Weather and Climate (1987) 7: 61-74 61

THE CANTERBURY PLAINS NORTH-EASTERLY

Ian G. McKendry Department of Geography, University of British Columbia, Vancouver, Canada Andrew P. Sturman and Ian F. Owens Department of Geography, University of Canterbury, , New Zealand

ABSTRACT Local airflow patterns over Canterbury are investigated using both standard observations and supplementary data. A comprehensive climatology of the north-easterly wind is presented and knowledge of its spatial and temporal variation extended. The diurnal periodicity of the north-easterly is examined in relation to the effects of both changes in atmospheric stability and the sea breeze influence. Disconnection of the north-easterly from the surface due to increased stability is slightly more frequent than the classical sea breeze effect. North-easterlies occur with a range of a synoptic circulation types, with a preference for anticyclonic situations. The north-easterly extends over the entire plains and interacts with katabatic flow of inland areas, a south-easterly sea breeze south-east of Banks Peninsula, and the foehn north-westerly. Possible zones of local wind interaction are suggested.

INTRODUCTION sponsible for the north-easterly regime is yet to Low level north-easterly winds have long be achieved. been recognised as the dominant feature of the wind regime in the vicinity of Christchurch city. Although there have been several analyses of The basic mechanism by which these winds the frequency and temporal variability of the develop has been known for some time (Watts, surface north-easterly at Christchurch Airport 1945; Kidson, 1932; Hutchings, 1944). Strong (Sturman and Tyson, 1981; McGann, 1983) the north-westerly airflow over the is spatial variability of the regime and its responsible for the deformation of the surface interactions with other local wind systems, pressure field (de Lisle, 1969) such that lower notably the sea breeze, the foehn north-westerly level flow funnelled through Cook Strait re- and the nocturnal katabatic wind, are still curves into the lee side trough as a cool moist poorly understood. These aspects of the north- north-easterly. More recently, the importance easterly have important implications in relation of thermal forcing (arising from differential to air quality, land-use management, fore- heating between the South Island land mass and casting and wind energy development. Given surrounding ocean) in contributing to the the site specific focus of previous studies this strong diurnal and seasonal variations in the paper aims to: "nor 'easter" has been recognised by Trenberth (1977), Sturman and Tyson (1981) and (1) provide a basic climatology of the north- easterly. McKendry (1985). Despite these revelations, a detailed understanding of the processes re- (2) describe spatial and temporal variations in 62 Canterbury plains north-easterly

/ Fig. 1: Oblique view of the Canterbury region illustrating major topographic features and the location of places mentioned in the text (modified from Sturman and Tyson, 1981). Canterbury plains north-easterly 63 surface winds over the plains during north- recorded at additional sites during an intensive easterly flow. study period January 1982 to August 1983 were (3) assess the role of boundary layer variations used to augment the standard Meteorological in contributing to the diurnal periodicity of Service data (Table 2). the north-easterly. BACKGROUND CLIMATOLOGY In Fig. 2, seasonal variations in north-easterly frequency (measured in terms of both the num- DATA ber of episodes of north-easterly flow and the Three major data sets were used in the total duration in hours) are presented. Although present study (locations are shown in Fig. I). surface north-easterly events (defined as a sus- (1) Christchurch Airport data for the period tained period of at least an hour of north-east- April 1967 to March 1972 were used for the erly flow identified from anemograph charts) basic climatology of north-easterly charac- occur with approximately equal probability in all teristics. This analysis was based on hourly seasons, there is a marked seasonal variation in observations of wind speed and direction, the total duration of north-easterly flow. This surface synoptic charts and 6-hourly pilot suggests that in winter, periods of surface north- balloon ascents, all of which were easily easterly flow are of shorter duration than in obtained for this period. summer when north-east winds are experienced at Christchurch Airport 51.4% of the time. This (2) Available computer coded hourly wind data result is confirmed in Fig. 3. In all seasons, (courtesty of the New Zealand Meteorological maximum frequencies are associated with event Service) were used in the investigation of spatial durations of 6-12 hours. However, winter and temporal variations in the local wind months appear to hold a disproportionate share regime (Table 1). of shorter duration events (less than 10 hours) (3) Hourly wind speed and direction data while the warmer months have more longer

TABLE 1: AVAILABLE METEOROLOGICAL SERVICE HOURLY SURFACE WIND SPEED AND aRECTION DATA.

