Southerly Buster Events in New Zealand

Weather and Climate (1990) 10: 35-54 35 SOUTHERLY BUSTER EVENTS IN NEW ZEALAND R. N. Ridley' New Zealand Meteorological Service, Wellington, New Zealand ABSTRACT The 'southerly buster' is a particularly intense form of southerly change which occurs along the New South Wales coast of Australia. In this study similar changes occurring in late summer in eastern New Zealand are identified and described, with emphasis on their synoptic environment. Typical features found include a sharp or sharpening trough approaching in a strong westerly flow, much warmer than usual air over New Zealand preceding the change, a shallow southerly flow following the change, and a complex meso-scale signature at the surface. The synoptic scale flow is found to have a significant influence on the formation and nature of the changes. On the meso- scale, the New Zealand southerly buster appears to have unsteady gravity current-like features similar to its Australian counterpart. A typical scenario for the occurrence of a southerly buster along the east coast of New Zealand is given. INTRODUCTION tries as marine, aviation, construction, forest- The Southern Alps are a major barrier to ry (including fire-fighting) and power supply. the prevailing westerly flow, occupying up to They pose a difficult forecasting problem due a third of the depth of the troposphere, and the to their occurrence on time and space scales structure of any front is expected, therefore, that are not well resolved by current numeri- to be greatly modified by flow over or around cal prediction models, and a lack of under- the mountains. Indeed, it is well known by standing of their dynamics. This, and the lack meteorologists in New Zealand that a critical of any previous detailed description or defini- factor in forecasting for areas east of the tion of the phenomenon in New Zealand, pro- Southern Alps is the prediction of a change vide major motivations for this study. from the predominant onshore winds on the The 'southerly buster' in Australia was de- west coast to onshore (southerly') winds on the scribed in some detail almost 100 years ago east coast. This transition often manifests by Hunt (1894), who used the term 'burster' itself on the east coast as a strong wind surge, rather than 'buster', suggestive of the phenom- in some cases lasting only a few hours, and enon manifesting itself as a dramatic 'burst' sometimes accompanied also by a dramatic of air from the south. He noted that in the fall in temperature, a rapid surface pressure early days of settlement it was in fact locally rise, and a spectacular roll cloud. Such a called the 'brickfielder' due to its arrival be- change is often referred to as a 'southerly ing heralded by a cloud of reddish dust from buster', consistent with its Australian counter- the great many brick fields in the area. We part (see e.g. Baines, 1980; Colquhoun et al., shall use the term 'buster' (and henceforth 1985; Coulman et al., 1985). These events have drop the quotes) in this paper to conform with significant safety implications for such indus- the predominant usage in recent publications. Consistent with the studies in Australia, including that of Hunt, we adopt a subjective Current affiliation: Centre for Dynamical Meteorology, definition that requires a southerly buster to Mathematics Dept., Monash University, Clayton, Vic., Australia be a gusty wind change that produces strong In this paper 'southerly' is used in a general sense to southerly winds near the coast soon after its mean a wind from between southeast and southwest passage, and which is not associated with a 36 Southerly buster events depression passing over or near the North as they passed Christchurch. Thus, precipitat- Island. We note that although rapid tempera- ing squall-line characteristics were not a pre- ture falls may occur in association with south- dominant feature of the changes. In some erly busters, these are not explicitly part of cases rain fell 6-12 hours later due to develop- the definition. We note also the distinction ments in the following airstream, consistent between the terminology 'southerly change' with the observation of Revell et al. (1987) which refers to the whole range of meteoro- that the wind changes often separate from the logically important phenomena, and 'souther- parent cloud body as they move north. ly buster' which is applicable to a relatively Revell et al. (1987) studied 10 southerly small percentage of the former. changes that occurred at various times of the There is little published material on south- year. They found that high pre-change tem- erly-buster events in New Zealand. Hutchings peratures induced by fohn winds or other (1944) illustrated the impact of topography on means lead to characteristically gusty the low-level air flow and surface pressure changes, but that the large density differences field over the country. He showed that the so implied