Atmospheric conditions influencing the spillover of rainfall to lee of the Southern Alps, New Zealand

INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 18, 77±92 (1998)

ATMOSPHERIC CONDITIONS INFLUENCING THE SPILLOVER OFRAINFALL TO LEE OF THE SOUTHERN ALPS, NEW ZEALAND

AMANDA M. CHATER* AND ANDREW P. STURMAN

Department of Geography, University of Canterbury, Private Bag 4800, Christchurch, New Zealandemail: [email protected]

Received 27 February 1996Revised 6 June 1997

Accepted 24 June 1997

ABSTRACT

Atmospheric conditions in¯uencing precipitation distribution over a large lee-side catchment in the Southern Alps of NewZealand are investigated. A signi®cant proportion of rainfall received in the upper Waimakariri catchment is generated by itsspillover from the windward side of the alps during conditions of westerly ¯ow. A transect of rain gauges was installed toexamine the distribution of rainfall across the mountains during northwesterly events, and an index was devised to quantify themagnitude of each spillover event, allowing comparison with other events that occurred within the study period. The spilloverindex combines a measure of the total quantity of rainfall reaching the ground with its distance east of the main divide.

Correlation and regression techniques were used to assess the role of various processes in determining the observedspillover distributions during 42 events. Several atmospheric parameters were considered, including measures of air-masstemperature, humidity, instability, windspeed and direction, and frontal intensity. It was found that spillover of rainfall into theupper Waimakariri catchment is largely determined by lower level wind speeds, latent instability and frontal intensity.Existing knowledge of atmospheric processes is used to explain how these factors in¯uence spillover activity. # 1998 RoyalMeteorological Society. Int. J. Climatol. 18, 77±92 (1998)

No. of Figures: 5. No. of Tables: 3. No. of References: 30.

KEY WORDS: orographic precipitation; Southern Alps; New Zealand; spillover precipitation; wind speed; latent instability; frontal intensity;correlation; regression.

INTRODUCTION

The effects of mountains on the distribution of precipitation are well illustrated in the Southern Alps of New

Zealand, where annual precipitation is as much as 12 m on the windward (western) side of the alps (Henderson,

1993), whereas on the lee side (east) of the mountains it is generally less than 2 m (New Zealand Meteorological

Service, 1984). The region is dominated by a prevailing westerly ¯ow, due to its location between the subtropical

anticyclones to the north and eastward moving low pressure systems to the south. The Southern Alps of the South

Island are the most signi®cant obstacle to this air¯ow, being aligned almost perpendicular to the prevailing

westerlies. The alpine terrain extends to altitudes greater than 1000 m over a length of 750 km, and the

interaction between terrain and synoptic-scale circulation results in the very strong precipitation gradients

observed.

The amount and distribution of orographic precipitation depends on interaction between prevailing air masses

and the topography. Wind speed, wind direction, air-mass temperature and humidity, convective activity,

air-mass strati®cation and cloud microphysics are all factors in¯uencing the nature of orographic precipitation

at any given time. Also, orographic in¯uences may occur in conjunction with other independent rain-forming

CCC 0899-8418/98/010077-16 $17.50

# 1998 Royal Meteorological Society

*Correspondence to: A. M. Chater, National Institute of Water and Atmospheric Research, P.O. Box 384, Greymouth, New Zealand.

processes, such as those associated with fronts or cyclonic weather systems. This research is concerned

with improving knowledge of the interaction of these in¯uential factors with the orographic component of

precipitation.

The occurrence of rainfall to the lee of an orographic barrier due to transport from the upwind side is termed

`spillover', and it may contribute signi®cantly to the total rainfall received in the upper parts of lee catchments

(Sinclair et al., 1997). The position of the boundary between rain and no rain (i.e. the rainfall frontier) is variable

during trans-mountain ¯ow. This boundary usually occurs in a zone to the immediate lee of the mountain barrier.

Ryan (1987) stated that this zone generally extends 10±20 km downwind for the case of the Canterbury high

country, where the present study was undertaken, and this is supported by Sinclair et al. (1997). In this study,

spillover is characterized both by the volume of rainfall occurring to the lee of a mountain range, and by its

distribution downwind from the crest of the range. Both of these characteristics have been shown to be quite

variable during single storm events and dependent on prevailing atmospheric conditions (Sinclair et al., 1997).

Previous research concentrating speci®cally on orographic spillover is limited, although spillover activity has

been referred to in more general studies of orographic precipitation (Bleasdale and Chan, 1972). Some research

has been concerned with understanding the in¯uence of prevailing atmospheric conditions on rainfall patterns

over elevated terrain. Peck (1972) found that precipitation patterns along a transect of four stations, lying

perpendicular to the Wasatch Mountains of Utah, were most signi®cantly correlated to the 700 hPa ± 800 hPa wet

bulb potential temperature difference and the 850 hPa lifting condensation level.

Givone and Meignien (1990) examined the correlation between meteorological factors (atmospheric thickness,

air-mass temperature, air-mass water content and wind speed) upwind of the French Alps and rainfall in the

alpine catchments with the aim of predicting rainfall amounts. A 50 km transect of rain-gauges was set up

perpendicular to, and on the windward side of the mountains. The strongest correlation was between wind speed

and rainfall, with this relationship dependent on wind direction. Winds less perpendicular to the Alps produced

less rainfall.

Related research in New Zealand has been limited, although the SALPEX (Southern Alps Experiment)

programme is currently addressing the problem (Wratt et al., 1996). Hill (1961) considered rainfall patterns

speci®cally to the lee of the Southern Alps, and found that to receive appreciable amounts of rain to the east of the

divide during west to north-west ¯ow, a disturbance such as a front or trough line is necessary. However, he

concluded that ìt is very dif®cult to ®nd satisfactory criteria for the determination of the eastward spread of rains'

(Hill, 1961, p. 4). It appears that high rainfall rates on the west coast during north-westerly ¯ows, produce little

spillover when they are the result of orographic lifting only. Penetration of north-westerly rain to the lee seems to

be associated with the passage of active fronts across the Southern Alps, and with thunderstorm activity about the

divide, indicating strong instability. Research so far has not been able to identify all the factors in¯uencing the

amount of spillover of rainfall to the lee of the mountain barrier.

