Energy balance and synoptic climatology of a melting snowpack in the Southern Alps, New Zealand

INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 17, 1595±1609 (1997)

ENERGY BALANCE AND SYNOPTIC CLIMATOLOGY OF A MELTINGSNOWPACK IN THE SOUTHERN ALPS, NEW ZEALAND

S. M. NEALE AND B. B. FITZHARRIS*

Department of Geography, University of Otago, Dunedin, New Zealandemail: [email protected]

Received 21 August 1996Revised 28 May 1997

Accepted 10 June 1997

ABSTRACT

Snow melt is calculated at 1780 m a.s.l., near the Main Divide of the Southern Alps, using a bulk aerodynamic energy balanceapproach. Results are related to melt measured directly using a lysimeter and to synoptic weather patterns. Measurements aretaken half hourly, over a 38 day period from the start of the spring melt season. Melt values at the site average 10 mm day71

but vary from less than 1 mm day71 to 63 mm day71. The energy balance model overestimated measured melt by 8 per cent.The synoptic situation exerts a strong in¯uence on the magnitude of melt. Melt is highest during north-westerly storms, andthree such days contributed one-third of the total melt recorded during the ®eld season. Melt is also high during anticyclones.Different synoptic situations generate distinctive energy budgets, with radiation dominating during large-scale anticyclonicpatterns, but sensible heat ¯ux also is important during north-westerly circulation patterns. Distinct pulses of melt, each lastinga period of about 1 week, re¯ect the cyclical passage of troughs and anticyclones across New Zealand. # 1997 RoyalMeteorological Society. Int. J. Climatol., 17, 1595±1609 (1997)

(No. of Figures: 7. No. of Tables: 3. No. of References: 37)

KEY WORDS: Southern Alps; New Zealand; lysimeter; snow melt; energy balance; synoptic climatology.

INTRODUCTION

The understanding of the in¯uence of synoptic weather systems on local and microscale climatology is crucial in

the determination of the resulting melt rate of the snow pack and the hydrological response of alpine catchments.

Better understanding of the links between these scales can be achieved only through a widespread set of

observations, both spatially and temporally, of synoptic conditions, and the resulting energy and moisture ¯uxes

over snow and ice in the alpine environment. Such knowledge has important implications for the down-scaling of

general circulation models and climate change studies. Little still remains known about the interrelationships

among large-scale atmospheric processes, the energy balance over snow, and the resulting melt rate (McGregor

and Gellatly 1996). The objective of this paper is to clarify these interrelationships for a site in the Southern Alps

of New Zealand, in spring melt conditions.

The forecasting of melt of seasonal snow is important economically in the Southern Alps because of its

contribution to hydroelectricity generation, which accounts for more than 75 per cent of New Zealand's total

electricity generation. Snow melt is also important for irrigation water and for recreation, but because the area of

snow cover is so vast (60 000 km2) and unpopulated, there are few measurements of melt rate or of the energy

sources involved. There is a need for more direct measurements of snow melt, and for more measurements close

to the Main Divide of the Southern Alps, because this is the key zone for generating river ¯ows and the main area

of the snow resource. Little information is presently available here.

CCC 0899-8418/97/141595±15 $17.50

# 1997 Royal Meteorological Society

*Correspondence to: Dr B. B. Fitzharris, Department of Geography, University of Otago, P.O. Box 56, Dunedin, New Zealand.

The Southern Alps of New Zealand are a major mountain system surrounded by vast areas of ocean in the mid-

latitudes of the Southern Hemisphere. On average, seasonal snow accumulates from April to reach a maximum in

October (Fitzharris and Garr, 1995). Snow melt is usually underway by September and at higher elevations

continues into summer. The volume of snow that seasonally accumulates and melts each year is estimated at

20 km3 water equivalent. As a result, snow melt exerts a strong in¯uence on the hydrology of alpine catchments.

Recent New Zealand studies have alluded to the role of synoptic weather patterns in in¯uencing the energy

balance over melting snow. Prowse and Owens (1982) showed that in the Craigieburn Ranges, which are on the

eastern fringe of the Alps (refer to Figure 1), sensible heat (QH) was the main contributor to snow melt, with net

radiation (Q*) being the second most important. Highest values of snow melt occurred during periods of rain

from the north-west, and were usually associated with warm and windy conditions and high sensible heat

transfers. The precipitation heat ¯ux (QP) was generally small. Moore (1983) compared the energy balance over

snow at Temple Basin, near the Main Divide of the Southern Alps, and Ski Basin in the Craigieburn Range (to the

east of the Main Divide). In Temple Basin, QE (latent heat ¯ux) and QP (precipitation heat ¯ux) were higher but

values for Q* and QH were lower, when compared with the Ski Basin. He attributed these ®ndings to the more

maritime in¯uence at Temple Basin. During the predominant westerly air¯ows it has more humid air and higher

levels of rain.

To the west of the Main Divide, energy balance investigations have concentrated on glacier surfaces, rather

than seasonal snow surfaces. Marcus et al. (1985) report short-term measurements (19±53 h periods) from the

lower reaches of Franz Josef glacier. This type of study has been followed up by Owens et al. (1992) and

Ishikawa et al. (1992), who also measured QE and QH using eddy correlation equipment. Surface ice melt occurs

during all seasons. Highest mean rates occur in summer, but intense warm storm events that occur even in winter

produced the maximum daily rates of ice melt. They found that the contributions of the radiative and convective

¯uxes to melt were strongly controlled by the prevailing synoptic situation.