are not necessary for this. They blocking of the cross-mountain low-level flow found also that most changes occur in associa- led to a characteristic 'S'-shaped pattern in tion with cold fronts originating over the the isobars over the country. Crawford (1977) southern ocean, and accompany the passage described a situation in which record high of short-wave troughs in the westerly flow. temperatures occurred in many parts of east- The latter is also confirmed by Ridley (1987), ern New Zealand, followed at Christchurch by who found that the general increase in wester- a drop in temperature of 20°C in two hours as ly flow at mid- latitudes during the Southern a southerly change occurred. oscillation event of 1982-1983 resulted in an A climatology of southerly changes at increase of about 30% in southerly changes at Christchurch was prepared by Ridley (1987). Christchurch during that period, compared Although this did not specifically identify with other years. Further, this increase was southerly-buster events, these evidently con- due almost entirely to a greater number of stituted a significant proportion of the includ- relatively intense events. ed changes; about 16% if a southerly buster is Baines (1980) proposed an analytical model assumed to be a 'strong' event according to his of the Australian southerly buster as a coast- definition (see section 2). This would give an ally-trapped gravity or density current (i.e. average of about 13 southerly-buster events the motion is driven primarily by the pressure per year at Christchurch over a seven-year gradient resulting from the density difference period. Using a more specific definition, across the interface at the leading edge of the Colquhoun et al. (1985) found about 10 souther- current), arising from blocking by the moun- ly busters per year on the New South Wales tain barrier of the along-front component of coast over a 10-year period. the wind in the cold air. The current is held The climatology found that relatively against the eastern side of the mountains by strong southerly changes were more likely in the coriolis force. The model is able to explain late summer and tended to pass through the observed 'S'-shaped distortion of fronts as Christchurch in the afternoon or evening they cross the dividing range, and predicts hours, whereas weak changes showed little that the depth of cold air should diminish with such dependence. Strong events, and those distance eastward from the mountains. Con- associated with large temperature falls, were firmation of the latter requires observations also more likely to have pre-change fohn con- over the sea which were not available at that ditions than other events. These observations time, however Gauntlett et al. (1984) demon- suggest dependence of the strength of the strated numerically such a decrease in depth. changes on the land/sea temperature con- Colquhoun et al. (1985) and Coulman et al. trast, the latter enhanced by a combination of (1985) described the results of southerly buster diabatic (surface) and adiabatic (fohn) heat- experiments on the New South Wales coast. ing. In only 3% of all cases included in the They concluded that topography and the land- climatology were thunderstorms reported. In- sea temperature contrast were important fac- deed, the majority of the relatively intense tors, however they found no observational changes, especially those in summer and au- evidence of the decrease in depth of cold air tumn, were without associated precipitation with distance eastwards from the mountain Southerly buster events 37 chain, at least over the ocean. They found stances some of the most dramatic southerly evidence of the existence of a roll vortex busters have occurred. behind the leading edge of the cold air such The importance of Cook Strait as a promin- that, except over a small low-level region, the ent break in the mountain chain must also be wind changes propagated faster than the ad- considered, as it provides a passage for much vection of cold air behind them. However, of the air that is forced to pass around rather they were unable to find a mechanism by than over South Alps. We would expect a which this process occurred. Their observa- current of air such as a southerly buster to tions suggest that the southerly buster, while have extreme difficulty in penetrating this having gravity current-like characteristics, is region unless the adverse pressure gradient fundamentally different in structure to steady there is reversed through suitable changes in gravity current flows simulated in laboratory the synoptic environment. tank experiments. A program of research on southerly Howells and Kuo (1988) carried out numeri- changes has been initiated recently in New cal simultations of a southerly buster event in Zealand, based largely around the SOUtherly Australia and concluded that several physical CHange EXperiment (SOUCHEX).

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