Sinclair (1994) applied a model for estimating the distribution of storm rainfall over orography in the New

Zealand Southern Alps region. The model used the horizontal upstream ¯ow and the height of the terrain to

estimate topographically forced vertical motion, and to calculate the condensation rate for a given air-mass

humidity. The condensation rate for different layers in the air-mass was integrated in the vertical to yield surface

rainfall rates (Sinclair, 1994). Once the model was adjusted to account for hydrometeor growth rates and fall

speeds, the spatial patterns of precipitation at the ground surface were quite well simulated, although the actual

amount of precipitation occurring at any one point was not so well reproduced.

Recent research by Sinclair et al. (1997) involved an intensive case study of a prolonged 4-day northwesterly

storm, which produced signi®cant short-term variability in the distribution of rainfall over the Southern Alps.

They concluded that changes in atmospheric stability and strength of the trans-mountain ¯ow were the main

contributing factors, so that rainfall remained on the upstream side during orographic blocking and shifted

downstream during more unstable ¯ow. Jowett and Thompson (1977) identi®ed similar variability in the location

of the edge of the rain-shadow area during westerly ¯ows.

The aim of the present study is to determine the relative importance of different meteorological parameters in

contributing to spillover into the Waimakariri River catchment. This catchment lies to the immediate lee (east) of

the Southern Alps (Figure 1) and receives signi®cant quantities of rainfall from spillover during westerly ¯ows

(actually between southwest and north-northwest). The study by Sinclair et al. (1997), also conducted as part of

78 A. M. CHATER AND A. P. STURMAN

Int. J. Climatol, Vol. 18: 77±92 (1998) # 1998 Royal Meteorological Society

the SALPEX programme, represented an initial analysis of a single extreme case, whereas the present study

extends their work by analysing data from more than 40 westerly wind events from the same year.

This study is expected to have practical applications. Understanding how upstream conditions affect rainfall in

mountainous areas, and the development of realistic models, would permit more accurate forecasts for remote

areas without the cost and impracticality of installing equipment in dif®cult terrain. Better forecasting has

bene®ts for human activities in alpine regions, such as ¯ood control, hydrolelectric power generation, agriculture

and recreation.

Figure 1. Map of New Zealand and the study area, including the locations of the Southern Alps, Hokitika, the meteorological buoy, Nelson,Invercargill, Paraparaumu and the Waimakariri catchment

SOUTHERN ALPS LEE-SIDE SPILLOVER RAINFALL 79

# 1998 Royal Meteorological Society Int. J. Climatol, Vol. 18: 77±92 (1998)

STUDY LOCATION

The upper Waimakariri catchment, where this study was undertaken, lies on the eastern side of the Southern Alps

(Figure 1). It occupies 2590 km2 (North Canterbury Catchment Board and Regional Water Board, 1986), with

wide valley ¯oors, steep scree slopes, glacial remnants, permanent snow®elds and exposed peaks all being

common to the area. Many of the peaks exceed 2000 m, although these are interspersed with numerous passes,

with Arthur's Pass along the main divide being at only 918 m. As for other areas to the lee of the Southern Alps,

the precipitation gradient across the upper Waimakariri catchment is very strong (Figure 2). Mean annual rainfall

decreases from over 5000 mm along the main divide, to less than 1000 mm only 30 km downstream (Moore and

Prowse, 1988), with this gradient being largely the effect of spillover activity (Ryan, 1987).

The Waimakariri catchment is similar to and therefore representative of other eastern catchments in the

Southern Alps, although the relief is not as great as that found in the Mount Cook region further south. It was

chosen because analysis of previous data suggests that it is typical of leeside catchments, and there is easy access

into the area.

Figure 2. Mean annual isohyets for the Waimakariri catchment (after New Zealand Meteorological Service 1973 and Hayward 1967), and thelocation of the Waimakariri transect gauges



SPILLOVER MEASUREMENT AND ANALYSIS PROCEDURES

The present study involved the establishment of a suf®ciently dense network of rain gauges to measure spillover

activity to the lee of the main divide; development of an index that could be used to quantify spillover activity;

and relating spillover activity to atmospheric parameters measured during northwesterly storms observed over a

6-month period in 1994.

A transect of eight rain gauges was set up in the upper Waimakariri catchment to enable trans-mountain

spillover of rainfall to be examined in detail. The transect was oriented in a general northwest-southeast direction

from Arthur's Pass on the main divide to the eastern foothills (Figures 1 and 2). The spacing between gauges was

as little as 3 km to the immediate lee of the divide, where most spillover activity occurs, and a maximum of

38 km between the seventh and eighth gauges in the transect. Past research and existing rainfall data from the

area were important considerations in siting the gauges. All the gauges were automatic tipping bucket gauges,

and the data were in the form of hourly rainfall totals.

Siting rain gauges in areas of complex terrain is dif®cult. However, it was hoped that rainfall measurements

would be more consistent if sites were con®ned to the valley ¯oors, rather than being spread over a range of

different elevations between valley ¯oors and high mountain slopes. The other advantages of measuring rainfall

patterns in valleys are that the likelihood of undercatch by wind effects is reduced by increased shelter, and less

snow is received at these lower levels.

The rain gauges along the Waimakariri transect were installed on the 12 July 1994 and removed on the 28

December 1994. Some 42 identi®able periods of westerly quarter ¯ow occurred over this time. On six of these

occasions no rainfall was received on, or east of the main divide (i.e. `non-spillover' events). The inclusion of

every period of westerly ¯ow in the analysis provided a wide range of spillover events, with variations in the

volume of rainfall and its extension eastward from the main divide.

Spillover index

An index was devised to quantify the magnitude of each spillover event. It is de®ned as the product of the

average downwind extent of rainfall (in kilometres) during the storm (termed here overreach) and the volume of

westerly rainfall occurring east of the main divide. This can be expressed simply as:

spillover index � total volume�mean overreach

Because this study involved data from a single transect of rain gauges, having a length but no true width

component, for the purpose of calculating the index the volume component (the product of amount of rain,

distance downwind and transect width) is derived by assuming a unit width.