Hay and Fitzharris (1988) measured the energy balance over the lower Ivory Glacier for two summer periods

totalling some 53 days. Ice-melt rates were also measured and related to the apportioning of the different energy

balance terms and to the synoptic weather patterns. They concluded that:

`. . . different synoptic weather situations generate distinctive energy budgets, with radiation (Q*)

important during southerly circulation patterns and the convective ¯uxes (QE and QH) relatively more

important with northerly circulation patterns.'

In general these studies parallel research elsewhere in the world (e.g. Hoinkes, 1968; Holmgren, 1971; Alt,

1978; Aguado, 1985; Karl et al., 1993; McGregor and Gellatly, 1996), which shows that the partitioning of

energy balance components over melting ice and snow surfaces is closely related to large-scale synoptic weather

patterns. Most of the studies mentioned do not measure melt directly, but infer it from river ¯ows or ablation

stake networks and density measurements.

This study further examines the linkages between large-scale atmospheric processes, the energy balance over

snow, and the resulting melt rate. The ®ndings are given strength by the use of direct measurements of snow melt

and the continuous measurement of energy ¯uxes over a 38 day period. A special climate station and snow melt

lysimeter were established and maintained over the study period at Mueller Hut (1780 m a.s.l.), near the Main

Divide of the Southern Alps. These local-scale measurements are used to calculate the energy balance, which is

then related to the large-scale synoptic weather situation.

THE SOUTHERN ALPS AND THE FIELD SITE

The Main Divide of the Southern Alps (Figure 1) has an elevation of 2000 m to over 3000 m and lies astride the

Southern Hemisphere westerlies. The climate near the coast is moist and equable. Annual mean temperature is

about 11�C and monthly averages have a seasonal range of less than 10�C. Precipitation in the Southern Alps

varies from about 10 000 mm or more near and west of the Main Divide, to less than 1000 mm, 100 km to the

east. There is only a small seasonal variation, with a winter minimum and spring maximum. Below 1000 m

elevation, almost all precipitation falls as rain. The end-of-summer snowline east of the Main Divide is at about

1596 S. M. NEALE AND B. B. FITZHARRIS

INT. J. CLIMATOL, VOL. 17: 1595±1609 (1997) # 1997 Royal Meteorological Society

Figure 1. Location of the Mueller Hut ®eld site, in the Southern Alps, South Island of New Zealand

MELTING SNOWPACK ENERGY BALANCE 1597

# 1997 Royal Meteorological Society INT. J. CLIMATOL, VOL. 17: 1595±1609 (1997)

2100 m. Because of the mild climate, rain can also occur at elevations above 3000 m on some occasions, but

above 2000 m the majority of winter precipitation falls as snow.

The ®eld site is within Mount Cook National Park and 2�4 km east of the Main Divide (Figure 1). The main

peak in the vicinity is Mount Sefton (3151 m). It lies within the catchment of the Hooker river, the waters of

which eventually ¯ow into Lake Pukaki, the major storage lake used for hydroelectricity generation in New

Zealand.

Mueller Hut, approximately 1780 m a.s.l., offers an established base for ®eldwork near the Main Divide of the

Southern Alps. It is located on a broad ridge in the major spillover zone of the predominant westerly

precipitation. Adjacent to the hut lies a snow®eld above the Mueller glacier. The snow®eld covers a smooth, ¯at

area of approximately 50 ha. Regular measurements of snow depth from a snow-pole located approximately

30 m below the hut (D.O.C., 1995), show that since 1983 the accumulation season near the Mueller Hut has

peaked between late September to early October, with snow depths reaching between 2 and 6 m. The snow®eld is

bordered by Mount Olivier (1912 m) to the south, the Mueller valley to the west and Hooker valley to the north.

METHODS

Measurements at the ®eld site took place from 16 October until 25 November 1995. An automatic weather station

was set up on the snow®eld, approximately 50 m west of the Mueller Hut. Instruments were attached to a tripod

and reset to level daily. They measured wet and dry bulb air temperature, net radiation, wind speed and wind

direction, pressure and snow temperatures. Any snowfalls were measured manually using three snowboards and a

ruler to obtain daily new snow depth. New snow density was obtained using a Taylor±Lachapelle snow density

kit. Snow water equivalent was calculated as the product of mean depth and snow density. A 1 m2, perspex

lysimeter was buried in the snowpack, approximately 1 m below the surface. A hose was attached to a tipping

bucket mechanism and measured percolated melt directly. Another tipping bucket rain-gauge also measured

precipitation falling as rain. This was subtracted from lysimeter measurements to give snow melt runoff only. All

instruments were attached to a Campbell logger. The logger sampled each instrument every 30 s and produced

half-hourly and 24-h totals or averages. Energy balance estimates for melt are compared with these direct

lysimeter measurements.

Calculation of the energy balance over melting snow

The energy balance over a melting snowpack is given by:

QM � � Q*� QH � QE � QG � QP �1�

where Q* is net radiation, QP is precipitation heat ¯ux, QH is the sensible heat ¯ux, QE is the latent heat ¯ux, QG

is the ground heat ¯ux, and QM is the energy used in snow melt.

