Meteorol Atmos Phys 89, 207–214 (2005)DOI 10.1007/s00703-005-0129-8

1 School of Meteorology, University of Oklahoma, Norman, OK, USA2 Centre for Environmental Modelling and Prediction, School of Mathematics,University of New South Wales, Sydney, Australia3 Climate Analyst, Insurance Australia Group, Sydney, New South Wales4 Bureau of Meteorology, West Perth, Western Australia

Climatology of meteorological ‘‘bombs’’in the New Zealand region

L. M. Leslie1;2, M. Leplastrier3, B. W. Buckley4, and L. Qi2

With 9 Figures

Received September 22, 2004; revised October 25, 2004; accepted December 8, 2004Published online: June 20, 2005 # Springer-Verlag 2005

Summary

The purpose of this paper is to present a recently developedclimatology of explosively developing south easternTasman Sea extra-tropical cyclones, or meteorological‘‘bombs’’, using a latitude dependent definition formeteorological bombs based on that of Simmonds andKeay (2000a, b), and Lim and Simmonds (2002). Thesehighly transient systems, which have a damaging impactupon New Zealand, are frequently accompanied bydestructive winds, flood rains, and coastal storm surges.Two cases are selected from the climatology and brieflydescribed here. The first case study is the major floodand storm force wind event of June 20 to 21, 2002that affected the Coromandel Peninsula region of theNorth Island of New Zealand. The second case was a‘‘supercyclone’’ bomb that developed well to thesouthwest of New Zealand region during May 29 to 31,2004, but which could easily have formed in the NewZealand region with catastrophic consequences. It waswell-captured by the new high resolution Quikscatscatterometer instrument.

This study extends the work of two of the authors(Buckley and Leslie, 2004; Buckley and Leslie, 2000; Qiand Leslie, 2000), on southern hemisphere meteorologicalbombs, to yet another geographical location. Here, we reliedheavily on surface observations, scatterometer and othersatellite-derived data, and weather radar imagery in develop-ing the climatology.

1. Introduction

The intensity of extra-tropical cyclones, as indi-cated by their central pressures, provides a goodprimary indication of the likely severity of theassociated weather. However, it does not providesufficient information to fully characterize theseverity of an intense extra-tropical cyclone. Thespeed of movement, physical size, and rate ofintensification (or weakening) should also beconsidered. In the analysis that follows, a clima-tology is developed of those lows that satisfythe criterion of some of the potentially mosthazardous extra-tropical cyclones – the so-called‘‘meteorological bombs’’. Simmonds and Keay(2000a, b) discuss the importance of includinga latitudinal discriminator in defining a meteo-rological bomb, so that the relative significanceof the event for a specified latitude can bemaintained. For an extra-tropical cyclone to beclassified as a meteorological bomb, using thisdefinition for the latitude band most relevant tothe main islands of New Zealand (from 35� S to47� S), a 24 hour pressure fall of at least 20 hPain a 24 hour period is required. It is noted that the

rate of intensification is important because thetime rate of change of the pressure gradient addsanother component to the strength of the nearsurface winds, making the weather potentiallyeven more severe, when compared to lows ofsimilar intensity but with a slower rate of devel-opment. Most bombs develop gale to storm forcewinds extending from the centre to distances ofthe order of 200 to 300 km. Those bombs, whichaffect the New Zealand region, occur most oftenin the southern and eastern sectors of the south-eastern Tasman Sea. However, gale or stormforce winds can occur in all sectors and theirlocation relative to the centre changes as the sys-tem evolves. Rainfall bands associated with thebombs intensify for two main reasons. First, animportant component in the explosive deepeningnormally is the amount of energy released bydeep convection, supplied to most bombs by adeep in-feed of tropical moisture on their easternflank. Second, the strengthened low-level windfield generates increased vertical motion, andhence heavier rainfall than from equivalent inten-sity but non-bombing lows.