This is illustrated in Figure 3, where typical average hourly rainfall recordings at each gauge are plotted against

the distance of that gauge downstream from the main divide. Pro®les generated for each of the 42 westerly ¯ow

events acted as the basis for indexing spillover. Average hourly rainfall over the course of the storm was used so

that the events were indexed on the basis of the average rainfall intensity, but not the storm duration. It can be

seen that a number of trapeziums are formed under the pro®le between adjacent rain gauges. The index for a

given storm was taken as the sum of the individual moments for each of the trapeziums. That is, the spillover is

equal to: Pni�1

xiyidxi

ÿ �� 112

dxi� �2dyi

ÿ �� 1�

where �xi is the distance of the mid-point between gauges xi and xi�1 from the divide (i.e. from site 1), �yi is the

average height of the trapezium formed under the rainfall pro®le between gauges xi and xi�1, dxi is the distance

between gauges xi and xi�1, dyi is the difference in average hourly rainfall between the adjacent gauges xi and

xi�1, with all measurements converted to metres.

Each individual index value is a product of the average distance of the trapezium downstream from the divide

and the volume and distribution of rain de®ned by its shape. Usually, more rainfall occurs closer to the divide

during westerly quarter storms, and thus the centre of mass of an individual trapezium is not exactly half-way



between xi and xi�1. The ( 112

(dxi)2dyi) factor in the equation is a small, usually negative, value that accounts for

this uneven distribution of rainfall between adjacent sites.

The simplicity of the spillover index unfortunately undermines its robustness, as the index is based on linear

interpolation between recording points, which assumes that rainfall drops off at a constant rate between sites.

This is unlikely to be the case in all circumstances. Also, it does not account for localized effects, such as local

precipitation enhancement and wind effects that may occur at speci®c sites. In spite of these limitations, the

results discussed later show that the index is a useful tool in the study of rainfall spillover.

For the purpose of this study, spillover was taken to be a product of both the amount of rainfall reaching east of the

divide (spillover volume) and its eastward extent from the divide (overreach). However, the individual volume and

overreach components were also separated because it was felt that different meteorological factors could be

important in determining the quantity and distribution of rainfall. These components can be derived from:

spillover volume �P yidxi �m3� �2�

mean overreach �in metres� � spillover index �m4�spillover volume �m3� �3�

Table I presents the spillover, volume and overreach indices for the average hourly rainfall over the course of

each westerly event. For the events where spillover of rainfall occurred, the start of the event was identi®ed as the

hour when rainfall was ®rst recorded on, or to the lee of, the main divide. The end of the event was normally

identi®ed as the hour when all rainfall had ceased along the transect. On the odd occasion rainfall occurred at the

easternmost site due to the onset of a cold front moving up the east coast of the South Island, displacing the north-

west ¯ow. However, it was easy to distinguish these situations and identify the true termination of rainfall

associated with westerly air¯ow. The start and end of the few `non-spillover' events that occurred were

determined more subjectively by identifying periods of westerly quarter ¯ow on the basis of surface synoptic

charts. The main problem encountered in indexing spillover was interpolating the downstream extent of rainfall,

especially if rain occurred a long way downwind of the divide where the gauges were most widely spaced.

Other atmospheric variables

An assessment was also made of the role of prevailing atmospheric conditions during each storm. An

indication of the atmospheric variables that are likely to have an in¯uence on orographic rainfall is provided by

Figure 3. Schematic representation of the series of trapeziums formed between rainfall gauges under a typical storm pro®le. The individualmoments of these trapeziums were summed to produce the spillover index



past studies (Hill, 1961; Elliott and Shaffer, 1962; Saker, 1966; Collinge and Jamieson, 1968; Nord, 1972; Peck,

1972; Badar and Roach, 1977; Hill et al., 1981; Carruthers and Choularton, 1983; Choularton and Perry, 1986;

Givone and Meignien, 1990; Haiden et al., 1991, 1992; Sinclair, 1994; Sinclair et al., 1997). However, most of

these studies are concerned with broader rainfall patterns or areas of maximum rainfall enhancement, which

generally occur on the windward side of the highest terrain. It was hypothesized that the factors in¯uencing

orographic spillover would be similar to those in¯uencing orographic rainfall in general, but may vary in their

relative importance. A set of parameters likely to affect spillover was therefore chosen mainly on the basis of past

research, but also on the availability of appropriate data.

The atmospheric parameters considered can be categorized into those of wind®eld characteristics, air-mass

Table I. Spillover index, spillover volume, and overreach indices for the average hourly rainfall recorded during each of the42 events over the 1994 study period

Eventnumber Date and time

Spillover index(103 m4)

Spillovervolume (m3)

Overreach(103m)

1 16 July 1200±17 July 1500 689088 43596 15�812 21 July 1100±22 July 1800 1764973 86262 20�463 28 July 1600±29 July 0600 17653 1900 9�294 1 August 2000±5 August 1800 646779 36072 17�995 7 August 1200±9 August 1900 1992187 58341 24�486 12 August 1200±15 August 0500 1097477 60483 18�177 15 August 0500±16 August 0000 795184 42794 18�698 23 August 1800±25 August 2000 54715 8203 6�679 29 August 0800±30 August 2100 756309 41674 18�15

