The Bomb

It’s been a while since a rapidly-deepening low passed close to, or over, New Zealand. I thought it might be interesting to take a quick look at why the “bomb” low of Saturday 03 March 2012 deepened so quickly and why the winds around it affected the areas they did.

First of all, here is a series of weather maps covering the period 1pm Friday 02 March to 1am Sunday 04 March.

Mean sea level analyses from 1pm Friday 02 March 2012 to 1am Sunday 03 March 2012, at six-hourly intervals.

A Bit of Background

Large-scale features on the weather map – that is, those systems which influence the day-to-day weather (highs, lows, fronts) – are driven by processes in the middle and upper parts of the troposphere. Lows become deep and anticyclones become intense when there is strong positive feedback between these processes.

In the case of lows, when the intensification (a change in central pressure) exceeds more than a certain amount over a given time, they are defined as “bombs”. Actually, it’s not quite that simple: if you really want to know more, see “Technical Stuff” at the end of this article.

Between 1am Friday 02 March and 1am Saturday 03 March, the central pressure of this particular low fell from 1002 hPa to 975 hPa: it qualifies.

The term “weather bomb” has come into popular usage in New Zealand to describe dramatic and/or destructive weather events – but very seldom is a “bomb” low the cause. “Bomb” lows aren’t all that common in the New Zealand area.

The Low of Saturday 03 March

Below is the weather map for 1am Saturday 03 March, superimposed on a satellite image. At this stage the low was still west of Taranaki, deepening rapidly and heading more or less straight for Palmerston North (but it never got there). I’ve drawn in some mauve arrowed lines to indicate the axis of strongest winds in the upper troposphere, known as the jet. The relative locations of the low and the jet, and the shape of the jet, strongly favour further deepening of the low and its movement towards the eastsoutheast – which is what happened. Note: this is a simplified explanation ahead of a bit more case work.

Mean sea level analysis 1am Saturday 03 March 2012, together with infra-red satellite image. Satellite image courtesy Japan Meteorological Agency.

A few hours later, at 6am Saturday 03 March, the low had a central pressure of about 971hPa and was not far south of Patea. Below is a portion of the working chart for 6am, drawn by one of MetService’s Severe Weather team. There’s a lot of isobars around the low – and over Taranaki, Wellington and the Marlborough Sounds in particular. The closer together the isobars, the stronger the winds. Also note that there’s a front drawn curling around the low. Tucked in on the southern and western side of this front is a zone of very strong winds, shown by the blue arrow. It’s mostly this zone which did the damage as it moved across the southern part of the North Island.

Hand-drawn mean sea level analysis for 6am Saturday 03 March 2012. This is an internal working chart, drawn by a member of MetService's Severe Weather team. Image copyright Meteorological Service of New Zealand Limited 2012.

Some Interesting Observations

At Hawera, the wind increased quickly during the early morning hours of Saturday 03 March.

Time Wind direction Mean speed (km/h) Highest gust last hour (km/h)
Midnight Fri-02-Mar Northnortheast 35 57
1am Sat-03-Mar North 26 44
2am Sat-03-Mar Northnortheast 24 43
3am Sat-03-Mar Northnortheast 53 83
4am Sat-03-Mar Northnorthwest 53 87
5am Sat-03-Mar Northwest 85 122

No observations were received between 6am and 11am because power to the reporting site was lost.

At Wanganui Airport, the temperature climbed steadily from 12.7 C at midnight Friday 02 March to 17.5 C at 5am Saturday 03 March. (The temperature behaved similarly at Hawera a few hours earlier). This happened because the relatively warm moist air flowing around the northern side of the approaching low was warmed and dried – the Foehn effect – as it passed across the high country to the north of Wanganui.

At Ohakea and Palmerston North, the winds were reasonably strong easterlies for some hours before and after dawn, but blew from the northwest for a while around dawn. Two switch-arounds (not quite 180 degrees) of steady to strong-ish winds in a short time is remarkable.

Between 4am and 3pm Saturday 03 March, the south to southeasterly wind at Brothers Island had a mean speed of 108 km/h. During this time, the highest gust was 142 km/h.

Crossing the Country

I mentioned earlier that at 6am the low was not far south of Patea and heading for Palmerston North but never got there. This is because it didn’t physically cross the North Island. Nevertheless, a graph of mean sea level pressure at Palmerston North (below) for Saturday 03 March 2012 shows that the pressure certainly reached a minimum around dawn before rising steeply again from mid morning.

Mean sea level pressure at Palmerston North.

While lows have distinct structures, they’re best not thought of as rotating solid bodies of air – because they’re not. Rather they are the manifestation, at the Earth’s surface, of processes which have produced a local minimum of pressure. Looking back to the working chart above, we see that there are two lows at 6am: one south of Patea, and another just east of Hawke’s Bay. The low south of Patea came ashore east of Wanganui and then decayed, while the “new” low continued to deepen and move away to the east. This decay/development happens because the “driving” atmospheric processes are largely above the Earth’s surface and moving with the general flow: they left the Patea low behind and powered the development of the “new” low east of the North Island.