Most of the extra-tropical cyclones in the NewZealand region that are categorized as ‘‘bombs’’spend much of their life cycles over the datasparse waters of the Southern Ocean or theTasman Sea. To increase the reliability of thisclimatological analysis, only the most recent15-year period, from 1990 through to the presenttime, were included in this climatology. The cov-erage of weather satellites and drifting oceanicbuoys was relatively consistent through the peri-od and the dataset can be considered essentiallyhomogeneous. In general, the analysis was basedon 24-hour pressure changes calculated from theofficial archived mean sea level pressure analysesof the Australian Bureau of Meteorology’sNational Meteorological and OceanographicCentre (NMOC), in Melbourne. In most cases,the pressure changes were based upon the mainanalysis time of the day, namely 00UTC,although charts from other times were alsoincluded in particular cases. It is noted that thethreshold pressure fall of 20 hPa used to identifycases for this climatology is likely to underesti-mate the true maximum 24 hour pressure fall.Moreover, the peak rate of change in centralpressure is unlikely to occur between 00UTCon one day and 00UTC on the following day

for most events. A relatively recent meteorologi-cal bomb that produced major insurance lossesacross the North Island of New Zealand, from20 to 21 June 2002, was one of these cases,and is discussed in Section 3, below.

2. The climatology

The figures that follow summarize the climatol-ogy. Figure 1 shows the number of meteorologi-cal bombs affecting the two main islands ofNew Zealand each year since 1990. There isconsiderable inter-annual variability, with multi-ple events in some years and none in others.The most active year for bombs was 1990, dur-ing which there were 6 events. In sharp con-trast, there were no events recorded in 1992,1993, 1995, 1997 and 1999. The average annualnumber of bomb events over the period is 1.7,but this number obviously has little meaning

Fig. 1. Number of meteorological bombs affecting theNew Zealand region on a yearly basis, with a five-yearaverage moving trend line also shown

Fig. 2. Frequency of occurrence of bombing lows bymonth with a smoothed fifth order polynomial curve of bestfit included. January is repeated at the end in order toachieve a more realistic trend line

208 L. M. Leslie et al

given the large variability in the yearly frequen-cies of occurrence.

Meteorological bombs are not evenly distrib-uted throughout the year. As Fig. 2 shows, thereis a bi-modal distribution with a major peak inJune–July and a secondary peak in November–December. Minima occur in the period fromJanuary to March and September–October, witha small minimum also observed in May.

The sizes of the meteorological bombs aresuch that large areas can be affected by individ-ual systems. The impacts are subdivided intothose that affect both of the main islands ofNew Zealand, those affecting the North Islandonly, or those affecting the South Island only.This classification is somewhat arbitrary giventhe large size of these systems, but is providedhere as a guide to the relative level of riskbetween the two islands. Either the North orSouth Islands is considered to have felt theimpact of the event if gale force winds and=orheavy rain associated with the lows were experi-enced. As seen in Fig. 3, the South Island isaffected by the majority, 88%, of all bombs,while the North Island feels the impact of lessthan half, namely 42% of all bombs. Both islandsfeel the effects of 31% of the events. Only 11%of bombs impact the North Island withoutsome of the effects also being felt on the SouthIsland.

The 24-hour rate of intensification provides agood indication of the relative severity of theevents, as shown in Fig. 4. Approximately half

of all the intense lows studied here barely quali-fied as meteorological bombs, including themajor destructive storm of 20 to 21 June, 2002.Only 8% of bombs experienced rates of deepen-ing in excess of 30 hPa per day, again underliningthe limitation imposed by the 00UTC to 00UTCtime frame used for the analysis. The most explo-sive development in the data set occurred on 28July 1990 when the central pressure of a low thatsubsequently impacted upon the North Island fellby a very large 36 hPa in the 24-hour period. Thenext most explosive event occurred on 24 May2001, this time affecting only the South Island.Although not included in this climatology,because it occurred just outside the region ofinterest here, an event of enormous significanceoccurred in May 2004, to the southwest of theSouth Island of New Zealand. It produced a hugepressure fall of 75 hPa, in the space of 48 hours,with a 24-hour pressure fall of 48 hPa. Thisevent, described as an extra-tropical ‘‘supercy-clone’’, is described more fully in Buckley andLeslie (2004).