Fluxes towards the snow surface are positive, those away from the surface are negative. Fluxes are expressed

either in W m72 for half-hourly or hourly values, or MJ m72 day71 for daily values.

Actual water yielded from snow melt (mm) is given by:

M � QM=�rLf � �2�

where M is measured snow melt water equivalent (mm), r is density of water (1000 kg m73) and Lf is the latent

heat of fusion (0�335 MJ kg71).

A net radiometer (REBS, model Q*6) measured Q*. A wire cage covered the radiometer to protect the plastic

hemispheres from damage by the destructive native mountain parrot, the kea. The well-spaced mesh is assumed

to have little effect on the sky view factor of the radiometer. Calculation of QP is after Male and Gray (1981),

using wet bulb air temperature and precipitation data. Variable QG is measured using a single soil heat ¯ux plate

buried 4 m below the snow surface at the snow±ground interface.



Calculation of QH and QE was based on the bulk aerodynamic equations of Prowse and Owens (1982), where:

QE � raLvDe�0�622=Pa��ea ÿ es� �3�QH � raCpDh�Ta ÿ Ts� �4�

where ra is air density (kg71 m73), Cp is speci®c heat of air at constant pressure (1004 J kg71 K71), Lv is latent

heat of vaporisation of water (2.5 MJ kg71), Dh is exchange coef®cient for sensible heat (m s71), De is exchange

coef®cient for latent heat (m s71), Ta is air temperature (�C), Ts is surface temperature (�C), Pa is near surface air

pressure (hPa), ea is vapour pressure at Ta (hPa), and es is surface vapour pressure at Ts (hPa).

A platinum-probed psychrometer measured wet and dry bulb air temperatures. When the air temperature (Ta)

over a snowpack is greater than 0�C, the snowpack is assumed to be melting and the surface temperature (Ts) is

assumed to be 0�C (Hay and Fitzharris, 1987). The study also assumes (after Price and Dunne, 1976) that if the

air temperature over snow is below 0�C, the surface temperature adopts the same value as the wet bulb. The

melting snow surface is assumed to be saturated (after Lowe, 1977).

Air density (ra) is calculated following Wallace and Hobbs (1977), and vapour pressure of the air (ea) is

calculated using the psychrometric approach.

Values for the exchange coef®cients for sensible (Dh) and latent (De) heat ¯uxes are assumed to equal that for

momentum (Dm). This is based on the similarity principle, as described by Sellers (1965), Priestley (1959), and

Anderson (1976) and used by Prowse and Owens (1982). Assuming neutral conditions, the exchange coef®cients

are expressed as:

Dh � De � Dm � k2u=�ln�z=z0��2 �5�where k is von Karmen's constant, u is windspeed (m s71) at height z (m), and z0 is roughness length (m).

Von Karmen's constant (k) is assumed to equal 0�4 (Businger, 1973; Prowse and Owens, 1982; Hay and

Fitzharris, 1987).

Measurement of the exchange coef®cients is sensitive to accurate evaluation of z0 (Lettau, 1969). The present

study assumes a mean momentum ¯ux over the measurement period of 0�0040, a value found by Hay and

Fitzharris (1987) elsewhere in the Southern Alps, and this is used to calculate a mean roughness length (z0).

The bulk aerodynamic model was adjusted in conditions other than neutral atmospheric stability by using the

Richardson number (Ri), following the practice of Price and Dunne (1976), Prowse and Owens, (1982), Hay and

Fitzharris, (1987):

Ri � gzTÿ1uÿ2z �Ta ÿ Ts� �6�

where g is acceleration due to gravity (9�81 m sÿ1), z is the height of the instruments (m), T is the absolute

temperature (K) in the z layer, and uz is windspeed (m s71) at height z.

This leads to the adjustment of stability dependent exchange coef®cients (D0h, D0e), as described in Oke (1987,

p. 382) using Ri calculated from equation (6), for stable situations:

D0h � D0e � Dm�1ÿ 5Ri�2 �7�and for unstable conditions:

D0h � D0e � Dm�1ÿ 16Ri�0:75 �8�If instrument errors are maximized and minimized in the same direction (i.e. no compensation of the individual

error terms), the errors in QM are estimated at � 9 per cent. However, in practice, measurement errors will

contribute less than this, because the signs of the individual measurement errors are unlikely always to be the

same direction.

Prowse and Owens (1982) note that these formulations assume steady, turbulent ¯ow over an in®nite, uniform

surface. These conditions are not met at Mueller Hut, where the terrain is smooth locally, but the surroundings are

mountainous. It is also assumed that the bulk exchange coef®cients for water vapour, heat, and momentum are

similar. Male and Gray (1981) show that the bulk aerodynamic equations can result in considerable errors, even

when corrected for stability. However, these errors are largest for unstable conditions, and become smaller for



stable conditions. As shown in Figure 3 , most days in this study had air temperatures at or above 0�C so that

neutral or stable conditions generally prevailed.

