Last Monday evening, having just arrived home after the short walk from Trentham Station, I remarked to my family that it was unusual for the wind to blow from the northeast in Upper Hutt. Five days later and it’s still blowing from there.
For over a week now, mean sea level pressure has been higher than usual south and east of New Zealand while pressures have been lower than usual over the Tasman Sea (to the northwest). This anomalous pattern is not confined to the surface – it extends up through the troposphere and further. This is known as a blocked pattern, where highs and lows become slow moving or stationary. Such slow moving frontal systems can result in heavy and sometimes intense rainfall, and we have seen some of that in the Severe Weather Warnings affecting south Canterbury, Waikato, Coromandel and Gisborne this week.
The animation here shows the high pressure to the southeast as either a ridge or an anticyclone. The depression over the Tasman Sea has gone through the complete life cycle from early stages of contrasting air masses on 28 and 29 July, through development and deepening on 30 and 31 July, to mature and filling in on 1 and 2 August. Today (3 August) there is a fairly uniform air mass of rotating belts of convection.
This simple map shows the meandering path of the centre of the depression during this time. The track starts near the north boundary of the map and ends towards the western boundary.
The satellite image below shows the old low centre this afternoon (3 August) with bands of moderate convectionspiralling out.
All this time, the wind flow over central and southern New Zealand has been from the northeast. For Wellington this has resulted in an unusually long period of wind from between east and northeast. Winds of this direction are fairly rare as shown in the diagram below, called a wind rose.
Diagrams such as these are called wind roses because, for most stations, the wind blows from many directions and the shape of the resulting diagram looks a bit like a flower. The wind rose here is for day-time in winter at Kelburn (central Wellington) and shows that the wind is mostly from the northwest to north or from between southeast and south. It very seldom blows from the east or west.
The graphic below traces the hourly wind speed and direction between midday 30 July and midday today 3 August 2012. The start point is at 030 13 km/h and the end point is at 080 22 km/h. Can you find the start and end points?
Anyway, you can see that a lot of the time during these last four days, the wind at Kelburn (and most of Wellington) was a moderate easterly. And you don’t often get that for such a long period.
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.
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.
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.
Some Interesting Observations
At Hawera, the wind increased quickly during the early morning hours of Saturday 03 March.
Mean speed (km/h)
Highest gust last hour (km/h)
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.
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.
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.
Yasi is a girl’s name and also the Fijian word for sandalwood.
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.
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.
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.
Today MetService has issued a severe weather warning for heavy snow over parts of South Island. Looking back over the past week, it is unusual for a single weather event to result in severe weather warnings for all of widespread heavy rain, severe gales and heavy snow. But this event has been far from typical in its severity and longevity.
The media have extensively reported the full impacts of the storm, so I won’t go into that here. Instead I’ll look a little more deeply into the meteorology of what’s been going on, starting from day one.
Here is an animation of the surface weather maps starting midnight on Friday 21 May – frames are every six hours throughout the event, and the isobars are at 2 hPa spacing:
There is a lot going on in these surface maps. To give a full account of what’s been happening I should really include a discussion about the weather in the upper air – but to keep this post short I’ll concentrate just on the surface here. Points to note from the maps are:
at the start of the loop, an old Low over the south Tasman Sea weakens
a new Low forms off the south Queensland coast, then moves southeastwards towards New Zealand and deepens
a second Low centre forms west of North Island, then a third forms over Bay of Plenty
the third Low takes a very unusual track, moving south then southwestwards towards Canterbury
a High moves onto the south Tasman Sea and stays there for the rest of the period
a very strong east to southeasterly airstream develops over the bottom half of South Island
the Queensland Low eventually moves eastwards over North Island
The centre of the Queensland Low took a rather convoluted track as it approached us:
Track of the low that formed on 21 May 2010
I guess you could say it “looped the loop” near longitude 170E. This was caused by the formation of the secondary and tertiary Low centres that shifted the “centre of gravity” of the broader system away from the originating Low.