Station Period of data used Missing data Anemograph

Christchurch Airport 1/1/65-31/12/78 Munro Winchm ore 1/8/70-31/12/75 Dines Bromley 1/4/67-31/3/72 Dines Lyttelton Harbour 1/6/74-30/4/77 Munro Airport 1/1/77-31/12/80 Munro Lincoln Oct-April 1969-80 Lambrecht July, August, Sept 1978 Sept 1979

TABLE 2: SUMMARY OF CONTINUOUS SURFACE OBSERVATIONS RECORDED SPECIFICALLY FOR THE PRESENT STUDY.

Station Period of data collected Missing data Anemograph type

Greendale 6/4/82-31/8/83 20/5/83-16/6/83 Lambrecht West Eyreton 23/7/82-31/8/83 Lambrecht 4/3 /82-3 1/8/83 16/7/83-16/8/83 Lambrecht Methven 18/9/82-25/2/83 Lambrecht Lincoln 1/3/82-31/8/83 Lambrecht 64 Canterbury plains north-easterly duration events (greater than 42 hours). January does there appear to be a tendency for Examination of the times of onset and later onsets. In these months the local cessation of the surface north-easterly (Fig. 4) frequency maximum 7-9 hours after sunrise further emphasises the strong diurnal (early afternoon) is strongly suggestive of the periodicity of the regime (McKendry, 1983; presence of a thermal forcing mechanism. Sturman and Tyson, 1981; Trenberth, 1977; McKendry (1985) suggests that in summer these McGann, 1983). Surface north-easterly flow is onsets may occur as a result of the development of a thermal low over the South Island. The usually initiated in daylight hours with 36.5070 of onsets occurring in the 4 hours following majority of events last beyond sunset with sunrise. Only in the months of December and maximum frequencies of cessation associated with the period up to 4 hours after sunset. As well as exhibiting strong diurnal 60 A periodicity at the surface, the north-easterly displays seasonality in both strength and depth (Fig. 5). At Christchurch Airport, maximum strengths occur in the summer months when the north-easterly typically attains speeds of 7-8 40 - ms—l. In winter, the north-easterly is less strong

30 -

20 - SUMMER 1 1 10 - *x. •z• •cr. 0 n n Ft n *Au .2 • , • z • •2- .1- 1 ;2. •I--• .z • • cr— •D •D• (17 • • • .o •

SEASONS

Ix 60

30 0 50 20- WINTER LL_ 1 1 40- '77-7 1 o - n n 11 0 1 1 D 30-

LLJ SPRING rE 20- •cr• •z. , cr. o. 10 .w. •2. • tLi I 0 - 2 • D • • 1-• • z . 'I'll'' I 1„I I 2. • 1— • • z • D• •• W, It .4! gji ow < vi •• 0 , - - - ..Z1 I 0 SURFACE NORTH-EASTERLY DURATION SEASONS (HOURS) Fig. 2: Percentage frequency of total surface north-easterly episodes by season and percentage of total hours in each Fig. 3: Variations in duration of surface north-easterly season associated with north-easterly surface flow at events at Christchurch Airport by season (1133 cases, Christchurch Airport (1133 cases, 1/4/67-31/3/72). 1/4/67-31/3/72). Canterbury plains north-easterly 65 with wind speeds typically in the range 5-6 I, ms-1. These seasonal variations in wind speed are matched by a strong diurnal variation whereby north-easterly winds are strongest in • , late afternoon and lightest in early morning (Trenberth, 1977). The mean depth of the • ' north-easterly layer (derived from pilot balloon soundings) also shows seasonal variability (Fig. gE 3 5b). Few events attain mean depths in excess of 1000m while the majority are less than approxi- JAN fEO MAR APR MAT JuN JUL AUG SEP OCT NOV OEC mately 750m. Increased depths occur in sum- MONTHS Of THE YEAR mer while lesser depths are associated with the cooler months. It is likely that these seasonal 1500 variations in both the strength and depth of the north-easterly are a response to seasonal 5 1250 variations in both low level stability and the 4 1000 thermal and dynamic forcing mechanisms responsibile for north-easterly flow. 750

