Monthly Archives: March 2011

The Eye of the Storm

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Here’s a couple of close-ups of Tropical Cyclone Bune as it passed near Raoul Island overnight Monday.

TC Bune, 0600UTC Mon-26-Mar 2011

7:00pm Monday 28 March 2011. Tropical Cyclone Bune was about 80km north of Raoul. Infra-red satellite imagery courtesy Japan Meteorological Agency.

TC Bune, 1200UTC Mon-26-Mar 2011

1:00am Tuesday 29 March 2011. Tropical Cyclone Bune was about 170km east of Raoul. Infra-red satellite imagery courtesy Japan Meteorological Agency.

The centre of Bune has passed a few tens of kilometres to the east of Raoul. The pressure at Raoul “bottomed out” at 972.5 hPa at 11:00pm Monday; at this time, Bune’s central pressure was around 967 hPa.

108 mm of rain was recorded at Raoul between midday Monday 28 March and 6:00am Tuesday 29 March. Most of this (86 mm) fell between 4:00pm and 8:00pm Monday, as Bune’s main feeder band (for more on feeder bands, see Bob McDavitt’s blog on tropical cyclone Yasi), wrapped around the south of its centre, passed across the island.

The graph below shows the hourly rainfall and mean sea level pressure from 9:00am Monday 28 March to 7:00am Tuesday 29 March.

Raoul MSLP and Rainfall

Hourly rainfall (red bars) and mean sea level pressure (blue line) at Raoul Island, 9:00am Monday 28 March to 7:00am Tuesday 29 March.

So … how windy did it get? The graph below shows mean wind and gust speed at Raoul from 9:00am Monday 28 March to 7:00am Tuesday 29 March. The wind was from the eastsoutheast through to about 10:00pm Monday, with mean speed increasing during the afternoon and early evening from about 15kt (about 30km/hr) to 39kt (about 70km/hr). Wind speed decreased after about 7:00pm Monday, with the winds becoming fairly light late in the evening when Bune passed closest to Raoul (and when, as noted above, Raoul’s pressure reached its minimum of 972.5hPa). This period of light winds late Monday evening also coincided with the cessation of the heavy rain: in other words, the feeder band has passed across the island, and Raoul was close to the eye for a few hours.

Mean wind speed (knots; blue line) and gust speed (knots; red line) at Raoul Island, 9:00am Monday 28 March to 7:00am Tuesday 29 March

After about 11:00pm Monday, when Bune was moving away from its point of closest approach to Raoul, the wind at Raoul tried to blow from the southeast, but was all over the place for a few hours, as shown in the table below. Of particular interest are the gust speeds: they are generally much larger than might be expected. Sometime in the hour between midnight and 1:00am, when the mean wind speed was a shade over 20 knots (about 35km/hr), there was a gust of 87 knots (about 160km/hr). Wow.

Time (NZDT) Wind Direction (degrees true) Mean Speed Gust Speed
11:00pm Monday 230 05 21
Midnight Monday 170 20 53
1:00am Tuesday 130 24 87
2:00am Tuesday 020 14 47
3:00am Tuesday 140 27 66
4:00am Tuesday 130 09 57
5:00am Tuesday 300 10 36
6:00am Tuesday 270 10 31
7:00am Tuesday 360 06 29

 

Bune is now undergoing extra-tropical transition. The next high seas warning on it, to be issued soon after 1:00pm Tuesday 29 March, will either be the last describing it as Tropical Cyclone Bune or the first describing it as “low, former tropical Cyclone Bune”.

As Bune moves further away from the tropics and takes on the characteristics of a mid-latitude depression, the area of gales and storms around it is expanding. It still remains a significant hazard to ships at sea. As stated in this MetService news release yesterday, ” … cyclone swells are expected to start arriving on  eastern shores from Northland to western Bay of Plenty, and in Gisborne and  Hawke’s Bay, on Tuesday.  On Wednesday and possibly also Thursday as well,  waves will probably be large enough in some places to pose a danger to  swimmers, cause coastal erosion and test the soundness of moorings”.

World Met Day 2011

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23rd March is World Meteorological Day.

Each year meteorologists around the world celebrate a chosen theme together to commemorate the anniversary of the founding of the World Meteorological Organization (WMO) on this day in 1950.

The theme this year is “Climate for You”.

[Click on the diagram to link to the WMO ‘Climate for You’ web site - where available information includes a brochure (2.7MB) and poster (0.4MB).]

WMO held an important Climate conference at Beijing in November 2005 — the WMO Technical Conference on Climate as a Resource — to consider how climate can be used as a resource, and how best to make climate data available to benefit society by improving economic decision-making.  These data are important for they shape the availability of natural and renewable energy resources.  A knowledge of rain and temperature trends is required to optimise agricultural performance, water management and food security.

