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.

Placeholder

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.

Placeholder

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.

Cloud structures over NZ on 26 July

On Thursday 26 July 2012 a cold southeasterly airstream flowed onto the North Island, around an anticyclone centred just east of the South Island. In this blog post we’ll look at some interesting small-scale cloud structures around the country on this day.

Below is the weather map at midday on Thursday 26 July. The red arrows show the sense of the broad-scale rotation around the anticyclone.

While the North Island was experiencing a southeasterly flow, the isobars were widely spaced over the South Island, indicating little wind there. Take a look at the animation below, based on visible light as received by the MtSat-2 geostationary satellite.

MTSAT-2 visible satellite images, each an hour apart, from 10am to 3pm NZST Thursday 26 July 2012. Images courtesy Japan Meteorological Agency.

As explained in the post on the effect of resolution, the visible satellite image shows all cloud as white or light grey, regardless of how high or low the cloud is. Most of the cloud over New Zealand is on the east coast of the North Island. Just off the Manawatu coast there is a plume-shaped area of cloud that extends northwestwards parallel to the flow. This is low-level cloud in a region where the air flow near the Earth’s surface is coming together, or converging. As the air convergences it is forced to rise and, with enough moisture, cloud forms.

There is a similar process occurring off eastern Bay of Plenty. In this case there is a zone of more concentrated convergence that shapes the clouds into a rope-like appearance, but the orientation of it is still towards the northwest and parallel to the flow.

Most of the South Island is cloud-free, but there is a patch of grey-looking cloud around Lakes Tekapo and Pukaki. This is also low cloud, but it has formed during the night in the valleys and basins. In the afternoon there’s been just enough heat from the weak winter sun to break up and disperse the cloud. The cloud base at Pukaki during the morning was reported as being about 600 metres above the ground – the air temperature had risen from minus 4 overnight to plus 3 by lunchtime.

The Terra polar-orbiting satellite has a very high resolution sensor. Terra passed over New Zealand within the period of the previous images, and I’ve reproduced the image below, split into two colour images.

Very high resolution satellite image within the period of the previous images. Image courtesy of MODIS Rapid Response Project, NASA/GSFC.

The features discussed above are very apparent. The area of convergence off Manawatu evidently has a double structure, and it is striking how this high resolution image shows individual cumulus clouds as white dots (as discussed in the previous post). There are cloud streets over the central North Island from the Kaweka to the Raukumara ranges.

Over the Pacific ocean there is a lot of cloud having a cellular structure. This is typical of a cold body of air that moves onto relatively warmer water. The air bubbles up into cumulus clouds that tend to clump together into ring-shaped clusters.

The low cloud over Lakes Tekapo and Pukaki (below) has a flat appearance typical of layered stratus cloud. It extends its fingers into the valleys between the peaks of the surrounding ranges.

As previous image, but for South Island.

The cloud over the ocean east of Canterbury is stratocumulus, a combination of the lumpy texture of cumulus cloud and the layering of stratus cloud. In many respects there was nothing particularly unusual about our weather on 26 July, but the satellite images were still able to reveal some fascinating and beautiful cloud structures.

 

A Winter Storm

Since Wednesday 6 July, stormy westerly conditions have affected New Zealand. In this blog, we’ll look at why.

The “Long Waves”

Below is the mean sea level analysis – the weather map – for 6am Sunday 10 July. In between big highs over the mid South Pacific and south of western Australia is a really large trough; it’s the area shaded light blue. The weather map has looked like this, more or less, since Wednesday 6 July: that is, the big features on it aren’t moving much.

MetService mean sea level analysis for 6am Sunday 10 July 2011.

There’s good reasons why these big features aren’t moving much. They reflect the so-called “long waves” in the troposphere (the troposphere is that part of the Earth’s atmosphere in which the weather occurs), which are stationary at the moment. Below is an image made from a Fourier analysis of the wave pattern in the Southern Hemisphere at midnight Saturday 09 July. There’s a trough (blue) in this wave pattern more or less in the same place as the one shaded light blue on the weather map above. And either side of this trough, there are ridges (pink) in about the same place as the big highs on the weather map above. For more about this, see my blog post on Wave Three.

The "long" and "medium" waves about half-way up the troposphere at midnight Saturday 09 July 2011.

Polar Air

Below is a plot of where the air arriving on New Zealand’s west coast at midnight Saturday 09 July came from. The air is from the Antarctic.

"Backward trajectories" of the air at mean sea level arriving at Cape Reinga, Farewell Spit and Secretary Island at midnight Saturday 09 July 2011. Each red line traces the path of the air over the period of a week. That is, the air arriving on New Zealand's west coast at midnight Saturday 09 July left the Antarctic about a week previously. Data courtesy NOAA Air Resources Laboratory.