Technical Stuff

Finally, the definition of a “bomb” low is technical and not whimsical. As far as I know, the first mention of the term “bomb” was in a paper by two distinguished meteorological researchers, Fred Sanders and John Gyakum, titled “Synoptic-Dynamic Climatology of the “Bomb””, published in the October 1980 issue of Weather and Forecasting, a journal of the American Meteorological Society. Because of their destructive potential, rapidly deepening lows have been the subject of many a research paper.

Tropical Cyclone YASI

Tropical Cyclone Yasi formed north of Fiji on Sunday 30 January 2011 and was named by the Tropical Cyclone Warning Centre of the Fiji Meteorological Service.

Yasi is a girl’s name and also the Fijian word for sandalwood.

Click to animate satellite loop showing 'Yasi' from 5pm Wed 2 Feb to 10am Thu 3 Feb 2011 (NZDT)

As Yasi moved westwards  and absorbed energy from the warmer-than-normal waters of the Coral Sea, its diameter expanded to 800 km: much larger than the 200-300km typical of tropical cyclones in this part of the world.  It intensified to Category 5 (on the Australian category scale) with gales out to 250km from its centre.

This was the largest category 5 tropical cyclone to cross the Queensland coast since 1918.

A circle of gales 500km wide is large enough to cover most of the North or South Island.  Even so, Yasi was not the largest cyclonic circulation on the planet at the time: over the last few days, a depression over the North Atlantic brought a major winter storm to the United States.

The conditions required for tropical cyclones to form are:

  • Warm seas. The flow of heat and moisture from the sea into the atmosphere powers the deep thunderstorms that form the building blocks of a tropical cyclone and enables the subsequent release of latent heat.
  • Converging winds. These draw the thunderstorms together and organize them.
  • Spin. The thunderstorms need to be drawn together and organized in such a way that they rotate cyclonically (clockwise, in the Southern Hemisphere). This creates a positive feedback, strengthening the winds and uptake of moisture, generating even more thunderstorms. To initiate spin, the developing circulation needs to be not too close to the Equator. (For a brief mention of the Coriolis Force, see the comments at the end of Chris Webster’s post titled “Year 12 Maths”).
  • Low wind shear. If the winds aloft are not too different from the winds near the Earth’s surface, we say that there is little vertical wind shear. Low wind shears allow the thunderstorms to grow unhindered; high wind shears stunt their growth.

Eye and eyewall: Click on this image to read more about the structure of low pressure systems.

Just outside the eye of a tropical cyclone is a region called the eye wall, where rain and wind are at their most intense. The eye of Yasi expanded to a diameter of 100 km at one stage, which is very large. Within Yasi’s eye wall, wind gusts of  up to 290 kph and rain amounts of 300mm were reported.

The track that a tropical cyclone takes is partly due to imbalances in its own symmetry and partly due to the environment around it. Yasi was embedded in a reasonably steady easterly flow aloft and this carried it along a fairly straight path towards northern Queensland.

Apart from wind and rain, the other destructive component of Yasi has been its storm surge. This in itself has several components. Firstly, lower than normal atmospheric pressure near the cyclone’s centre raises the sea level. Secondly, strong winds blowing from the sea to the land generate large waves and push the sea onto the land. These latter effects are most pronounced to the left of the tropical cyclone’s track (in the Southern Hemisphere), where the movement of the cyclone adds to the wind strength. If all these coincide with high tides, the effect of storm surge is increased.

Yasi weakened quickly as it moved inland. This is mostly because it became cut off from its energy source, the warm sea. But also, the low-level air in the circulation encounters more friction over land than over sea, and this slows everything down.

Summer in the Australia – New Zealand region continues in a remarkable way under the influence of La Nina.

A Lot of Rain

There has been a lot of rain in many parts of New Zealand over the past two weeks. One of the few places to escape this was the sunny West Coast of South Island – the days are clear and stunning there when the flow is southeasterly.

If you’ve been following the surface weather maps during May you would have noticed a lot of low pressure over the Tasman Sea and northern NZ, and periods of relatively high pressure farther south. This pattern has also been the case aloft, where upper level processes have been driving what has happened at the surface. Take a look at the two charts below – they’re similar to the coloured charts from an earlier post, showing the variation of upper level pressure heights. You can think of them as showing where the  flow was cyclonic and anticyclonic in the upper atmosphere.