The WMO Conference on Living with Climate Variability and Change: Understanding the uncertainties and managing the risks (held in Espoo, Finland in July 2006) underscored that while climate is indeed a critical resource, we are also especially vulnerable to its variability and change.  Some actions need be taken urgently to manage the risks of climate variability and change impacts.  Others should be increasingly implemented to harvest the benefits to be derived from climate information and services, by helping socio-economic sectors maximise their efficiency and productivity.

In March 2007, WMO organised in Madrid the International Conference on Secure and Sustainable Living: Social and Economic Benefits of Weather, Climate and Water Services, which provided an outstanding opportunity for a wide exchange of views, expectations and knowledge across various societal sectors to optimise the decision-making process, and formed the Madrid Action Plan.

Moreover, 2007 was the year when the WMO co-sponsored IPCC, released its fourth Assessment Report and received the prestigious Nobel Peace Prize, as well as when the fifteenth World Meteorological Congress agreed to convene with partners at a World Climate Conference-3, or WCC-3, in the spirit of the previous two historic World Climate Conferences which WMO had organised in 1979 and 1990.

Not long before the WCC-3 began, the WMO Executive Council decided at its 61st session in June 2009 that the theme of the World Meteorological Day for 2011, commemorating the coming into force of the WMO Convention on 23 March 1950, would be “Climate for You”.

At WCC-3, a High Level Taskforce was mandated to prepare a report which will be among the key issues to be considered by WMO Members during the sixteenth World Meteorological Congress, to be held at Geneva in May 2011.  This report includes a proposal for establishing a Global Framework for Climate Services, or GFCS.  The New Zealand delegation to Congress will be led by Dr. Neil Gordon from MetService, who is the Permanent Representative of New Zealand with WMO.

Dr. Neil Gordon from MetService, the Permanent Representative of NZ with WMO.

As WMO has recently reported, 2010 ranked together with 1998 and 2005 as the warmest year on record, which only confirms the observed long-term warming trend highlighted by the IPCC report.  All of the ten warmest years on record were experienced since 1998.  Additionally, over the ten years elapsed since 2001, global temperatures averaged almost half a degree above the 1961-1990 mean, the highest ever recorded for any 10-year period since the beginning of instrumental climate observations.

WMO activities in the area of climate are focused on human safety and well-being and the realisation of economic benefits for all.   This is in line with the spirit of the WMO Convention which came into force on 23 March, sixty one years ago, as well as the patrimony of the former International Meteorological Organization IMO established by the First International Meteorological Congress in Vienna, Sept 1873.

Please join with the meteorological community around the world in a round of applause and a toast to celebrating World Meteorological Day 2011.

————————————————————————————————————————————————————————————

The above text is based on a special message from Michel Jarraud, Secretary-General of WMO,  to mark World Meteorological Day 2011.

Michel Jarraud, Secretary-General of WMO

 

Information about WMO activities related to climate is available at: http://www.wmo.int/pages/themes/climate/index_en.php .

The WMO online youth corner, created recently, also includes a section on climate relevant to the theme of World Meteorological Day 2011: http://www.wmo.int/youth/climate_en.html

 

Storm Surge

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On 23 January 2011, widespread flooding affected places from Wanganui to Hawke’s Bay northwards. Coastal sea inundation affected several parts of Auckland including Queen Street, Tamaki Drive, the northwest motorway, Herald Island, Maraetai and Miranda (western side of Firth of Thames).

SH16 Auckland NW motorway city-bound buslane, Sun 23 Jan 2011. Photo courtesy of Peter Mitchell, Auckland Motorways.

This was a case of storm surge associated with a passing low pressure system. The system formed in the tropics around New Caledonia, but left the tropics before it had time to deepen into a tropical cyclone.


Weather map for noon on Sunday 23 January 2011. The region marked DQ is explained in the text below. Click on the map for a loop of animated maps in 12 hour time steps covering Sun-Mon 23-24 January 2011.

Storm surge is the name given to the situation when the sea floods inland along the coast. It has three components.

1. Wind setup.  When strong winds blow from the sea to the land (“onshore”), a wind setup is formed: under some circumstances, the sea is pushed onto the land faster than it can drain away, and waves penetrate beyond the high water mark. Wind setup depends on the size and shape of the strongest wind zone and the land it encounters. It is accentuated in shallow basins such as the Firth of Thames.

Strong winds are often associated with low pressure systems. Storm movement – that is, the movement of the low pressure system itself – influences the wind strength.

In the Southern Hemisphere, where the wind flow around a low is clockwise, storm movement adds to wind strength on the left side of the storm track.  For a low travelling southwards, this means that winds are stronger in its eastern semicircle.  This is why many sailors call this the dangerous semicircle and its front-left quadrant the dangerous quadrant.

