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A Review of Topoclimatology in New Zealand

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A Review of Topoclimatology in New Zealand Weather and Climate (1989) 9: 7-13 7 A REVIEW OF TOPOCLIMATOLOGY IN NEW ZEALAND B. B. Rtzharris Department of Geography, University of Otago, P.O. Box 56, Dunedin, New Zealand ABSTRACT Because New Zealand is such a hilly country it contains a wide range of topoclimates but these have received surprisingly little attention. Development of horticulture, continued hydroelectric power developments, irrigation planning, large industrial proposals, and investigations into local air pollution problems have stimulated interest in topoclimates. Studies which examine the spatial variation of climate by using the network of climate stations in New Zealand are reviewed, but it is concluded that these give only a general indication of topoclimates. Some papers attempt to fill in the gaps between climate stations with temporary special purpose measurements, or by finding relationships that allow mapping. The main topics discussed in the New Zealand literature are local airflows, wind and shelter, growing degree days, effectiveness of various frost fighting techniques, measurement of vertical temperature structure of the boundary layer, and winter chill units. Unlike the case for other natural resources, such as soils, there is no systematic programme for the mapping of topoclimates at scales that might be useful to horticulture. INTRODUCTION areas of the South Island the climate is dis- This review first outlines the topography tinctly continental in character, despite the and climate of New Zealand and their control fact that they are no more than 130 km from on topoclimates, then notes an increasing in- the ocean. In addition, abrupt changes in to- terest in the topic. The few attempts to map pography, and the strength of the westerlies topoclimates are outlined, together with other ensure that many places are windy. studies on local climate and associated pro- In his review of New Zealand mountain cesses, although the latter survey is not ex- climate, Coulter (1967) notes "a rather large haustive. Finally, some ideas for the future average lapse rate of temperature with height are presented. and occasional periods of low temperatures in Because of their latitude between 34'S and summer" as being distinctive. Certainly the 47'S, the climate of the main islands of New juxtaposition of snow covered mountains and Zealand is determined by their intermediate snow free valleys produces large altitudinal position just south of the sub-tropical high contrasts in climate. The snow also generates pressure belt and on the northern fringe of the cold air drainage into the valleys and hemispheric westerlies. Other main factors intermontane basis where it is frequently that influence the climate are the country's trapped. For example, at Earnscleugh within position in the midst of a vast ocean and its an Otago intermontane basin, there are 180 shape and topography. The oceans ensure that ground frosts and 113 air frosts per year on there is abundant moisture for precipitation average, whereas on the eastern coast of the and equable temperatures. On the other hand, province at Dunedin, which is isolated from the mountains generate sharp contrasts in snow covered hills and where the maritime climate from west to east, and in some inland influence is strong, the number of ground 8 Topoclimatology in New Zealand frosts average 78 and air frosts just 10 per relocated because of the demands of alternat- year. ive land uses such as is caused by the en- The net effect of these factors is that there croachment of hydro lakes or urban areas. are at least 15 distinct macro-scale climate For example in Central Otago, there was a regions in New Zealand. However, the topog- need to identify new orchard sites to replace those flooded by the Clyde high dam raphy creates a highly varied landscape so that a wide range of topoclimates are pro- (Fitzharris 1977a, 1977b, 1979, Beecroft et al., duced that can change rapidly over short 1983). There is also quickening interest in distances. The main controls on these finding new wine growing areas (e.g. Franklin 1968, 1969, Hutchinson 1969, Marris 1978, topoclimates are elevation, receipt of solar radiation, shelter from wind, distance from Milne 1978, Steans and Jackson 1978). In these circumstances, maps of local scale climate the ocean, and propensity for cold air ponding. have to be produced within a very short time, sometimes within a year. In addition, siting BACKGROUND TO MAPPING OF TOPOCIAMATES and construction of large industries lead to detailed investigations of likely air pollution The need for topoclimate maps in New dispersion and of the boundary layer in their Zealand is increasing because the pace of land vicinity, so that maps and statistics of use change is quickening. Historically, pasto- topoclimatic phenomena, such as the depth of ral agriculture has been the dominant land cold air ponding