,6J 500

250 JAN JAN TES MAR APR MAT JUN JUL AUG SEP ACT NOV DEC

FEB MONTHS Of THE YEAR MAR Fig. 5: Seasonal variations in (a) daily maximum mean APR hourly surface windspeeds (10 m) and (b) mean depths MAY (average of wind soundings during each event) of north- easterly events at Christchurch Airport (1/4/67-31/3/72). JUN Contours represent percentages per month. JUL

AUG SEP The relationship between north-easterly flow OCT and the synoptic scale circulation has been NOV noted previously by Summers (1954), Neale DEC (1959), Trenberth (1977), and Sturman and Tyson (1981). In this analysis, sea level 2 3 4 5 6 7 8 9 ID II IS i3 i4 atmospheric circulation over the South Island SUNRISE for the period 1967-1972 was classified at the ONSET TIME OF NORTH EASTERLY (HOURS AFTER SUNRISE) time closest to north-easterly onset in an attempt to identify those situations most JAN frequently associated with the development of FEB north-easterly flow. This subjective technique MAR produces 27 classes based on isobaric APR orientation and curvature and is described by MAY Sturman et al. (1984). JUN

JUL The high frequency of surface north-easterly winds on the Canterbury Plains suggests that AUG north-easterlies are associated with a broad SEP OCT range of synoptic circulation types. Examin- ation of Fig. 6 reveals that this is indeed the NOV DEC case. At the time of onset, north-easterly events at Christchurch Airport are most frequently -4 ,3 -2 -i I I 2 3 4 5 6 7 8 9 ID Ii 12 characterised by anticyclonic curvature, al- SUNSET though a significant proportion of situations in- CESSATION TIME OF NORTH EASTERLY (HOURS AFTER SUNSET) corporating a westerly component show Fig. 4: Contoured frequencies (percentage per month) of cyclonic or unspecified isobaric curvature. onset and cessation times of surface north-easterly events at Christchurch Airport (1/4/67-31/3/72). The most important gradient wind direction 66 Canterbury plains north-easterly

n_ {Inn

AUTUMN

fin - n n n

20- WINTER

10-

rl, ,n _Ilnll n n - Hn

_1ln 3 z on !, 3

ANTICYCLONIC CYCLONIC UNSPECIFIED

SYNOPTIC CIRCULATION CLASSIFICATION Fig. 6: Frequencies of surface synoptic circulation types at time of onset of north-easterly events at Christchurch Airport (1/4/67-31/3/72). at the time of north-easterly onset is classified south-east (Fig. 7b). Often, these anticyclones as south-westerly (28.807o of events), with a are characterised by twin cells either side of the strong bias toward anticyclonic isobaric main divide resulting in easterly flow in the curvature. Such situations usually herald the vicinity of the Canterbury Plains. approach of an anticyclone from the central Westerly or north-westerly gradient wind Tasman Sea (Fig. 7a) and are characterised by directions are also often associated with north- the initial development of a pressure gradient from north to south along the Canterbury easterly onset. These types show no particular tendency in respect to isobaric curvature and coast. This pressure gradient is responsible for are characterised by strong flow across the the development of north-easterly flow and mountain barrier and through the straits to the may be regarded as the beginning of lee trough north and south. Usually, such flow is associ- development as the upper flow turns more ated with appreciable lee trough development westerly with anticyclonic passage (Trenberth, on the eastern side of the Alps (Fig. 7c). 1977; McKendry, 1985). North-easterlies also tend to be associated with north-easterly For all seasons, the maximum frequency of gradients or slack gradients (ND) with anti- north-easterly events is associated with anti- cyclonic curvature. These situations frequently cyclonic south-westerly situations. No marked occur with an anticyclone positioned directly seasonal variation is apparent in the data, over the South Island or moving off to the although in spring there is a tendency for north- Canterbury plains north-easterly 67