10 31 August 0100±31 August 1500 60371 9984 6�0511 1 September 0500±2 September 0500 31176 4718 6�3612 4 September 0200±4 September 1100 2185 1008 2�1713 6 September 0700±7 September 1000 3762707 132994 28�2814 11 September 2300±12 September 0500 103716 16001 6�4015 15 September 0900±18 September 0200 59804 8043 8�0416 18 September 2000±21 September 1300 281818 34002 16�3817 27 September 1200±28 September 0900 4637 1048 4�4318 8 October 2300±10 October 0700 112291 11201 10�0219 11 October 0200±12 October 1200 768253 39273 19�5620 13 October 1200±14 October 0300 235304 20927 11�0421 15 October 0000±15 October 1500 0 0 022 18 October 0100±18 October 1400 128639 20198 6�3723 19 October 0400±19 October 2100 70970 9171 8�3724 26 October 1200±27 October 0600 0 0 025 29 October 1400±29 October 2400 637145 68354 9�3226 4 November 1600±5 November 1500 61498 10715 5.7427 5 November 1800±9 November 1600 4805201 204047 24�3528 11 November 0600±11 November 2200 0 0 029 11 November 2300±12 November 1700 645239 58240 11�0730 13 November 1400±14 November 0300 832030 63436 13�1231 14 November 0000±16 November 0700 1116040 64651 17�2632 17 November 0100±18 November 1000 376036 23881 15�7533 20 November 2200±21 November 2100 1830050 119102 15�3634 22 November 1500±23 November 0100 259124 20474 12�5935 23 November 1100±24 November 0400 49556 7206 6�8836 25 November 1200±28 November 1300 0 0 037 28 November 1400±29 November 0800 6625 2078 3�1938 30 November 0400±30 November 1200 1286 593 2�1739 1 December 1200±1 December 1800 0 0 040 7 December 1200±9 December 0200 0 0 041 18 December 2300±19 Deceember 1000 38643 7875 4�9142 20 December 0800±20 December 1600 603370 46432 11�97



characteristics and frontal intensity. The wind characteristics upstream of the Southern Alps were obtained from

radar-tracked balloon ¯ights and surface measurements from Hokitika Airport (38 km upwind of the main

divide), and from a meteorological buoy that was located 30 km off the coast near Hokitika (Figure 1). Wind

speed and direction at the 850 hPa, 700 hPa and 500 hPa levels were examined for the ¯ights carried out during

each storm event (three times daily at 0600, 1200 and 2400 NZST). Average hourly surface wind speed and

direction at Hokitika and the meteorological buoy were also examined, as was the average hourly Nelson minus

Invercargill pressure difference. The latter was used as a general measure of the strength of the geostrophic ¯ow

perpendicular to the mountains. It has the advantage of not being affected by local site conditions, unlike surface

wind measurements.

The air-mass characteristics considered were those of air-mass temperature, humidity and instability. As there

are no upper air measurements from the region upstream of the divide, daily soundings from Invercargill and

Paraparaumu were used to give some indication of the nature of the air mass that was impinging on the Southern

Alps during westerly ¯ows (Figure 1). Although the soundings made at these stations are unlikely to exactly

replicate air-mass conditions directly upstream of the divide, they were the best data obtainable. Lag times were

used to make allowance for the locations of these sites away from the upstream region, by assuming that, for

example, due to the westerly progression of weather systems across the New Zealand region, air arriving at

Paraparaumu at a given time would be similar in character to that arriving over the upstream area several hours

earlier. The 1000±500 hPa thickness values obtained from soundings made at Paraparaumu and Invercargill over

the course of each event, were used to indicate the relative heat content of the air-mass, and the freezing level was

also used as a check.

Measures of air-mass moisture used here were the 850 hPa lifting condensation level (LCL) temperature and the

precipitable water between the surface and 300 hPa. The difference between the 850 hPa LCL temperature and the

850 hPa dry bulb temperature is a measure of air-mass humidity and indicates the amount of lifting required for

condensation to occur. Because of seasonal changes in average wet bulb potential temperatures, the 850 hPa LCL

temperature was used as a measure of air-mass moisture in preference to the wet bulb potential temperature.

Total precipitable water was calculated using the method described by Tomlinson (1973). Precipitable water

was calculated and summed for ®ve layers between the surface and 300 hPa (above which atmospheric moisture

is negligible), providing an indicator of atmospheric moisture content for the whole atmosphere up to that level.

Measures of both latent and potential instability were derived from soundings from Paraparaumu and

Invercargill. The Totals Index was used as a measure of latent instability between the 850 hPa and 500 hPa levels

(Miller, 1972). This index is obtained from:

Totals Index � T850 � Td850 ÿ 2T500 �4�

where T is the dry bulb temperature, Td is the dew point temperature, and 850 and 500 are the 850 and 500 hPa

levels respectively. The index estimates the stability of the atmosphere on the basis of the environmental lapse

rate between 850 and 500 hPa, and the humidity of the air at 850 hPa.

It is thought that the realization of potential instability due to orographic lifting is an important process in the

generation of rainfall over mountains. Thus differences in equivalent potential temperature (ye) with height (for

the layers 850±700 hPa, 700±500 hPa and 850±500 hPa) were used as an indication of the existence and strength

of potential instability during the westerly events in question. The equivalent potential temperature for the

relevant soundings was calculated using the equation derived by Bolton (1980).

It was hypothesized that convective activity may also be related to spillover activity. Warming at the base of an

air mass, by warmer surfaces over which the air mass is passing, can provide the necessary heating for the release

of such convection. Therefore the sea-surface/air temperature difference at the meteorological buoy was used as

an indication of the amount of warming that potentially occurred at the base of the approaching air masses. Data

were available for only the ®rst 17 spillover events of the ®eld study because the buoy was removed at the

beginning of October 1994.

Frontal activity is believed to have a strong in¯uence on the rainfall distribution to the lee of the Southern Alps

(Hill, 1961), so an attempt was made to quantify the intensity of fronts passing over the study area during

spillover events. Frontal intensity is a function of the temperature difference between the two air masses on either



side of the front. Thus the change in the 1000±500 hPa thickness (a generalized measure of air-mass temperature

change) as the front passed over Invercargill, was used as an index of frontal intensity.

RESULTS

Correlation analysis

Correlation analysis provided a preliminary appraisal of the atmospheric parameters most in¯uencing spillover

activity. The results of correlating each individual atmospheric variable with the spillover indices (spillover

index, spillover volume and overreach) are presented here.

Wind speed. The average hourly pressure difference between Nelson and Invercargill (Figure 1) over the

course of each storm was correlated against spillover. To allow for time lag effects, the pressure difference for the

period starting 12 h ahead of the start of the spillover event and ending 12 h ahead of the end of the event, was

also considered. The average hourly surface wind speed at Hokitika over the course of the storm was also

considered. Wind speeds at the 850 hPa, 700 hPa and 500 hPa levels were derived by averaging the wind speeds

recorded during all ¯ights that were carried out during a given spillover event.