The central pressures of the bombs studiedhere, following the period of rapid intensificationand which may not be the actual minimum pres-sures of the lows, is shown in Fig. 5. The mostintense bomb that occurred in the period coveredby the climatology reached a central pressure of965 hPa, with the median central pressure being980 hPa. All of the bombs in the climatologydeveloped large circulations spanning severalhundred kilometers, following their rapid inten-sification periods.

Fig. 3. Impacts of meteorological bombs in the NewZealand region

Fig. 4. Maximum rate of intensification of the meteoro-logical bombs over the 24 hour period of their rapid inten-sification, together with a trend line that provides asmoothed representation of the likely range of intensifica-tion rates (measured 00UTC to 00UTC for most cases)

Climatology of meteorological ‘‘bombs’’ in the New Zealand region 209

3. Two examples of bombs in or nearthe New Zealand region

The large ocean areas surrounding New Zealandmeans that it is not possible to accurately docu-ment the actual physical characteristics ofbombing lows in the New Zealand region in atrue climatological sense. The Quikscat scatte-rometer instrument, on board polar orbitingsatellites, has been providing data routinely avail-able since mid 1999. This instrument has madeit possible to investigate, far more accuratelyand comprehensively, the occurrence, size, tracksand intensity of these lows. Two recent examplesare provided that have served as modellingcase studies of New Zealand region meteoro-logical bombs. The results of these modellingstudies are to be reported on elsewhere in thenear future.

3.1 The bomb of June 20–21, 2002

This low developed over the northern TasmanSea, then moved rapidly south east across theNorth Island of New Zealand. Central pressuresfell by 24 hPa in 24 hours as it crossed the NorthIsland. The Coromandel Peninsula was particu-larly hard hit by this system. In Fig. 6a, the scat-terometer wind field ahead of the bombing lowcan be seen, with sustained 10 minute northeast-erly winds to 45 knots shown. In Fig. 6b, thewind field following the bomb’s passage areshown. Sustained storm force winds were pro-duced by this intense low pressure system duringthe period of its intensification and movementacross the North Island. Full details of thismeteorological event are available from Munro

Fig. 6a. Average 10 metre wind field at 0519UTC 20 June2002 as determined by the satellite based Quikscat scatte-rometer ahead of the meteorological bomb. Mean windspeeds of 45 knots are shown. Applying a 30% gust factorwould indicate 60 knot wind gusts were possible. (Imagecourtesy of NOAA). (b) Average 10 metre wind field at1920UTC 20 June 2002, as determined by the satellitebased Quikscat scatterometer, in the wake of the meteoro-logical bomb. (Image courtesy of NOAA)

Fig. 5. Central pressures of the bombing lows at 00UTCfollowing their period of rapid intensification

210 L. M. Leslie et al

(2002). The following is a general outline of theevent, sufficient to demonstrate the impact of thestorm on the Coromandel Peninsula region ofNew Zealand.

A low-pressure system developed over thenorthern Tasman Sea, deepening explosivelyover a 24 hour period as it traversed the NorthIsland of New Zealand. The central pressure ofthe low fell by approximately 24 hPa over a 24-hour period, leading to its classification as ameteorological bomb. Figure 7a and b showsthe broad-scale low pressure system movementand intensification that produced this extremeweather event, as depicted by analyses fromthe official archives of the Australian Bureauof Meteorology. The most extreme weatheroccurred in a rainband with embedded deep con-vection (thunderstorms) that formed along a con-vergence zone between gale force northeasterlywinds and northerly winds that crossed thePeninsula between 10UTC and 13UTC on 20June. The convergence zone was well-capturedby weather radar.

Severe flash flooding was caused by the shortduration, very heavy rainfall at numerous loca-tions across the Coromandel=Waikato region.There was a very high degree of variability inthe reported rainfall, even over very short dis-tances, indicating that the rainfall was producedby very small scale, transient convective cellswithin the larger thunderstorm complexes. Someof the most significant very intense short durationrainfalls reported from this event included: 15minute duration rainfall of 31 mm at Waiomu;hourly rainfalls of 125 mm at Coromandel,97 mm at Te Aroha, 83 mm at Tapu and 80 mmat Waiomu, and two hour rainfalls of 120 mm atPutaruru and in excess of 150 mm at ThorntonBay. Wind gusts to 120 km=h (65 knots) werereported from the Coromandel Peninsula regionin the northeasterlies immediately ahead of thechange.