Analysis of synoptic situations

A synoptic classi®cation is applied to each of the 38 daily surface weather maps of the ®eld measurement

period. The classi®cation is based on that used by Hay and Fitzharris (1988) and divides daily surface weather

maps into the following classes:

(i) south to south-westerly ¯ow over the Southern Alps;

(ii) north to north-easterly ¯ow over the Southern Alps;

(iii) north-westerly ¯ow over the Southern Alps;

(iv) an anticyclone over the Southern Alps, or weak easterly ¯ow;

(v) a trough or front over the Southern Alps.

The mean melt for each synoptic class is calculated, and Mann-Whitney tests undertaken to ascertain whether

these melt values are signi®cantly different amongst the classes.

RESULTS

Snow melt and the sequence of weather

Over the total 38 days of the ®eld season, melt calculated from the energy balance showed good agreement with

that measured by the lysimeter. The energy balance overestimated measured total melt by 8 per cent (406 mm

compared with 375 mm). When comparisons are made for individual days when air temperature was above

freezing, the correlation between the two is more moderate (r� 0�59, refer to Figure 2), with some scatter about

the 1:1 line. The scatter seems to be due mainly to undermeasurement of actual melt from refreezing of melt

water in the snow layer between the surface and lysimeter (A points), or to underestimation by the energy balance

model during warm windy conditions (B points). If A and B points are excluded from Figure 2, r� 0�75.

Thermocouples were buried in the snowpack to record internal temperatures, but these failed to record properly in

the ®eld and attempts to calibrate them were unsuccessful. As a result, any further inferences made about

Figure 2. Daily melt measured by the lysimeter compared with that estimated by the energy balance model. Both terms are millimetres waterequivalent. A points are due to undermeasurement of actual melt due to refreezing of melt water. B points are due to underestimation by the

energy balance model due to warm windy conditions



snowpack temperatures are based on lapsed air temperatures. Lysimeter melt was generally less than 20 mm

day71, but exceeded this on 3 days, reaching 63 mm day71 on 9 November. Together these 3 days accounted for

one-third of all melt of the ®eld season.

From observations of lysimeter melt, temperature, and windspeed (Figure 3), four main melt pulses are

identi®ed during the ®eld season (A, B, C, and D). The lysimeter-measured melt follows a similar pattern to air

temperature. Each melt pulse builds up over about a week (but may vary between 4 and 9 days), with maximum

melt occurring near the end. Pulses are separated by intervals of about 2 days when no melt occurs. During non-

melt intervals minimum air temperatures fall below ÿ2�C, which creates a cold content in the snowpack.

Melt pulses and excessive melt days are related to distinct patterns of synoptic weather. The synoptic weather

pattern during part of melt pulse B (5±11 November) is shown in Figure 4. Between 5 and 7 November, a trough

Figure 3. Mean daily temperature, windspeed and lysimeter-measured melt during the ®eld season, with the four main melt pulses shown(A, B, C, and D)



Figure 4. The sequence of synoptic weather patterns during melt period B



lying over the Southern Alps was replaced by an anticyclone approaching from the south-west. As a result, skies

became clearer and solar radiation levels increased, but cooler air advected from the south reduced the air

temperature. The net result was low levels of snow melt.

Between 8 and 10 November, the anticyclone moved north and east. Wind direction changed to the north and

then north-west. An increase in air temperatures and windspeed was associated with the advance of occluded

fronts. This, combined with clear weather, greatly increased daily snow melt.

Passage of a cold front on the 11 November produced a change to west±south-west ¯ow, which lowered air

temperatures, and created a cold content in the snowpack. Snow melt therefore ceased.

This sequence of weather patterns was typical of the ®eld season. Easterly migrating anticyclones dominated

weather for 3±4 days and were replaced by frontal troughs, which lasted 2±3 days, when a similar cycle began

again. These patterns, typical of New Zealand's 4±10 day weather cycle, exerted a strong in¯uence on

temperature and energy balance regimes and produced distinct snow melt pulses during the ®eld season.

Energy balance analysis

Figure 5 shows the daily contribution of individual ¯uxes (Q*, QH, QE, Q and QP) to the overall energy

available for melt (QM). There are large variations in the contribution that each ¯ux provides for melt on different

days and in the total energy available for melting snow. Table I gives the mean, standard deviation, maximum,

and minimum values of each of the ¯uxes over the study period.

Over the 38 days, Q* provided the dominant source of energy for QM (63 per cent). Sensible heat (QH)

averaged 27 per cent of QM, followed by QE (4 per cent), QG (3 per cent) and QP (3 per cent). The contribution of

Figure 5. Partitioning of the energy ¯uxes for snow melt over 38 days

Table I. Energy budget for the Mueller Hut ®eld site for 19 October 1995 to 25 November1995 (MJ m72 day71)

Q* QH QG QE QP QM

Mean 2�2 0�9 0�1 0�1 0�1 3�5SD 1�8 0�8 0�0 0�4 0�2 2�4Maximum 5�7 3�5 0�1 1�7 0�9 9�9Minimum ÿ1�4 0�0 0�1 ÿ0�4 0�0 0�0



QG to the daily energy balance was negligible, remaining consistently small throughout at 0�5 MJ m72 day71, or

about 0�15 mm day71. On a couple of days, Q* was negative, and a small amount of snow loss by sublimation

occurred (i.e. QE was negative).