and inversion frequency, use and topoclimates were relatively unim- have often been produced. A summary by portant, because free-ranging livestock inte- topic area of the known papers published on grated any differences in grass production by topoclimates in New Zealand is given in Table grazing over the variety of terrain on the 1. farm. As a result, even agricultural research stations made little effort to identify topoclimates, although the measurements of TABLE 1: SUMMARY OF MAIN TOPIC AREAS IN THE Chiew (1976) and Tuckey (1976) at Invermay NEW ZEALAND LITERATURE ON TOPOCLIMATES near Dunedin, are notable exceptions, and Topic area Number of Papers some attempt to redress this deficiency is suggested in the guidelines of McAneney and 11 Kerr (1984). However, now that there is a Investigations for growing of grapes Investigations for growing of other crops 6 decline in the economic returns from grass- 9 land farming and a change to higher value Study of climate parameters (eg frost, wind) Study of alpine climate processes 8 crops based on horticulture, orchards and 8 vineyards, identification of the best Study of inversions, air pollution dispersion topoclimates becomes more critical because Investigations related to hydroelectric development 4 wise choices give large economic advantages to the grower. Total 46 Recent discussion of enhanced greenhouse warming brought about by an anthropogenic increase in the concentration of gases such as MAPPING OF TOPOCIAMATES carbon dioxide has raised the question of fu- Mapping of topoclimates should be at the ture climates for horticulture and other crops local scale, defined after Oke (1987), as areas (for example see Ministry for the Environ- between 0.1 and 100 sq km. In New Zealand ment 1988). Various scenarios have been pro- there are about 300 climate stations with posed, with the Royal Society of New Zealand longterm records, or one for every 850 sq km, (1988) suggesting a warming of up to 3°C. In so that although these stations record infor- climatic terms this is a large change that will mation that is useful for selection of horticul- greatly affect the geographical distribution of tural sites, the network is too coarse to crops. It is now realised that a sound know- identify local scale topoclimates. For exam- ledge of present topoclimates greatly helps in ple, in the fruit growing area of the "Central deducing future limits for crops in a green- Otago warm zone", which represents land house warmed New Zealand. highly suitable for production of export quali- From time to time orchards have to be ty stone fruit, there is only one climate station Topoelimatology in New Zealand 9 for every 2600 hectares. This is inadequate beause most orchards operate on less than ten hectares of land, often taking advantage of local climates in sheltered vales or on slopes where the risk of frost is diminished. 111. All criteria root Nevertheless, data from climate stations 2 Less suitable (rainfall <1200mm) have provided the starting point for 3 Marginal (GOD >900) topoclimatic mapping, and the New Zealand Meteorologial Service has produced a series of climatological maps at a scale of 1:2,000,000 which show the distribution of such things as climate regions, soil temperatures, air temperatures (several parameters), and rainfall (several parameters). Hurnard (1982) used the combined criteria of growing degree days, frosts, and rainfall, as recorded at cli- mate stations, to identify areas at the meso- scale that may be suitable for riesling grapes (Fig 1), although there were similar attempts as early as last century (Bragato 1895, 1906, Palmer 1898). Kerr et al. (1981) used data from 74 climate stations, 20 sunshine stations and 437 rainfall stations from the lower North Island to begin the process of matching horti- cultural crops and climates. A sample of their results are shown in Fig 2. Normally growing Fig 1. Areas mapped by Hurnard (1982) as climatically degree days are accumulated for the warmest suitable for reisling white wine grapes. GDD is growing six months of the year (1 November to 30 degree days (base temperature 10°C) for October to April inclusive. Napier Fig 2: Isolines of accumulated Apo° growing degree days above 10C for October-April inclu- sive which are exceeded in 8 out of 10 years. Area shown is the southern part of the North Island and is from Kerr et al (1981) 1000 Wellington • Climate station Land below 200m 10 Topoclimatology in New Zealand April), and tables for various base tempera- tures have been prepared for 130 climate stations by the New Zealand Meteorological Service (1978). However, Jackson and Cherry (1988) do not favour the use of growing degree days and propose an index based on latitude and mean temperature of the warmest month. In order to calculate and map average de- gree days at the local scale, Turner and Fitzharris (1986) developed a technique which they illustrated with the example of the Ban- nockburn district, a 21 sq km area of diverse terrain in Central Otago.
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