The structure of the rain band is best shown bylooking at the weather radar images (not shownhere) that captured its passage across theCoromandel Peninsula. The radar is C-Band ra-dar and hence any echoes that lie to the east ofthe closest line of intense storms are likelyto under-represented (attenuated). The extremerainfall was largely confined to one very intensefractured line of thunderstorms that crossed the

Fig. 7a. Official Australian Bureau of Meteorology meansea-level pressure analysis chart for 00UTC 20 June 2002.(b) Official Australian Bureau of Meteorology mean sea-level chart analysis valid 00UTC 21 June 2002

Climatology of meteorological ‘‘bombs’’ in the New Zealand region 211

Peninsula between 1030UTC and 1300UTC. It isalso noted that the observed rainfalls could wellunder-represent the actual rainfalls across the keycatchments for two main reasons. Firstly theobservations are primarily from in or near themain towns, which tend to be located on rela-tively low-lying terrain. Topographic maps showthat the river catchments which produced theflash flooding extend inland over sharply risingterrain that would be expected to increase rainfallacross the elevated portions of the catchments.They are particularly exposed to weather systemsmoving in from the north northwest, as wasthe case for this rain band. The lower land tohighland rainfall variation is illustrated by com-paring the observed rainfalls between the coastallocation of Thames and inland location ofKauaeranga River (Pinnacles). Second, the mea-surements themselves are likely to under-recordthe true rainfall received. During very heavy rainevents the buckets of tipping bucket rain gaugestend to overflow between tips with the problemworse in the smaller bucket sizes, thereby under-recording the true rainfall. In extreme eventssuch as this the buckets can actually stall duringa tip if the rain is heavy enough. The rain alsofell during a period of very strong winds. Co-incident strong winds are known to decreasethe rainfall catch in gauges of the Nylex 1000type and all standard tipping bucket rain gauges,which comprise a large part of the official observ-ing network. This under-recording can be asmuch as 30% in exposed sites. Many of theobservations were also from unofficial sources,increasing the probability of sub-standard instru-ment exposure.

3.2 The bomb of May 29–31, 2004

This ‘‘supercyclone’’, described in detail byBuckley and Leslie (2004), did not affect theNew Zealand region. However, it was closeenough to provide a demonstration of the poten-tially ‘‘worst case’’ meteorological bomb thatcould impact upon the New Zealand islands, withthe South Island being most vulnerable to thistype of development. However, as all the ingre-dients that led to its development can exist overthe southern Tasman Sea (Buckley and Leslie,2000), the possibility exists that such a low couldimpact upon the North Island, although with alow probability of occurrence. The west coast-lines of both islands would face the highest riskfrom such a system. The data sparse nature of theSouthern Ocean makes it very unusual for theintensity of very strong southern cyclones to beverified. However, during the period from 29 to31 May 2004, a mid latitude cyclone deepenedexplosively over the waters to the south ofAustralia. Mean sea-level pressures fell 75 hPaover a 48 hour period, from around 1004 hPaon 00UTC 29 May to a very low 929 hPa on00UTC 31 May 2004. The low then weakenedto 942 hPa during the following 24 hour period.The life cycle of this low was well captured bythe Quikscat scatterometer. The initial low devel-opment started as a wave feature on a large coldfront to the south of Western Australia. The scat-terometer pass at 0630UTC 30 May 2004 iden-tified a rapidly enlarging area of 50 to 60 knotunidirectional northwesterly winds that extend-ed 1000 km along and up to 600 km to the northeast of an almost linear shear line that markedthe location of the front. Although most vectorsare rain affected, they are considered realistic tobe estimates of the surface conditions. The scat-terometer data clearly showed the greatest long-itudinal extent of storm force winds almostperpendicular to the first signs of a wave on thefrontal boundary at 47� S. There was no identifi-able surface low in this region with the parentlow some 1200 km further south. Winds to thewest of the front were generally only around 30to 35 knots, which is common at these latitudes.