ANTICYCLONIC SOUTH -WESTERLY (A-SW)

-Ns,'.241-_111111r2 0 I? 2,0 Km

WEST EYREION

40-2%

244% C.RISTCHuRCHAIRRORT @BROMLEY (11) GREENS ALE • ‘14:4 28.0'4

23-0% EESTO

INCRMORE KEY , Percentage of total hours surface X% airflow from north-easterly quadrant

0 Percentage of Occasions north-easterly event recorded at site the same day as recorded at Bromley

ANTICYCLONIC NO DIRECTION (A-ND) Fig. 8: Spatial variations in the frequency (daily) and total duration (hours) of north-easterly surface flow (August 1982-August 1983).

easterlies to be associated with higher fre- quencies of cyclonic westerlies, a reflection of the seasonal movements in the mid-latitude high pressure belt.

SPATIAL AND TEMPORAL VARIATIONS In Fig. 8 the spatial variations in the frequency of north-easterly flow are presented. Clearly, north-easterly flow is most frequent at those sites to the north of Banks Peninsula. For example, at Bromley north-easterly winds are experienced 40.207o of the time while at Winch-

UNSPECIFIED NORTH WESTERLY (U-NW) more north-easterlies are only half as frequent. However, 76% of north-easterly events occur as far to the south-west as Winchmore on the same day that they are apparent at Bromley. From these patterns it is apparent that for a high proportion of north-easterly events the flow extends across the entire plains at some time during the day. There are two likely explanations for this pattern. Either the north- easterly invades the plains in a sea breeze fashion giving shortest durations inland and longest durations at the coast, or the north- easterly predominates over the entire plains simultaneously but is displaced at the surface in some areas by other local wind systems. It is difficult to confirm the first explanation without intensive studies utilising vertical wind profiles at numerous sites. However, consid- Fig. 7: Synoptic scale circulation types associated with erable evidence exists for the interaction of the north-easterly airflow on the Canterbury Plains: a) anticyclonic south-westerly, b) anticyclonic non- north-easterly with the other local wind systems directional, c) unspecified north-westerly. (McKendry, 1985; Ryan, 1980). The spatial and 68 Canterbury plains north-easterly

Fig. 9(a): Three-hourly frequencies of windspeed and direction for January, April, June and October for (a) 0000-0200h. (Data periods for each site are shown in Tables 1 and 2). Canterbury plains north-easterly 69

Fig. 9(b): Same as Fig. 9(a) except for 1200-1500h. 70 Canterbury plains north-easterly