Snow melt energy balance and synoptic weather patterns

The partitioning of energy balance components for melt according to weather class is given in Figure 6. Table

II provides statistics of snow melt by weather class.

Class (i) days are typi®ed by anticyclones centred over the Tasman Sea and depressions lying to the south of

the South Island. Air¯ow is from the south or south-west. The energy balance is dominated by Q* (70 per cent of

QM). However, daily values of Q* are relatively low (averaging 2.1 MJ m72 day71) and this is re¯ected in the

daily melt rate of 9�2 mm day71. Low temperatures and moderate windspeeds (averaging 6 m s71) combine to

keep humidity gradients over the snowpack down and reduce the magnitude of the convective ¯uxes. Variables

QH and QE still make up 29 per cent of QM but this adds to less than 0�9 MJ m72 day71. Brief frontal activity

associated with class (i) weather patterns brings short periods of rain but, QP is less than 1 per cent of QM.

Class (ii) days typically have an anticyclone to the north-east of New Zealand and a north or north-east ¯ow

over the Southern Alps. Melt averaged 11�3 mm day71. Clear skies associated with anticyclonic subsidence

allow high levels of incoming solar and net radiation. The resulting energy balance is dominated by Q* (71 per

cent). Warm northerly air¯ows lead to modest contributions from QH. The drier air makes QE negligible, but still

positive, as some condensation occurs. The forcing of air over the ranges of the South Island causes heavier

precipitation, often as rain, and hence QP can be 5 per cent of QM.

Class (iii) days have north-westerly circulation patterns over the Southern Alps, with an anticyclone to the east.

There is a strong pressure gradient over the South Island, which results in high windspeeds. Average daily melt

Figure 6. Partitioning of the energy ¯uxes for snow melt according to weather classes

Table II. Snow melt variations with synoptic weather class

Class Number of daysQM

(MJ m72 day71)Mean melt

(mm day71)Standard deviation

(mm day71)Standard

error

Class (i) 11 3�1 9�2 5�1 1�5Class (ii) 4 3�8 11�3 1�8 0�9Class (iii) 7 5�3 15�9 9�8 3�7Class (iv) 7 5�1 15�3 5�7 2�2Class (v) 9 1�5 4�6 6�6 2�2



was high at 15�9 mm day71 but varied considerably (standard deviation is 9�8 mm day71). The energy balance is

still dominated by Q* (59 per cent), but QH increases its importance (34 per cent), as does QP (6 per cent). The

presence of an anticyclone results in clear skies and high levels of net radiation. The other effect of increased net

radiation and warm wind is to dry the air. As a result of this QE values are low, but positive, because a little

condensation continues.

Class (iv) days have anticyclones over or near the Southern Alps. Average daily melt is high at 15�3 mm

day71. Average daily values for Q* are also high (3�4 MJ m72 day71), due to the clear skies, and Q* makes up

68 per cent of total daily QM. Also important is QH (1�5 MJ m72 day71), which contributes 29 per cent of QM.

The drier air means that QE and QP play minimal roles in melt.

Class (v) days have troughs or fronts covering the Southern Alps. They exhibit low daily melt values (4�6 mm

day71). This is a re¯ection of the low overall energy balance. Variable Q* still dominates (66 per cent), but

averages less than 1�0 MJ m72 day71. The cooler air temperatures associated with the troughs and fronts reduce

humidity gradients over the snowpack and decrease the magnitude of QH and QE. In fact on over half of these

days, the air was so dry that evaporation occurred (negative QE). Frontal precipitation leads to some small

contribution from QP (3 per cent) of QM.

Comparisons of the mean melt by synoptic class using Mann±Whitney tests and 95 per cent con®dence levels

showed no signi®cant difference between class (iii) and (iv) days. Melt between classes (i), (ii) and (v) days also

showed no signi®cant difference. The statistical analysis suggested that melt on class (i), (ii) and (v) days was

signi®cantly different from that on class (iii) and (iv) days. Other differences may become apparent over a longer

®eld season.

DISCUSSION

Previous research both in New Zealand and elsewhere (Hoinkes, 1968; Holmgren, 1971; Alt, 1978; Aguado,

1985; Hay and Fitzharris, 1988; Ishikawa et al., 1992; Owens et al., 1992) show that large-scale weather patterns

are intricately related to the partitioning of energy balance components for ice melt. This study also shows that

the synoptic weather patterns control snow melt. Weather patterns over the Southern Alps, which lie within the

westerlies, are dominated by the easterly migration of a sequence of ridges or anticyclones separated by troughs

or depressions about once a week. Each sequence gives rise to a pulse of seasonal snow melt. This pattern could

be disrupted by blocking from anticyclones, but this is very uncommon in New Zealand in spring.

Within each sequence, different synoptic classes generate distinctive energy budgets. Radiation (Q*)

dominates in all classes, followed by QH. Variables QE, QG and QP make up only a small percentage of energy for

melt. During anticyclonic situations, Q* and QH are large and melt is at its maximum. When the north-westerly

¯ow over the Main Divide is strong, temperatures rise and energy ¯uxes are higher and melt becomes large.