Upper flows from 1 to 14 May 2010: blue/purple = cyclonic, yellow/red = anticyclonic

As previous, but for 15 to 28 May 2010. Images courtesy U.S National Oceanic & Atmospheric Administration Earth System Research Laboratory

During the first half of May (top chart) there was a strong tendency for anticyclonic conditions southeast of NZ. For the second half of May (bottom chart), this anticyclonic anomaly shifted westwards while a marked cyclonic tendency formed over the Tasman Sea, North Island and much of the South Island. This combination generated not only a lot of rain, but also a powerful easterly flow that amplified the rain in many eastern parts of the country.

Northern districts have had a lot of rain too, and the intense falls that were reliably recorded in Whakatane on Tuesday evening (1 June) were phenomenal. In just one hour 89.8 mm of rain fell, plus there was heavy rain either side of that hour. Falls of that intensity are fortunately rare in New Zealand, and it’s hard to imagine what they’re like unless you’ve experienced them. I did once, when visiting Nadi, Fiji many years ago…

It was the heaviest rain I’d ever seen, and it set a local record.  I recall having to leave a building and get into a car, and needing to go through the rain for just 2 or 3 metres. But that was enough for my clothes to be soaked. Driving was virtually impossible until it had eased off.

Returning to the weather in May, the storm I wrote about in my previous post was unusual in many respects, but the way the depression formed off the east coast of Australia was actually quite common. The sea off the eastern Australian seaboard is a favoured area for depressions to form, as shown here: 

The reason is to do with the temperature and moisture contrast between the Australian continent and the sea – these contrasts are the food of developing lows, and are perhaps the main way that Australia influences our weather and climate. Once the lows have formed, they then preferentially move towards the east-southeast – you guessed it, straight towards New Zealand.

Keep an eye out, if you can, on the weather maps this Queen’s Birthday weekend. Another low is forming – in the favoured area – and looks like it will bring more rain to the north, starting Saturday night. Our forecasters will be monitoring it closely to give you the best possible forewarning.

On the Cusp

MetService issues severe weather warnings to New Zealanders whenever widespread heavy rain, heavy snow or damaging winds are expected. There are formal criteria that events have to meet or exceed for the forecast to be called a success; the wind criteria are in a previous post, while the rain and snow criteria relate to the total falls required within set periods (write a comment below if you want to ask for details).

Sometimes a warning is considered a false alarm because the event was “on the cusp” of meeting the criteria but its intensity fell just short. These marginal events can be trying for forecasters. However, there has been nothing marginal about the rainfalls that have occurred over the West Coast of the South Island last week. Rainfall amounts have far exceeded warning criteria, and there have been some very large totals. For example:

rainfall Sun 25 Apr
(midnight to midnight)
Mon 26 Apr
(midnight to midnight)
West Homer (Fiordland) 213 mm 119 mm
Cropp Hutt (Westland) 382 mm 155 mm

The rainfall has been associated with an active front along which a large depression (or Low) subsequently formed over the Tasman Sea. Here is a sequence of satellite pictures showing the Low’s development and movement. The enhancement is the same as in an earlier post:

Infra-red satellite image, noon NZ Standard Time, 25 Apr 2010

Initially the band of deep clouds (purple, red and yellow) extend almost in a straight line from New South Wales to Fiordland.

As previous but midnight 25 Apr 2010

As time passes the cloud band perturbs and eventually takes on a comma shape.

Noon 26 Apr 2010
Midnight 26 Apr 2010
Noon 27 Apr 2010

Note how the clouds spiral in towards the centre of the depression in the last two pictures. The pointy part of the cloud near the centre is called a cusp. Its shape results from the flow that is rotating clockwise around the developing depression, with a maximum speed just out from the centre. To the southeast of the centre the flow is splitting and then moving in the opposite direction, creating an abrupt southwestern edge to the cloud there.

In the last satellite picture I’ve drawn arrows to show how part of the flow in the upper troposphere is wrapping around the depression while another part is being drawn into the westerly airstream at higher latitudes.

The animation below illustrates how a depression shapes the air around it, with the red and purple areas resembling the areas of cloud in the satellite pictures.

How regions of cold dry air (blue) and warm moist air (red) are shaped by a depression

Once depressions reach the stage of forming a cusp they are becoming mature. There is little further scope for them to develop, although a new development often occurs downstream as the older system fizzles.

By looking at animations of satellite images on TV or on the internet you may be able to identify systems that complete a full life cycle, and others that are disrupted in some way.

The Structure of Lows – part II

In my previous blog post I pointed out that tropical lows and cyclones don’t have fronts like the lows we’re used to around NZ, but rather, a core of warm air near the centre. I’d like to follow up by further contrasting tropical and mid-latitude lows, and looking a bit more closely at tropical cyclones and how they can affect our weather in New Zealand.

When tropical lows fully develop into cyclones they become the most damaging of tropical weather features. There are three main reasons for this:

  1. damaging winds at, or very close to the Earth’s surface
  2. intense rainfall leading to flooding
  3. near the coasts, the sea can be driven inland as a storm surge.