CHRISTCHURCH AIRPORT 0 360 CHRISTCHURCH AIRPORT 90 1E310 270

CHRISTCHURCH AIRPORT CHRISTCHURCH AIRPORT

CHRISTCHURCH AIRPORT CHRISTCHURCH AIRPORT

Fig. 10: Simultaneous comparison of hourly surface wind directions at Christchurch Airport with Methven (18/9/82-25/2/83), Leeston (4/3/82-31/8/83), Bromley, Winchmore (1/1/82-31/8/83), Greendate (6/4/82-31/8/83) and West Eyreton (23/7/82-31/8/83). (Contours are based on 10' intervals and represent frequencies per thousand.) Canterbury plains north-easterly 71 temporal characteristics of these local wind Airport. The first is the daytime south-easterly systems are evident in Fig. 9. sea breeze identified in Fig. 9. The second is the The period 0000-0300 h shows several north-westerly regime which includes both the features typical of nocturnal airflow over the foehn north-westerly (a daytime maximum fre- Canterbury Plains. For each seasonally repre- quency) and the light nocturnal katabatic. The foehn and nocturnal katabatic north-westerlies sentative month, winds are typically light with a tendency toward downslope flow from the also occur at West Eyreton during north- north-west. Only at sites nearest the coast easterly flow at Christchurch. (Bromley, Christchurch Airport, Lincoln, Comparison of the airflow regime at Leeston and Lyttelton Harbour) does north- Methven with that at Christchurch Airport re- easterly flow still dominate. In contrast, veals several features not apparent at the other regional daytime patterns (1200-1500 h) sites, notably, the very high frequency of north- strongly reflect the north-easterly influence westerly winds. The relatively low frequency of especially in the warmer months. A possible north-easterly flow at Methven and the associ- explanation of reduced north-easterly fre- ation of a wide range of flow directions with quencies to the south-west of Banks Peninsula north-easterly flow at Christchurch Airport at this time can be seen in the tendency for suggests that Methven is close to the inland strongly diurnal south-easterly flow at both the limit of the zone influenced by the north- Leeston and Winchmore sites. McKendry easterly circulation. (1985) has shown that these localised south- In the above discussion, emphasis is placed easterly winds represent true sea breezes and are on surface observations and vertical variations often embedded in the low level north-easterly in airflow are largely ignored. However, sur- flow. In January and October the high face observations may be strongly influenced frequency of north-westerly winds at sites by diurnal and seasonal variations in low level adjacent to the Alps (Greendale, West Eyreton stability such that increased stability at night and Methven) reflects the importance of foehn due to nocturnal cooling may result in de- influence at inland locations. At Methven, the coupling of the prevailing wind from the predominance of daytime north-westerly flow surface (Oke, 1978). At the surface, the north- may be attributed to the proximity of this site to easterly flow may therefore be replaced by Gorge which funnels the large scale calms or shallow katabatic winds (Ryan, 1980). westerlies onto the western plains. To investigate the frequency of this The extent to which these different local wind phenomenon 204 occasions (January systems (the north-easterly, the sea breeze, the 1982-August 1983) were identified when surface katabatic and the foehn) may interact is north-easterly flow was apparent on con- apparent in Fig. 10 when hourly wind directions secutive days at Christchurch Airport. In Table at Christchurch Airport are compared with 3 these onsets are classified according to the simultaneous observations at six other nocturnal presence/absence of continuous sur- locations. Cursory examination of the patterns face north-easterly flow and the presence/ for each site suggests that surface airflow over absence of continuous north-easterly flow the plains is relatively homogeneous when aloft. North-easterly flow was deemed to be winds from the north-west and south-west continuous aloft if a layer of north-easterly quadrants are experienced at Christchurch flow could be detected in both the 0000 h and Airport. As might be expected from the dis- 0600 h rawinsonde soundings at Christchurch cussion above, greatest variations in wind Airport. Of the three types, type 3 has the direction over the plains occur in conjunction highest frequency (37o) suggesting that with north-easterly flow at Christchurch Air- decoupling of the surface flow plays an port. Only at Bromley is north-easterly flow at important role in "masking" the true north- Christchurch Airport not associated with easterly frequency at Christchurch Airport. airflow from other directions. This is clearly a result of the close proximity of the two sites. In Fig. 11 absolute frequency windrose diagrams showing gradient wind directions Although north-easterly flow still pre- (derived from surface isobaric orientation) for dominates at Leeston, Winchmore and Green- each type for each season are presented. dale, two other regimes are evident at these sites Diurnal circulations (type I) are primarily during north-easterly flow at Christchurch associated with synoptic airflow from a 72 Canterbury plains north-easterly

TABLE 3: CLASSIFICATION OF NOCTURNAL NORTH-EASTERLY FLOWS AT CHRISTCHURCH AIRPORT ACCORDING TO SURFACE FLOW AND FLOW ALOFT. EVENTS COMPRISE NORTH-EASTERLY FLOWS ON CONSECUTIVE DAYS.