Variable QH becomes a more important part of melt. North-westerly events are important catalysts in priming the

snowpack, removing any cold content and inducing spring snow melt. When a depression lies to the south of New

Zealand, ¯uxes are much smaller owing to the cooler temperatures associated with the southerly air¯ows. At least

in the early phases of spring snowmelt, southerly air¯ows and nocturnal frost appear brie¯y to create a cold

content in the snowpack. These sequences of weather patterns are related directly to local pulses of snow melt

and suggest that synoptic climatology could lead to useful rules for forecasting in¯ows to hydroelectricity lakes.

Other New Zealand investigations report higher convective heat ¯uxes and rates of melt during anticyclonic

conditions (Moore, 1983; Prowse and Owens, 1982; Hay and Fitzharris, 1988) than those reported at the Mueller

site. Kuusisto's (1986) summary of global energy balance studies concludes that energy balance compositions are

site-speci®c. This is supported by the data in Table III, but due to differing methods and conditions in the

different studies, the energy balances may not be strictly comparable. The energy balance also changes through

the ®eld season (Ishikawa et al., 1992). The Mueller study began early in the melt season and the ®eld site was at

a relatively high elevation (1780 m a.s.l.), only 300 m below the end-of-summer snowline. The lower melt rates

and different energy balances at the Mueller site compared with others, may be a re¯ection of these two factors.

This view is also supported by Moore (1983) in his study of two alpine basins in New Zealand, who found spatial

variations in melt rates and in the energy balance.



Figure 7 compiles the average daily energy balances over snow and ice, at ®ve South Island sites, in a transect

across the Southern Alps (Prowse and Owens, 1982; Moore, 1983; Hay and Fitzharris, 1988; Ishikawa et al.,

1992). Values from the Mueller site are included. The relative partitioning of the energy ¯uxes varies across the

Alps. The Mueller site shows the highest contribution by Q* to melt (63 per cent). This is at the expense of the

convective ¯uxes (QH and QE), which contributed 32 per cent of snow melt energy at Mueller compared with 76

Figure 7. Relative energy ¯uxes (¯ux density as a percentage of QM with respect to position across the Southern Alps

Table III. Summary of average snow melt and of relative contributions of different components of the energy balance to snowmelt (adapted from Kuusisto (1986) and listed in order of melt). Negative signs indicate ¯uxes are directed away from the

snowpack

Average meltPercentage

contribution to QM

Reference Site (mm dayÿ1) Q* QH QE

Eaton and Wendler (1981) Open ®eld (Alaska) 3 67 33 ÿ68Gold and Williams (1960) Open ®eld (Canada) 7 75 25 ÿ25Kuusisto (1979) Open ®eld (Finland) 7 46 53 ÿ1Present study (1996) Mountains (New Zealand) 10 63 28 5Kuusisto (1982) Open ®eld (Finland) 14 48 47 2Treidl (1970) Open ®eld (Michigan) 15 17 47 36De la CasinieÂre (1974) Mountains (France) 16 85 15 ÿ15Clarke (1995) Mountains (New Zealand) 17 57 32 9Braun and Zuidema (1982) Mountains (Switzerland) 23 8 65 20Prowse (1981) Mountains (New Zealand) 28 33 57 8Moore (1983) Mountains (New Zealand) 32 16 57 25Prowse and Owens (1982) Mountains (New Zealand) 35 30 57 13Hay and Fitzharris (1987) Glacier (New Zealand) 38 52 30 16Moore (1983) Mountains (New Zealand) 40 18 78 4Ishikawa et al. (1992) Glacier (New Zealand) 42 45 35 20Marcus et al. (1985) Glacier (New Zealand) 44 26 73 1Dewalle and Meiman (1971) Forest opening (Colorado) 50 56 44 ÿ3McGregor and Gellatly (1996) Snow covered glacier (France) 134 67 33 0Owens et al. (1992) Glacier (New Zealand) 173 25 54 22



per cent (at Ski basin), 72 per cent (at Temple Basin), 55 per cent (at Franz Josef), and 46 per cent (at the Ivory

Glacier). Figure 7 also shows that Q* is the dominant energy ¯ux at West Coast sites. At more eastern sites QH is

the dominant ¯ux, probably due to the in¯uence of north-west foehn winds.

The high altitude of the Mueller ®eld site and the dry nature of the alpine air leads to dominance of Q*. The

value of QH varied throughout the ®eld season but was usually constrained by cool temperatures at that altitude. It

became more important during days with warmer air temperatures. These usually occurred with anticyclones,

when the freezing level rose above the site due to heating by large-scale subsidence, or with a north-west air¯ow,

when foehn conditions occurred. Because of the location of the Mueller site, east of the Main Divide and the dry,

high altitude air, QE was small during the majority of the measurement season.

Neither Prowse and Owens (1982), Moore (1983) or Hay and Fitzharris (1988) measured QG in their

investigations. The present study shows that in terms of daily contributions to QM, QG plays a minimal role,

averaging 0�05 MJ m72 day71, although this was measured by one ¯ux plate only. This value is lower than that

found by the US Army of Corps of Engineers (1956) of 0�27 MJ m72 day71 and the 0�86 MJ m72 day71 found

by Gold (1957), but is similar to previous research by Male and Gray (1981). The Mueller snowpack is much

deeper than in these studies. Accumulated over the ®eld season QG caused 3 per cent of seasonal snow melt at the

Mueller ®eld site.

