21 August – Anniversary of Meteorological Service in NZ

On 21 August in 1861, Dr. Charles Knight was appointed the first Director of Meteorological Stations in New Zealand.

His appointment marked the founding of the New Zealand Meteorological Service – this country’s oldest continuous scientific institution. Early missionaries and settlers quickly realised our coasts were subject to rapid changes of weather, with frequent violent storms. In the 1840s and 1850s weather studies were made by military officers, but in 1859 the government put weather stations on a more formal footing. Ten permanent meteorological stations were established in 1861 and Dr. Charles Knight was appointed to control the new service.  By 1867, 26 telegraph stations were exchanging regular weather reports. In May 1874, after a spate of shipwrecks, a storm warning system was established by Commander R. Edwin RN (retd) and the first public weather forecasts were issued.

1930s

Forecasting remained a marine service until 1936 when it was attached to the newly formed Department of Scientific and Industrial Research. During World War II weather forecasting became the responsibility of the Royal New Zealand Airforce.  After the war, the weather forecasting service became a semi-autonomous branch of the Air Department.  In 1964, the passing of the Civil Aviation Act meant that the Service became one component of a new Civil Aviation Department.  A few years later, the passing of the Ministry of Transport Act 1968 meant that the Service was included in the then new Ministry of Transport.

In 1991 there was a restructuring of New Zealand’s scientific institutions.  As a result, on 1 July 1992, the New Zealand Meteorological Service was split into research and forecasting.  The research component joined the National Institute for Water and Atmospheric Research (NIWA), a crown research institute specialising in scientific management of New Zealand’s atmospheric, marine and freshwater systems.  The forecasting component became MetService, a state owned enterprise specialising in operational meteorology.

Today, weather forecasters have access to up-to-the-minute reports from many dedicated weather reporting stations throughout New Zealand and various offshore islands.  Along with satellite images, weather radar and regular output from several global computer models, forecasters produce a wide range of weather forecasts and warnings supporting the various sectors of the New Zealand economy.

Acknowledgments:

“On this day: NZ Historic Moments”, by Lorain Day and Tim Plant (2002, Reed).

“Sails to Satellites: a history of meteorology in New Zealand”, by John de Lisle (1986.New Zealand Meteorological Service).


The Thunderstorm in History

One of the pleasures of reading history is coming across stories about the weather. Thunderstorms often figure in these. One of the most dramatic examples was recorded in the sixth century AD, by Gregory, Bishop of Tours, in his Historia Francorum (The History of the Franks).

In AD 536 there were three rulers of Frankish kingdoms: Childebert, the king of Paris; his brother Lothar, the king of Soissons; and the brother’s nephew Theudebert, the king of Metz. Childebert and Theudebert joined forces and set out with a large army to attack Lothar, who retreated to a fortified position on a hilltop. Hearing of the imminent battle, Queen Clothild, mother of Childebert and Lothar, went to the tomb of Saint Martin and prayed through the night for divine intervention to prevent her sons fighting.

The next morning, before battle preparations had been completed, a terrific thunderstorm laid waste to the aggressor’s camp. Tents were blown down, gear was scattered and horses driven away by hail and lightning. The hailstones were so large and pelted down with such force that many soldiers, including the two kings, were cut by them, driven to the ground and forced to shelter beneath their shields. Meanwhile, Lothar and his army were untouched by the thunderstorm. Accepting the event as divine chastisement, Childebert and Thuedebert did penance to God begging forgiveness for attacking their own kith and kin, then sued for peace and concord, which Lothar granted. Lothar’s dynasty prospered, leading eventually to the unification of France and the rule of Charlemagne.

The role of weather is also given a prominent place in The Oxford History of the French Revolution by William Doyle. Repeated drought during the 1780s caused soaring grain prices leading to repeated civil disturbances in many parts of France. Then, in July 1788, on the eve of the harvest, widespread hail storms devastated hundreds of square kilometres of crops in the Paris Basin, which was one of the most productive agricultural areas in France. Hailstones were so large they killed men and animals. The inability to gather tax revenue on the destroyed harvest bankrupted the French Government and the price of grain rose to almost 90% of a workers salary.

