A new dish

A new satellite receiver (“dish”), for improved reception of data from polar-orbiting weather satellites, was installed at MetService in early February.

Benefits

Polar-orbiting weather satellites yield rich information about the atmosphere, valuable for New Zealand weather forecasting.

Benefits of faster access to more data, and sharper identification tools, include:

  • Better detection of airborne volcanic ash
    MetService operates the Wellington Volcanic Ash Advisory Centre (VAAC), on behalf of the Civil Aviation Authority of New Zealand. The better the tracking and forecasting of volcanic ash, the safer and more efficient is aviation in the New Zealand region.
  • Better detection of low cloud and fog
    Fog and low cloud – which can be particularly disruptive to aviation – are difficult to detect in the overnight and early morning hours, the time when airlines are planning the coming day’s flights. In the overnight and early morning hours, there are at least four polar-orbiting satellite passes over the New Zealand area.
  • Better analysis of severe weather
    Polar-orbiting satellite passes complement weather radar, providing high-resolution information about incoming weather features beyond the footprint of the radar network and which are too small to resolve using imagery from geostationary satellites.

An example

The two images immediately below demonstrate the difference in resolution, of visible imagery, between two of the weather satellites that view New Zealand. As explained later in this article, data from each type of satellite has its advantages.

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Visible image for 2300UTC 26-Feb-2013 (midday Wednesday 27 February, New Zealand Daylight Time) from the geostationary meteorological satellite MTSAT-2.
Data courtesy Japan Meteorological Agency.

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Visible image for 2244UTC 26-Feb-2013 (11:44am Wednesday 27 February, New Zealand Daylight Time) from the polar-orbiting meteorological satellite Terra.
Data courtesy National Oceanographic and Atmospheric Administration.

Satellites available

Using the new receiver, MetService currently gathers data from the Terra, Aqua and NOAA series of satellites. Their passes across the New Zealand area are generally clustered around early morning and late afternoon. Data from other polar-orbiting weather satellites is a work in progress.

More about polar-orbiting satellites

The animation below, from the National Oceanographic and Atmospheric Administration (NOAA), nicely demonstrates the differences in the orbits of polar-orbiting and geostationary satellites.

Polar-orbiting satellites continuously orbit the Earth from pole to pole at low altitude (about 800 km). Because they are much closer to the Earth than geostationary satellites (about 36,000 km), they yield data at much higher resolution – that is, information from them is much more detailed.

On the other hand, information from any given sun-synchronous polar-orbiting satellite is available much less frequently than from a geostationary satellite and covers only a limited area. This is because each polar-orbiting satellite crosses the New Zealand area twice per day and is too close to the Earth’s surface to take a “full disk” view.

To receive data from a polar-orbiting satellite, the receiver must point at the satellite as it moves across the sky. This requires a precise knowledge of the orbit and being able to finely control the movement of the receiver. It’s a bit more complicated than setting up a satellite dish to receive a signal from a television satellite.

Installation

Final assembly of the satellite antenna, receiver and dome was carried out by MetService and SeaSpace engineers in the MetService head office car park over a period of three days.

The lift of equipment onto the roof of the MetService building required an early morning start and very little wind. Things went extremely well and the HeliPro helicopter successfully lowered the 364 kg load accurately onto the roof platform.

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.

Story behind polar outbreak of August 2011

August 2011′s polar outbreak was a major weather event that drew media interest from around the world.

This event was notable in recent history, in terms of the coldness of the air and extent of its spread across New Zealand.  During the week of the ‘big chill’, MetService’s Chief Forecaster kept up an in-depth explanation of the polar outbreak as it happened.

Behind every forecast is a lot of work.

The starting point for an accurate forecast is an accurate representation of the atmosphere’s current state. This is why MetService operates extensive observing and weather modelling programmes and runs a large 24/7 forecasting operation. For much more about this, see the blog on MetService’s Investment in Forecasting and TVNZ’s recent Breakfast programme: “How does the weather work”.

The 7-day rain forecast on www.metservice.com is very useful for planning purposes, with the maps showing pressure, direction and strength of wind, and where rain is expected to fall. The forecast is in the form of a series of maps, which are also provided in a player, so you can animate the series of maps to see how the situation could change over the period of time.

Rainfall forecast (yellow shades) in the six hours from 6:00pm to midnight Sunday 14 August; forecast wind speed and direction (barbs) at midnight Sunday 14 August; mean sea level pressure (blue lines) at midnight Sunday 14 August, as displayed on www.metservice.com on Friday 12 August.

Our 3-day model provides a closer look at New Zealand. On these charts, the thick purple line is a guide to where precipitation may fall as snow.

Rainfall forecast (shaded) in the three hours from 9:00pm to midnight Sunday 14 August; forecast wind speed and direction (barbs) at midnight Sunday 14 August, as displayed on www.metservice.com on Friday 12 August. Snowfall is likely in the coloured areas enclosed by the thick purple line.

However, computers deal only in numbers and don’t give the complete story – this is where human expertise comes into play. Trained forecasters do things that weather models cannot, for example:

  • Consider, in real time, conflicting information – from models and observations – and determine which outcomes are more likely, based on an understanding of the weather situation
  • Steer a steady course when the model forecasts are “jumping” from run to run
  • Reconcile observed and forecast weather and recognise when the forecast needs changing (regardless of what the models say)
  • Explain the weather, particularly to those who are managing weather-related risks (people talk to people; computers talk to computers)

It is vitally important for forecasts (and updates as they happen) to be communicated in a measured, timely and credible manner.

On Wednesday 10 August, days before the outbreak, MetService published a media release and contacted farming organisations, to alert those with a real need to plan in advance about what forecasters were anticipating. Snow and cold would have a large impact to farmers in the lambing and calving season so this was highlighted in the news release.