For many alpine environments, anticyclonic weather types and their associated air enthalpy and

high radiation levels are responsible for much ablation, as noted by McGregor and Gellatly (1996). However,

this study also demonstrates that, at least for maritime mountains, advective in¯uences can be equally

as important, as demonstrated by north-westerly circulation regimes over the South Island of New Zealand.

The spectrum of atmospheric circulation types over the Southern Alps appears to determine the amount of snow

melt. With enhanced greenhouse climate changes, any changes in this spectrum will affect rates of melt. Such

issues need to be better understood if the impacts of climate change on snow resources are to be assessed and

modelled.

CONCLUSIONS

Snow melt of a high mountain snowpack is measured with a lysimeter and calculated using an energy balance

model for a 38 day period. The model provides reasonable estimates of measured snow melt. Over the study

period, the model overestimated measured total melt by 8 per cent (406 mm compared with 375 mm). Net

radiation (Q*) provided the dominant source of energy for melt (63 per cent of QM). Sensible heat (QH) provided

27 per cent, followed by latent heat (QE, 4 per cent), ground heat ¯ux (QG, 3 per cent), and precipitation heat ¯ux

(QP, 3 per cent).

Synoptic climatology greatly in¯uenced both the magnitude and partitioning of the energy ¯uxes that

contribute to snow melt. Snow melt was greatest during anticyclones and north-westerly air¯ows. The average

daily melt for north-west air¯ows (class (iii) events) was 15�9 mm day71, but varied considerably. This was

similar to an anticyclone lying over or near the Southern Alps (class (iv) events), which produced a daily average

melt of 15�3 mm day71. Class (ii) events, when a north to north-east ¯ow covered the Southern Alps, resulted in

an average daily melt of 11�3 mm day71. Class (i) events (south to south-west ¯ow) produced an average of

9�2 mm day71. The situation of a trough or front crossing the Southern Alps (class (v) events) resulted in the

lowest melt, with an average daily melt of 4�6 mm day71.

The synoptic situation is also re¯ected in the partitioning of the energy balance components for melt. The ®ve

classes of synoptic situation each produced a different mix of the contribution of energy balance components to

melt. On south to south-westerly days, Q* dominated the energy balance (70 per cent of QM). With north to

north-easterly circulation patterns, Q* accounted for 71 per cent of QM, QH made only a modest contribution, the

dry air made QE negligible, and QP was up to 5 per cent of QM. North-westerly days showed the lowest relative

contribution of Q*, at 59 per cent of QM. The contribution of QH was 34 per cent of QM, with QP contributing 6

per cent, and QE was low. Anticyclone days showed Q* to be 68 per cent of QM, with QH at 29 per cent, whereas

QE and QP were minimal due to the dryness of the air. On trough or frontal days, the energy available for melt

was low.



The cyclic nature of the sequence of synoptic weather patterns tracking across New Zealand and

the changing energy balance during the sequence can be seen as troughs and anticyclones move over

New Zealand. As a result, the seasonal melting of the snowpack close to the Main Divide of the Southern Alps

occurs in distinct pulses. Four such pulses of melt were observed in the study period. The melt builds up over

about a week, with maximum melt occurring near the end of the sequence as the anticyclone moves away to the

east.

This study shows that there are strong and discernible links between large-scale circulation patterns, the energy

balance and snow melt. Such studies may be used for down-scaling of general circulation models to alpine

environments, where estimating snow melt for climate change scenarios is an important part of assessing future

changes in water resources and the magnitude of ¯oods.

REFERENCES

Aguado, E. 1985. `Snowmelt energy budgets in southern and east-central Wisconsin', Ann. Assoc. Am. Geogr., 75(2), 203±211.Alt, B. T. 1978. `Synoptic climate controls of mass-balance variations on Devon Island ice cap', Arctic Alpine Res., 10, 61±80.Braun, L. and Zuidema, P. 1982. `Modelling snowmelt during advection melt situations in a small basin', Proceedings, Symposium on

Hydrological Research Basins, Bern, Switzerland, Vol. 3, pp. 711±780.Clark, M. P. 1995. Investigations in snow hydrology: an examination of energy balance in¯uences on snow melt and characteristics of water

movement through snow in the Craigieburn range, New Zealand. Unpublished MSc thesis, University of Canterbury, Christchurch, NewZealand, 99 pp.

Dewalle, D. and Meiman, J. 1971. Ènergy exchange and late season snowmelt in a small opening in Colorado subalpine forest', WaterResourc. Res., 7(1), 184±188.

De la CasinieÂre, A. 1974. `Heat exchange over a melting snow surface', J. Glaciol., 13(67), 55±72.Eaton, F. and Wendler, G. 1981. `The heat balance during the snowmelt season for a permafrost watershed in interior Alaksa', Arch. Meteorol.