The analysis produced signi®cant correlations (at the 99 per cent signi®cance level) between spillover and

strength of the ¯ow measured at the surface. It can be seen in Table II that the correlation coef®cients generated

for the Nelson/Invercargill pressure difference were larger when the values 12 h prior to the spillover event were

considered. The correlations with upper level wind speeds were weaker, probably as a result of using only a small

number of readings over the storm to estimate the average wind speeds aloft for the entire storm. The results

suggest that the 850 hPa and 500 hPa wind speeds were more in¯uential on spillover activity than the 700 hPa

wind speeds (Table II). In all cases wind speed was better correlated with spillover volume than with the

downstream distribution of rainfall (i.e. overreach).

Wind direction. To measure the in¯uence of the wind direction on spillover activity the average deviation of

the wind from a given direction was taken over the course of the storm. Thus, if wind direction is a determining

factor for spillover, the correlation of spillover with the deviation of wind from a given direction should vary for

different directions. Those directions that are associated with the strongest spillover activity should produce the

largest negative correlation (i.e. the smaller the deviation from this direction the greater amount of spillover).

Table II. Correlation coef®cients for spillover indices and various atmospheric parameters for 42 cases (unless otherwiseindicated): ** denotes 99 per cent signi®cance, * denotes 95 per cent signi®cance

Spilloverindex

Spillovervolume Overreach

Wind speed Hokitika surface (average over storm) 0�69** 0�63** 0�58**N/I pressure difference (average over storm)a 0�55** 0�52** 0�45**N/I pressure difference (12 h ahead of the storm)a 0�69** 0�72** 0�64**Hokitika 850 hPa 0�63** 0�64** 0�56**Hokitika 700 hPa 0�31* 0�39* 0.26Hokitika 500 hPa 0�45** 0�54** 0�51**

Measures of air-mass Air-mass thickness (1000±500 hPa at Invercargill) ÿ0�02 0�00 ÿ0�32*temperature Freezing level (Invercargill) ÿ0�06 ÿ0�12 ÿ0�32*Measures of air-mass Average precipitable water at Paraparaumu 0�35* 0�41** 0�27humidity Maximum precipitable water at Paraparaumu 0�48** 0�53** 0�33*

Average T850ÿLCL850 at Paraparaumu ÿ0�25 ÿ0�24 ÿ0�45**Minimum T850ÿLCL850 at Paraparaumu ÿ0�33* ÿ0�36* ÿ0�52**

Measures of instability Average Totals Index (at Invercargill, 41 events) 0�28 0�25 0�60**Average ye850ÿye500 (at Invercargill, 40 events) 0�24 0�20 0�46**Average TssÿTa (at the meteorological buoy, 17 events)b 0�42 0�42 0�55*

Frontal intensity Change in atmospheric thickness at Invercargill (33 cases) 0�59** 0�60** 0�57**

aN/I represents Nelson minus Invercargill.bTssÿTa represents the sea-surface/air temperature difference.



At Hokitika (Figure 1), the strongest correlations for surface wind direction and spillover (spillover index,

spillover volume and overreach) were found for deviations of ¯ow away from the north-northwest. However,

correlations of wind direction at the meteorological buoy with spillover were strongest for the northwest and

west-northwest directions (Table III). This was the same pattern at 750 hPa, although the correlations were not

nearly as strong. It was evident that due to local topographic effects, surface wind directions recorded at Hokitika

(not shown in Table III) were an inferior measure of general upstream wind directions, compared with

measurements made at the meteorological buoy and upper levels.

Wind direction aloft was more strongly correlated with overreach than spillover volume, but the reverse was

true for wind direction at the surface (meteorological buoy). The results in Table III also show that spillover

activity is more sensitive to wind direction at upper levels, although the correlations are weaker. This is especially

true at the 750 hPa level, where a positive correlation between spillover and deviation of winds away from the

southwest shows that southwest ¯ows aloft (parallel to the mountain range) are associated with a lack of

spillover.

Air-mass temperature. Generally weak correlations were produced when spillover was correlated against the

average 1000±500 hPa thicknesses at Paraparaumu and Invercargill. The correlation of the Invercargill 1000±

500 hPa thickness with overreach produced the strongest correlation coef®cient of ÿ0�32 (Table II). The freezing

level above Paraparaumu and Invercargill also produced weak correlations with spillover, with the strongest

correlation again being for the Invercargill freezing level and overreach (r�ÿ0�32).

The problem with considering variations in air-mass thickness (representing temperature) between the 42

events is that the events occurred over a period from mid-winter to mid-summer so that the average air-mass

temperatures would have increased over this time. To account for the seasonal ¯uctuation in air-mass

temperatures, the 42 storms were divided up chronologically into four subgroups containing around 10 events

each, and the effect of temperature changes was considered for each of these subgroups. The resulting

correlations showed no consistent pattern, suggesting that the samples were probably too small.

Air-mass humidity. On initial investigation, it was found that the correlation between air-mass humidity and

spillover activity was very weak for Invercargill, but more signi®cant for the case of Paraparaumu. This is due

most likely to the loss of moisture from the air mass as westerly quarter winds are lifted over the mountains

upwind of Invercargill (Figure 1). Consequently, only the results from Paraparaumu are presented here.

Spillover was correlated with the average and maximum precipitable water that occurred at Paraparaumu over

the course of the storm. Precipitable water showed positive correlations with all spillover indices (Table II). The

correlation was de®nitely stronger for the case of maximum precipitable water than for that of average

precipitable water. This is because the averaging of air mass characteristics tends to generalize the largely

¯uctuating conditions that occur over the course of the storm, as noted by Sinclair et al. (1997). The maximum

and average precipitable water both produced the strongest correlation with spillover volume, and the weakest

correlation with overreach (Table II).

