Wave three

New Zealand has been in a strong west to southwest flow for a few days now. This weather regime looks as though it will continue for the next few days.

There are reasons why the weather gets into regimes like this. It has to do with what meteorologists call the “long waves”.

The map below is one way of depicting what the weather pattern about half-way up the troposphere (the troposphere is that part of the atmosphere in which the weather occurs) looked like at midday Thursday 16 September 2010 New Zealand Standard Time, just before the westerlies broke out across the country. In this pattern, there’s nothing that looks like a high, or low, or front. But there is a bunch of wavy lines … which look kind of regular, but not quite.

NCEP 500hPa reanalysis valid 0000UTC 16-Sep-2010
Image provided by Physical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, from their Web site at http://www.esrl.noaa.gov/psd/

Now what if those waves could be separated out into longer ones and shorter ones, so we could see what was going on “under the bonnet”? There happens to be a way of doing exactly this; it’s called Fourier analysis. (Here we stand on the brink of numerical weather prediction, but we’ll go there some other time).

The picture below is made from a Fourier analysis of the pattern 12 hours further on from the one shown above. It reveals that, in New Zealand’s latitudes, it’s dominated by a wave which has three troughs (blue) and three ridges (pink) wrapped around the hemisphere. Most important to us is the big area of blue shading – a trough – near New Zealand.

Hemispheric wave analysis for 1200UTC 16-Sep-2010
The "long" and "medium" waves about half-way up the troposphere at midnight Thursday 16 September 2010

The animation below shows how this trough in the three-wave pattern edges only slowly onto New Zealand during the few days following Thursday 16 September 2010. “Wave three”, as it’s often called by meteorologists, is one of the long waves. It tends to move only slowly – which is why, once it’s gathered a bit of energy, it can influence the weather over a period of days.

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What the "long" and "medium" waves did (and are forecast to do) for the few days following Thursday 16 September 2010

We live on the Earth’s surface, at or near the bottom of the troposphere. Down on the Earth’s surface, the isobar maps of the last few days (see animation below) have a certain sameness – they all show

  • Tightly packed west to southwest isobars over New Zealand, which is why the wind has been more or less from the same direction
  • Fronts whizzing across the country, which is one of the reasons why showers have come and gone, and the wind shifted around a bit in direction, and gone up and down a bit in strength.
Isobar maps, at 6-hour intervals, from midnight Thursday 16 September 2010 to 6am Monday 20 September 2010

Breaking waves in the sky

Here is an image taken by Norman Robinson’s cellphone of some unusual clouds seen from Ophir in Central Otago last  May.  Norman adds “We have some really interesting cloud patterns here at times, but this was one of the more unusual ones.”

Taken by Norman Robinson, 8 May 2010

MetService’s Consultant meteorologist, Ross Marsden, has been able to ascertain from metadata encoded in the iPhone image file that it was taken on 8 May 2010 at 16:16 at latitude 45.11S, longitude 169.60E at altitude 302 metres,  which is, of course, Ophir.

Here is a colour-enhanced detail from the above image:

This image shows some very nice Kelvin-Helmholtz instability made visible by cloud.

The weather map for that day shows a large low pressure system slowly making its way across the north end of North island, and a rather flat ridge of high pressure on the south of this cradled across Central Otago.

Courtesy of NASA, the MODIS satellite image (for close to the time the cloud image was taken) shows some filaments of cirrus clouds streamed out by the upper winds which were from the northwest over Central Otago.    Click on the image here to link to the full scale image

Satellite image from MODIS, 8 May 2010

Here are the zoomed-in details over Central Otago with a yellow arrow showing the vicinity where the image was taken.

These clouds show waves rolling along and breaking in the sky,  similar to the way that waves behave on the sea.  It occurs when the change in wind across the boundary between two fluids is so much that steady simple ‘laminar’ flow breaks down and becomes turbulent, and the two fluids mix, with regularly spaced eddies.   The name that has been given to this phenomenon is Kelvin-Helmholtz instability, after Lord Kelvin (William Thomson, 1824-1907)  and Hermann von Helmholtz (1821-1894).   This is just one of many types of turbulence in the atmosphere,.

The following section is for meteorological boffins:

The weather balloon released from Invercargill at noon that day took a sounding that measured the temperature, moisture, density and wind profiles.  MetService aviation forecasters use a specially designed diagram called a Tephigram to help grasp the impact of all this data, and the one attached below has a yellow-red vertical bar attached.  This bar uses a derived parameter, Richardson’s Number or Ri, to show the likelihood of turbulence. Red zones have low Ri and here the wind changes are sufficient to dislodge stratified layers and thus be turbulent.  It is unlikely that the balloon went near the cloud seen in the image above, but the sounding can be taken as representative of the whole region, and so the red bar near 350 hPa can be taken as a indicator that the Kelvin-Helmholtz instability layer was at a height of around 8km above ground level.

Invercargill weather balloon noon 8 May 2010 NZST. Temperature trace on right, dewpoint to left, wind plot is north up, each barb=10 knots, pennant=50 knots.

These breaking waves were made visible thanks to the layer of cirrus cloud.  They can occur whenever the wind and air density change fast enough, as found near a jetstream –  a river of rapid moving air aloft.  Often these breaking waves occur in areas which are cloud free.  Airline pilots have a healthy respect for such areas and call them CAT, or Clear Air Turbulence.   Special aviation weather maps are routinely prepared by MetService to forecast hazards such as CAT, and when a pilot encounters a CAT area a report is sent to alert all aviators.

Thanks to Norman Robinson for agreeing to share this image.  Cloud-watching can be a fascinating and useful hobby.  Identifying the processes at work in the changing sky, and linking these to changes in the barometer and isobars on the weather map, allows us to understand what is happening now, all the better to predict what will happen next.


Here is another wonderful image of breaking waves in the sky – this was sent in by Carol Diehl and seen from west Timaru on 22 Sep 2010

22 Sep 2010