In an earlier blog post we saw that, on a weather map, the closeness of the isobars is related to the strength of the wind. There is a latitude effect too: for a given isobar spacing, if you were nearer the equator the wind would be stronger than if you were nearer the poles. This is an important point, because a tropical cyclone has very closely packed isobars near its centre and is relatively close to the equator.

Also, as you get nearer the equator the wind blows more across isobars, in contrast to the mid-latitudes where the wind blows approximately parallel with the isobars (like slot cars on a track). So in tropical cyclones there is a strong inflow into the centre as in the diagram below.

Inflow into a tropical cyclone (southern hemisphere). The innermost circle represents the "eye".
Inflow into a tropical cyclone (southern hemisphere). The innermost circle represents the "eye".

I don’t want to overcomplicate things, but there is a further effect based on the curvature of the isobars. If you’re interested send me a comment and I’ll explain it.


At the innermost circle in the diagram, the air is racing so fast that it doesn’t make it all the way into the centre. Inside this circle is the eye and a phenomenal contrast between the wind and rain at the outer wall and the relative calm of the interior.

The rainfall is usually at its most intense just outside the eye, and the cloud is very deep with extremely cold tops – see for example the infra-red satellite pictures described in the previous post. I remember being in Darwin many years ago when Cyclone Rachel was over the Timor Sea. We were some distance away from the centre of Rachel, but the cloud overhead was so dense that we needed lights on during the daytime if we wanted to read. We set a 24hr rainfall record for Darwin of 290mm, although the wind wasn’t particularly strong (compared with what I’m used to in Wellington).

Tropical cyclones sometimes affect New Zealand during our warmer months – a couple of examples were mentioned here. You may well recall some more recent examples, e.g. Drena and Fergus. After they form in the tropics they typically move quite slowly and erratically, then usually curve southeastwards into the mid-latitudes.

Once they move over colder water in mid-latitudes they undergo a transition in structure from the tropical to the extra-tropical (meaning outside the tropics, like “extraordinary” means “outside the ordinary”).

At this stage the cyclone tends to expand by drawing in colder contrasting air and evolving into a more typical mid-latitude storm complete with warm and cold fronts. On a few occasions (e.g. the Wahine Storm in 1968) they interact with a vigorous mid-latitude low to form a new system, complete with active fronts. Either way, the new storm isn’t a tropical cyclone anymore in terms of structure but can still be very damaging as a major mid-latitude depression.

The Structure of Lows

You are probably familiar with seeing lows and highs on our weather maps around New Zealand. See, for example, previous blog posts on a mid-July northern lowHow Lows and Highs move and the satellite loop in Winds Aloft.

The lows or depressions that affect us in the mid-latitudes are accompanied by warm and cold fronts with marked contrasts between the warm/moist air and cold/dry air that follow behind these two types of front, respectively. See our Learning Centre for more.

Occasionally, at favourable times of the year, lows form in the tropics that, on the face of it, may seem to be quite similar to the lows around New Zealand. But they are actually very different. If you looked at only 2-D surface weather maps you might think that tropical lows and cyclones are like our mid-latitude lows, but their structures differ markedly.

In different parts of the world these tropical cyclones are given other names. Here is a satellite image of a typhoon affecting the Philippines (in middle-right of picture).

Infra-red satellite image, 10am NZ Daylight Time, 2 Oct 2010
Infra-red satellite image, 10am NZ Daylight Time, 2 Oct 2009

This typhoon, called “Parma”, was following hot on the heels of Typhoon Ketsana that caused serious flooding in the Philippines and then Vietnam in late September.

The image is specially enhanced to highlight the deepest and most active cloud, shaded white and blue in this case. The temperature of these coldest cloud tops is -90C. This is perhaps an example of a paradox: In the global troposphere the coldest air is at upper levels in the tropics, not in the mid-latitudes and not at the poles.

How colours relate to temperature in the previous infra-red satellite image

Also note that with this enhancement there is a double entry for black: cloud tops between -20 and -30C appear similar to warm tropical sea (high 20s) between the clouds.

As I implied above, these tropical lows don’t have fronts associated with them. They normally have a warm core with a much more symmetric structure than mid-latitude lows. This core of warm air near the centre also means that their strongest winds are near the ground.

Another difference is a region of light winds at the centre, called an eye. The eye can’t always be detected in satellite pictures, but does show in the recent example below. Typhoon Melor (upper right of picture, encircled by purple and blue) was curving from westwards to northwards over the North Pacific Ocean. Parma was just north of the Philippines at this time. 

As previous satellite image but exactly four days later

When the tropical lows fully develop they become the most damaging of tropical weather features. I will explain the reasons for this in my next blog post, with a focus on how they can affect our weather in New Zealand.