A better view out to sea in the far North

MetService’s newest long-range weather radar, situated near Kaeo in Northland, officially commenced operations on Monday 28 July 2014, with early imagery from the radar enabling MetService forecasters to provide very valuable information to Civil Defence and the public ahead of the June storms experienced in the region.

Although Northland is also covered by MetService’s Auckland radar (on Mt Tamahunga, near Warkworth), by the time that radar’s beam reaches the Far North it is several kilometres above the ground, meaning that a lot of rain quite literally falls ‘under the radar’ and is not accurately detected as a result.

The view from the radar atop Te Huia hill out to Whangaroa Harbour.

The view from the radar atop Te Huia hill out to Whangaroa Harbour.

MetService Chief Executive Peter Lennox looks on as local school children sing at the opening ceremony.

MetService Chief Executive Peter Lennox looks on as local school children sing at the opening ceremony.

The new Northland radar has an expansive view of the Far North region, which will provide excellent weather detection across a range of 300km from the site. It is the ninth radar in MetService’s network, and the fifth of five new radars installed over the last six years to support expanded thunderstorm warning services. It is a powerful weather forecasting tool providing detailed depiction of torrential rain, hail and snow storms.

Northland radar now on MetService mobile website

Northland radar now on MetService mobile website

The Northland radar imagery as it appears on metservice.com

The Northland radar imagery as it appears on metservice.com

 

 

 

 

 

 

 

 

 

MetService worked closely with Te Runanga o Whaingaroa and local marae groups, as well as the Shareholders of the Te Touwai land block on which the radar is built, to gain the support of the local community. Another great outcome of this engagement is an annual scholarship set up by MetService to support local students with an interest in meteorology or related sciences; a MetService meteorologist has also visited schools in the area to talk about weather and the role of the weather radar.

New Zealand’s first modern weather radars were installed over 20 years ago to provide coverage of the Auckland, Wellington and Canterbury regions, followed some time later by a Southland radar. The current project has seen new radars established in Taranaki (2008), the Napier/Gisborne region (2009), Bay of Plenty (2010) and Westland (2011).

Bugs in the weather radar

If you were looking at radar imagery overnight Thursday 21 February 2013 or this morning (Friday 22 February 2013), you could be forgiven for thinking that there was quite a lot of light precipitation over the northern half of the North Island and west of Auckland.

MetService radar imagery from 7:00pm Thu-21-Feb 2013 to 11:20am Fri-22-Feb 2013

Except for parts of Gisborne and Hawkes Bay, we know that there was almost no precipitation over the North Island in the period covered by the radar imagery above.

So what was all that yellow radar echo over parts of the central North Island and around Waikato and Auckland?

We’re not completely sure, but we strongly suspect the echo is swarms of insects. To show up in radar imagery like this, they must have some of the characteristics of precipitation particles: similar size, fairly numerous, enough water content (or perhaps coating).

The overnight period Thursday 21 February / Friday 22 February 2013 isn’t the only time lately radar echoes like this have been observed over the North Island. They’ve shown up, to a greater or lesser extent, most evenings in February. The weather over the North Island has been settled — and very dry — during this time.

Interestingly, there is quite a lot of literature on the subject of radar entomology.

A Southerly ‘Buster’

On Monday 28th November, a south to southwest change swept its way northwards across Otago and Canterbury during the afternoon.   Temperatures soared to 28 C preceding this change then rapidly plummeted to around 16.  This was a good example of what is known in Australasia as a ‘buster’.

Temperature traces on 28 November in degrees Celcius. DNA=Dunedin, OUA=Oamaru, TUA=Timaru, CHA=Christchurch and KIA=Kaikoura. Timestamp is in UTC; 27 0000 is 1pm NZDT.

The weather map for 1pm Monday 28 November 2011 showed a typical trough moving across New Zealand. The last of a series of fronts within this trough was the one responsible for this dramatic drop in temperature.

Weather map for 1pm NZDT Monday 28 November

The reasons for temperatures soaring to between 26 and 28 C ahead of this southerly change are:
•    Northwest winds ahead of the trough warmed by around 5 to 10 degrees Celsius as they descended down the eastern slopes of the Southern Alps
•    Sunny conditions in the relatively clear skies over the Canterbury Plains – on a date less than one month ahead of the longest day.

These warm temperatures combined with falling air pressure to produce a zone of relatively low density. Higher density air in the cooler southerly flow that followed this cold front accelerated into this zone of low density air producing a squally “gust front” with the wind change. This “gust front” built in size and intensity during the afternoon as can be seen from the tweets sent from @metservice during the afternoon

  • Southwest change arrived Dunedin Airport around 11:50am. Temp dropped from 22 to 14 C , gusts to 50 kph , and its on its way north. ^BM
  • Southerly change got to Oamaru about 1:30 pm, temp. dropped from 22 to 14 C, initial gusts were 54 kph . South Canterbury’s next ^BM
  • Southwest change arrives in #Timaru just before School’s out, Temperature drops from 28 C at 2:30 pm to 16 C by 3 pm , Gusts to 70 kph ^BM
  • Southerly change reached #Ashburton between 3:30 and 3:45 pm, temp. dropped from 26 to 16 C, gusts 70 kph see http://t.co/YhuAEZ4L ^BM
  • Southwest reached #Christchurch at 5 pm in time for evening commute, temps 28 C to 17 C in 20 minutes. with wind gusts 75 kph , ^BM
  • Hi #Christchurch be quick and look at wind blown dust of that southerly change on MetService radar at http://t.co/lmw4BcYA past hour ^BM