During the following 14 hours the transforma-tion of this system was substantial, as can be seenfrom the scatterometer pass at 2040UTC 30 Mayin Fig. 8a. A significant low pressure system

Table 1. Reported 24 hour rainfalls for some of the moresignificant locations across the Coromandel Peninsula andsouthern Waikato region for varying times ending between5.30 am and 9 am local time

Station Rainfall (mm)

Manaia 230Coromandel 205 (270 in 48 hr)Tapu 200–215Thames 130Kauaeranga River=Pinnacles 200Te Aroha 115–157Putaruru 150–260Tirau 133–200

212 L. M. Leslie et al

formed, with storm force winds in all quadrants,although the storm had a relatively asymmetricalstructure. Maximum sustained wind speedsappear to have remained close to 60 knots,although their geographical distribution haschanged dramatically during the 14 hour period.Further intensification of the system occurredduring the next 10 hours. It was during this pe-riod that the low began to affect the Bureauof Meteorology’s weather station on MacquarieIsland, located near 54.5S 159E. Wind speedsmeasured on the island rapidly increased to a

peaked of 62 knots from the northwest at08UTC, a speed that validates the accuracy ofthe nearby Quikscat wind vectors from the0630UTC 31 May pass (Fig. 8b shown). Furtherto the west, wind vectors of up to 80 knots can beseen with the zone of storm force winds arcingaround the low for a distance of approximately1200 km.

4. Conclusions

A climatology of rapidly developing extra-tropi-cal cyclones that meet or exceed the criteria forclassification as meteorological bombs has beencarried out for the New Zealand region. The cli-matology shows that these weather systems arerecurring events, with a large range in tracks.There are two preferred periods of the year thatfavour the development of meteorological bombevents, but they can occur in all months of theyear. All parts of both the North and SouthIslands of New Zealand are exposed to severeweather phenomena, notably storm force winds,flood rainfall totals, and coastal storm surges,associated with these systems. Two exampleswere examined in brief, to illustrate some ofthe characteristics of these destructive events.

It is vitally important that the strength and lifecycle of meteorological bombs that form overand traverse the South Indian and Tasman Seabe far better understood, analysed and predicted.This is true on all time scales, from hours anddays through to climate scales, as there are majorimplications at all of these scales. In particularthe detailed structures of the most rapidly devel-oping of these powerful of these storms mustcontinue to be investigated in current and future

Fig. 8a. QuikSCAT scatterometer 12.5 km resolution oceanwind vectors centred near 55S 143E at 2050UTC 30 May2004. (Image courtesy of NOAA.) (b) Quikscat scattero-meter 12.5 km resolution ocean wind vectors centred near55� S 157� E at 0630UTC 31 May 2004. (Image courtesyof NOAA)

Fig. 9. Graph of the 10 minute mean 10 metre wind speedfor Macquarie Island over the period from 0000UTC31 May 2004 to 0000UTC 1 June 2004

Climatology of meteorological ‘‘bombs’’ in the New Zealand region 213

climate scenarios. Follow-up studies are under-way in two areas and will be reported on in thenear future. First, high-resolution modelling stud-ies are being carried out to assess whethernumerical models are now able to simulate thekey meteorological features of explosively devel-oping extra-tropical systems with a high degreeof accuracy. The expectation is an improvingpotential to model the impacts of these stormswith a greater level of skill than hitherto. Im-proved planning and community preparednessdecisions will result from an improved under-standing of the dynamics of these recurring,severe weather systems. Second, work has justbeen completed which will be used to estimatehow the climatology and intensity of these severeextra-tropical systems, and their tropical counter-parts, might change in an increasing greenhousegas world.

Acknowledgements

This research has been substantially funded by the IAG NewZealand. The support of Environment Waikato, ThamesCoromandel District Council, NIWA, IAG New Zealandand IAG Australia were gratefully received.

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Corresponding author’s address: Lance M. Leslie, Schoolof Meteorology, University of Oklahoma, Norman,Oklahoma 73019, USA (E-mail: lmleslie@ou.edu)

214 L. M. Leslie et al: Climatology of meteorological ‘‘bombs’’ in the New Zealand region