In order to try to gather tax from the nobility, who were largely exempt, the French King was forced to call the Estates General for the first time in over a hundred years. Once assembled the Estates General moved beyond the King’s control, passing laws he neither wanted nor anticipated. Within the year, the struggle for power escalated into violence, the Bastille was stormed, and the French Revolution was underway.

More intriguing is the story of Martin Luther and the thunderstorm. Having completed a masters degree and a visit home to his parents, Martin Luther was returning to University in Erfurt to study law when, on July 2 1505, near Stotterheim, he was caught in a thunderstorm. Thrown to the ground by a lightning bolt striking near him, he called out to St Anne, promising to become a monk if his life was spared. Two weeks later he abandoned law studies and entered a monastary, starting down a path that eventually changed European history for ever, splitting the church and triggering decades of war.

Told this way, there is a hint of myth about the story. In fact, Martin Luther seems not to have been too keen on a law career and to have been thinking about joining the church anyway, but this was bitterly opposed by his father. Perhaps the weather provided Martin Luther with an alibi. “ Sorry Dad, a thunderstorm made me do it.”

How Lows and Highs move

In my blog post about winds aloft there is a loop of satellite images for a week in winter 2008.  It shows that the big cloud features in the mid-latitudes typically travel from west to east. In other words, the features you see on weather maps affecting New Zealand have usually started out roughly in the area of southern Australia.

There are exceptions to this, and a notable one that comes to mind is when tropical cyclones or sub-tropical depressions move southwards onto the Tasman Sea/New Zealand area. A couple of famous examples are tropical cyclone Giselle in April 1968 (the “Wahine” Storm that devastated Wellington and many other parts of NZ) and tropical cyclone Bola in March 1988 (that badly flooded the Gisborne region).

Sometimes, too, the Highs or Lows seem to grind to a halt and this is another variation on the westerly pattern. But despite these exceptions it is true that most of our weather does come from the west. This result also relates to the “red sky at night” saying that Ross Marsden discussed a couple of months ago.

I’d like to present you with another interesting aspect to this typical movement, and if you’ve looked at a lot of sequences of weather maps you may have spotted it. Have you ever noticed that the depressions (“L”s) usually drift slightly southwards within the westerly pattern? Conversely, anticyclones (“H”s) usually drift slightly northwards within the westerlies.

I’ve put together an animated sequence of weather maps for you to see for yourself. There’s some jumpiness due to small scale effects, but I hope you can pick up the general trend.

 

The maps are Mean Sea Level analyses (fronts omitted) from 14 to 20 August 2009.

There are exceptions; e.g., small depressions that wrap around bigger ones and therefore take a path like a loop around the larger Low. But the drift I’ve mentioned does generally occur.

The first and best explanation I have found of this effect comes from a brilliant Englishman called Reginald Sutcliffe. He lived in the middle of the 1900s and, as well as being an active researcher, applied his scientific nous during the second World War. You can find out more about him on the internet; e.g., on Wikipedia.  He died in 1991, and I would have loved to have had the privilege of meeting him.

The slight drift comes about by first noting how a developing depression in the middle latitudes has colder air on its western side and warmer air to the east. You’re probably already familiar with this notion, since it explains why we tend to have milder weather as depressions approach us (when we are on the eastern side of the L) and colder weather as they move away (when we are on the western side).

Sutcliffe showed that this typical temperature pattern steers the depression towards the southeast - the precise direction depends on just how much cold air there is relative to warm air (which can also be deduced). For an anticyclone, the converse applies and it is steered towards the northeast.

Of course Sutcliffe’s research was originally based in the northern hemisphere, but the result is just as valid in the southern hemisphere which is how I’ve stated it here by applying a mirror image of the effect. With his explanation, Reginald Sutcliffe devised one of the first objective methods for weather forecasting.

Many of the depressions that affect NZ form near the New South Wales coast. This typical drift within the westerlies takes the developing systems on a direct path towards NZ!

As with the example in my post about maths and meteorology, the steering result can be demonstrated using equations. But the beauty and simplicity of it can also be observed on most sequences of weather maps.