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September to November is the season for the strongest of these southerly busters (but they can occur at any time of the year).  Spring brings the strongest westerly winds of the year to South Island and it is also a time of relative cold offshore sea temperatures.  The temperature difference between the heated air over the Canterbury Plains and seas in the Canterbury Bight is what feeds the wind gusts of a buster. The coldest sea temperatures of the year occur in early spring, and they only just start rising in November.   You can find the latest reading from metservice.com by clicking on ‘marine’ and then ‘beach’ and selecting a suitable site.  The one shown below is Jack’s Point near Timaru (timestamp is 10am Wed, 30 Nov 2011).

Jack's Point on the metservice.com Marine & Surf section. Wind, Wave ands Sea conditions are now available on metservice.com

 

The showers with this buster occurred mainly along the coast and at sea.   In the drier air over the Canterbury Plains the southwest wind change picked up dust and dirt, especially over the Rakaia River, and lifted and blew these as a “dust storm” into Christchurch.  This can clearly be seen on the animation below, taken from the high frequency Christchurch rain radar site at metservice.com.

Rain or Showers

We had an enquiry recently from an astute member of the public asking about the comings and goings of rain.  

They had noticed that in southerly weather the rain has a tendency to “come in bands (e.g., 20 minutes rain, 20 mins dry, 20 mins rain etc.) rather than as a more constant rain that comes with northerlies”. They were wondering why this was. This is a good question and I will try to answer it here.  

Radar examples

First, let’s have a look at examples of northerly and southerly precipitation. Below are radar images that show rainfall-sized drops as explained in an earlier post on the storm of late May. As before, light falls are yellow and heavier falls blue.  

1.  Rain approaching Taranaki from the north on 5 June 2010. In this animation the precipitation is relatively uniform, suggesting more continuous rain:  

New Plymouth radar imagery, 6pm to 10:30pm NZ Standard Time, 5 June 2010

As an aside, the semi-circular area of yellow over the North Taranaki Bight (towards the end of the animation) is caused by cluttering reflection off the sea.  

2.  Showers moving onto the Canterbury coast and plains from the south on 8 June 2010. Here the precipitation is speckled indicating showers (with breaks in between):  

Christchurch radar imagery, 6pm to midnight NZ Standard Time, 8 June 2010

The key to unlocking the cause of the difference between rain and showers lies in the nature of vertical motion. In the post on The Structure of Highs I explained the importance of up and down motion of air. Even though vertical motion is usually much weaker than horizontal motion of air (wind), it really dictates the state of the sky:  

  • upward motion of moist air favours the formation and maintenance of cloud, and possibly precipitation too,
  • downward motion of air inhibits cloud, favouring clear skies.

Relating this to our original question, northerly flows over New Zealand occur ahead of approaching depressions or troughs of low pressure. These flows are characterized by gently rising warm moist air covering a large area. In these situations you’re more likely to get continuous rain.  

In contrast, southerly flows occur immediately behind departing depressions or troughs, where there tend to be pockets of more vigorously rising air surrounded by generally sinking clear air. In addition, if the air flows over the sea, the colder drier air picks up extra moisture and becomes unstable so that the rising air gets extra buoyancy. In these situations the precipitation is likely to be punctuated with breaks, i.e. you’re more likely to get showers.  

Combinations and exceptions

There are some complications to this. Occasionally within the gently rising northerly airstream there are pockets or bands of vigorous rising air that can even generate embedded thunderstorms. These are caused by subtle instability mechanisms. They are of great interest to pilots, because the surrounding rainy areas can make the embedded and hazardous thunderstorms difficult to detect.  

It’s also possible to have an approaching front generating a broad area of rain as it flows over the top of an older shallow flow generating light showers. I can show you an example if you’re interested - just leave a comment below. 

Topographic effects can lead to preferred places for showers in southerly flows. For example, have another look at the radar animation of showers above. Note the band of showers over the Canterbury Plains at the western edge of the radar echoes. This is caused by a low-altitude convergence effect that favours upward motion there. There are no showers farther to the west due to sheltering from the eastern Otago ranges. If you are beneath that band of showers, then it will seem more like continuous rain than passing showers. 

Another common situation for showers to behave more like continuous rain is when an unstable airmass flows onto the West Coast – in these conditions the cloud is often continuous, and the precipitation is too, with alternating periods of heavier and lighter rain.  

If you look very closely at the second animation you may detect a slight change in the orientation of the band of showers – it turns a few degrees of angle clockwise and crosses Christchurch. This turning is caused by a small clockwise shift in the direction of the wind flow. Subtle changes like this are a real challenge for our forecasters, but our growing network of weather radars is increasing our ability to forecast them.