Surface North-easterly North-easterly aloft Type (anemograph) (000 h and 0600 h soundings) No. of events

Discontinuous (diurnal) Discontinuous 65 2 Continuous Continuous 64 3 Discontinuous (diurnal) Continuous 75

TYPE I TYPE2 TYPE3 ations although highest frequencies occur with autumn and winter gradient north-easterly situations. This is to be expected since SUMMER nocturnal decoupling of the surface flow is most likely to occur during winter when clear skies promote strong nocturnal cooling and stable conditions. As mentioned earlier, gradient north-easterly winds are often

AUTUMN associated with such anticyclonic clear sky conditions (Fig. 7b).

DISCUSSION Despite the rather simplistic nature of the WINTER analysis this study has revealed several previously undocumented features of the low level north-easterly in Canterbury. De Lisle's (1969) classical explanation of the northeasterly as being mainly trough induced is SPRING largely supported by the results presented here, although the strong association of anticyclonic VALUES IN CENTRE REPRESENT NO DIRECTION southwesterly conditions with the onset of 0 5 10 northeasterlies suggests that their initial

FREQUENCY development may be thermally induced. In regard to the spatial extent of the north- Fig. 11: Absolute frequency windroses showing nocturnal gradient airflow (0600 h) for cases of surface north- easterly, results are also in agreement with de easterly flow on consecutive days at Christchurch Airport Lisle's (1969) suggestion that the north-easterly for TYPE 1 (diurnal north-easterly), TYPE 2 (continuous north-easterly at surface and aloft) and TYPE 3 is well developed in the vicinity of Christchurch (continuous north-easterly aloft but not at surface), and may extend inland beyond Ashburton. January 1982-August 1983). However, the present results indicate that the north-easterly is less of a coastal phenomenon westerly quarter with maximum frequencies than inferred by de Lisle (1969) with episodes of associated with south-westerly flow in autumn north-easterly flow on the majority of months. On contrast, continuous surface occasions extending at least to Winchmore in north-easterlies tend to be associated with the south-west and as far as the foothills of the north-westerly or northerly flow in all seasons. Alps in the west. The extent of inland Such flows are typically characterised by a penetration will undoubtedly be related to the pronounced lee trough, strong winds and location of the lee side trough to the east of the cloudy skies, factors which would tend to mountains. Investigation of the degree to maintain nocturnal north-easterly flow and which the reduced surface frequency of north- prevent the development of a very stable easterly flow at inland sites is due to nocturnal boundary layer. Events exhibiting a undercutting by katabatic winds or the local sea diurnal periodicity at the surface, but not aloft, breeze should be a primary goal of future are associated with a range of synoptic situ- research efforts. Canterbury plains north-easterly 73 actions between the various local wind systems. stability variations. The identification of For example, it is likely that there is a preferred probable zones of interaction between the zone of confluence between the south-easterly north-easterly and the sea breeze, katabatic and sea breeze and the north-easterly in the vicinity foehn north-westerly also suggests that the of Lincoln. This may have important concentration of observational studies in areas implications for cumulus development and not previously instrumented may be a fruitful hence severe storm forecasting. The presence of avenue of research. For example, it is likely that marked spatial and temporal variation in wind the region of confluence between the south- direction over the plains highlights the need for easterly sea breeze and the north-easterly in the adoption of a three dimensional perspective vicinity of Lincoln is of applied significance, (e.g. Surridge, 1980) when investigating airflow while investigation of vertical variations in in this region. airflow adjacent to the foothills of the Alps Previous studies which have documented the could confirm the presence or absence of north- strong diurnal periodicity of the surface north- easterly flow overlying the nocturnal katabatic easterly (Trenberth, 1977; Sturman