The New Zealand *** Southern Alps Experiment ^ D. S. Wratt,* R. N. Ridley,* M. R. Sinclair,* H. Larsen,* S. M. Thompson,* R. Henderson/ G. L. Austin,* S. G. Bradley,* A. Auer,@ A. P. Sturman,& I. Owens,& B. Fitzharris,** B. F. Ryan,++ and J.-F. Gayet** ABSTRACT The Southern Alps Experiment is being mounted to study the influence of New Zealand's Southern Alps on local weather and climate. This paper describes these alpine influences and outlines proposed field and modeling experi- ments. Experiment goals include understanding and quantifying factors that govern the intensity and spatial distribu- tion of heavy rainfall, the west to east distribution of precipitation across the mountains, and the intensity of lee wind storms and warming. Linked research will explore the use of deterministic rainfall models to predict river flows from mountain watersheds. 1. Introduction 95), mountain influences on South Island weather have been explored using archived weather data and The Southern Alps Experiment (SALPEX) is a new mesoscale models, and two initial field campaigns study of the processes through which the Alps influ- have been mounted. Results from this first phase are ence the weather and climate of New Zealand. The being used to refine experimental plans and develop Southern Alps form a long, narrow barrier across the hypotheses for testing during the second phase, which predominantly western flow over New Zealand's will include the main field work component (1996- South Island. In the first phase of SALPEX (1993- 97). In the final phase the experimental and model- ing results will be consolidated into an improved quan- *National Institute of Water and Atmospheric Research, titative understanding of mountain influences on New Wellington, New Zealand. Zealand weather. +National Institute of Water and Atmospheric Research, Orographic influences on weather have been ad- Christchurch, New Zealand. dressed in several major international research cam- # Physics Department, University of Auckland, Auckland, New paigns. For example, lee cyclogenesis, local winds, Zealand. @Meteorological Service of New Zealand, Wellington, New mountain drag, and effects of differential heating were Zealand. examined in the European Alps during the Alpine &Geography Department, Canterbury University, Christchurch, Experiment (Kuettner 1982; Smith 1986). Orographic New Zealand. influences on flow, mountain waves, wave breaking, **Geography Department, Otago University, Dunedin, New and fohn effects are topics for the Pyrenees Experi- Zealand. ment (Bougeault et al. 1993). The new European Me- ++Division of Atmospheric Research, CSIRO, Aspendale, Aus- tralia. soscale Alpine Program (Kuettner 1995) will inves- ##Laboratoire de Meteorologie Physique, Universite Blaise Pas- tigate orographically influenced precipitation events, cal, Clemont-Ferrand, France. and the three-dimensional structure of fohn-related Corresponding author address: Dr. David S. Wratt, National In- flow and wave features. stitute of Water and Atmospheric Research, 69-B Gracefield Rd., While results from such experiments provide valu- P.O. Box 31311, Lower Hutt, New Zealand. E-mail: [email protected] able guidance for New Zealand, regional research on In final form 13 November 1995. mountain meteorology is still required because of © 1996 American Meteorological Society particular aspects of the local geography and environ- Bulletin of the American Meteorological Society 683 Unauthenticated | Downloaded 10/04/21 09:37 AM UTC SW-NE axis for a distance of 800 km. Cook Strait forms a break between the two islands, with a further mountain chain extending from southwest to north- east up the North Island. The North Island mountains are not generally as high as those in the South Island, but there are still extensive areas above 1000 m and many peaks over 1500 m. New Zealand's topography has an obvious effect on the climate and weather. Figure 2 (Henderson 1993) shows how annual rainfall varies across the mountains. Annual average rainfall is 3.5 m at Haast on the west coast, peaks at around 11 m in the moun- FIG. la. Map showing eastern Australia, the Tasman Sea, and tains, and is only 0.7 m at Christchurch on the east New Zealand. coast. In 1777, an early European explorer, Captain James Cook, noted that northwesterly storms in the Cook Strait region were (Kerr 1974), ment. For example, the upwind fetch for many of the severe storms in New Zealand's mountains comprises ... sometimes violent and troublesome, owing at least 2000 km of ocean, almost devoid of direct to the neighbouring mountains (always at these meteorological observations or soundings. Before times laden with vapour's) which, not only in- crease the force of the wind but alter its direc- SALPEX, almost no