The mid-July northern low

On the night of 17th July and early on the 18th, New Zealand was affected by a fast-moving and rapidly deepening depression originating in the north Tasman Sea. Sustained southwesterly winds of more than 60 knots were recorded in Colville Channel as the low passed by. Severe Weather Warnings were issued for wind in Coromandel/Great Barrier Island and rain in the eastern North Island.

This post will have a look at some of the reasons that the low deepened so rapidly, and whether the computer models did a good job of predicting its path. We’ll also see that weather is a lot more complicated than simply following what happens at the surface only.

MODIS imagery of the low as it moved away from the country on Saturday 18th July
MODIS imagery of the low as it moved away from the country on Saturday 18th July

On Friday 17th July at 6PM, a low was analysed just west of Northland with a central pressure of about 997 hPa.

24 hours later the low lay a few hundred kilometres east of the North Island and had deepened to around 975 hPa. It continued deepening as it headed southeastwards – New Zealand was only affected by the first stages of the rapid development.

Midday 17th July
Midday 17th July – a “slack low”
Midnight 17th July
Midnight 17th July – 12 hours later the low has quickly evolved

The rapid development of this low cannot be adequately explained by considering surface conditions only. Chris Webster here states “Per­haps we inad­ver­tently rein­force a per­cep­tion that our weather is 2-D by pub­lish­ing lots of weather maps that are valid only at the Earth’s sur­face (e.g., see Weather Maps). Rarely do we show what’s hap­pen­ing higher up through the atmos­phere or, more pre­cisely, the tro­pos­phere —?the part of the atmos­phere that con­tains our weather.”

Our low was in fact strongly influenced by what was happening at the highest reaches of the troposphere – the regions where jet streams tend to be present.

Upper levels

ECMWF forecast for 3PM on the 17th. Green contours are surface isobars. Shaded regions are those of high upper level winds (black being the strongest).

ECMWF forecast for 3PM on the 17th. Green contours are surface isobars. Shaded regions are those of high upper level winds (black being the strongest).

This forecast chart is entirely computer generated, but shows why the low began organising itself and deepening from the Friday evening. The surface low pressure centre lies near the equator-ward entrance region of an upper level jet stream. This is a particularly favourable position for cyclogenesis (meaning development of a low). This is because it is an area of upper level divergence, which favours convergence at the surface and upward motion  – both of which are conducive to developing depressions.
The other favourable position is the pole-ward exit region. Where these coincide (the exit of one jet, and the entrance of another), and where a low lies downstream from a sharpening upper trough then a recipe exists for “explosive cyclogenesis”:

uppertrough

Here are the comments made by senior forecasters regarding the development of this low:

“A sharpening upper trough over the Tasman Sea with double jet structure should provide strong upper level divergence for the low developing there.”

“Deepening caused by very strong divergence and vorticity associated with a broad very strong subtropical jet with winds reported 150-200 kt by aircraft”

…………………………………………………………………………………………………………………

The actual behaviour of the low

06z18z

21z00z

Analysis charts above, drawn every 1 hPa, show the actual track of the low as analysed by a senior meteorologist. Chart times are 6PM 17th (top left); 6AM 18th (top right); 9AM 18th (bottom left) and Noon 18th (bottom right).

Contrast the 6AM analysis with these forecast model prognoses for that same time:

UKMO Unified Model and ECMWF IFS model output - runtime midnight 15th July

Surface pressure contours. UKMO Unified Model (red) and ECMWF IFS model (blue) output for 6AM 18th July. Model runtime was midnight 15th July

Analysis for 6AM on the 18th July

Surface analysis for 6AM on the 18th July

There was considerable disagreement between models as to the track and depth of this low,  and, as suggested by the analysis, no single model got it quite right.
Even though computer models are becoming ever more sophisticated, it’s important not to slavishly follow their predictions. As we have seen with this fairly brief event, it’s crucial – especially for high-impact weather events – to have professional meteorologists monitoring the situation and applying their judgement and conceptual knowledge of meteorology to the output of the models. This is something that is not likely to change in the future.