The average and minimum difference between the 850 hPa lifting condensation level temperature and the

850 hPa dry bulb temperature (denoted as T8507LCL850) at Paraparaumu also produced signi®cant correlations

Table III. Correlation coef®cients for the spillover index and deviation of wind away from given directions at themeteorological buoy (®rst 17 events only), and above Hokitika at 750 hPa (42 cases): ** denotes 99 per cent

signi®cance; * denotes 95 per cent signi®cance

Deviation of wind from given directionWind direction at meteorological buoy

(17 events) Wind direction at 750 hPa

North ÿ0�10 ÿ0�18North-northwest ÿ0�35* ÿ0�24Northwest ÿ0�72** ÿ0�34West-northwest ÿ0�68** ÿ0�32*West ÿ0�47** ÿ0�01West-southwest ÿ0�29 0�16Southwest ÿ0�05 0�18



with spillover. Again, the averaged data produced weaker correlations than minimum T850 7LCL850 recorded

over the course of the storm. The minimum T850 7LCL850 represents the most humid situation because air

approaches saturation as the dry bulb temperature approaches the lifting condensation level temperature. Whereas

precipitable water through the air column to 300 hPa showed the strongest correlation with spillover volume,

humidity at the 850 hPa clearly showed a stronger correlation with overreach (Table II)

Air-mass instability. It was found that the correlations between spillover and air-mass instability were stronger

for the Invercargill soundings than the Paraparaumu soundings, so that only the Invercargill results are presented

here. Also, as the results were similar whether averaged or maximum measures of instability were used, only the

averaged results are presented.

Larger values of the Totals Index and larger differences in ye with height indicate more unstable situations.

Table II shows that latent instability (the Totals Index) and potential instability (change of ye over the 850±

500 hPa layer) are positively related to overreach, whereas weak correlations were found with spillover volume.

Analysis of potential instability over sublayers (e.g. 850±700 hPa and 700±500 hPa) showed that there was some

variability in the strength of the correlation, although correlations with spillover were strongest when using the

whole 850±500 hPa layer.

The results show that larger sea-surface/air temperature differences (as a measure of convective activity) are

associated with stronger spillover activity (Table II). Convective activity appears to in¯uence both spillover

volume and overreach, although the results here are less signi®cant because of the smaller number of events

considered in the analysis.

Frontal intensity. By referring closely to the synoptic charts for each event, two soundings were chosen to

represent air-mass temperatures ahead of and behind the front, as a measure of frontal intensity. For simplicity

only cold fronts were considered. For this reason, and the exclusion of events where too much subjectivity was

involved, the total number of events incorporated in the frontal analysis was reduced to 33. Events that involved

no frontal activity were assigned a frontal intensity of zero. Strong positive coef®cients were produced for the

correlation of frontal intensity with all three spillover indices; with spillover volume having a slightly stronger

correlation than the spillover index and overreach (Table II).

Regression analysis

Multiple linear regression was carried out on the selected atmospheric parameters so that spillover activity

could be attributed to a combination of atmospheric conditions, replicating better the processes actually occurring

in the real world. Multiple regression is based on the assumption that data are approximately normally

distributed, although it is fairly robust under conditions of departure from normality. In considering the selected

atmospheric data and the spillover indices, it was found that the distributions of some data sets were skewed,

whereas others were approximately normal. In an effort to normalize the data, square-root values were taken for

the spillover index and spillover volume, and for the measures of humidity and frontal intensity.

Spillover volume. Approximately 81 per cent of the variation of square-root transformed values of spillover

volume was explained by a combination of a strong Nelson minus Invercargill pressure difference (12 h ahead of

the storm), high 850 hPa wind speeds at Hokitika, and strong latent instability at Invercargill alone. Precipitable

water in the air mass (square-root values) at Paraparaumu was the next most in¯uential parameter in terms of

spillover volume, although its addition to the analysis only increased the value of r2 to 84 per cent.

Overreach. Just under 81 per cent of the variation in overreach could be explained by the Nelson minus

Invercargill pressure difference (12 h ahead of the storm), latent instability and square-root values of frontal

intensity measured at Invercargill. These three factors also produced the strongest individual correlation

coef®cients with overreach of 0�64, 0�60 and 0�57 respectively. Of these three factors, frontal intensity proved to

be the least determinant in the regression, with its removal from the analysis reducing r2 to 77 per cent.

The spillover index. When considering spillover as a product of both the volume of rainfall and its downwind

extension, three factors in combination with one another were found to explain as much as 76 per cent of

spillover activity. They were the Nelson minus Invercargill pressure difference (12 h ahead of the storm); the

average 850 hPa wind speed at Hokitika during the storm; and latent instability of the air mass at Invercargill. In



terms of statistical signi®cance, the Nelson minus Invercargill pressure difference was the most important factor

by far, although the other two factors were all associated with a high level of signi®cance.

DISCUSSION

The relationships identi®ed between spillover and selected atmospheric parameters strengthen existing ideas of

orographic rainfall processes, and the results indicate that some atmospheric parameters have a greater in¯uence

on orographic spillover of rainfall than others. The major atmospheric parameters and their effects on orographic

spillover of rainfall into the Waimakariri catchment can be discussed in relation to wind®eld and air-mass

characteristics, while the effects of their interaction have also been shown to be important.

Wind®eld characteristics

Wind speed was found to be highly correlated with spillover activity, especially spillover volume. The positive

relationship between overreach and wind speed is due to higher wind speeds distributing precipitation further

downwind due to wind-drift effects, which determine the fall trajectory of hydrometeors. However, it appears that

strong low-level winds are also associated with high volumes of spillover due to their role in advecting moisture

from the Tasman Sea on to the Southern Alps. Strong ¯ows play a crucial role in maintaining a high moisture ¯ux

into the alpine region, and consequently signi®cant rainfall rates. They also provide the necessary momentum and

vertical lift, as the air approaches the terrain, to carry the moisture laden air upwards to its condensation level, and

then eastwards to the lee of the mountains.

