;";.'' , 7/0'' Pa,* • .1,, • ••

Fig. 5: Contour map (above) and eastern perspective (below) of Mt. Egmont. The 516 September 1981 avalanche zone is shaded on both maps. Snowpit locations are shown on the contour map. 48 Snowpack Structure and Climate, Mt. Egmont

Tc'C 30

600 200 /4 (kg r11-3)

0°C Iso hermoi K = Hardness S Soft M Medium H Hard ELEV 1980ro ✓ Very

— E • = Groin Diameter (aim;

• Snow Groin Type it Early E. T. partially settled • Late E.T. ; rounded • Melt - Freeze ; rounded ww• Ice O.0 Isotherm& • C Conglomerated Crystals ELEV, 1370rn

600 200

i /r2 ( kg ro-3 ) • 600 200

( kg )

Fig. 6: Mt Egmont snowpit data, 29 August - 1 September 1981.

SNOW CHARACTERISTICS grains, on 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. At all relatively hard when frozen (Perla and four sites, the snowpack is characterised by Martinelli, 1976). Evidence of another process, alternating layers of melt-freeze (MF) ice temperature gradient (TG) metamorphism, was grains and ice bands, with the upper layers not present. Temperature gradient meta- containing some relatively fresh snow crystals morphism involves the vertical migration of which have undergone to varying degrees equi- water molecules through the snow pack in 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 or cess by 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 in the pack can of water molecules from areas of the snow form from two complementary processes. The crystal having high free energy, such as the first is the freezing of condensation onto the convex points on a classic stellar crystal, to 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 (MF) the 1935 Ski Year Book. What is required is Snowpack Structure and Climate, Mt. Egmont 49

a cold, clear night during which the snow sur- The temperature profiles at all sites are face cools radiatively, followed by 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, in which the snowpack the snow surface. The second process involves rarely undergoes prolonged periods of net heat rain or meltwater which percolates down loss. The sub-zero temperatures in the upper through the pack until it 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 in 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 with elevation (E) (though the small from liquid and vapour transfers until the sample number limits the use of the relation- bands decay during spring thaw. ships in Table 1 for other than description). However, this is not due to increasing snow density with either elevation or snowpack Density profiles are also similar for all four depth; rather, it reflects an altitudinal increase sites. They consist of the layers of MF grains in the mass fraction of the snowpack which is having densities from 350-450 kgm-3 sand- wiched between the ice bands of about 800 composed of high density ice bands. This fact is apparent from the strong relationship be- kgm-3 density. The ET grains in the upper tween integrated ice band thickness (IT) (the layers have densities of 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 of the layers of MF 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 with ele- snow is subjected to ET and MF 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 of precipitation

TABLE I: ALTITIIDINAL VARIATION OF 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 62.5.5

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 = 243 -I- 0.129E r2 = 0.804 S.E. = 36 IT = —92.4 4- 0.0771E r' = 0.979 S.E. = 6.2 (SE. = standard error of the estimates) 50 Snowpack Structure and Climate, Mt. Egmont