On its way to New Zealand, this air travelled over a long stretch of ocean. It will have been colder than the surface of the sea it passed over, and will therefore have taken up heat from the sea surface. As heat transfers from the sea surface to the air immediately above it, “blobs” of air become warmer than their surroundings and rise upwards. This process is known as convection (see Chris Webster’s blog post about predictability and popcorn). If convection continues for long enough, showers and/or thunderstorms are the result. In the satellite picture below, more or less all of the clouds over the Tasman Sea, New Zealand and the seas to the south of the country are “blobs” of air which is rising (or has recently risen) convectively.

MTSAT-1R infra-red satellite image for midnight Saturday 09 July 2011. Image courtesy Japan Meteorological Agency.

The Jet

A major flood of air out of the Antarctic region, like this, has other consequences. The northern boundary of the cold air pushes against the warmer air further north. Thus, the north-south temperature contrast increases and simultaneously the strength of the westerly winds increases – not just at the Earth’s surface, but throughout the depth of the troposphere. (How this works might be the subject of a future blog post). This is at the heart of why many places have been windy over the last few days.

The axis of the strongest winds, in the mid- to upper troposphere, is known as the jet stream. In the plot below (for midnight Saturday 09 July), the polar jet has two branches (arrows in black): one curving across the south Tasman Sea and over the South Island, and the other crossing the south of the North Island. Over central New Zealand, wind speeds above 10,000ft were generally 80 kt (150 km/hr) or more.

Wind speed at 500 hPa (approximately 18,000 ft) in the New Zealand region at midnight Saturday 09 July 2011. Warmer colours are stronger winds. Data courtesy European Centre for Medium-range Weather Forecasting.

Thunderstorms and Tornadoes

Air ascending into convective clouds (showers and thunderstorms) comes down again. When a convective cloud collapses, the resulting downdraft hits the Earth’s surface and spreads out, much like water does when tipped from a bucket onto the ground. When the showers and thunderstorms are themselves fast-moving (on the afternoon of Saturday 09 July, storm motions on the Kapiti Coast were 70 to 90 km/hr), the winds near the Earth’s surface can become very strong when downdrafts occur: this is very likely to be the cause of some of the wind damage on the Kapiti Coast on the afternoon of Saturday 09 July. And this is one of the reasons why the various Severe Thunderstorm Outlooks, Watches and Warnings issued over the last few days have included the mention of damaging wind gusts.

Imagery (see below) from the Wellington radar for 4:00pm Saturday 09 July shows a line of thunderstorms extending across Cook Strait onto the Kapiti Coast. The tornado is very likely to have been associated with the strong thunderstorm shown just east of Waikanae at 4:00pm; eye witness reports suggest that the tornado crossed State Highway 1 around 3:55pm. The tornado’s genesis remains unclear: it may be that a low-level vortex was “spun off” the northern end of Kapiti Island just at the time that this thunderstorm passed by, and the strong ascending motion in the thunderstorm developed it into a tornado between there and landfall on the Kapiti Coast. Once again, it is remarkable that there was no loss of life.

Reflectivity image from the Wellington radar, 4pm Saturday 09 July 2011. Colours represent how strongly precipitation bounces the radar signal back to the radar.

Snow

The Southern Alps are a significant barrier: in westerly airstreams, generally only a moderate amount of precipitation falls any distance east of the Divide. How much precipitation falls east of the Divide, and how far east of the Divide it reaches, depends on a number of factors. Not the least of these is the Foehn Effect. Overnight Saturday 09 July and on the morning of Sunday 10 July, a reasonable amount of snow fell east of the Divide, in parts of Otago, to below 500 metres, in a northwesterly airstream. This is an uncommon occurrence, and reflects how deeply cold and showery the air passing across Otago was at the time.

Large Sea Waves

Since about the middle of the first week of July, sea waves arriving on New Zealand’s western coasts have been notably large. In some places, they have probably attained heights observed only once every year or two – and will remain high until late in the week ending Fri-15-Jul. The weather map below, from about the time large waves began arriving, shows why:

  • The fetch – that is, the expanse of ocean over which waves arriving on New Zealand’s western coasts have been generated (pink arrow) – is very long
  • The waves are still growing as they reach New Zealand’s western coasts, because the winds across New Zealand are themselves strong.

MetService mean sea level analysis for midnight Thursday 07 July 2011. The red arrow shows the approximate path sea waves arriving on New Zealand's western coasts have travelled.

Winds Aloft

In my previous blog post I wrote about how the winds high in the sky differ from the winds we usually experience on the surface of the Earth.
 
We can get an appreciation of the winds aloft by looking at a loop of images from a geostationary satellite. The Japan Meteorological Agency launched the satellite we use most in New Zealand, and it orbits the earth at an altitude of 35,000 kilometres above Papua New Guinea. It’s specially configured to move in an orbit synchronized with the rotation of the Earth, hence the name geostationary (meaning Earth-stationary).
 