Geophys. Bioclimatol., 31, 19±33.Fitzharris, B. B. and Garr, C. E. 1995. `Simulation of past variability in seasonal snow in the Southern Alps, New Zealand', Ann. Glaciol., 21,

377±382.Fitzharris, B. B., Stewart, D. and Harrison, W. 1980. `Contribution of snow melt to the October 1978 ¯ood of the Pomahaka and Fraser Rivers,

Otago', J. Hydrol. (NZ), 19(2), 84±93.Gold, L. and Williams, G. 1960. Ènergy balance during the snowmelt period at an Ottowa site', General Assembly of Helsinki: Commission of

Subterranean Waters, IAHS, Publication 51, pp. 288±294.Hay, J. E. and Fitzharris, B. B. 1987. À comparison of energy balance and bulk aerodynamic approaches for estimating glacier melt',

J. Glaciol., 34(117), 1±9.Hay, J. E. and Fitzharris, B. B. 1988. `The synoptic climatology of ablation on a New Zealand glacier', J. Climatol., 8, 201±215.Hoinkes, H. C. 1968. `Glacier variation and weather', J. Glaciol. 7, 3±19.Holmgren, B. 1971. Climate and Energy Exchange on a Sub-polar Ice Cap in Summer, Parts A±F, Uppsala Universitet, Meteorologiska

Institutionen, Meddlelande, pp. 107±112.Ishikawa, N., Owens, I. F. and Sturman, A. P. 1992. `Heat balance characteristics during ®ne periods on the lower parts of the Franz Joseph

Glacier, South Westland, New Zealand', Int. J. Climatol. 12, 397±410.Jowett, I. G. 1979. `Clutha ¯ood of 1978, J. Hydrol. (NZ), 18(2), 121±140.Karl, T. P., Groisman, P. Ya., Knight, R. W. and Hein, R. R., Jr. 1993. `Recent variations of snowcover and snowfall in North America and

their relation to precipitation and temperature variations', J. Climate, 6 1327±1344.Kuusisto, E. 1982. `Temperature index, energy balanceÐor something else'? Symposium on the Use of Hydrological Research Basins in

Planning, Bern, Switzerland.Kuusisto, E. 1986. The Energy Balance of a Melting Snow Cover in Different Environments, IAHS, Publication 155, pp. 37±45.Lettau, H. 1969. `Note on aerodynamic roughness-parameter estimation on the basis of roughness-element description', J. Appl. Meteorol.,

18(5), 828±832.McGregor, G. R. and Gellatly, A. F. 1996. `The energy balance of a melting snowpack in the French Pyrenees during warm anticyclonic

conditions', Int. J. Climatol., 16, 479±486.Male, D. M. and Granger, R. J. 1979. Ènergy and mass ¯uxes at the snow surface in a prairie environment', in Modelling of Snow Cover

Runoff.Male, D. M. and Gray, D. H. 1981. `Snowcover ablation and runoff', in Gray, D. H. and Male, D. M. (eds), Handbook of Snow; Principles,

Processes, Management and Use, Pergamon Press, Toronto, pp. 360±436.Marcus, M. G., Moore, R. D. and Owens, I. F. 1985. `Short-term estimates of surface energy transfers and ablation on the lower Franz Joseph

Glacier, South Westland, New Zealand', N. Z. J. Geol. Geophys., 28, 559±567.Moore, R. D. 1983. À comparison of the snowmelt energy budgets in two alpine basins', Arch. Meteorol. Geophys. Bioclimatol. Ser. A, 33,

1±10.Moore, R. D. and Prowse, T. D. 1988. `Snow hydrology of the Waimakariri catchment, South Island, New Zealand', J. Hydrol. (NZ), 27(1),

44±68.Oke, T. R. 1987. Boundary Layer Climates, 435 pp.Owens, I. F., Sturman, A. P. and Ishikawa, N. 1992. `High rates of ablation on the lower part of the Franz Joseph Glacier, South Westland',

Proceedings of the Inaugural Joint Conference of the New Zealand Geography Society, Institute of Australian Geographers, Auckland,1992. New Zealand Geographical Society Publication, pp. 576±582.



Price, A. G. and Dunne, T. 1976. Ènergy balance computations of snowmelt in a subarctic area', Water Resour. Res., 12(4), 686±694.Priestly, C. H. B. 1959. Turbulent Transfer in the Lower Atmosphere, University of Chicago Press, Chicago.Prowse, T. D. 1981. The snow environment of the Craigieburn Range, Unpublished PhD thesis, University of Canterbury, New Zealand, 359

pp.Prowse, T. D. and Owens, I. F. 1982. Ènergy balance over melting snow, Craigiburn Range, New Zealand', J. Hydrol. (NZ, 21(2), 133±147.Sturman, A. P., Owens, I. F., Kelliher, F. M., Clark, M. P., Byers, J. N., Hunt, J. E. and McSeveny, T. M. 1994. Èstimation of the heat budget

over a glacier snow®eld, Franz Joseph, New Zealand', Ann. Meteorol., 30;, 299±302.Sverdrup, H. U. 1936. `The eddy conductivity of the air over a smooth snow ®eld', Geofys. Publikasjoner, 11(7), 1±69.Treidl, R. 1970. À case study of warm air advection over a melting snow surface', Boundary Layer Meteorol., 1, 155±168.US Army Corps of Engineers 1956. Snow Hydrology, Summary of the Snow Investigations, US Army Corps of Engineers, North Paci®c

Division, Portland, Oregon.Wallace, J. M. and Hobbs, P. V. 1977. Atmospheric Science; an Introductory Survey, Academic Press, New York, 467 pp.



Documents

Energy balance and synoptic climatology of a melting snowpack in the Southern Alps, New Zealand