with elevation; the variation of melt with ele- Knowledge of snowpack structure and the vation; and elevation of the rain-snow effects of weather are important for assessing boundary, which is correlated with the freez- avalanche hazard (Fitzharris et al., 1983). ing level (although falling snow can pene- Although the snowpack on Mt Egmont did trate some distance below the freezing level). not contain any TG crystals, the shovel tests For the Mt 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 to the underlying ice were fitted, and our transect did not extend bands, presenting another form of potential to the mountain's summit. The scouring of failure plane. In 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 of Mt Egmont on the As discussed previously, evidence does not evening of 5/6 September and ran down the support the possibility of precipitation increas- Manganui Gorge. The zone affected by the ing with elevation on Mt Egmont. Therefore, avalanche is shown in Fig. 5. The runout the increase in WE and HS with elevation is length and the depth of debris in the gorge probably caused by Fitzharris' latter two suggest that more than just fresh snow was factors. That is, the greater the elevation, the involved in the event. A failure probably greater the amount of precipitation which falls occurred in the old snow on one of 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 in IT with elevation. Since less ice and snow melt at higher elevations, more of the ice bands are preserved in the pack. ACKNOWLEDGEMENTS We wish to express our appreciation to Julie DISCUSSION Gardner for field assistance; to Snow Mace The Mt 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 of TG meta- Tomlinson of the Christchurch Weather Office; morphism, indicates that 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 indicate that rain and/or meltwater probably REFERENCES run through the upper snow layer following Anderton, P. W., 1974: Estimation of snow storage each major snow event. These inferences from and melt in the catchment of Lake Pukaki. N.Z. Hydrological Society Annual Symposium, observed snowpack structure agree with avail- University of , Dunedin. able meteorological records and accounts of Chinn, T. J., 1969: Snow survey techniques in the skiers. Waitaki Catchment, South Canterbury. N.Z. Journal of Hydrology, 8: 68-76. Contrasting situations are given by Mc- Coulter, J. D., 1975: The Climate. In Biogeography and Ecology in New Zealand (ed. G. Kuschel). Nulty and Fitzharris (1980) and Weir and N.Z. Meteorological Service Meteorological Owens (1981), who respectively studied snow- Note, No. 80. pack structure at Porter Heights and Mount Eitzharris, B. B., 1976a: Spatial variations in snow Hutt, both areas in the eastern Canterbury accumulation on Central Otago Mountains. Pro- ceedings, N.Z. Hydrological Society Symposium, mountains. In both cases, periods of cold, Rotorua: 165-177. clear weather produced TG crystals in the Eitzharris, B. B., 1976b: An avalanche event in the snowpack. In the colder, more continental seasonal snow zone of the Mt Cook region. climate of these sites the TG crystals persisted N.Z. Journal of Geology and Geophysics, 19: 449-462. until late in the season, and probably com- Eitzharris, B. B., 1978: Problems in estimating snow prised weak layers which failed, resulting in accumulation with elevation on New Zealand avalanche activity. If TG crystals did form in mountains. N.Z. Journal of Hydrology, 17: Mt Egmont's snowpack, subsequent warm 78-90. Fitzharris, B. B. and Owens, I. F., 1981: Avalanche temperatures and the percolation of rain atlas of the Milford Road and an assessment of through the pack would cause a change from the hazard to traffic. New Zealand Mountain TG to MF grains. Safety Council Report, No. 4, Snowpack Structure and Climate, Mt. Egmont 51

Fitzharris, B. B., McNulty, D., Owens, I. F. and avalanche problems in New Zealand. New Zea- Miller, I. D., 1983: A pilot avalanche forecast- land Mountain Safety Council, Avalanche Com- ing project for the Craigeibum Range, New mittee Report, No. 2. Zealand. Weather and Climate, 3: 52-58. McNulty, D. and Fitzharris, B. B., 1980: Winter Gamier, B. J., 1958: The Climate of New Zealand. avalanche activity and weather in Canterbury Edward Arnold, London. alpine basin. N.Z. Journal of Geology and Geo- Heine, A. J., 1962: Snow structure at Mount Rua- physics, 23: 103-111. pehu. N.Z. Journal of Geology and Geophysics, Perla, R. I. and Martinelli, M. Jr., 1976: Avalanche 5: 573-583. Handbook. USDA Forest Service, Agriculture Heine, A. J., 1963: ice and snow Handbook, 489. photo survey. N.Z. Journal of Geology and Prowse, T. D., 1981: The Snow Environment of the Geophysics, 6: 261-267. Craigieburn Range. Unpublished Ph.D. thesis. Kells, B. R. and Thompson, R. D., 1970: The University of Canterbury, Christchurch. Whakapapanui Glacier, Mt Ruapehu. Soil and Thompson, C. S., 1981: The climate and weather of Water, 6: 34-38. the Taranaki region. N.Z. Meteorological Ser- Krenek, L. O., 1959: Changes in the glaciers of Mt vice Miscellaneous Publication, No. 115(9). Ruapehu in 1955. N.Z. Journal of Geology and Weir, P. L. and Owens, I. F., 1981: Snowpack Geophysics, 2: 643-653. structure at , Canterbury, in 1979. La Chapelle, E. R., 1979: An assessment of New Zealand Journal of Science, 24: 95-102.

SPECTACULAR LIGHTNING DISPLAY OVER ASHBURTON

Photograph by Nigel Yates, Ashburton Guardian.

This spectacular display of lightning, photographed at Ashburton about 9.40 p.m. on 21 November 1982, was associated with one of a group of thunderstorms which advanced along the Canterbury coast that evening. These storms, accompanying a southerly change, also brought considerable amounts of hail which damaged crops, and there were reports of power failures caused by lightning strikes.