and Tyson, westerly. More detailed analysis of the 1981) have attributed this to the diurnal relationship between northeasterly wind speeds periodicity of the forcing mechanism. How- and associated atmospheric circulation systems ever, results of this study suggest that on may prove useful in local forecasting. Only approximately half of the occasions when with such approaches can a full understanding surface north-easterlies are interrupted of the north-easterly be gained and the applied nocturnally by a period of "calms" or light significance of its interactions with other local north-westerlies, the north-easterly appears to wind systems be assessed. continue aloft. This decoupling of the surface north-easterly is most likely to occur under ACKNOWLEDGEMENTS conditions of strong nocturnal cooling (clear The authors are grateful for the financial skies in winter) or when the north-easterly support of the N.Z. Meteorological Service, circulation is relatively weak. This result University Grants Committee and the Uni- emphasises the need for considerable care when versity of Canterbury. Thanks also go to the interpreting the nature of forcing mechanisms N.Z. Meteorological Service and the Crop on the basis of surface wind observations alone. Research Division of D.S.1.R., Lincoln, for the For example, the strong diurnal periodicity of provision of data. The Geography Department the north-easterly may be attributed to sea also supported this research in many ways, breeze or atmospheric tidal effects when in fact including draughting and technical assistance. much of the apparent diurnal periodicity is a result of low level stability variations. REFERENCES de Lisle, J.F., 1969: The climate and weather. In: Knox, CONCLUSIONS G.A. (ed), The natural history of Canterbury. Reed, Wellington: 68-76. Previous investigations of the Canterbury Hutchings, J. W., 1944: Orographic disturbance of the Plains wind regime have tended to concentrate pressure field over New Zealand. N.Z. Met. Office Rep. on surface observations from one or two Ser. A. No. 7. Wellington. Kidson, E., 1932: The Canterbury "northwester". N.Z. J. locations. Consequently, the marked spatial Sci. and Tech. 14: 65-75. and temporal variations in flow associated with McGann, R. P., 1983: The climate of Christchurch. N.Z. the north-easterly have remained poorly Met. Seri). Misc. Publ. 167(2), Wellington. documented. McKendry, I. G., 1983: Spatial and temporal aspects of the surface wind regime on the Canterbury Plains, New By emphasising the marked spatial and Zealand. J. Climatology, 3: 144-166. temporal variations in flow over the plains McKendry, I. G., 1985: An empirical and numerical during periods of north-easterly flow, this modelling analysis of a complex mesoscale wind field, study highlights the need for future Canterbury Plains, New Zealand. Ph.D. thesis, University of Canterbury, Geography Department, observational programmes to adopt a three Christchurch, 388pp. dimensional perspective in order to provide an Neale, A. A., 1959: Surface wind and weather conditions improved understanding of the local wind at Wigram related to general airflow and pressure dis- regime. Such an approach alleviates the tribution, N.Z. Met. Seri). Tech. Note 129, Wellington. Oke, T. R., 1978: Boundary layer climates. Methuen, problems associated with the "masking" of the London. 327pp. north-easterly at the surface by boundary layer Ryan, A. P., 1980: Northwest drifts and night cooling in 74 Canterbury plains north-easterly

winter in Christchurch. N.Z. Met. Sery. Tech. Note 243. Wellington. Sturman, A. P. and Tyson, P. D., 1981: Sea breezes along the Canterbury coast in the vicinity of Christchurch, New Zealand. J. Climatology 1: 203-219. Sturman, A. P., Trewinnard, A. C. and Gorman, P. A., 1984: A study of atmospheric circulation over the South Island of New Zealand (1961-1980), Weather and Climate, 4: 53-62. Summers, P. W., 1954: The relation between the surface wind at Wigram and the main sea level distribution. N.Z. Mel. Serv. Tech. Note 116. Wellington. Surridge, A. D., 1980: Examples of wind shear and tenlper- ature inversion surfaces over Christchurch. N.Z. J. Sci. 23: 283-288. Trenberth, K. E., 1977: Surface atmospheric tides in New Zealand. N.Z. J. Sci. 20: 339-356. Watts, 1. E. M., 1945: Forecasting New Zealand Weather. N.Z. Geogr., 1: 119-138.