The results presented here show that not only is wind speed important, but also wind direction relative to the

orientation of the mountains. Warm, moist air is advected at low levels by northwesterly ¯ow ahead of cold fronts

as they move across New Zealand (Figure 4), and this appears to be an important factor in the generation of

strong spillover activity. Wind direction at upper levels seems to be more in¯uential on overreach, whereas

surface wind directions are more in¯uential on spillover volume, although the former relationship appears to be

stronger. In addition to the wind-drift effect already mentioned, the role of both upper level wind speed and

direction in determining spillover activity may also be related to effects on upper level divergence. In the

Southern Hemisphere, Rossby wave development contributes to regional ascending motion due to upper level

divergence on the poleward limb of the waves, when upper level northwesterly air¯ow occurs. This ascending

motion enhances that produced by such mechanisms as orography, instability and frontal activity to generate

widespread rainfall that extends well into the lee of the divide. Inspection of upper level synoptic charts showed

that these zones of upper level divergence were present when the downwind distribution of spillover was great.

Air-mass characteristics

The correlation of spillover with air-mass temperatures produced quite weak correlations, with the strongest

correlation between air-mass temperature and overreach. The effects of seasonal variation of air temperature

tended to produce inconsistent and weak relationships. The weak association of colder air masses with stronger

overreach can be explained most directly by the lower freezing level and thus greater proportion of ice crystals in

colder air masses. Ice crystals being lighter and having a larger surface area than raindrops, are carried further

downstream during their descent. Ice crystals also act as important condensation nuclei for precipitation

enhancement in low-level orographic clouds, via the seeder±feeder mechanism, due to their large surface area

and low terminal velocity (Choularton and Perry, 1986). However, it is felt that the importance of air mass

temperature lies in its effect on other more in¯uential parameters, such as air-mass stability, regional-scale lifting

and frontal development. Colder air masses often become unstable as they are warmed at the base by the surfaces

over which they are travelling, for example, when cold polar air masses are advected northwards by cyclonic

circulations across the relatively warm waters found at mid-latitudes. The correlation between air-mass

temperature and air-mass instability (Totals Index) for 41 of the events that occurred over the study period,

produced a coef®cient of ÿ0�59. Advection of cold air is often associated with cyclonic systems over the

Southern Ocean. Conversely, warmer westerly ¯ows are generated by more stable anticyclonic circulations to the



north and are therefore associated with regional descending motion. Cold fronts were also found to be an

important factor in generating spillover activity in the Southern Alps, as observed in previous studies (Hill, 1961).

The precipitable water at Paraparaumu (in the surface to 300 hPa layer) showed a good correlation with

spillover volume, but not with overreach. A high air-mass humidity implies that potentially signi®cant amounts

of rainfall can occur if this air is cooled to its dew point. Thus spillover volume is also dependent on moisture

upwind of the alps being raised to its condensation level and transported at least into the very upper reaches of the

Waimakariri catchment. On the basis of past studies it was expected that air-mass moisture would be a more

signi®cant factor for spillover activity. For example, Peck (1972) found moisture at the 850 hPa level to be the

most signi®cant factor (from a selection of 12 parameters) in determining the distribution of precipitation across

the Wasatch Front Range of northern Utah. These ranges are subject to ¯ows that are quite variable in terms of

their moisture content. However, westerly ¯ows impinging on the Southern Alps are consistently moist, as is

characteristic of maritime regions. Thus humidity becomes a less important discriminating factor in explaining

rainfall variability over the mountains, compared with more continental situations. The fact that humidity upwind

of the Southern Alps was estimated by soundings carried out at Paraparaumu to the north of the region (see

Figure 1) is also likely to have had some effect on the results.

The total precipitable water at a given time is largely a function of humidity in the lowest kilometre. When

considering humidity at the 850 hPa level only (about 1500 m) it was found that moisture at this level had a good

correlation with overreach, but not with spillover volume, which is opposite to the case of precipitable water. This

suggests that signi®cant amounts of moisture must be available at the height of the crest of the alps (about

1500 m), if spillover from the west is to extend very far into the lee.

Latent and potential instability both showed strong positive correlations with overreach, but much weaker

correlations with spillover volume. Instability is a determinant of overreach because it enhances vertical lifting

and results in the formation of precipitation at high levels in the atmosphere. In a very unstable situation, if the

in¯ux of moisture is suf®cient, saturation may occur to high altitudes. This results in a long trajectory taken by

the hydrometeors as they fall from their place of origin to the ground. This, coupled with stronger winds at higher

levels results in precipitation being carried further east during its descent, particularly as it is often in the form of

ice crystals. As already stated, ice crystals have a greater potential to be wind-drifted eastward than raindrops

because of their size, shape and density. These crystals grow by collision and coalescence with cloud droplets as

they fall through great depths of the atmosphere, resulting in signi®cant amounts of rainfall well downwind from

the crest of the mountains.

Ascending motion due to unstable conditions is not only important in enhancing orographic lifting to the

windward side of the alps, but also in counteracting the descending motion of the air to the lee. Thus in unstable

Figure 4. Typical mean sea-level isobaric chart showing north-westerly ¯ow ahead of a cold front moving across the South Island



situations spillover may be a result of both rain formation to the lee of the divide and its carry over by wind drift

effects from the west. This is illustrated during large spillover events, when well-developed cumulonimbus

clouds can be seen to be forming east of the main divide.

The correlation results also showed that the upwind sea-surface/air temperature difference (as a measure of

convective instability) has an in¯uence on overreach for the 17 events considered, although the small number of

events used in the analysis limits the interpretations that can be made. Also, it cannot be ascertained to what

degree this result occurs indirectly due to the association of spillover with colder air masses. However, it is likely

that the release of large-scale convection due to warming at the base of the air mass does in¯uence spillover

activity, and this is often illustrated by diurnal ¯uctuations in spillover intensity.