Severe Weather Outlook map issued at 1:43 pm on Thursday 11 August.

To ensure that all sectors of New Zealand were kept up to date, a Special Weather Advisory was also issued (click here to see pdf) on Thursday 11 August. This is a special news release that draws people’s attention to particularly significant or widespread weather to come. On Friday 12 August another media release was issued to give a further update on the developing polar blast.

Because of the chaotic nature of the atmosphere (and the variation of predictability with time), the forecast for a given day (or time) in the future can change as we get closer to it. This is why the forecast issued on a Monday for the weekend to come might be very different to the forecast issued on, say, the Thursday of the same week.

In the days that followed Friday 12 August, Severe Weather Watches and Warnings (click here for a detailed account – a pdf document) were issued and updated as conditions warranted. As more data became available, site-specific urban forecasts were updated to include the risk for snow.

Mean sea level analysis for midnight Sunday 14th August, near the beginning of the coldest period during this event.

Snow fell in episodes over several days. There were some notable snow amounts – and reports of snow in places that had not seen it in a very long time.

Snow map showing reports of the depth of snow (in centimetres) from various parts of the country, during the event.

And here is a photo gallery showing images that were sent in from around New Zealand.

This polar blast was long lived and delivered snow to many regions of the country. While heavier snowfalls have been recorded before in some regions, this is undoubtedly the most widespread and prolonged event since 1939 – the subject of a blog by Erick Brenstrum.

Forecasting for the Summer’s Day Sweepstakes

This blog post is about the MetService Summer’s Day Sweepstakes. I’d like to give you some background on how to use the material on the MetService website to give yourself an advantage in the Sweepstakes. The prizes are fantastic so it’ll be worth your effort.

The first thing you should do when you start thinking about your Sweepstake picks is to try and find out what’s happening with the weather now around NZ. To achieve this, look at recent weather mapssatellite imagery, rain radar and latest NZ observations. There’s information on how to read weather maps in our learning centre. Try and extrapolate what’s been happening, to give you a first estimate of what might occur in the future. Extrapolation is a valid method of forecasting if things advance consistently, but doesn’t work if they become chaotic.

Next you should look at the rain forecasts for 3 Days and 7 Days ahead. Draw on your knowledge and experience of New Zealand weather to infer from these maps what the general weather conditions are likely to be. Then, for each location, think carefully about how to adapt what the model is telling you for your picks.

A look through the MetService Blog may be useful if you expect something to occur in the weather that’s related to one of the posts.

There are three elements for you to forecast: temperature at noon, wind direction at noon, and rainfall for the day (24 hour total). Let’s look at these in turn:

Temperature at noon

The temperature at noon generally won’t be the highest for the day (the max usually occurs a couple of hours later). Try and estimate the direction from which the air will arrive – northerly winds are warmer, southerly winds are colder (as explained in the post on Rain or Showers). If the wind comes in from the sea, e.g. as a sea-breeze, it won’t be as warm by day as if it had originated over land. If the air is flowing over a mountain range, allow for warming due to the Foehn effect.

Take care to account for the exact position of the weather station where the forecast is being recorded. Big differences can occur in New Zealand, sometimes over a distance of a kilometre or less.

Wind direction at noon

On weather maps the wind blows approximately parallel to the isobars, anticlockwise around anticyclones and clockwise around depressions (lows). But in the summer, coastal locations often have a sea-breeze during the daytime that blows in from the sea; a similar but weaker effect occurs near lakes. The terrain can channel the wind too.

Winds in New Zealand can vary a lot from one place to another and, at any location, can blow from several different directions throughout the day.

Rainfall for the day (24 hour total)

Because you’ll be forecasting for a 24-hour period, any uncertainty about timing of a rainband shouldn’t affect your forecast unless you’re expecting it to come through close to either end of the period.

The 3 Day data give you rainfalls for the 3-hour period previous to each map time, and the 7-Day data give rainfalls for the corresponding 6-hour periods.

 

There’ll be five locations for you to forecast for: Whangarei, Taupo, Hokitika, Christchurch and one location of your choice. Some specific points to note about the four common locations:

Whangarei: north of Auckland, near the coast and away from large mountain ranges.

Temperature variations are moderated by the sea, and the temperatures are fairly high because of the latitude. Winds come from several directions, but a southwesterly is the most common; the sea-breeze is a southeasterly. Rain generally comes from moist northeasterly flows ahead of fronts crossing from the Tasman Sea. As fronts cross there’s typically a period of heavier rain, typically followed by showers arriving from the southwest.

 

Taupo: in the central North Island, next to the Lake.

At higher altitude than the other stations (400 metres above sea-level), but has a continental climate in summer with typically warm days. Not a great deal of wind because of sheltering from the surrounding mountain ranges, but any wind tends to be westerly. In summer Taupo can get heavy convective showers.

Hokitika: immediately west of the Southern Alps and near the coast.

Temperature variations are usually moderated by the sea, but can get a warm southeasterly Foehn. Winds are often northerly or westerly. Rain is strongly enhanced by uplift as a northwest flow approaches the Alps ahead of a front. Heavy showers can occur behind a front if the flow comes from a westerly direction.

Christchurch: east of the Southern Alps, near the coast and Banks Peninsula.

Often gets a northeasterly wind off the sea, but a nor’wester Foehn wind can push the temperature over 30C in summer. Doesn’t get a southeasterly because of obstruction from Banks Peninsula. Most rain comes from the east and south. Can get heavy convective showers in unstable southwesterly flows.

I hope this post gives you something useful to work with. Best of luck for the Sweepstakes!