measurements had been made of tion in such a manner that no two blasts follow the physical properties of the clouds impinging on the each other from the same quarter. Southern Alps, or the precipitation physics taking place within them as they travel over the mountains. The Southern Alps lie within the northern periph- Since mesoscale numerical weather prediction ery of the midlatitude Southern Hemisphere wester- models have not yet been used operationally for lies. Migrating anticyclones, typically in the 33°-40°S weather forecasting in New Zealand, tests are needed latitude band, pass eastward at 3-7-day intervals of model predictions against observed rainfall, snow- (Coulter 1975; Sinclair 1996). Low pressure "troughs" fall, and winds. We need to understand the processes pass over New Zealand between pairs of anticyclones. occurring when moist oceanic air is forced up abruptly These troughs are often associated with low pressure over our mountains, to investigate the density, type, centers that migrate eastward at latitudes south of New and mix of observations (including satellite sound- Zealand (Sinclair 1994b). Meridionally or NW-SE- ings) that should be assimilated into mesoscale mod- oriented fronts in the troughs are a common weather els for the New Zealand region, and to determine the disturbance in New Zealand. As these fronts approach suitability of large-scale prognoses from global mod- the country from the west, they are commonly pre- els for initializing and guiding mesoscale models ceded by northerly winds, increasing cloud, and oro- nested over New Zealand. graphic rain upwind of and near to the main alpine divide. This rain in the mountains generally occurs for a. Geography and weather at least several hours up to and including the passage New Zealand comprises two long, narrow moun- of the front, which is generally followed by colder tainous islands extending from 34° to 47°S, together southerly to southwesterly winds and showery with various small offshore islands (Fig. 1). The domi- weather. More details of typical New Zealand weather nant mountains in the South Island are the Southern sequences are documented by Ryan (1984). Alps, with many peaks exceeding 2500 m and the The storms, which give heavy rainfalls on the west- highest (Mt. Cook) extending to 3754 m. In the cen- ern side of the Alps, generate mountain waves (Cherry tral part of the South Island the main range is within 1972; Auer 1992) and often cause strong northwest- 40 km of the west coast, and a substantial plain oc- erly winds in the lee of the mountains. Northwesterly curs to the east of the mountains. Farther north and flows across the mountains also give rise to high fohn south, the mountains extend across more of the island. temperatures in the east (Ryan 1987). While north- The Southern Alps, and the associated mountains westerly winds are common in the inland parts of the that extend to the top of the South Island, run along a South Island, they do not always reach the east coast. 684 Vol. 77, No. 4, April 1 996 Unauthenticated | Downloaded 10/04/21 09:37 AM UTC Air is deflected around the top of the South Island and through Cook Strait by the mountains, and a lee trough develops downwind of the mountains as part of the mountain wave structure. These fea- tures, together with land-sea tempera- ture differences, often lead to a north- easterly flow over coastal Canterbury (Coulter 1975; McKendry et al. 1986). b. Economic impacts Interactions between the mountains and the large-scale atmospheric circula- tion patterns and weather systems have important economic impacts. For ex- ample, northwesterly storms interact best with the mountains to produce spillover of heavy precipitation across the main alpine divide into New Zealand's major hydroelectricity watersheds. A reduction in the frequency of upper-level north- westerlies through 1992 and 1993 asso- ciated with a persistent El Nino led to ^ South Island, New Zealand, showing the topography and the South considerabl• i e stres, s on ,hydroelectricit i , , . y Island locations mentioned in the text, generation (Fitzharris 1992). Agricultural activities and forestry are affected by rainfall patterns, floods, strong winds, and disrupt transport. They also provide stored water for high temperatures. Heavy snowfalls in the mountains hydroelectricity generation, irrigation for agriculture, cause problems for farmers, produce avalanches, and and snow for the tourism industry. Therefore, better weather forecasts, improved seasonal outlooks, and guidance on possible future regional manifestations of global climate change would all be of considerable practical benefit. These depend on better understand- ing and modeling of alpine effects on New Zealand's weather and climate. 2.