Take a look at this loop of satellite pictures for the week 28 July to 4 August 2008:

 

The images are similar to what you see on the Maps & Rain Radar page (by selecting Satellite Imagery), TV or newspaper, but with added colour. Firstly, you must be aware that the satellite is “seeing” in the infra-red, like wearing special night-goggles to look at the Earth. We’ve then applied a colour enhancement to highlight what’s happening. With this particular enhancement, cold surfaces are shown as light grey or white and warm surfaces are a darker grey or black; very cold surfaces, typically clouds high in the sky, are coloured red (-50 C) or green (-60 C); there are even a few patches of blue at -70 C, indicating extremely cold cloud that is high above the Earth.

We can learn so much by looking at a loop such as this!
 
First let’s look at the area across central Australia. Over the dark background (the warmer Australian landmass) there are bits of white and red cloud high up above the land, caught up in a very strong westerly jet-stream of air that’s racing towards the Tasman Sea and New Zealand. A weather balloon flight at a height of 10 km above Wagga Wagga (New South Wales) reported a wind of 356 km/hr around the time of the middle of the loop (1 August 2008). Winds over Sydney peaked at 302 km/hr. You almost never get winds that strong near the Earth’s surface.
 
Over the Top End of Australia there are filaments of high cirrus that originated near the equator and are moving towards the jet-stream. A bit farther down to the south there are the comma-shaped clouds of lows, plus intervening highs, that we see on weather maps trundling through our patch of the Earth, the mid-latitudes.
 
If you look at the loop several times, you’ll notice patches of dark grey cloud (near the ground or sea) swirling around in quite different directions (and speeds) from the white or coloured cloud higher up.
 
As an interesting aside, notice how the Australian landmass seems to pulse from dark to light each day? This happens as the land warms from the sun in the daytime (dark shading) and cools at night (light grey shading). The effect is strongest over the southern states.
 
The jet-stream isn’t apparent at the very top and bottom of the images, and this is quite typical. Near the equator and near the poles the winds aloft are not as strong as in-between.
 
I think this loop gives us insight into why the wind is different high in the sky. I’ll continue this thread in a later post.

Mt Taranaki Kármán Vortex street

MetService weather forecasters naturally spend a lot of time looking at satellite imagery and every so often are treated to some fascinating cloud patterns in the airflows around New Zealand. One pattern I’ve always liked seeing is the Kármán Vortex street, most frequently observed near our shores to the west of the North Island, generated by Mt Taranaki in a south to southeast flow.

Put simply, a Kármán Vortex street is a series of vortices (or eddies) generated in the flow past an obstacle. When wind, cloud and stability conditions combine “just right” over the west of the North Island, the result can be a spiral pattern in the cloud moving away from Mt Taranaki. Yesterday (Sunday, 21 June) we saw an example of this:

Kármán Vortex street west of the North Island, 10:15am 21 June 2009.<br /> (Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Kármán Vortex street west of the North Island, 21 June 2009 (Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

For more information about Kármán Vortices, head over to this Wikipedia page where you’ll also find this nice little animation demonstrating the phenomenon:

Karman Vortex street animation

Kármán Vortex street animation (image courtesy Cesareo de La Rosa Siqueira, via Wikipedia)

Note in the animation above that successive vortices are spun off each side of the obstacle and then move downstream in the flow (left to right in this case).

To help visualise the large scale weather pattern yesterday, here’s the analysis map from midday showing a high southwest of the South Island and a generally southerly flow over the country:

MSL analysis, midday 21 June 2009.

MSL analysis, midday 21 June 2009.

And to dive further into the situation, here’s a “QuikSCAT” image from earlier yesterday morning showing wind barbs over the ocean – depicting the strength (in knots) and direction of the sea-surface winds downstream (north and northwest) of Mt Taranaki:

QuikSCAT image showing surface winds. (Image from NOAA/NESDIS)

QuikSCAT image showing surface winds. (Image from NOAA/NESDIS)

While these winds were measured around two hours before the satellite image the situation didn’t change much so will be representative of the low level wind flow resulting in the Kármán Vortex street. In this case the surface wind downstream of Mt Taranaki was south-southeast at around 20 to 25 knots.

For some more dramatic examples of cloud vortices, head over to the MODIS Rapid Response System website where their handpicked gallery features a number of vortex images.

Canterbury Snow, 10 May 2009

With clear skies over most of Canterbury on Monday, we got a good look at the fresh snow that fell on Sunday (10th May).  Here’s the view late Monday morning (around 10:30am) from NASA’s Earth Observing System Terra Satellite,

2009-05-10-2225

Fresh snow on the Alps and Canterbury foothills - Monday 11 May 2009. (Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Based on the coverage in that image and reports from snow observers, the bulk of the snow in South Canterbury fell above about 300 metres, although some places lower down, especially near the foothills, may still have had light snow that didn’t settle appreciably.

While this wasn’t the first cold outbreak of the year, Sunday’s snow event over the lower South Island (including Fiordland, Southland and Otago) was certainly the most significant of 2009 to date.  If you have any tales of how you were affected that you’d like to share, please feel free to leave a comment below.