If you look again closely at the loop of weather maps above, you’ll notice that just after halfway through, the second and third Lows are dumb-belling cyclonically around each other over New Zealand. This motion occurs when a multi-centred Low develops – we sometimes call these systems complex lows. The combined motion plus the spiralling bands of precipitation were well captured by our current radar network.
The following animation shows where our weather radar is detecting rainfall-sized drops, and the shape of the rain areas illustrates the position and movement of the Lows. Pictures are seven minutes apart and cover the period from the evening of 25 May to the early hours of 26 May. Light falls are yellow and heavier falls blue.
Click to view animation. Note – the animation is a large gif file: 2.9MB
The radar pictures also show the change in texture from north to south, with the northern precipitation looking more speckly (showery) and the southern precipitation more uniform (rainy).
As I mentioned at the beginning, this storm is not over yet, with snow still to come in the south. You can keep up to date with the latest weather by continuing to check out metservice.com.
On the night of 17th July and early on the 18th, New Zealand was affected by a fast-moving and rapidly deepening depression originating in the north Tasman Sea. Sustained southwesterly winds of more than 60 knots were recorded in Colville Channel as the low passed by. Severe Weather Warnings were issued for wind in Coromandel/Great Barrier Island and rain in the eastern North Island.
This post will have a look at some of the reasons that the low deepened so rapidly, and whether the computer models did a good job of predicting its path. We’ll also see that weather is a lot more complicated than simply following what happens at the surface only.
MODIS imagery of the low as it moved away from the country on Saturday 18th July
On Friday 17th July at 6PM, a low was analysed just west of Northland with a central pressure of about 997 hPa.
24 hours later the low lay a few hundred kilometres east of the North Island and had deepened to around 975 hPa. It continued deepening as it headed southeastwards – New Zealand was only affected by the first stages of the rapid development.
Midday 17th July – a “slack low”
Midnight 17th July – 12 hours later the low has quickly evolved
The rapid development of this low cannot be adequately explained by considering surface conditions only. Chris Webster here states “Perhaps we inadvertently reinforce a perception that our weather is 2-D by publishing lots of weather maps that are valid only at the Earth’s surface (e.g., see Weather Maps). Rarely do we show what’s happening higher up through the atmosphere or, more precisely, the troposphere —?the part of the atmosphere that contains our weather.”
Our low was in fact strongly influenced by what was happening at the highest reaches of the troposphere – the regions where jet streams tend to be present.
ECMWF forecast for 3PM on the 17th. Green contours are surface isobars. Shaded regions are those of high upper level winds (black being the strongest).
This forecast chart is entirely computer generated, but shows why the low began organising itself and deepening from the Friday evening. The surface low pressure centre lies near the equator-ward entrance region of an upper level jet stream. This is a particularly favourable position for cyclogenesis (meaning development of a low). This is because it is an area of upper level divergence, which favours convergence at the surface and upward motion – both of which are conducive to developing depressions.
The other favourable position is the pole-ward exit region. Where these coincide (the exit of one jet, and the entrance of another), and where a low lies downstream from a sharpening upper trough then a recipe exists for “explosive cyclogenesis”:
Here are the comments made by senior forecasters regarding the development of this low:
“A sharpening upper trough over the Tasman Sea with double jet structure should provide strong upper level divergence for the low developing there.”
“Deepening caused by very strong divergence and vorticity associated with a broad very strong subtropical jet with winds reported 150-200 kt by aircraft”
The actual behaviour of the low
Analysis charts above, drawn every 1 hPa, show the actual track of the low as analysed by a senior meteorologist. Chart times are 6PM 17th (top left); 6AM 18th (top right); 9AM 18th (bottom left) and Noon 18th (bottom right).
Contrast the 6AM analysis with these forecast model prognoses for that same time:
There was considerable disagreement between models as to the track and depth of this low, and, as suggested by the analysis, no single model got it quite right.
Even though computer models are becoming ever more sophisticated, it’s important not to slavishly follow their predictions. As we have seen with this fairly brief event, it’s crucial – especially for high-impact weather events – to have professional meteorologists monitoring the situation and applying their judgement and conceptual knowledge of meteorology to the output of the models. This is something that is not likely to change in the future.