On 23 January, when the low on the weather map above approached Northland, its dangerous quadrant (labelled DQ on the map above) brought the most pronounced onshore winds and highest wind setup to the Coromandel – Bay of Plenty area. NIWA measured storm surges of 370 mm around Coromandel and 590 mm at Moturiki Island near Tauranga.

2. Low air pressure. The inverse-barometer effect occurs whenever air pressure over the ocean differs from normal. Each hectoPascal of air pressure below the norm (of 1013 hPa) may raise the sea level by ten millimetres. So, a large low pressure system is accompanied by a dome of elevated  sea surface.

On 23 January, when the pressure dropped to 986 hPa around Auckland, the maximum inverse barometer effect was around 270mm.

3.  The tide comes and goes around once every 12 hours 20 minutes. The tidal range – that is, the difference in height between low and high tide – is largest a few days after the moon reaches its perigee (closest point of its orbit to earth) within a day or so of reaching a full or new phase. This happens only a few times per year,  and is sometimes called a King tide.

Auckland has a reasonably large tidal range of around 3 m. On 23 January the high tide height was 3.5m – only about 100mm lower than a King tide.

Strong onshore winds from the tropics often produce heavy rain on any hills and ranges. If this drains off quickly (as is usually the case in New Zealand) and arrives at the coast at the same time as a high tide,  the chances of flooding near river mouths and estuaries is increased.

Auckland/ mm/23 Jan ~noon ~midnight
1. Wind setup

449

270

2. Inverse barometer

 180 250
3. Astronomical tide

3500

3400
Total =  measured storm tide 4129 3920

Auckland’s tide gauge showed a peak of 4.129m between 11am and noon on 23 Jan 2011.  Data are courtesy of Ports of Auckland.

When Tropical cyclone Yasi made landfall onto Queensland,  Townsville (with a two-metre tidal range) was in the dangerous quadrant.  Yasi’s two-metre storm surge arrived on the outgoing tide, reducing its impact.  The highest-ever  measured storm surge was 8.5 metres at St Louis Bay in Hurricane Katrina (2005).  The worst storm surge, in terms of loss of life, was the Bhola cyclone which hit East Pakistan (now Bangladesh) on 12 Nov 1970, here an estimated 500,000 people perished.

The online nautical almanac published by Land Information New Zealand explains more about how tidal predictions only give the astronomical component,  and how strong meteorological effects need to be taken into account.  During a period of King tides, if there is an anticyclone around or strong offshore winds, extra care is needed around rocks, reefs and sandbars, as the actual water level may drop to near or below chart datum – which is the level that charted depths displayed on a nautical chart are measured from.  Each nautical chart carries a note defining its chart datum and most New Zealand nautical charts use the lowest astronomical tide computed over the period of one entire SAROS cycle.

The  next  King tides are on 21-22 March and 19-20 April.

NEW dust graphs for Christchurch city

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Over the past couple of days we’ve been working on an easy-to-understand indicator of the risk of wind-driven dust clouds in and around Christchurch city. To our knowledge, no-one’s tried this before.

These ‘dust graphs’ are now live on metservice.com so that the people of Christchurch, emergency services, lifelines and others have more information to help them deal with the very trying circumstances they continue to face.

As these graphs are a totally new feature on our website, please bear with us while we monitor them over the weekend for any technical problems. This shouldn’t affect any other information on metservice.com.

Besides wind speed, there are many factors that influence how much dust gets into the air and how long it stays there. Most of these are not easily quantified; however, we can identify (a) when the wind gets strong enough to raise dust (gusts of about 28km/h) and (b) when rain is likely.

The new dust graphs show hourly forecasts, for the next 24 hours, of:

Wind (10m) gust speed
Wind (75m) direction
Rainfall

… for the following locations:

Christchurch City
Lyttelton
New Brighton

We hope you find this new information useful and we look forward to your feedback to @MetService on Twitter or to MetService New Zealand on Facebook.

FitzRoy: Inventor of the weather forecast

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This article was originally published in New Zealand Geographic, issue 99 September-October 2009 

Robert FitzRoy is famous as captain of the Beagle on the voyage when Darwin made his discoveries, although many New Zealanders also know FitzRoy as Governor of New Zealand before George Grey. But to meteorologists, FitzRoy is famous as one of the pioneers of weather forecasting; indeed, FitzRoy coined the term “weather forecasting”. He was also a superb navigator and surveyor whose charts of South America were still in use more than 100 years after he made them. 

FitzRoy’s career got off to a strong start at 14 when he topped his class at the Royal Naval College then, after four years at sea as a midshipman, he became the first candidate ever to pass the Lieutenant’s exam with perfect marks. 