A strong positive relationship has also been identi®ed between frontal intensity and spillover, especially for the

case of spillover volume, with a correlation coef®cient of 0�60. For all the spillover events that involved frontal

activity, the maximum rainfall intensity and maximum overreach coincided with the passage of the front over the

region. The role of fronts in contributing to spillover activity is much like that of instability. The lifting created

along the frontal zone acts in conjunction with orographic lifting to enhance the vertical motion on the windward

side of the alps. This enhanced lifting is important in supplying moisture to higher levels in the atmosphere,

which seems to be an important control on the downwind extension of spillover. Frontal lifting may also be

suf®cient to counteract the descent of air in the lee of the alps, resulting in the continued formation of rainfall

over the eastern catchments. Frontal activity is often associated with strong winds (due to the formation of frontal

jets) and instability, which are also major determinants of spillover activity. Strong northwest winds that often

precede cold fronts approaching from the southwest, are important in supplying moisture to the frontal zone. This

moisture is driven upwards along the frontal zone as the warm air in the northwest ¯ow is replaced by cooler air

from the south. The combined effects of frontal and orographic lifting result in signi®cant amounts of moisture

being transported to catchments on the lee side of the divide.

Interactions between atmospheric parameters

In the correlation analysis it was shown that most of the parameters selected had some impact on spillover

activity. However, using regression it was found that several of these factors in combination with each other

explained a signi®cant percentage of spillover activity (both volume and overreach), and the addition of other

factors to the analysis improved the regression results only slightly. Strong air¯ow at the surface and at 850 hPa,

combined with air mass instability to explain 76 per cent of the variation in the spillover index (volume and

overreach) for the 42 events considered. These factors provide the necessary in¯ux, lifting and transportation of

moisture over the alps to generate signi®cant amounts of spillover into the Waimakariri catchment (Figure 5).

Separate analysis of the factors in¯uencing overreach showed the important effects of the strength of air¯ow

across the mountains, the instability of the air mass and frontal intensity. Spillover volume also proved to be very

dependent on wind speeds at both lower and upper levels, instability and to a lesser extent precipitable water. These

factors control the rate at which moisture is supplied to the region, as well as the transport of precipitation across the

mountains.

It is clear from this study that a number of atmospheric parameters interact to create the spillover effect. For

example, it appears that the in¯uence of instability on spillover volume is dependent on moisture availability.

Strong low-level winds will result in signi®cant in¯ux of moisture, with instability being an important mechanism

for enhancing the transportation of this moisture into the lee of the alps. However, in the event of weak westerly

quarter ¯ows, and therefore a low moisture in¯ux, instability is ineffective in the generation of high volumes of

spillover. Similarly, the amount of precipitable water measured at Paraparaumu was quite an important factor in

explaining spillover volume when it was considered in conjunction with lifting mechanisms. Hill et al. (1981)

also recognized the combination of wind speed, humidity and stability to be important in generating orographic

rainfall over smaller terrain in South Wales.

Although the correlation results suggest that wind direction has a signi®cant in¯uence on spillover, it was

found to be less in¯uential when considered in conjunction with other parameters in the regression analysis. For

example, the deviation of wind away from the west-northwest at the 500 hPa level had a correlation with

overreach of 0�51. However, its removal from the regression analysis barely reduced the value of r2. It appears



that once the air¯ow is passing over the alps from the westerly quarter, and other conditions are conducive, subtle

changes in wind direction are not likely to have a signi®cant effect on spillover to the lee. Factors such as the

strength of the ¯ow, instability and frontal intensity are obviously more important controls. Northwesterly and

west-northwest winds, which show the strongest correlation to spillover activity, are often associated with strong

wind speeds ahead of cold fronts. It is uncertain to what extent the relationship between spillover and wind

direction exactly normal to the orientation of the alps is a direct one, and to what extent the relationship arises

because of the association of winds from these directions with strong ¯ows and frontal activity.

CONCLUSIONS

The interaction of the Southern Alps of New Zealand with a prevailing westerly ¯ow, produces an extremely strong

rainfall gradient across the South Island of the country. Studies of processes responsible for this rainfall gradient have

been few to date, compared with research into more general aspects of orographic precipitation undertaken

internationally. This paper is therefore an initial attempt to explain atmospheric controls of rainfall gradients across a

large catchment to the lee of the mountains. The results show that although individual meteorological factors are

important in the spillover process, the interaction of a number of factors explains a large amount of the variation in

downwind distribution of precipitation. The most signi®cant factors were found to be low-level wind speed upwind

of the mountain barrier, 850 hPa wind speed, air-mass instability and frontal intensity.

In addition to the more general signi®cance of this research to knowledge of orographic precipitation, the

results presented here have practical signi®cance for regional weather forecasting in this alpine region. Although

the period of analysis (July±December 1994) is limited and the study con®ned to only one lee-side catchment, the

set of events used in this study covered a good range of situations in relation to both the volume of rainfall

recorded and its distribution along the transect, and the Waimakariri catchment is representative of other eastern

catchments of the Southern Alps.

The meteorological parameters used in the analysis were chosen on the basis of past research and the

availability of data. However, a number of other atmospheric parameters also may be in¯uential on spillover

activity. In future studies, it would be valuable to consider factors such as the rate of uplift, air-mass strati®cation,

cloud microphysics and rates of rainfall enhancement, as well as some measure of upstream blocking. Satellite

data could be used to characterize upstream air-mass conditions with regard to the upstream temperature,

moisture and cloud cover, more detailed synoptic information would allow investigation of the importance of

divergence and vertical motion ®elds for rainfall distribution during trans-mountain ¯ows.

ACKNOWLEDGEMENTS

The authors are grateful to the National Institute of Water and Atmospheric Research (New Zealand) for the

provision of data, ®eld equipment and ®nancial support, as well as to the Canterbury Regional Council for rainfall

Figure 5. A schematic representation of the major factors in¯uencing the spillover of orographic rainfall into the Waimakariri catchment



data and funding. Thanks are also due to the Department of Conservation, Cora Lyn Station and Flock Hill

Station for allowing access to ®eld sites and installation of equipment. Special thanks are due to Dr Murray Smith

for his assistance with the statistical analysis, to Mark Pascoe of the New Zealand Meteorological Service for

access to unpublished material, to Professor Bob Kirk and Dr Hamish McGowan for useful comments on the

manuscript, and to Michelle Rogan for completion of the ®nal ®gures.

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Atmospheric conditions influencing the spillover of rainfall to lee of the Southern Alps, New Zealand