Aged 23, he was given command of the Beagle, assisting Captain King surveying South America. Off the coast of Patagonia, a few months later, hurrying to a rendezvous, FitzRoy ignored the danger signs of a plunging barometer and threatening sky, with fatal consequences. A violent pampero wind caught the Beagle with too much sail and threw the vessel on her side. Topmasts and jib-boom were blown away, along with two sailors, who drowned. 

FitzRoy ordered both anchors dropped which righted the Beagle and pulled her head into the wind, saving her from foundering. Praised for his seamanship in saving the vessel, FitzRoy felt he could have done better in anticipating the sudden onset of the gale. 

The coastline of Tierra del Fuego and Magellan Straits proved so complex that the survey was broken off and the expedition returned to England. The following year, the navy hydrographer, Captain Beaufort (now known for the Beaufort Scale for measuring wind strength) persuaded the Lords of the Admiralty to resume the survey and FitzRoy was ordered back to South America. 

On this voyage, FitzRoy took Charles Darwin as a gentleman naturalist and companion. Constrained from conversing freely with officers under his command, FitzRoy needed someway to mitigate the loneliness of his position on such a long and stressful expedition. In fact, the previous commander of the Beagle had committed suicide after a breakdown. 

The voyage, lasting from 1831 to 1836, was hailed as a triumph and FitzRoy honoured with the Gold Medal of the Royal Geographical Society Medal. But the voyage eventually became famous for finding evidence for the theory of evolution, something that bitterly disappointed FitzRoy late in his life when Darwin’s Origin of the Species was finally published. By that time, FitzRoy’s religious views had hardened into a literal interpretation of the bible and he argued that large animals like dinosaurs had become extinct because the doors of Noah’s Ark were too small. 

In 1841, FitzRoy was elected to parliament then, two years later, offered the position of Governor of New Zealand. Backed with few troops and little money, the job was a poisoned chalice. 

FitzRoy arrived in time to adjudicate the Wairau massacre, finding that the New Zealand Company settlers had no right to try to arrest Te Rauparaha but that Te Rangihaeata was wrong to have executed the prisoners taken in the affray. This infuriated the settlers while Te Rauparaha sent a message that FitzRoy should not trouble to send soldiers to find him at Waikanae as he would be happy to turn up in Wellington with a thousand warriors on any date FitzRoy cared to name. In less than two years, political allies of the New Zealand Company engineered FitzRoy’s recall while the settlers in Nelson burnt him in effigy. 

FitzRoy’s voyage home was notable for a storm he forecast when the ship was anchored for the night in calm weather in Magellan Straits. Both FitzRoy’s barometers were falling fast but the captain ignored his warnings and retired below deck. Eventually, a young officer agreed to put out a heavy anchor on a heavy chain. When the storm struck at 2am, the heavy chain broke but tangled around the lighter chain. This was all that saved the ship from crashing into rocks and foundering with the loss of all onboard. 

The chance to develop his ideas on weather forecasting came to FitzRoy in 1854 when he was appointed Meteorological Statist to the Board of Trade. Initially tasked with compiling weather statistics for each part of the globe from ship’s logs, he took advantage of the invention of the telegraph to initiate forecasting. Barometers were distributed around the coast and their readings telegraphed to FitzRoy’s office every morning along with wind and temperature observations. Within hours a forecast would be telegraphed back. If FitzRoy thought a storm imminent, warning symbols of drums and cones were displayed from a mast. 

The storm warnings proved both successful and popular, although not infallible. FitzRoy also sent forecasts to the daily papers. Whereas his storm warnings were displayed as soon as the telegram arrived, the newspaper forecasts usually took 24 hours to appear and their success judged on how well FitzRoy’s more speculative outlook day turned out. 

In 1863, FitzRoy published The Weather Book containing his theories on how wind patterns and storms evolved as well as advice on how to use a barometer and a litany of horror stories of shipwrecks in storms. 

Click to expand: 

FitzRoy’s illustration of the interaction of polar and tropical air masses looks remarkably like a modern satellite photo. From FitzRoy’s The Weather Book: a manual of practical meteorology, published in 1863.

FitzRoy was working 13 days a fortnight to improve his forecasts when criticism came to a head in parliament. FitzRoy had a nervous breakdown and took his own life on 30 April 1865. Born rich, FitzRoy died poor having expended much of his wealth advancing his work. A collection was taken up for the support of his widow and children. Notable among the contributors was Charles Darwin, who praised FitzRoy as ” an ardent friend to all under his sway.” 

The storm warnings issued by FitzRoy’s staff were discontinued but soon reinstated by public demand and remain his greatest legacy. Also testament to his abilities is the number of people who served under him on the Beagle who went on to distinguished careers. 

Today FitzRoy lives on in the name of one of the sea areas in the British marine forecasts, a mountain in Patagonia and numerous streets in New Zealand.